interactions among freshwater macrophytes and …...interactions among freshwater macrophytes and...

Interactions among freshwater macrophytes and

physical-chemical-biological processes in Lake Chini,

Malaysia

Zati Sharip,

B. Chem. Eng. (Hons.) & M.Sc.

Universiti Teknologi Malaysia

This thesis is submitted in fulfilments of the requirement for the degree of

Doctor of Philosophy of The University of Western Australia

School of Environmental Systems Engineering

School of Plant Biology

December 2011

iii

Abstract

Aquatic vascular plants that grow within the littoral and pelagic environment are an

important element of shallow lake and wetland ecosystems. Alteration of the natural

environment, mostly human-induced, is rapidly changing the natural ecological

processes and altering biotic factors including aquatic plant community composition in

wetlands all over the world. This thesis examined the macrophyte communities in Lake

Chini, a shallow floodplain wetland in Malaysia: specifically how physical-chemical and

biological processes were related to plant community dynamics and changes of

macrophyte dominance, from the native floating-leaved species (Nelumbo nucifera) to

non-native submerged macrophytes (Cabomba furcata).

Whole-lake vegetation surveys and statistical models were used to analyse

spatially and temporally the macrophyte community composition and the environmental

variables that influence their structure. The lake can be divided into distinct quasi-

independent sub-basins. Temperature and dissolved oxygen measurements were

conducted in benthic chambers and free water at two separate sub-basins and

mathematical calculations were employed to describe (1) the influence of convective

circulation, driven by horizontal temperature gradient and thermal structure on the

nutrient transport in the system, and (2) oxygen dynamics and differences in primary

production among habitat and macrophyte communities.

Overall, this study found that the macrophyte community in Lake Chini is highly

dynamic with possible alternate states dominated by floating-leaved and submerged

species. The flood regime has a strong effect in controlling the variation in plant

community dominance. Spatial variation in plant community composition was influenced

by total depth, nutrient concentration and substrate. C. furcata appears adapted to

annual floods, and its invasion affected the diversity and plant community composition in

this wetland.

Additionally, both floating-leaved and submerged vegetation contributed to thermal

structure and water exchange dynamics in the system. Weak density-driven flow was

induced by the differential temperature gradient between the open water and the littoral

areas dominated by floating-leaved plants. Additionally, depth variation between the near

littoral zone and the open pelagic region induced physical circulation that could improve

nutrient delivery to the submerged C. furcata bed. In addition to the effects of the

macrophyte, lake physical characteristics such as shape and surrounding topography

iv

influenced the variation of thermal stratification and mixing dynamics between the lake

segments at different water levels.

Oxygen dynamics differed between the two plant communities. Gross primary

production rates and biomass accumulation were higher in C. furcata sites and

ecosystem production contributed to increased carbon fixation in the system. High

consumption of DO by sediment communities and microflora associated with C. furcata

beds increased respiratory activities. An increase in abundance of the invasive

submergent C. furcata throughout the pelagic zone affected net ecosystem production

and biomass accumulation. The potential for shifts in macrophyte dominance from

floating-leaved to submergent species has important implications for the overall

dynamics of the lake.

v

Statement of originality

The research presented in this thesis is an original contribution to the field of limnology,

invasion and vegetation ecology. I hereby declare that all materials presented in this

thesis are original except where due acknowledgement has been given, and have not

been submitted for the award of any other degree or diploma. The body of this thesis

(Chapter 2-4) is presented as a series of papers for journal publication, and some

repetition of the literature review, details of study area and methodology have therefore

been necessary. As the author of all material within this thesis, I am completely

responsible for all data analysis and mathematical calculation, presentation and written

text contained herein. In addition, I performed all vegetation surveys, field

experimentation and secondary data collection.

My supervisors have been included as authors as they provided supervision and

reviewed all manuscripts preparation. The first manuscript has been reviewed and

revised, so the material presented herein incorporates some of the reviewers‟

suggestions. Shon Schooler also contributed in the conceptual design of the vegetation

survey and participated in the first fieldwork in September 2009. Justin Brookes from

University of Adelaide contributed in the configuration, calibration and deployment of

the benthic chamber experiments in April 2010. Matthew Hipsey also contributed in the

calibration of sensors and participated in the field experiments in April 2010.

vii

Acknowledgements

In the name of Allah the most gracious the most merciful. All praise is due to Allah for

giving me the inspiration to start this PhD journey and providing His blessings and

guidance by bestowing all the strength that I need to persevere and finish this thesis.

First and foremost, I am grateful to Malaysian Ministry of Natural Resources (MoNRE)

and Environment and Public Service Department of Malaysia for providing me the

opportunity to do this PhD through the MoNRE PhD and Masters Program Scholarship.

I would like to thank National Hydraulic Research Institute of Malaysia (NAHRIM) for

their full support and funding for all my fieldworks. Thanks to NAHRIM for permitting me

to use their data.

I would like to thank Dr. Shon Schooler, Dr. Matt Hipsey and Prof. Richard Hobbs for

their willingness to accept me as their student and continuously supported me by

reviewing all my work. It was a privilege to learn from experts in their respective fields

and to be able to reconcile both worlds of limnology and ecology. I sincerely appreciate

their trust in me by allowing me a large degree of freedom to define and undertake my

research. I thank Dr. Shon Schooler, who not only provided valuable supervision but

gave me the first insight on invasive species and provided indispensable suggestion

concerning the biological analysis and statistical validity of my research work. I thank

Dr. Matthew Hipsey and Prof. Richard Hobbs, who not only served as coordinating

supervisors and managing my academic supervision arrangements but provided

invaluable guidance and direction on my thesis.

I would like to acknowledge the continuous and unconditional support, love and prayers

I received from my dad and mum, Sharip Mamat and Tejah Husop. Thank you for

providing the moral support and taught me about faith, patience and perseverance

during the difficult periods of my candidature. Thanks to my sister for being a constant

reader of my work. Also thanks to my brother for all his help.

I am grateful to Ir. Dr Salmah Zakaria who has encouraged, inspired and given me the

opportunity to continue my PhD. Also thanks to Juhaimi Jusoh for his personal support

and Ir. Ahmad Jamalluddin Shaaban who has continuously supported my work.

viii

I would like to thank the Economic Planning Unit, Prime Minister‟s Department of

Malaysia and Pahang State Government for providing me the permission to conduct

this research in Lake Chini, Malaysia.

I want to acknowledge the technical support team of National Hydraulic Research

Institute of Malaysia, particularly Ahmad Taqiyuddin Ahmad Zaki for all his coordination

and assistances in the field. Also, the personal effort, in relation to field data collection,

by Shon Schooler, Matthew Hipsey, Juhaimi Jusoh, Justin Brookes, Brendan Busch,

Bashirah Mohd Fazli, Fadhil Mohd Kassim, Farhan, Ab Hadi, Haffiz and Jeyaprakash is

highly appreciated.

I am grateful to Prof. Marti J. Anderson for her statistical advise and Prof. Bob Clarke &

Ray Gorley for providing the multivariate statistic workshop using PRIMER-

PERMANOVA which forms the analysis for the first paper. I thank Dr. Joanne

Edmonston and Dr. Krystyna Haq for their invaluable feedback on the academic writing

of the introduction and conclusion chapters.

I am thankful for many useful discussions with Sutomo, Revalin Herdianto, Zhenlin

Zhang, and Ana Ruibal regarding various statistical analysis and MATLAB simulation.

Thanks to all those students whom I shared office, and for their friendship and

interesting conversation. Thanks also go to the Centre for Ecohydrology for the

administration of my study. Special thanks to Ecological Restoration and Intervention

Ecology Group (ERIE) led by Prof. Richard Hobbs for all the interesting discussion and

presentation on the research and management challenges in dealing with restoration

ecology and invasive species. Appreciation also goes to every individual, who has

helped and supported me, either through provision of papers, data and ideas.

ix

Dedication

This work is dedicated to my dad and mum, Sharip Mamat and Tejah Husop, whose

trust, love and support throughout the years have been unbounded.

xi

Preface

The main body of this thesis is comprised of three chapters, (Chapter 2-5) each of

which is a paper written with the intention to be suitable for publication in peer-reviewed

scientific journals. Each chapter is a stand-alone manuscript, which includes abstract,

independent introduction, literature review, methods, results, discussions and

conclusions sections. In addition, each chapter is independently referenced to

acknowledge the contributions of previous related works. Each paper is presented with

the original internal headings, figures and tables however, for ease of reading and flow,

the formatting in the thesis is uniform. Headings, figures and tables are numbered in

the thesis style.

The introductory Chapter 1 presents the motivation, background and scope of the study

and links the individual publications.

Chapter 2 has been published in Biological Invasion as

Sharip, Z., Schooler, S.S, Hipsey, M.R. and Hobbs, R.J. (2011) „Eutrophication,

agriculture and water level control shift aquatic plants in Lake Chini, Malaysia‟,

Biological Invasions. DOI: 10.1007/s10530-011-0137-1

Chapter 3 is the expanded version of the paper that has been accepted for oral

presentation at the Lake Sustainability Conference 2010 and has been accepted for

publication in International Journal of Design, Nature, and Ecodynamics under the title

„Physical circulation and spatial exchange dynamics in a shallow floodplain wetland‟.

Appendix 2 provided the abridged version of Chapter 3 that has been peer-reviewed

and presented at Lake Sustainability Conference 2010 with the appendix is formatted

according to the original final proof.

Chapter 4 will be submitted for publication in Hydrobiologia under the title „Metabolism

differences among habitat and macrophyte communities in a tropical lake’

The major outcomes of this work are summarized and discussed in Chapter 5, which

include recommendation for future works.

The material presented in this thesis is a synthesis of my own ideas and the work

undertaken by myself in consultation with my supervisors Dr. Shon S. Schooler, Dr.

Matthew R. Hipsey and Prof. Richard J. Hobbs.

Sharip, Z., Interactions among freshwater macrophytes and physical-chemical-biological processes in Lake Chini, Malaysia

xiii

Contents

Abstract .................................................................................................................. iii

Statement of originality .......................................................................................... v

Acknowledgements .............................................................................................. vii

Dedication .............................................................................................................. ix

Preface ................................................................................................................... xi

Contents ............................................................................................................... xiii

Chapter 1 - Introduction ......................................................................................... 1

1.1 Background and motivation ................................................................................... 1

1.2 Lake Chini, Malaysia .............................................................................................. 6

1.3 Research objectives and approach ...................................................................... 7

1.4 Logistical considerations ........................................................................................ 9

1.5 Thesis outline ........................................................................................................10

Chapter 2 - Eutrophication, agriculture and water level control shift aquatic plant

communities from floating-leaved to submerged macrophytes in Lake Chini,

Malaysia ................................................................................................................ 11

2.1 Abstract ..................................................................................................................11

2.2 Introduction ............................................................................................................12

2.2.1 Study Area ...........................................................................................................13

2.2.2 Study organisms .................................................................................................15

2.3 Methods and materials .........................................................................................16

2.3.1 Overview ..............................................................................................................16

2.3.2 Plant community sampling ................................................................................16

2.3.3 Environmental variables ....................................................................................17

2.3.4 Statistical Analysis..............................................................................................17

2.4 Results ....................................................................................................................19


xiv

2.4.1 Catchment and hydrological changes............................................................. 19

2.4.2 Spatial distribution of aquatic plants ............................................................... 20

2.4.3 Environmental variables potentially influencing macrophyte distribution .. 26

2.5 Discussion ............................................................................................................. 29

2.6 Acknowledgement ................................................................................................ 34

Chapter 3 – Physical circulation and spatial exchange dynamics in a shallow

floodplain wetland ................................................................................................36

3.1 Abstract .................................................................................................................. 36

3.2 Introduction ............................................................................................................ 37

3.3 Methods ................................................................................................................. 38

3.3.1 Study site ............................................................................................................. 38

3.3.2 Field measurements .......................................................................................... 39

3.3.3 Analysis of physico-chemical structure and heat budgets calculation ....... 41

3.4 Results ................................................................................................................... 43

3.4.1 Meteorology and physical-chemical structure ............................................... 43

3.4.2 Heat budget and water exchange ................................................................... 51

3.5 Discussion ............................................................................................................. 54

3.5.1 Temperature structure and water exchange .................................................. 54

3.5.2 Implications of physical-chemical coupling to macrophyte dynamics ........ 57

3.6 Conclusion ............................................................................................................. 58

3.7 Acknowledgements .............................................................................................. 58

Chapter 4 – Metabolism differences among habitat and macrophyte communities

in a tropical lake ....................................................................................................59

4.1 Abstract .................................................................................................................. 59

4.2 Introduction ............................................................................................................ 60

4.3 Methods and materials ........................................................................................ 62

4.3.1 Experimental design and water quality measurements ......................... 62


xv

4.3.2 Benthic metabolism (specific processes, small scale) ...........................63

4.3.2.1 Benthic chamber deployments ...............................................................63

4.3.2.2 Estimates of diurnal productivity ............................................................63

4.3.2.3 Estimates of MPB respiration and production ......................................63

4.3.3 Net ecosystem metabolism .........................................................................64

4.3.3.1 Diel oxygen measurements ....................................................................64

4.3.3.2 Estimates of free water metabolism ......................................................64

4.3.4 Data analysis .................................................................................................65

4.4 Results ....................................................................................................................65

4.4.1 Oxygen fluxes and water quality measurement .......................................65

4.4.2 Diurnal productivity .......................................................................................68

4.4.3 MPB and microflora respiration and production ......................................69

4.4.4 Free water productivity ................................................................................70

4.5 Discussion...................................................................................................................71

4.5.1 Oxygen fluxes and water quality dynamic ................................................71

4.5.2 Diurnal productivity .......................................................................................73

4.5.3 MPB and microflora respiration and production ......................................73

4.5.4 Free water productivity ................................................................................74

4.6 Conclusion .............................................................................................................75

4.7 Acknowledgements ..............................................................................................76

Chapter 5 – General discussion and conclusion ................................................ 77

5.1 Summary of research findings ............................................................................77

5.2 Discussion ..............................................................................................................79

5.3 Implications for wetland and lake management ...............................................82

5.4 Original contributions ............................................................................................84

5.5 Study limitations and recommendations for future work .................................85


xvi

5.6 Conclusion ............................................................................................................. 88

References ............................................................................................................89

Appendix 1: Supporting document for Chapter 2 ............................................. 108

Appendix 1a: List of methods for laboratory analysis .............................................. 108

Appendix 1b: Supporting information.......................................................................... 109

Appendix 2: Conference Paper .......................................................................... 112


1

Chapter 1 - Introduction

1.1 Background and motivation

Aquatic macrophytes, including submergent, emergent, floating and floating-leaved

species (Cronk and Fennessy 2001; Schulthorpe 1967), not only characterise shallow

floodplain riparian systems, lakes and wetlands but also play a central functional and

structural role in these endangered ecosystems (Jeppesen et al. 1997; Scheffer 2004).

Macrophytes shape aquatic habitats by affecting physico-chemical properties of the

water, metabolic production, biotic interactions and community structures (Jeppesen et

al. 1997). Worldwide, the composition of many of these endangered ecosystems is

rapidly changing as a result of environmental stressors, mostly human-induced, which

have led to the degradation of the natural environment and altered their resilience to

change. These environmental changes make wetlands particularly more vulnerable to

invasive species.

Research in invasion ecology has shown that the expansion of invasive species can be

a product of environmental changes (Didham et al. 2005; MacDougall and Turkington

2005). Naturalised plants start to produce reproductive offspring in large numbers when

certain environmental requirements are met, and if there are no inhibitory factors they

become invasive (Richardson and Pysek 2006). The presence of invasive species in

aquatic systems affects both ecosystem functioning and abiotic-biotic interactions

(Chapin et al. 1997), that eventually alters community structure and ecosystem function

(Didham et al. 2005; Ehrenfeld 2003; Farrer and Goldberg 2009; MacDougall and

Turkington 2005). The invasive species could also provide competitive pressures that

cause a decline in native species abundance and diversity (Hobbs and Huenneke

1992; MacDougall and Turkington 2005; Zedler and Kercher 2004).

A recent review by Ehrenfeld (2010) has also shown that the impacts of certain

invasive species on ecosystem dynamics are complex and involve multiple interacting

pathways and site-to site differences. Increasingly, research shows that modification of

physical environments by invading species leads to their dominance (Crooks 2002;

Cuddington and Hastings 2004; Farrer and Goldberg 2009). A small number of studies

have also shown that invasive species alter biogeochemistry by modifying soil

environments and nutrient pools (Ehrenfeld 2004; Farrer and Goldberg 2009;


2

Weidenhamer and Callaway 2010) or accumulating litter (Farrer and Goldberg 2009;

Vaccaro et al. 2009).

Wetlands, are particularly susceptible to invasion (Zedler and Rhea 1997). Being the

low-lying component in any catchment, wetlands become a basin for accumulation of

nutrients and sediment entrained in surface run-off from the surrounding catchment,

which contributes to invasibility by the invaders (Zedler and Kercher 2004). Numerous

studies have reported the factors and processes that affect macrophyte composition

and dominance in wetlands in temperate climates (Baldina et al. 1999; Brinson and

Malvarez 2002; Busch and Smith 1995; Coops and Doef 1996; Lougheed et al. 2001;

Nilsson and Svedmark 2002; Tallis 1983). Most studies have shown that macrophyte

composition and dominance is linked to eutrophication (Egertson et al. 2004;

Gunderson 2001; Lougheed et al. 2008), water level regulation (Catford et al. 2011;

Van Geest et al. 2005; Whyte et al. 2008), drought (Greening and Gerritsen 1987),

water level fluctuations (Baldina et al. 1999; Hudon 1997, 2004; Kunii and Maeda

1982), and thermal pollution (Allen and Gorham 1973; Gallup and Hickman 1975).

Other studies have shown that macrophyte composition and dominance can change as

a result of factors such as sedimentation and fertilizers (Kercher and Zedler 2004),

stabilised water levels and phosphorus addition (Boers and Zedler 2008; Herrick and

Wolf 2005), eutrophication and acidification (Roelofs 1983), and eutrophication and

invasion (Kercher et al. 2007).

Less is known, however, about the driving mechanisms and processes that are able to

cause a shift in macrophyte dominance in wetlands in tropical settings. Invasion by

invasive Mimosa pigra have been reported to affect biodiversity in Kakadu National

Park, Australia as a result of high dispersal rates (Cook et al. 1996; Cowie and Werner

1993). Invasion of African grass has also been reported in a tropical freshwater marsh

in La Mancha, Veracruz, Mexico as a result of the species inherent ability to use water

efficiently and produce high amounts of biomass (Rosas et al. 2005). A few invasive

plant species have also caused serious problems in many rivers, reservoir, lakes and

wetlands in Malaysia including Salvinia molesta (Baki et al. 1991; Mansor 1996),

Eichhornia crassipes, Lemna perpusilla, and Pistia stratiotes (Mansor 1996), Hydrilla

verticillata (Baki 2004), and Mimosa pigra (Mansor and Crawley 2011). Massive growth

of these aquatic weeds has been associated to high nutrient concentrations (Mansor

1996), which occur as a result of agriculture and aquaculture activities (Baki 2004).


3

Understanding how ecosystem processes influence macrophytes is complicated by the

fact that macrophytes are highly variable elements within aquatic systems (Carpenter

and Lodge 1986), and differences in site availability, species availability, and species

performance alters vegetation dynamics (Pickett and Cadenasso 2005; Pickett and

McDonnell 1989). The pathways and causal relationships between invasive species

and changes in ecosystem processes and properties, are still poorly understood

(Levine 2003).

Cabomba spp. is one of the macrophytes that have become invasive in many parts of

the world and presents a serious threat to the health of rivers, lakes and wetlands

(Hogsden et al. 2007; Mackey and Swarbrick 1997; Schooler et al. 2009; Wilson et al.

2007; Xiaofeng et al. 2005; Zhang et al. 2003). The ecological impact of the invasion of

this species, if not controlled, includes disruption of navigation and recreational

activities, replacement of native species, reduction of biodiversity and water quality

(Hogsden et al. 2007; Mackey and Swarbrick 1997; Wilson et al. 2007; Zhang et al.

2003). As cost-effective measures to control Cabomba populations are not currently

available (Wilson et al. 2007), further understanding of the ecological niche of this

species is imperative.

There is limited understanding of Cabomba populations in introduced areas, in

particular how this invasive macrophyte affects, and is itself affected, by the area‟s

physical processes and abiotic regimes. There is also limited understanding of the

production rates of Cabomba in introduced areas and how they compare with the

native communities they are displacing. Past studies have shown that Cabomba

populations are species that have high ability to adapt to new environments, are

persistently competitive and able to spread rapidly (Hogsden et al. 2007; Zhang et al.

2003). With the increased in interest in invasion by introduced species, a better

understanding of Cabomba invasion would assist managers in managing this aquatic

plant. The aim of this study is to gain a broader understanding of the mechanisms and

processes driving Cabomba invasion and subsequent shifts in plant community

dominance in an invaded ecosystem, and to ultimately facilitate better management

and restoration of wetland systems.

Studies of species dominance are receiving increasing interest due to their influence on

community assemblages and environmental conditions (Frieswyk et al. 2007;


4

McNaughton and Wolf 1970). Dominance, defined in this context as the most abundant

species in an ecosystem, can exert control over the other species present

(McNaughton and Wolf 1970). MacDougall and Turkington (2005) argues that to

understand species dominance requires an understanding of the connection between

the mechanisms triggering the abundance of the dominant species and the mechanism

limiting the occurrence of other species. Understanding the causes of dominance is

important as dominant species not only influence environmental conditions, but also

affects the physical and biological processes that characterise ecosystem functioning

(Frieswyk et al. 2007).

Numerous physical processes causing transport and mixing in lakes have been well

documented (Imberger and Patterson 1990; Imboden and Wuest 1995; Socolofsky and

Jirka 2004). However, studies that examine how biological-physical coupling or

chemical-physical coupling relate to aquatic macrophytes are limited. In recent years,

there has been an increasing amount of literature that shows that thermal gradients

drive convective exchanges in lakes, in particular as a result of depth variation (Forrest

2008; Horsch and Stefan 1988; James and Barko 1999; James et al. 1994; Monismith

et al. 1990; Palmarsson and Schladow 2008). Studies by James and Barko (1991)

have highlighted the importance of night time convective circulation in transporting

littoral phosphorus to the pelagic zone. Schladow et al. (2002) have also shown that

vertical penetration of cold-dense littoral water is an important mechanism in

transporting oxygen from the surface of the lake to the bottom.

Vegetation also alters the hydrodynamic motions induced by physical processes and

affects the overall physical circulation of lakes. Floating plants were found to modify the

intrusion of convective motions (Coates and Ferris 1994), and submerged macrophytes

were found to shape the thermal structure (Rooney and Kalff 2000). Macrophyte beds

also strengthen temperature stratification and reduce the mixed layer depth (Herb and

Stefan 2005). Herb and Stefan (2004) have shown that natural convective mixing

during night-time cooling was not affected by the presence of submerged macrophytes,

while wind-driven mixing during the day was significantly attenuated in dense, full-

depth macrophyte beds resulting in higher maximum surface temperatures. Studies of

the influence of natural convection and water exchange on macrophyte species

dominance are rare, and in particular the influence of submerged macrophytes

compared to the influence of floating-leaved plants on this process is largely unknown.


5

Studies on biological processes such plant productivity, which are the essence of plant

physiology, have been very well established (Talling 1961). The majority of plant

productivity measurements have been based on standing crops and biomass

production (Westlake 1963). Since its application to the study of rivers, spring and coral

reefs by Odum in the 1950s (Odum 1956), diel changes in dissolved oxygen (DO)

concentrations have been used to estimate rates of production and respiration in

aquatic ecosystems (Staehr et al. 2010a) in both temperate and tropical regions

(Melack and Fisher 1983; Melack 1982; Talling 1957; Talling 2004). Diel DO changes

have also been used to assess the productivity in the Malaysian lakes, Lake Bera

(Ikusima and Furtado 1982) and Paya Bungor (Yusoff et al. 1984). However, these

studies did not address how the metabolism of the benthic communities is impacted by

a shift in macrophyte assemblages.

As the growth rate of species in communities depends on the rate of their primary

production or metabolism (Westlake 1963), it therefore follows that alteration in species

composition would lead to changes in productivity (Chapin et al. 1997). Lauster et al

(2006) showed metabolic differences between littoral and pelagic habitats when

different species were present. In Hanson et al. (2003), increases in gross primary

production (GPP) and respiration of plants were shown to be influenced by dissolved

organic carbon and nutrient enrichment. However, to date no study has ever been

conducted to assess how the metabolism of invasive plant communities compares to

that of native communities in tropical wetland systems, and this could potentially be an

important mechanism that contributes to their dominance.


6

1.2 Lake Chini, Malaysia

The study site for this research is Lake Chini, a shallow floodplain wetland in the state

of Pahang, Malaysia. The system, divided into twelve small lake sub-basins with a total

surface area of about 2 km2, is the second largest freshwater lake in the country. The

lake changed its size and depth seasonally; remaining shallow with mean depth around

1.9 m during the dry period throughout most of the year and expanding to double its

normal size during the monsoon months (Nov – Jan). The wetland was dammed in

1995 by the construction of a weir at the confluence of the Chini and Pahang Rivers

which altered the natural flow pattern. The wetland experienced a larger flood, where

the mean depth tripled in 2007.

Past studies have shown this lake is ecologically and social-economically important

because it is an eco-tourism destination and home to a number of endemic and

endangered species (Idris et al. 2005; Sharip and Jusoh 2010). However, a number of

environmental changes have been documented including land use and hydrological

alteration (Idris et al. 2005; Sharip and Jusoh 2010), which has affected water quality

and the hydro-ecological dynamics of the ecosystem.

Initial studies indicated that the wetland‟s ecosystem is degraded (Sharip and Jusoh

2010). Numerous studies in this floodplain wetland, however, have been focused on

the water quality issues specifically in relation to eutrophication (Idris and Abas 2005;

Muhammad-Barzani et al. 2008; NAHRIM 2007; Sharip and Jusoh 2010; Shuhaimi-

Othman and Lim 2006; Shuhaimi-Othman et al. 2007). These water quality changes

are thought to have contributed to the replacement of native floating-leaved species of

macrophytes (Nelumbo nucifera) by an introduced submerged species (Cabomba

furcata) (Idris 2007; Sharip and Jusoh 2010; Shuhaimi-Othman et al. 2007).

However, no studies have been carried out to assess the processes involved in the

observed shift of N. nucifera to C. furcata, or the processes that are likely to control

macrophyte domination in this system including the role of the introduced species (C.

furcata). It is of paramount importance to investigate the nature of the degradation and

the underlying dynamics of the ecosystem in order to suggest effective management

and restoration measures (Hobbs and Harris 2001; Hobbs and Richardson 2011; King

and Hobbs 2006). As the Cabomba population, in general, has been naturalised in


7

Malaysia (Chew and Siti-Munirah 2009), this is particularly important as invasion by this

species may have far-reaching ecological impacts for the region.

Studies on the hydro-ecological dynamics of this lake, however, have been very

limited. In a study by NAHRIM (2007), it has been shown that the main driving forces of

water flow in a shallow system such as Lake Chini were wind, river through-flow, and

the density gradient formed by differences in water temperature distribution. Density

gradients and wind motion were important mechanisms during both the normal and dry

seasons (NAHRIM 2007). However, further studies have not been carried out to

examine the exchange mechanisms induced by horizontal density gradients that affect

the patterns of observed water quality and nutrient concentrations. It is hypothesized

that these mechanisms may contribute towards the shift in macrophyte dominance in

the system and further work on understanding these processes is required. As this

wetland is very shallow with macrophytes present in a large proportion of the area,

such information will provide a better understanding of the importance of this physical

process towards the dynamics of the macrophyte community composition.

Further, despite the significance of macrophytes in wetlands, few studies have shown

the importance of macrophytes in contributing to primary productivity in shallow

swamps (Barko et al. 1977; Ikusima 1978; Ikusima and Furtado 1982; Wetzel 1992a),

and in particular how this may vary between an invasive or a native population. The

abundant growth of submerged C. furcata in many of the sub-basins of Lake Chini

could drive the primary productivity in the wetland (Sharip and Jusoh 2010). However,

no studies have been carried out on the primary production of the macrophyte

communities in Lake Chini, specifically the production rates of submerged C. furcata

and the associated benthic community changes. Differences in plant community growth

could contribute to further shifts of macrophyte composition and dominance.

1.3 Research objectives and approach

This study was broadly focused on providing a better understanding of vegetation and

invasion ecology, and freshwater ecology and limnological processes. Specifically, this

study aims to better understand the factors and processes controlling macrophyte

dominance in a shallow tropical floodplain wetland, with the ultimate goal of informing

management of the introduced species (C. furcata). It specifically explores the

dynamics of the abiotic-biotic interactions in the system and the physical processes


8

contributing to the transport of resources and biological processes. Further,

understanding the abiotic-biotic interaction and the ecosystem processes allows

wetland administrators and managers to more effectively restore and manage the

wetland ecosystem.

The main research questions, specific to Lake Chini Malaysia, addressed in this thesis

were:

Which environmental variables are associated with patterns of N. nucifera and

C. furcata abundance?

Does the invasion by C. furcata negatively affect the diversity of N. nucifera

dominated communities?

How do the N. nucifera and C. furcata communities affect the thermal structure

and convective circulation, and the subsequent patterns of nutrient distribution?

How do the circulation patterns and phosphate concentrations change as the

water levels change in response to seasonal flooding?

How do the rates of primary production and metabolism between communities

dominated by N. nucifera and C. furcata differ?

How does C. furcata affect the overall net ecosystem metabolism?

Overall, what factors control the shift in aquatic plant communities from

dominance of floating-leaved macrophytes N. nucifera to submerged

macrophytes C. furcata?

In order to answer these questions, the overall aim of the present study was therefore

to provide a comprehensive assessment of the water quality and macrophyte

community in Lake Chini and to characterise the ecosystem processes that contribute

to the changes in macrophyte composition and structure observed in the system. This

overall aim was addressed by three studies. In the first study, the system as a whole

was examined by investigating the historical context linked with environmental changes

in the system. The existing macrophyte communities within the studied area and the

environmental processes that affect the community composition were also analysed.

Whole-lake vegetation surveys, water quality testing and sediment sampling were

carried out in September 2009 prior to the monsoon. Vegetation surveys and water

quality testing was repeated in April 2010 after the monsoon. The vegetation surveys

addressed the temporal and spatial changes of plant communities within the whole lake


9

system, and examined the role of the monsoon on the wetland communities and the

environment.

In the second and third study, the two processes that affect the two main macrophytes

species during their initial growth stage were addressed. The processes were studied

at two separate lake sub-basins, one dominated by N. nucifera and another dominated

by C. furcata. High frequency data loggers were deployed in April 2010 to understand

the physical structure and community metabolism of the site. As thermal induced

forcing has been identified as an important physical driver in this shallow system

(NAHRIM 2007), this physical process was specifically examined in the second study.

Additional measurements of temperature were taken in March 2011 to explore the

temperature gradient and circulation pattern due to unexpected flooding. In the third

study, the ecosystem metabolism and primary production and respiratory rates of the

communities dominated by the two macrophytes were examined. These biological

processes were explored using oxygen data from two benthic chambers experiments

and sonde measurements deployed in free water in the littoral and pelagic zones of the

lake.

In all studies, water samples, biomass measurement and sediment were analysed at

an external analytical laboratory. The first study was followed up by multivariate

statistical analysis while the second- and third studies were supported by mathematical

calculations and data analysis techniques.

1.4 Logistical considerations

Future research of changes in macrophyte composition in floodplain wetlands may

focus on the hydro-ecological dynamics of the system associated with the main river. In

the present study it was not feasible to undertake this research due to time constraints

and the scope of the PhD studies. The remoteness of the study site placed a number of

additional logistical constraints on the study. Long-term experiments could not be

conducted as the study site was a long way from analytical facilities. Instrument

malfunction as a result of the challenging tropical environment, unexpected flooding

and human-interference also limited the studies. Vegetation surveys were only carried

out twice, before and after monsoons. Benthic experiments were only able to be

conducted for about 4-days during a dedicated campaign, with one-day diurnal primary

productivity measurement ultimately used due to difficulties in deployment of


10

instruments and limitations in instrument availability and functioning. Yet, the available

data collected over the study period still enabled investigation of the ecological

processes possibly controlling the shift of macrophyte dominance in this system.

1.5 Thesis outline

This thesis is presented as a series of scientific papers and consists of five chapters.

Chapter 1 presents the context for the study, the objectives, and the approach

used and the background review of the current and historical literature for the research

paper presented in Chapter 2-4.

Chapter 2 presents a comprehensive assessment and statistical

characterisation of the observed pattern of the macrophyte communities in Lake Chini

with a focus on the distributions of N. nucifera and C.furcata and the environmental

changes. The chapter develops a conceptual model of the ecological changes that

shape the potential macrophyte alternate stable states.

Chapter 3 focuses on the physical processes in particular the physical

circulation induced by the thermal structure and how it influences the water quality in

the habitat and control nutrient delivery for submerged macrophyte production. The

purpose of this chapter is to illustrate the importance of exchange dynamics between

lakes dominated by different vegetation.

Chapter 4 focuses on the biological processes associated with dissolved

oxygen dynamics, and in particular the metabolic differences between the habitats and

the associated communities dominated by N. nucifera and C.furcata.

The final synthesis chapter (Chapter 5) summarises and discusses all findings

of Chapter 2-4. The chapter also provides concluding remarks on the management

implications arising from the findings of this study, study limitations and

recommendations for future work.


11

Chapter 2 - Eutrophication, agriculture and water level control

shift aquatic plant communities from floating-leaved to

submerged macrophytes in Lake Chini, Malaysia1

2.1 Abstract

In this study we: 1) present a quantitative spatial analysis of the macrophyte

communities in Lake Chini with a focus on the biogeographical distributions of the

native Nelumbo nucifera and the invasive Cabomba furcata; 2) examine the

environmental changes that affect plant community composition; and 3) outline a

conceptual model of the variation of ecological processes that shape the macrophyte

communities. Plant species cover, biomass of C. furcata and N. nucifera, and water

quality and environmental variables were measured before and after monsoonal floods

in September 2009 and April 2010. Permutational multivariate analysis was used to

examine the significance of the invasion of C. furcata at different spatial scales.

Relationships between plant species cover and environmental variables before and

after flooding were examined using principal coordinates analysis and non-parametric

multivariate multiple regressions. Our findings suggest that 1) Variation in plant

communities was significant at the lake scale and the distribution of plant species

changed after annual floods. 2) Invasion by C. furcata significantly affected the overall

plant community composition. 3) C. furcata biomass increased after the monsoonal

season, which indicates that C. furcata is adapted to flooding events and that it is

becoming increasingly abundant. 4) In addition to the strong monsoonal effect, total

depth, nutrient concentration, and sediment type were important environmental

variables that significantly affected plant community composition. The macrophyte

community in Lake Chini is highly dynamic. The spatial and temporal plant community

dynamics are associated with flood regime, water quality, and substrate. Human-

induced changes in these parameters are likely shifting the macrophyte dominance

from floating-leaved to submerged species.

Keywords

Ecological alteration, eutrophication, floodplain wetland, invasive aquatic plant, lotus,

water fanwort

1 Have been published in Biological Invasions Journal


12

2.2 Introduction

Floodplain lakes and wetlands are continuously subjected to environmental stressors

that affect their plant community composition and structure. Human-induced alterations

such as eutrophication have been reported to result in change of dominance from

rooted and floating-leaved plants, (which typically characterise undeveloped wetlands

(Lougheed et al. 2008) and natural environments (Radomski 2006)), to assemblages

made up of floating plants and more tolerant emergent species (Lougheed et al. 2008),

from submerged plants to emergent species (Egertson et al. 2004) or from submerged

plants to floating plants (Bini et al. 1999). Changes in the hydrological regime have also

been responsible for transition in wetland macrophyte communities. Van Geest et al.

(2005) and Whyte et al. (2008) reported that floodplain lakes along the Lower Rhine

and Lake Erie coastal wetlands switched from floating-leaved dominance to emergent-

dominance following decline of the water level. In the Murray-Darling Basin, Australia,

regulation and damming of the river changed the hydrology of fringing floodplain

wetlands and subsequently altered ecological processes that affected macrophyte

diversity and species composition (Kingsford 2000).

Infestation of non-native species in such changing environments alters community

structure and ecosystem function (Ehrenfeld 2003; Farrer and Goldberg 2009;

MacDougall and Turkington 2005) and patterns of succession. The dynamic patterns of

vegetation and community succession following transition from a native species

assemblage to an invaded community are influenced by site availability and the

species differential availability and performance (Pickett and McDonnell 1989). Multiple

perturbations, such as nutrient enrichment and alteration of food-web structure, reduce

ecological resilience, alter feedback mechanisms, and may subsequently induce a

sudden shift of dominance (Carpenter and Cottingham 1997; Folke et al. 2004).

Understanding the key mechanisms that drive shifts of plant community dominance is

crucial to facilitate management and restoration.

This study reports the plant community alteration that has occurred in a tropical

floodplain wetland system, Lake Chini, Malaysia, in response to a shift in plant species

dominance from a floating-leaved species (Nelumbo nucifera) to a submerged species

(Cabomba furcata), as previously observed by Shuhaimi-Othman et al. (2007). The

appearance of C. furcata, a non-native species, was first reported in 1998 (Wetland

International 1998) and its abundance was noted to have spread to other lakes within


13

the wetland basin in 1999 (Gan and Bidin 2005). It has currently reached infestation

proportions (Chew and Siti-Munirah 2009) and is severely impacting on the natural and

cultural heritage values of the wetland. What has triggered this infestation and shift of

plant community dominance away from the once stable Nelumbo community is not

known. Unfortunately, past studies of the plant communities in Lake Chini were not

based on quantitative data, which hinders the understanding of how the changes in the

macrophyte community composition and structure relate to variability in environmental

properties. This is compounded by limited hydrological and water quality data available

from which to build a baseline understanding of the system. To the authors‟ knowledge,

there is also no specific research that has been carried out elsewhere in the world on

the change of dominance between Nelumbo and Cabomba species. Clearly, a

thorough understanding of different plant response to ecological changes needs to

consider alterations to key environmental attributes that characterise the ecosystem. It

is therefore the aim of this paper to: 1) to present a quantitative spatial analysis of the

macrophyte communities in Lake Chini with a focus on the biogeographical

distributions of N. nucifera and C. furcata; 2) to examine the environmental changes in

Lake Chini that may affect the plant community composition; and 3) to outline a

conceptual model of the variation of the ecological processes that shape macrophyte

distribution to assist in management of Lake Chini to meet conservation goals. The

work is based on review of data from the available literature and a comparative

analysis of baseline ecological survey work conducted in September 2009 and April

2010.

2.2.1 Study Area

Lake Chini is the second largest freshwater lake in Malaysia. It is a small (~2 km2)

alluvial riparian swamp system that is located within the Pahang River floodplain (Fig.

1). The lake is comprised of 12 small lakes that are inter-connected by natural

channels. Due to its close proximity to the main river, the lake‟s size and depth

changes significantly seasonally, remaining shallow (mean depth 1.9 m) during the dry

period throughout most of the year and expanding to double its normal size during the

monsoon months (Nov – Jan). Extensive beds of floating-leaved and emergent

macrophytes form an important feature of the lake, and similar to other wetlands,

account for a large proportion of the total primary production (Ikusima and Furtado

1982). N. nucifera once covered a substantial part of the surface water and large

blooms of its lotus flower led to the lake becoming a tourist attraction. Due to its


14

ecological importance and ecotourism value, Lake Chini was designated as a

UNESCO Biosphere reserve in 2009. However, the extensive presence of C. furcata in

Lake Chini in 2007 disrupted navigation and has since diminished its tourism potential

(Chew and Siti-Munirah 2009). There is a need to understand the factors that influence

the differences in macrophyte dominance in order to rehabilitate and manage Lake

Chini‟s environment for eco-tourism purposes, and concomitantly retaining its

ecological diversity.

Figure 1: Location of Lake Chini in peninsular Malaysia with inset showing the lake

system. Lake outlet is located between Lakes 4 and 5

Alteration of the land use in both the catchment and the lake environment, facilitated by

socio-economic pressure over the past 20 years, has affected the lake ecosystem.

Logging and conversion of forested area to agricultural land has increased (Shuhaimi-

Othman et al. 2007), and such practices are known to affect hydrological processes

(Bruijnzeel 1991). In 1995, a weir was constructed at the downstream end of Chini

River, at the confluence of the Chini and Pahang Rivers, in order to maintain higher

lake water levels for year-round navigability (Shuhaimi-Othman et al. 2007). This

altered the natural flood pulse and pattern of water level fluctuation and may affect the

macrophyte community structure.


15

2.2.2 Study organisms

Nelumbo spp. and Cabomba spp. come from different life form groups which differ in

structure, morphology, physiology, and habitat (Cooke et al. 2005; Schulthorpe 1967),

and subsequently their adaptation to the environment. N. nucifera is an Asiatic floating-

leaved plant that is native to Malaysia (Schulthorpe 1967). N. nucifera and its American

relative, Nelumbo lutea, comprise the only two species in the family Nelumbonaceae.

They have roots and rhizomes in the sediment, connected by petioles to floating and

emergent leaves (Cronk and Fennessy 2001). Rapidly growing petioles, attached in the

middle of the leaf blade, allow the leaves to rise with increasing water levels (Nohara

and Tsuchiya 1990). Nelumbo spp. grows well at depths of 1.5-2 m (Baldina et al.

1999; Schulthorpe 1967) up to a maximum depth of approximately 3 m (Kunii and

Maeda 1982; Unni 1971). They can grow in both acidic and alkaline water (Meyer

1930). They absorb nutrients from sediment and propagate by seeds and rhizomes.

The preferred substrate is benthic soil comprised of mainly gray, loamy mud (Kunii and

Maeda 1982).

In contrast, Cabomba is a genus of submerged macrophytes consisting of five species

that originate from the Americas (Orgaard 1991). Several species in the genus have

been introduced to water bodies around the world and the distribution exhibits a wide

latitudinal range from cold temperate to tropical waters (Schooler et al. 2009). As for

most introduced Cabomba populations, C. furcata was probably introduced to Malaysia

through the aquarium trade (Orgaard 1991) and has become naturalised in Lake Chini

within the last decade (Chew and Siti-Munirah 2009). C. furcata, in its native range,

grows in an environment that experiences fluctuating water levels, from dry periods to

periods of high water levels (Orgaard 1991). It commonly grows rooted at 2-3 m water

depth (Orgaard 1991), although Cabomba caroliniana has been found rooted at depths

up to 6 m (Schooler and Julien 2006). It grows well in acidic water, preferably at pH 4-6

(Orgaard 1991; Sanders 1979). Cabomba spp. uptake nutrients directly from the water

through shoots, leaves, and stems (Wilson et al. 2007) and have been observed to

grow well in nutrient rich water (Oki 1994). Cabomba spp. can also exist in turbid water

(Sanders 1979), prefer fine and soft substrates, and primarily propagate through

vegetative reproduction by means of stem fragmentation (Schooler et al. 2009).


16

2.3 Methods and materials

2.3.1 Overview

In order to assess changes within the Lake Chini catchment, data on land use and

surface phosphate concentration were collected from the available literature (Division

of Agriculture 1966; DARA 1992 unpubl. data, Shuhaimi-Othman et al. 2007). Data on

adjacent land use were available in the literature from 1975 to 2007 while data on

surface phosphate concentration were available from 1994 to 2007. Water level data

were obtained from the Drainage and Irrigation Department of Malaysia and the

National Hydraulic Research Institute of Malaysia. Hydrological records were available

from 2007-2010. Field measurements were carried out at Lake Chini on 26-28

September 2009 involving a survey of plant community abundance and composition

and measurement of water quality and sediment properties. Twelve main stations were

selected at each lake for intensive water quality and sediment sampling. Similar plant

abundance and water quality surveys were undertaken on 12-14 April 2010 to examine

the after-effect of the monsoonal flood on the abiotic variables and biotic communities.

2.3.2 Plant community sampling

The abundance of aquatic plant species within the lakes was assessed as plant cover.

Plant cover was visually estimated from a boat by an experienced observer within 1 m2

quadrats at each site along 2-4 transects within each lake. Two quadrats were

recorded at each site. Three to five sites were assessed in each transect, depending

on the transect length which range between 70 to 500m. Transect positions were

randomly selected, beginning in the centre of each lake, and arranged perpendicular to

the shoreline. The coordinates for each site were marked using Garmin GPSMap 60

model CSx (Garmin International, Kansas, USA) and downloaded into the Garmin

MapSource ® mapping software version 6.13.7. The surveyed positions were overlaid

on a satellite image (IKONOS 2006) using GIS software (Arcview ver. 3.3, ESRI,

Redlands, CA, USA). Specimens of plant samples were sent to the local herbarium

(Forest Research Institute of Malaysia and UKMB) for taxonomic identification.

Biomass measurements of C. furcata and N. nucifera were carried out where dense

beds of the respective plants grew along gradual slopes. One transect was selected at

each aquatic plant bed, where the quadrats were haphazardly positioned through these

beds, beginning at the shoreline and extending to the centre of the lakes. Using

SCUBA, three samples were taken, first at 0.25m, then at each successive 0.5m depth


17

increment, from 0.5m to either: a) the depth where the plants no longer grew or b) the

deepest point in the lake. Plant samples were collected using a quadrat (0.25 m2), then

washed and coiled, and placed into individual plastic bags. The plants were then stored

in cool container, and transported to the laboratory within 24 hours. Plant samples were

dried to constant weight in a drying oven at 103oC and the dry weights were recorded

to 0.01g.

2.3.3 Environmental variables

In-situ profiles of temperature, pH, dissolved oxygen (DO), turbidity, specific

conductivity and depth were taken at all surveyed sampling points using a YSI 6600

multi-parameter sonde. Water samples were collected using a Van Dorn sampler at 12

main stations located around the centre of each lake at 2-depths (surface and bottom

levels). Additional samples were taken near the surface (0.5 m) at selected sites having

dense macrophyte beds, mainly either C. furcata or N. nucifera. Water samples were

analysed for phosphate (PO43-), ammonium (NH4

+) and nitrate (NO3-) in accordance

with standard methods (APHA 1992) as listed in Appendix 1b. All the water samples

were placed in a cooler box (temperature ≤4oC) and transported to the laboratory within

24 hours. Sediment samples were collected using a Van Veen Grabber, stored in a

transportation box, and sent to the laboratory for sediment texture analysis (British

Standards Institution 1990). An additional distance measure, the distance from the

centre of each lake to the weir, was obtained from the coordinates and the satellite

image. Distance from the weir was ranked prior to analyses.

2.3.4 Statistical Analysis

To provide a quantitative spatial analysis of the macrophyte communities in Lake Chini,

we first used non-metric multidimensional scaling (NMDS), based on the Bray-Curtis

dissimilarity matrix (Clarke 1993) to visualise multivariate patterns among the plant

species cover within the 12 lakes, across the two seasons. To extract meaning from

the axes produced in NMDS, we performed principal coordinates analysis (PCoA) in

PERMANOVA to correlate environmental variables with the ordination axes (Anderson

et al. 2008; Anderson and Willis 2003). In this analysis, we used Pearson correlation to

highlight linear relationships between individual variables across the plot. Principal

component analysis (PCA), which projected samples to maximise their variance, was

applied to reduce the number of observed variables and summarize the pattern of the

correlation between total depth, temperature, pH, dissolved oxygen, turbidity,


18

conductivity, phosphate, nitrate, ammonium, %silt/clay content and rank distance from

weir. NMDS and PCA were based on lake averages. We then used linear regression to

illustrate the relationship between C. furcata cover and diversity. The Shannon-Weiner

Index of Diversity, calculated from mean lake abundance (cover) in September and

April, was used as the measure of plant diversity.

A hierarchical experimental design composed of three factors, i.e. lake (L: fixed),

transect (T: random, nested in L) and site (S: random, nested in L and T) and percent

C. furcata cover as covariate was used to characterize the effects of invasion by C.

furcata on the macrophyte community composition in Lake Chini. Permutational

multivariate analysis of variance, PERMANOVA (Anderson 2001a; Anderson et al.

2008) was performed on a basis of zero adjusted Bray-Curtis dissimilarity matrix

(Clarke et al. 2006) for the rest of the other species cover dataset to test whether

invasion by C. furcata significantly affected plant community composition at the spatial

scale of lake, transect and site (quadrat). The complex nature of the wetland

morphology and the subsequent plant assemblages and hydrological restriction during

our study led to an un-balanced replicated study design. All the analyses were

performed separately for data collected in September 2009 and April 2010, which

characterise the community assemblages before and after the monsoon season,

respectively.

Nonparametric multivariate multiple regression, DISTLM was employed to model the

relationship between the multivariate species matrix and the 11 environmental

variables. The individual variable was initially examined in the marginal test (excluding

other variables) and then subjected to a forward selection procedure (BIC selection

criterion) with sequential tests to determine the predictor variables that explain the

variation in the data. Significance was performed by 9999 permutations of residuals

under the reduced model (Anderson 2001b).

Wilcoxon Signed Rank Tests were performed to determine significant differences

between biomass harvested in September and April. Plant cover data were square-root

transformed, while biomass values and the water quality variables were natural log

transformed to improve normality. All Wilcoxon Signed Rank Tests were performed in

SPSS 16.0 (SPSS Inc.). Multivariate methods (NMDS, PCA, PCO, PERMANOVA and


19

DISTLM) were conducted in PRIMER 6 (Plymouth Marine Laboratory, Plymouth, UK)

and PERMANOVA+ (Plymouth Marine Laboratory, Plymouth, UK).

2.4 Results

2.4.1 Catchment and hydrological changes

Landscape changes within the Lake Chini catchment were apparent, changing from a

forested area in 1966 (Division of Agriculture 1966) to agricultural land with patches of

deforestation. Agricultural practices in the form of oil palm plantations and rubber

estates accounted for only 280 ha (5.6%) of the catchment area in 1992 (DARA 1992

unpubl. data) and this increased to a total of 807 ha (16.2%) in 2004 (Shuhaimi-

Othman et al. 2007) (Fig. 2a). Increased water surface phosphate concentration was

evident, from average of 0.01 mg/L in 1998 to around 0.06 mg/L in 2010 (Fig. 2b). A

decrease in surface phosphate between yr 2008-2010 could be associated to flushing

of nutrient from the lake by a large flood event in 2007.

0

5

10

15

20

1960 1970 1980 1990 2000 2010 2020Ag

ricu

ltu

ral l

and

-use (

%)

Year

0

0.02

0.04

0.06

0.08

0.1

1960 1970 1980 1990 2000 2010 2020

Me

an

ph

osp

ha

te

co

nce

ntr

atio

n (

mg

/L)

Year

(a)

(b)

Figure 2: Historical changes in (a) agriculture land use fraction and (b) mean

phosphate concentration (mg/l) at the water‟s surface. Phosphate concentrations from

2009 and 2010 were taken during the present survey. Historical data sources are listed

in the method


20

Historical data on water levels (Fig. 3) showed the influence of the Pahang River on

Lake Chini surface water level during monsoon months. The variability of the lake

water level in relation to fluctuation of the river water level was reduced by the weir

structure with the lake level increasing slightly with the Pahang River level. Further rise

in Pahang River level inundated the wetland and subsequently increased the lake

surface level.

9

10

11

12

13

14

15

16

17

18

25/10/2007 14/11/2007 4/12/2007 24/12/2007 13/01/2008 2/02/2008 22/02/2008

Wate

r le

ve

l (m

)

Pahang River level 2007-2008 Lake Chini level 2007-2008

Pahang River level 2009-2010 Weir height

Figure 3: Comparison between the Pahang River level and Lake Chini water level. Both

water levels were approximately referred to mean sea level

2.4.2 Spatial distribution of aquatic plants

A total of 17 plant species, native and non-native, were recorded from the 137

surveyed sites over the 12 lakes (Table 1). We found that Pandanus helicopus, C.

furcata, N. nucifera, Scirpus spp. and Lepironia articulata are the most abundant

species in the lake system. Comparison between prior surveys of macrophyte diversity

in the literature and the present survey indicate the presence of previously undetected

species (see Table S1 in supporting information).


21

C. furcata and N. nucifera coverage differed between the 12 lakes (Fig. 4 and Fig. 5). C.

furcata remained widely established in the absence of N. nucifera in Lake 12 and Lake

10, and appeared in another lake (Lake 2), where it was previously not found during

the September (pre-flood) sampling. The N. nucifera population has increased in all

lakes where it was present in 2009, and in particular in lakes that were located closer to

the Pahang River. Diversity in lakes decreased with increasing C. furcata abundance

(Fig. 6) in September (t12=10.276, P<0.001).

Table 1: List of macrophytes recorded in the present survey in September 2009

LAKE

Species (scientific name) 1 2 3 4 5 6 7 8 9 10 11 12

Submerged macrophyte

Cabomba furcata* + + + + + + + + + +

Utricularia gibba + + + + + + + + +

Chara sp. +

Floating macrophyte

Salvinia molesta* +

Rooted, Floating-leaved

Nymphea pubescens lotus + + +

Nelumbo nucifera + + + + + + + +

Emergent

Lepironia articulata + + + + + + + + + +

Eriocaulon sexangulare +

Scirpus grossus + + + + + + +

Pandanus helicopus + + + + + +

Eleocharis ochrostachys + + +

Lycopodiella cerniea (cernua) +

Cyperus pilosus +

Pragmites communis + +

Lygodium microphyllum +

Panicum Repens + + + + + + + + + + * Introduced species


22

Figure 4: Comparison of C. furcata and N. nucifera coverage before and after

monsoonal flooding


23

C. furcata

N. nucifera

Figure 5: Mean cover of C. furcata and N. nucifera by lake and by sampling date. Error

bars are ± SE


24

Y =-0.049x+0.675;

R2=0.130

Y+=-0.110x+1.344;

R2=0.507

Mean C. furcata cover (sqrt)

Figure 6: Regression of C. furcata cover against Shannon-Weiner Diversity Index (H‟).

Symbols indicate sampling month: +September, ● April

The PERMANOVA detected significant variability at the “lake” spatial scale. The factor

“transect” and “site” were pooled since they were the factors with negative estimates

for component of variance and a higher p-value (Anderson 2001b; Anderson et al.

2008). PERMANOVA tests (Table 2) showed that differences between invaded and un-

invaded plant assemblages were highly significant, both in September and April.

Differences in the composition of invaded and un-invaded sites were stronger in

September compared to April, indicating the influence of the monsoonal flood on the

macrophyte community composition.


25

Table 2: Permutational multivariate analysis of the effects of invasion, lake, transect

and site on plant community abundance

PERMANOVA tests

Source of variation df SS MS F P(perm)

September 2009 Invasion 1 19590 19590 12.475 0.0001*** Lake 11 46222 4202 2.676 0.0001*** Pooled 134 210430 1570.3 Total 146 276240 April 2010 Invasion 1 13991 13991 11.202 0.0003*** Lake 11 50843 4622.1 3.701 0.0001*** Pooled 134 167370 1249 Total 146 232200 df degree of freedom, SS sum of squares, MS mean squares, ***=p<0.001

Mean C. furcata biomass from both sampling periods was correlated with depth (r = -

0.773, P=0.042) (Fig. 7). A Wilcoxon Signed Rank Test indicated an increase in

biomass from Sept to April; z=-2.201, P=0.028 (two-tailed). The magnitude of

differences in the means was large and the median score on the biomass increased

from Sept (15.1 g/m2) to April (318 g/m2). The highest C. furcata and N. nucifera

biomass were sampled in April at depth of 0.5 m and 1.5 m with biomass 934.7 g/m2

and 62.7 g/m2 respectively.

0

5

10

15

20

0.25 0.50 1.00 1.50 2.00 2.50 2.80 3.00

Me

an

bio

mass (

g/m

2)

Depth (m)

(a) Mean C. furcata biomass in Sept 2009

0

200

400

600

800

1000

0.25 0.50 1.00 1.50 2.00 2.50 2.80 3.00

Me

an

bio

ma

ss (

g/m

2)

Depth (m)

(b) Mean C. furcata biomass in April 2010

Figure 7: Relationship between depth and mean C. furcata biomass. Biomass is the

mean dry weight sampled in all quadrats at that depth


26

2.4.3 Environmental variables potentially influencing macrophyte

distribution

PCA, employing Euclidean distance, indicated a consistent linear combination of

variables making up the principal components. The first three eigenvalues explained

69% of the variance; the first component corresponded to 34% variance and second

component explained 20% of the variance. Plots of the first two components showed

separation of the dataset before monsoon to the dataset after the monsoon, which

indicated the influence of flooding on the water quality (Fig. 8). The separation of sites

by sampling date along axis 1 was primarily correlated with nitrate concentration and

total depth and along axis 2 was correlated to phosphate, turbidity and pH. Details on

the environmental variables are given in supporting information (Table S2).

-4 -2 0 2 4

PC1

-4

-2

0

2

4

PC

2

MonthSept

April

S1

S2

S10

S11

S12

S8

S9

S3

S4

S5

S6

S7

A1

A2

A10

A11

A12

A8

A9

A3A4

A5

A6

A7

Total depth

Temperature

Conductivity

pH

Turbidity

DO

PO4

NH3NNO3

%silt/clay

Distance to weir

Figure 8: PCA biplots of the average values of the environmental variables by sampling

month. Number indicates lake

The results of DISTLM tests (Table 3) revealed a positive match between the

environmental variables and species assemblages in the Lake Chini dataset. Total

depth explained ~42.6% of the variation in the composition of the plant assemblages in


27

September 2009 and 26.5% of the community variation in April 2010. Phosphate,

temperature, nitrate, %silt/clay and conductivity contributed a further 26.2% of the

variance in September 2009, while distance from weir, dissolved oxygen, ammonium

and temperature together explained about 24% of the variance in April 2010.

The phosphate concentrations sampled in all areas having dense C. furcata and N.

nucifera beds were very low (non-detectible) in September, while in April, nitrate

concentrations were low in the plant beds (Fig. 9). PCoA results indicate C. furcata was

associated with lakes having a higher silt/clay percentage and increasing distance from

the weir, whereas N. nucifera was found in lakes having lower silt/clay content (Fig. 10).

0

20

40

60

80

100

0.0 0.1 0.2 0.3 0.4

Co

ve

r (%

)

PO4 concentration (mg/L)

(a) C. furcata

Sep-09 Apr-10

0

20

40

60

80

100

0.0 0.1 0.2 0.3 0.4

Co

ve

r (%

)

PO4 concentration (mg/L)

(b) N. nucifera

Sep-09 Apr-10

0

20

40

60

80

100

0.0 1.0 2.0 3.0 4.0 5.0

Co

ve

r (%

)

NO3 concentration (mg/L)

(c) C. furcata

Sep-09 Apr-10

0

20

40

60

80

100

0.0 1.0 2.0 3.0 4.0 5.0

Co

ve

r (%

)

NO3 concentration (mg/L)

(d) N. nucifera

Sep-09 Apr-10

Figure 9: Relationship between dominant species cover and phosphate and nitrate

concentration


28

Table 3 Results of the DISTLM analysis on the Lake Chini dataset in September 2009

and April 2010

Sequential tests

Variable BIC Pseudo-F P % Var %Cum

September 2009

+Log(Total depth+1) 418.50 42.957 0.0001*** 42.55 42.55

+Log(PO4+1) 416.35 6.2481 0.0019** 5.68 48.23

+Log(Temperature+1) 411.99 8.4811 0.0003*** 6.81 55.04

+Log(NO3+1) 410.58 5.2837 0.006** 3.94 58.98

+%silt/clay 408.41 5.9403 0.0039** 4.07 63.04

+Log(Conductivity+1) 402.51 9.6054 0.0001*** 5.67 68.71

April 2010

+Log(Total depth+1) 438.49 20.917 0.0001*** 26.505 26.51

+Rank distance to weir 435.97 6.6387 0.0008*** 7.667 34.17

+Log(DO+1) 435.90 4.0264 0.0151* 4.416 38.59

+Log(NH4+1) 434.72 5.0482 0.004** 5.163 43.75

+Log(Temperature+1) 431.25 7.2604 0.0008*** 6.667 50.42

BIC Bayesian Information Criterion, %Var percentage of variance in species data

explained, % Cum cumulative percentage of variance explained *=p<0.05, **=p<0.01,

***=p<0.001


29

-60 -40 -20 0 20 40 60

PCO1 (61.2% of total variation)

-80

-60

-40

-20

0

20

40

PC

O2

(1

8.2

% o

f to

tal

var

iati

on

)

Dominant species

0

1

2

3S1

S2

S10S11

S12

S8S9S3

S4

S5

S6 S7

A1A2A10A11

A12

A8

A9

A3

A4

A5

A6

A7

Depth

Temp

CondpH

Turb

DO

PO4

NH4NO3

%silt/clay

RDis

Figure 10: PCoA plot of plant species abundance and environmental variables.

Symbols represent main species: triangles, N. nucifera; diamonds, N. nucifera and C.

furcata; squares, C. furcata, circles, other species. Number indicates lake; Letter

indicates sampling month: S=September, A=April. Water quality variables were natural

log-transformed

2.5 Discussion

Similar to other lakes and wetlands around the world, human land-use practices to

address socio-economic needs are affecting Lake Chini, resulting in alteration of

habitat and biotic communities. The results of this study document landscape and

water quality changes in Lake Chini. Change in the surrounding terrestrial landscape

from forested areas to agricultural land could alter runoff quantity and quality, and in

particular may possibly increased nutrient input into the lake and the suspended

sediment load and subsequent deposition. Hydrological alteration further induces

ecological changes by stabilising the lake water levels, reducing the flood disturbance

regime, and affecting the flood timing and duration (Nilsson and Svedmark 2002).


30

Spatial variation in the abundance and distribution of the studied plant species between

the lakes was apparent. The cover of C. furcata observed in September 2009 was

lower compared to observations in 2007 (before the intermediate size flood event)

(Sharip pers. observation, Chew and Siti-Munirah 2009, Sharip and Jusoh 2010).

However, both cover and biomass increased from September 2009 to April 2010 which

indicated that it may be in the process of re-spreading. Dissimilarities between lakes

within invaded and un-invaded sites could result from the complex interaction among

different forcing mechanisms and ecological processes that dominate during the

different seasons. This invasion is having a significant impact on the overall plant

community diversity. Early growth phase after seasonal disturbance could explain the

weak correlation between C. furcata abundance and diversity in April. Increased C.

furcata biomass after the monsoonal season indicates that the plants survived from the

previous year and then grew rapidly, probably due to the availability of resources

during and after the floods. The sustained growth of such invasive species could affect

the plant community structure and reduce the establishment of other species (Santos

et al. 2011).

The spatial and temporal variation of water quality parameters described here are

consistent with the results of prior studies that examined the water quality in Lake Chini

(Shuhaimi-Othman et al. 2007). The nutrient levels were high, suggesting that nutrient

enrichment is a concern and may have a strong impact on the composition of the plant

community and the population dynamics of C. furcata and N. nucifera in Lake Chini.

The mean phosphate concentration in Lake Chini was high in both sampling months.

Shuhaimi-Othman et al. (2007) suggest that the increase in phosphate concentration at

Lake Chini is attributed to run-off from agriculture areas and settlement (Shuhaimi-

Othman et al. 2007). Higher phosphate concentration in the water column after flooding

could also be attributed to P-release from sediments during high water (Boers and

Zedler 2008).

Trace values of phosphate in areas where C. furcata was abundant suggest that it may

have been consumed by the plants. Studies have shown Cabomba spp. absorb

nutrients directly from the water through shoots, leaves, and stems (Wilson et al. 2007)

and nutrient rich water may have promoted high C. furcata abundance in Lake Chini.

There was a decreasing trend of phosphate concentration from pelagic to littoral

regions in September, while in April, the decreasing trend from pelagic to littoral


31

regions was apparent for nitrate concentration. Physical processes that control the

exchange of these constituents from the pelagic to the littoral zone should be further

investigated, since differential temperature gradients between the zones were noted

and may control nutrient delivery to dense C. furcata beds. In addition, the switch from

detectable nitrate and undetectable phosphate in September, before the monsoonal

floods, to undetectable nitrate and detectable phosphate after the floods in dense

macrophyte beds suggests a change in nutrients limiting macrophyte growth.

A change in sediment characteristics in Lake Chini was apparent with increased silt

content compared to earlier findings (Abas et al. 2005). Deposition of silty materials is

most likely enhanced from run-off and erosion from an increasingly cleared catchment.

An increase in silt content in the substrate makes the site more suitable for Cabomba

spp. growth as it more closely resembles the substrate characteristics of both its native

range and areas where it is highly invasive (Schooler et al. 2009). N. nucifera

preferance for mud loamy subtrate (Kunii and Maeda 1982) explains the negative

correlation between N. nucifera abundance and %silt/clay content. Negative correlation

between N. nucifera abundance and distance to weir and positive correlation between

N. nucifera abundance with turbidity may be associated with floods. The Pahang river,

like many tropical rivers, contains high suspended sediment ranging from 1-152 mg/L

(Sulaiman and Hamid 1997). The suspended solid, derived from erosion of the alluvial

and podzolic soil in the river basin (Panton 1964), is composed largely of loamy clay,

which is not only suitable for N. nucifera growth but also easily resuspended when

disturbed. Depending on the timing, duration and flood amplitude, the floating leaves of

N. nucifera elongate petioles to extend above the turbid water to reduce the impact of

flooding (Nohara and Kimura 1997; Nohara and Tsuchiya 1990).

The statistical analysis clearly indicated that water depth was an important

environmental variable for both submerged C. furcata and floating-leaved N. nucifera.

The association of depth to variation in C. furcata abundance and biomass agrees with

the finding of previous studies of another Cabomba species, C. caroliniana (Schooler

and Julien 2006). High C. furcata cover and density were found in shallower areas and

abundance decreased with depth. The decrease in lake water level during the dry

season (as apparent in April sampling), which happens after the monsoonal flood,

coincides with the initial plant growth phase and may contribute to the re-zonation of

plant establishment towards the centre of the lake where a more suitable water depth is


32

apparent. Water depth is strongly associated to light penetration (Spence 1982), which

is essential for submerged plant growth. A decrease in water level alters the light field

across the benthos and can lead to an increase in temperature as more heat is

absorbed by smaller volume of water. As the system becomes shallower, the

hypolimnion may disappear and result in reduced stratification and increased dissolved

oxygen at depth. The high concentration of ammonium in April could be associated

with the flood (Shuhaimi-Othman et al. 2007) and result in the observed increases in C.

furcata biomass. C. furcata showed higher primary production in rainy seasons where

waters contains high ion, nutrient concentrations and low transparency (M. Pezzato,

pers. comm.).

The change in floristic communities may be the result of the large flood event that

occurred in December 2007. The influence of inflow from the Pahang River and the

different flooding effect during the monsoon season may have affected the ecological

processes and subsequently the composition and distribution of the plant community

since significant differences in plant abundance was apparent between the two

seasons. Different magnitudes of floods have been reported to exhibit different effects

to wetland ecosystems (Hughes 1997; Richardson et al. 2007); intermediate size floods

influence plant community distribution while small annual floods affect plant species

abundance (Nilsson and Svedmark 2002). The changes in distribution and abundance

of C. furcata and N. nucifera indicates dynamic population shifts which may be largely

influenced by the annual flooding cycle. Flooding events may disrupt the substrate and

dislodge plants. This could increase the spread of C. furcata, which can grow from

stem fragments. Results from this study, in conjunction with historical observations,

suggests that the distribution and abundance of N. nucifera is constantly shifting and is

currently expanding, despite recent changes in sediment structure and eutrophication.

Based on our results we have developed a conceptual model of ecological variation in

Lake Chini that describes how alteration of the lake ecosystem is associated with the

combination of disturbances (Fig. 11), which are known to enhance biological invasion

(Hobbs 1991). The installation of the weir structure has changed the biological and

physical characteristics of the lake by altering the hydrological processes and

disturbance regimes. Together with continuous alteration of the surrounding landscape,

and eutrophication and siltation pressures increasing the availability of nutrients in the

water and silt content in the sediment, the ecological community is adapting to the new


33

state. Where non-native species are involved, research has shown that disturbance will

enhance invasions when it is also associated with an increase in the availability of

limiting nutrients (Hobbs 1989). The stabilised water depth, and increasing nutrient

availability in water and silt content in the subtrate has facilitated the invasion of C.

furcata in the wetland. The extent of the effects of engineering intervention (damming)

in causing a rapid proliferation of the invasive species in Lake Chini is consistent with

changes in riparian vegetation (Richardson et al. 2007). In this shallow floodplain

wetland, the change of the dominant aquatic vegetation from N. nucifera to C. furcata

has been dramatic and has occurred within a period of less than 10 years.

The occurrence of regime shifts or alternative stable states are acknowledged in

different kinds of freshwater ecosystems (Dent et al. 2002; Folke et al. 2004). Floating

plant dominance has been recognised as a new stable state associated with

eutrophication over a state characterised by submerged plant in tropical lakes (Scheffer

et al. 2003). In contrast to shallow lakes where alternative stable states are primarily

between macrophytes and algae, wetland systems where macrophytes dominate

primary production exhibit different multiple states. Nutrient enrichment has shifted

freshwater marshes in the Everglades from wetlands dominated by sawgrass to cattail

marshes (Gunderson 2001). Changing patterns of macrophyte dominance between N.

nucifera and C. furcata implies the possibility of two alternate stable states in Lake

Chini. Floating-leaved plants, such as N. nucifera, are better competitors for light,

carbon and nutrients from the sediment, and once re-established after intermediate

size floods, they may have competitive advantage to spread when the natural flow

regime is re-established due to flooding by the Pahang River as a results of frequent

high intensity rainfall in the Pahang river basin. However, submerged species, such as

C. furcata, are superior competitors for nutrients in the water column (Gunderson

2001), and continuing eutrophication will likely increase its abundance and spread in

the system and eventually could cause a shift of dominance.

Macrophyte occurrence and dominance is characterised by species life history traits

and environmental conditions (Pickett and McDonnell 1989). Being able to vegetatively

propagate abundantly via fragmentation increases spread rates for Cabomba spp.,

particularly after disturbance events where plants are broken (such as from propellers).

This, along with eutrophication and river damming, contributes to making Cabomba

spp. invasive in many parts of the world (Mackey and Swarbrick 1997; Schooler et al.


34

2009; Wilson et al. 2007). This study, which based on one seasonal cycle, indicates

dynamic patterns of aquatic vegetation with possible multiple states in the Lake Chini

ecosystem with the floods and nutrient dynamics as important drivers for shifts in

dominance. Continuing observation of the aquatic plant communities and the

environmental variables, however, will further illustrate the successional patterns in

this lake system. This is particularly relevant to C. furcata, as it appears to have the

ability to re-colonize and grow rapidly following a flood event, which may be linked to its

observed proliferation.

There is no prior knowledge of C. furcata being invasive in other countries. The

infestation of C. furcata in Lake Chini, if not controlled, may disrupt its ecosystem

function and impact its natural diversity. Continued research is essential in order to

recommend proper management measures. Further research should concentrate on:

1) nutrient cycling and exchange of nutrients between pelagic and littoral zones and

between lakes, (2) primary production of the main macrophytes and their relationship to

water quality, (3) role of flood variability on plant community structure, and (4)

alternative measures for sustainable rehabilitation and management of the lake

ecosystem to prevent further loss of plant diversity.

2.6 Acknowledgement

Funding for Z. Sharip was provided by the Malaysian Ministry of Natural Resources

and Environment scholarship. We thank Juhaimi Jusoh and Ahmad Taqiyuddin Ahmad

Zaki for acquisition of data, assistance during field experiments and the coordination of

financial support. The National Hydraulic Research Institute of Malaysia (NAHRIM)

provided data, and financial assistance for the field experiments. We thank Dr. Marti J.

Anderson for her statistical advice. Two anonymous reviewers contributed constructive

and informative comments and suggestions.

35

Figure 11: Conceptual model of ecological changes, feedback mechanisms and multiple states in Lake Chini. Dotted line indicate processes that can

move the system from one state to the other


36

Chapter 3 – Physical circulation and spatial exchange

dynamics in a shallow floodplain wetland2

3.1 Abstract

This paper examines spatial patterns of water exchange based on water temperature

variation between littoral and pelagic zones and compares the patterns in a series of

shallow lakes at different water levels. Exchange patterns were assessed by

developing isotherms along the transects and estimating the surface energy budget

using the vertical temperature profile and time-series measurements. Our results

indicate the presence of density-driven flow induced by the differential temperature

gradient between littoral areas, which are dominated by either floating-leaved or

submerged vegetation, and the open pelagic region. Persistent stratification was noted

in the narrower lakes, which is thought to be due to the presence of dense submerged

vegetation that attenuates wind-driven turbulence. In addition, variation of thermal

stratification and mixing dynamics between these lakes at different water levels has

corresponding effects on the biological and chemical regime. The circulation

contributes to increased transport of the phosphate that we believe could favour

submerged species and subsequently induce shifts of macrophyte community

composition. The results of this study have implications for the rehabilitation and

management of lake ecosystems.

Keywords: convective circulation, density driven flow, floating-leaved plant, Lake Chini,

shallow wetland, submerged macrophytes, thermal stratification, water exchange

2 Have been accepted in International Journal of Design, Nature, and Ecodynamics


37

3.2 Introduction

Convective circulation induced by density gradients resulting from horizontal depth

variation has been demonstrated to be an important exchange mechanism of

phosphorus (Horsch and Stefan 1988; James and Barko 1991), and mercury

(Palmarsson and Schladow 2008) between the littoral and pelagic zones, nutrients

between side embayment and the main basin of reservoir (Monismith et al. 1990), and

pollutants between a fringing wetland and lake (Nepf and Oldham 1997). A

temperature gradient develops when shallow littoral areas and deeper pelagic zones

receive the same incoming solar radiation but distribute it differently over their

respective volumes (Dale and Gillespie 1977b; Horsch and Stefan 1988; MacIntyre and

Melack 1995; Monismith et al. 1990). Several studies have also shown that aquatic

plants are able to induce convective motion by promoting differential shading and by

attenuating wind in shallower regions, which can further lead to horizontal and vertical

gradients in temperature (Coates and Ferris 1994; Dale and Gillespie 1977a; Herb and

Stefan 2004; Lovstedt 2008; Lovstedt and Bengtsson 2008; Nepf and Oldham 1997;

Waters 1998).

Studies on the convective exchange between vegetated littoral regions and pelagic

zones have been focused on emergent vegetation communities (Nepf and Oldham

1997) such as reed beds (Lovstedt 2008; Lovstedt and Bengtsson 2008) and Typha

stands (Waters 1998), and floating species such as Azolla spp. and Lemna spp.

(Coates and Ferris 1994). In all of these studies, a horizontal density gradient develops

as the surface water underneath or within the vegetation becomes colder than the

adjacent open water due to shading, and subsequently induces overflow of warmer

pelagic water along the surface towards the plant bed during the day. The resulting

horizontal temperature gradient induced by differential shading also differs with the

density of the vegetation bed (Lovstedt and Bengtsson 2008; Waters 1998).

From an ecological perspective, exchange flows generated by differential heating can

contribute to replacement of nutrients taken up by the macrophytes (Coates and Ferris

1994). However, despite the potential significance of this process in shaping the

ecological dynamics of wetland macrophyte communities, studies on the effect of

convective exchange induced by different types of macrophytes, for example rooted

floating-leaved plants or submerged species, have been limited. In an earlier study, we

identified differential nutrient gradients between the littoral and pelagic zones in Lake


38

Chini, a tropical floodplain wetland in Malaysia (Sharip et al. 2011a), and it was

hypothesized that this may have been influenced by temperature differences between

the two zones and associated exchange between them. In particular, the circulation

patterns induced by the thermal gradient could potentially regulate transport of

nutrients to dense submerged beds of the highly invasive macrophyte, Cabomba

furcata (Sharip et al. 2011a), and subsequently promote the proliferation of this species

due to its ability to uptake nutrients through stems and shoots (Wilson et al. 2007).

The aim of this study is to examine the convective exchange pattern resulting from

differential cooling and heating in a shallow floodplain wetland and its effect on water

quality dynamics in the system that could contribute to the shifts of macrophyte

community composition that have been previously documented (Sharip et al. 2011a).

Specifically, we identify how horizontal exchange flows are affected by dominance of

two different types of aquatic plants along the littoral areas and also investigate the

variation in the circulation pattern in two sub-basins in the wetland at different water

levels. This is achieved by (1) describing the spatial and temporal variations in thermal

structure, light climate and phosphate concentrations in the two lakes of the wetland

system during low- and high- water level or flooding, (2) estimating the surface energy

budgets and the convective flow rates between the littoral zone and open water at two

sites, and (3) using this information to assess the implications of physical-chemical

coupling to macrophyte responses and the density gradient induced by differential

shading by vegetation between the littoral and pelagic zones as an important

mechanism for transport of nutrients to C. furcata beds, and subsequently the

promotion of invasion of the species in the wetland system. This study also provides an

improved understanding of the water circulation induced by differential heating and

cooling in areas dominated by different types of aquatic plants, namely emergent

floating leaved plants versus submerged macrophytes.

3.3 Methods

3.3.1 Study site

Lake Chini, Malaysia, is a shallow wetland located within the Pahang River floodplain,

which exhibits characteristics of a system that suggest multiple ecological regime shifts

driven by flood and nutrient delivery (Sharip et al. 2011a). The wetland comprises a

series of 12 inter-connected lakes, has a total surface area of 1.69 km2 and an average

depth of 1.9 m [16], with its littoral areas extensively dominated either by stands of the


39

floating-leaved plant (Nelumbo nucifera) or submerged species (C. furcata). To

maintain water level for year-round navigability, a weir was constructed downstream

end of Chini River in 1995, which connects the wetland with the main river (Sharip and

Jusoh 2010; Shuhaimi-Othman et al. 2007).

3.3.2 Field measurements

Surveys were carried out on 19-24 April 2010 to investigate the exchange flows

between the littoral and pelagic region. Transects were positioned in two lakes; Lake 1

linking the open pelagic region to littoral area dominated by floating-leaved vegetation,

and Lake 12 linking pelagic zone to a littoral area dominated by submerged vegetation

(Fig. 1).

1

23

4

12

34

Lake 1

Lake 12

Lake 5

Figure 1: Lake Chini and location of temperature monitors (star)

Thermistor chains were deployed at two locations within the pelagic zone (Station 3

and Station 4) in Lake 1 and Lake 12 and at one location in Lake 5; each anchored at

the top on buoy and kept taut to the bottom by a weight. The three sites were selected

to represent lakes dominated by N. nucifera (Lake 1), dominated by C. furcata (Lake

12) and dominated by both species and subjected to direct flooding due to its close

proximity to the weir (Lake 5). Four TPS WP82 temperature-dissolved oxygen sensors

(TPS Pty Ltd, Brisbane Australia) were deployed at the littoral zone; one sensor was

deployed at the open water near the edge of the macrophyte bed at approximately 0.2-


40

m depth (Station 2), and another three sensors inserted at the top of benthic chambers

(Station 1) with a 12 V submersible pump used to re-circulate the water within the

chamber to maintain similar water conditions as outside. Thermistors recorded

temperatures at intervals and depth as in Table 1. Vertical profiles of photosynthetically

active radiation (PAR) and temperature-depth were made with a LiCOR sensor (Li-

192SA) and a YSI 6600 multi-parameter profiler. The temperature sensors were

calibrated together in a container at varying temperature, either before or after field

experiments. Temperature data were corrected where necessary with the mean

temperature so that the maximum differences between any loggers do not exceed

0.15oC. Linear interpolation was used to calculate hourly interval values where

necessary. Water samples were collected at surface- and bottom levels at each lake

for analysis of phosphate, nitrate and dissolved organic carbon in accordance with

standard methods (APHA 1992). Additional vertical profiles of temperature and light

were also carried out using YSI 6600 multi-parameter probe and LICOR sensor on 29-

30th March 2011 during a high water level period. The profiling was carried out around

mid-noon and early morning where differential heating and differential cooling would

likely to occur (Smith et al. 1995).

Table 1: Temperature monitors in Lake Chini

Sensor Parameter Recording

interval

(min)

Depth (m) Lake

Hobo

RBR

D-Opto

TPS sensors

Hydrolab

Temperature

Temperature

Temperature, DO

Temperature, DO

Temperature, DO, pH,

Chlorophyll

15

1

1

10-20

15

0.5, 1.0, 2.0

1.0

0.5

0.15-0.25

1.0

1,5,12

1,12

1,12

1,12

1,12

Hourly meteorological data were obtained from a weather station positioned in the

centre of Lake 1, and the Muadzam Shah meteorological station located about 40 kms

south of the study site. The on-lake weather station recorded solar radiation, wind

speed, air temperature, relative humidity, and precipitation. Due to the failure of the

wind speed and air temperature sensors, the wind speed and air temperature data

were collected from the Muadzam Shah Station. To include the localized lake-effect, air


41

temperature and wind speed values at the lake were calculated from the Muadzam

Shah Station using a linear regression between data from the two sites at times when

both were available. The air temperature and wind speed data between the on-lake

weather station and Muadzam Shah meteorological station were highly correlated

(r2=0.87, n=2880, p<0.001 and r2=0.53, n=356, p<0.001). Mean air temperature at

Lake Chini (26C) was significantly lower by 4.5% (t=15.1, df=5338.5, p<0.001) than

mean air temperature the Muadzam Shah Station, and this could be due to

microclimatic differences caused by the surrounding topography and the mediating

effect of the water-body.

3.3.3 Analysis of physico-chemical structure and heat budgets calculation

Spatial distribution of water temperature, and phosphate concentrations along the

transects were generated using SURFER (version 9.0; Golden Software, Inc.) with

kriging as the gridding method. Convective exchange patterns were assessed by

generating contours over the transects using the temperature data recorded by

thermistor probes and temperature-depth profile data measured over 45-min intervals

or less as described in Smith et al. (1995).

The atmospheric heat fluxes calculation were computed in Matlab R2008a (The

MathWorks) using a bulk aerodynamic approach applied to the available

meteorological data (MacIntyre et al. 2002; Tennessee Valley Authority 1972; Verburg

and Antenucci 2010). The two lakes were assumed to have similar meteorological

parameters. The overall heat budget calculation was calculated according to Lovstedt

and Bengtsson (2008):

.0

0 QsensibleLatentnet

h

P HHHRzt

TC

(1)

where, ρ0 is the density of water (kg/m3), Cp is the specific heat capacity for water,

4.18x103J/(kg K), h is the water depth (m), ∂T is the average temperature change (oC)

over the time step, ∂t (s), z is the vertical distance from surface (m), Hlatent is the heat

loss by latent heat fluxes (W/m2), Hsensible is heat loss by sensible heat transfer (W/m2),

and HQ is the lateral heat flux between the open water and the vegetation bed. The

incoming net solar radiation, Rnet (W/m2), was calculated according to:

.ionbackradiatlongwaveshortwavenet HHHR (2)


42

where the short wave radiation is estimated as:

.)1( inshortwave SH (3)

Here Sin is the short wave incoming radiation (W/m2), α is the albedo or the reflectance

of the solar radiation and was taken as 0.05 from water surface (Shaw 1994) and 0.16

from the floating-leaved, water lily (Cooley and Idso 1980). The incoming long wave

radiation and the long wave back radiation was calculated according to:

.)17.01(42

awalongwave TCH (4)

.)/(642.0 7/1

aaa Te (5)

.4

wionbackradiat TH (6)

where σ is the Stefan-Boltzman constant (=5.67x10-8 W K-4 m-2), εa is emissivity of air,

ea is the vapour pressure of the air (Pa), C is the fraction of cloud cover, Ta is the air

temperature (K), Tw is the surface water temperature (oK) and ε is the surface

emissivity (=0.97 for water (Shaw 1994) and =0.98 for the aquatic plant vegetation

(Lovstedt and Bengtsson 2008)).

The sensible and latent heat fluxes, Hsensible and Hlatent, were estimated using the bulk

aerodynamic transfer method based on the wind speed and specific humidity gradient.

To address the strong effect of the thermal inertia of the water over the diel scales

(MacIntyre et al. 2002), neutral values of the transfer coefficients at 10 m,

CDN=1.0x10-3, CHN=1.35x10-3 and CEN=1.35x10-3 (MacIntyre et al. 2002), were

corrected for non-neutral atmospheric stability effects at the relevant measurement

height following Monin-Obukhov similarity theory (Amorocho and DeVries 1980;

MacIntyre et al. 2002). In this study, sensible heat transfer coefficient (CH) was

assumed the same as latent heat transfer coefficient (CE) in keeping with the literature

(Imberger and Patterson 1990; MacIntyre et al. 2002; Verburg and Antenucci 2010).

The roughness length for momentum was considered in the formulation of the stability

functions for stable conditions (Imberger and Patterson 1990) with a cut-off of stability

parameter, |zL-1|<15 imposed following the limits set in Imberger et al. (1990). The

stability calculations were iterated until the Monin-Obukhov length scale (L), which is a

ratio of the reduction of potential energy as a result of wind mixing and the


43

development of atmospheric stratification due to the heat flux, converged to within

0.001% between iterations (MacIntyre et al. 2002; Verburg and Antenucci 2010).

The exchange flow, Q (m3 s-1m-1) is derived from Lovstedt and Bengtsson (2008):

.. 0 QTTCLH plantopenPQ (6)

We assume the energy fluxes within the littoral bed were distributed evenly. To

eliminate the influence of density gradient induced by depth variation, heat fluxes were

calculated based on temperature measurements between Station 1 and Station 2 in

each lake. The estimation of the heat fluxes covered the whole one-day cycle for Lake

1 and only the differential heating (day) phase in Lake 12 due to sensor failure. To

check the reliability of the lateral heat flux estimation, the averaged-temperature as a

function of time was calculated at Station 2 using the atmospheric surface fluxes for the

open water and lateral heat fluxes transported from Station 1, and compared with the

measured temperature data (20-min interval) in the area (Lovstedt and Bengtsson

2008).

The dissolved oxygen dynamic pattern will be described elsewhere. Vertical

attenuation coefficients were calculated using the formula in Kirk (1983) to evaluate

light availability for submerged macrophytes. Analysis of means and T-Tests were

performed in PASW Statistic 18 (SPSS Inc.).

3.4 Results

3.4.1 Meteorology and physical-chemical structure

The weather during the April 2010 experiment was at its driest condition with no outflow

as lake level was lower than the weir height. The skies were occasionally cloudy but no

rainfall was recorded during the two days thus inflows into the lakes were negligible. No

information of groundwater was available to account for the temperature differences

however this is thought to have a negligible influence. Wind speed was generally low

during the days with an average wind speed of around 0.8 m/s. Wind speeds were near

zero throughout the nights (Fig. 2a).


44

19/04 20/04 21/04 22/04 23/040

1

2

3

4

5

Win

d s

peed (

m/s

)

19/04 20/04 21/04 22/04 23/04

25

30

35

40

Tem

pera

ture

(oC

)

Lake 1 Lake 12

Figure 2: Variations in (a) wind speed (b) air temperature and surface water

temperature at Lake Chini during the experiment period in April 2010.

Air temperature cooled from a maximum of 30.4C at 1600 h on 20 April to 23.7C at

0600h on 21 April (Fig. 2b). Air temperature was lower than the surface water

temperature (measured at 0.5 m from surface) in both lakes, which has been frequently

observed in tropical lakes and wetlands (Furtado and Mori 1982; Ganf 1974; MacIntyre

and Melack 1984; MacIntyre et al. 2002; Verburg and Antenucci 2010). According to

Verburg and Antenucci (2010), the lower air temperature than the surface water can

cause an increase in atmospheric instability and subsequently an increase in sensible

and latent heat loss and affect differential heating and cooling of the surface the water.

High temperature within the littoral zone vegetation dominated by the floating-leaved

plant, N. nucifera, was consistent with past studies (Coates and Folkard 2009). The

surface water temperature in the littoral area dominated by N. nucifera (Station 1)

reached 1.3C warmer than the adjacent open water (Station 2), which could be due to

sparse N. nucifera cover (13%) in the area and shallower depth for heat absorption.

Mean surface water temperatures were higher than the mean air temperature at the

littoral and pelagic zones in both lakes (p<0.001). Mean temperature differences

between the water-plant surface and the air were higher in the littoral zone of Lake 12

compared to Lake 1, with values of 5.4 and 4.9C, respectively. Higher surface

temperatures above submerged beds could be associated to absorption of solar

radiation at the upper part of the water column. Differential heating during the day was


45

apparent along both transects generating overflows from shallow littoral region to the

pelagic zone as shown by the slanting contour in Fig. 3a and Fig. 3b. A weak

metalimnion, as represented by the 31.8C to 32.6C contours, develop in Lake 1 at

the pelagic region between the 0.5- and 1.0-m depths. Around noon on 20th April,

differential heating began to develop in Lake 12, as the water temperature at the littoral

(Station 1) was nearly 2C greater than those in the upper 0.5 m layer of the pelagic

water at Station 4 (Fig. 3b).

Figure 3: Longitudinal and vertical variations in water temperature on 21st April in (a)

Lake 1 at around 1535, (b) Lake 12 at around 1220, (c) Lake 1 at around 0954, (d)

Lake 12 at around 1750. Numbers and bars at top temperature profile correspond to

station.


46

The surface water temperature cooled more rapidly at the littoral site compared to the

pelagic zone in both lakes during the night between 20 - 22 April 2010. Movement of

cooler littoral water as an underflow current into the deeper pelagic zone is shown by

the position of the 30.7C isotherm (Fig. 3c). In Lake 12, the littoral area cooled sooner

than the pelagic water which could be due to shading by the high-canopy forest where

movement of cooler water starts to develop by late afternoon. Intrusion of cool water

appeared to move away from the lake bed in Lake 12 as shown in the slanting contour

at the surface (Fig. 3d), which could be associated to the presence of a dense

monospecific submerged macrophyte bed (Station 3). The water at the surface and

bottom layer also remained separated at Station 3 (Fig. 4c), which was in contrast to

the pelagic zone (Station 4) (Fig. 4d) where the surface layer was completely mixed

with the bottom layer.

16:48 19:12 21:36 00:00 02:24 04:48 07:12 09:36 12:00 14:24 16:4828

30

32

34

T (

oC

)

Lake 1 - Station 3

Surface (0.5 m)

Bottom (1.0 m)

16:48 19:12 21:36 00:00 02:24 04:48 07:12 09:36 12:00 14:24 16:4828

30

32

34

T (

oC

)

Lake 1 - Station 4

Surface (0.5 m)

Mid- (1.0 m)

Bottom (2.0 m)

16:48 19:12 21:36 00:00 02:24 04:48 07:12 09:36 12:00 14:24 16:4828

30

32

34

T(o

C)

Lake 12 - Station 3

Surface (0.5 m)

Bottom (1.0 m)

16:48 19:12 21:36 00:00 02:24 04:48 07:12 09:36 12:00 14:24 16:4828

30

32

34

T (

oC

)

Lake 12 - Station 4

Surface (0.5 m)

Mid- (1.0 m)

Bottom (2.0 m)

(c)

(d)

Figure 4: Variation in water temperature between the littoral zone and pelagic zone on

20-21 April 2010 (Lake 1) and on 21-22 April 2010 (Lake 12)


47

Over the daily cycle, stratification and vertical mixing patterns were clearly different

between the lakes (Fig. 5). A pattern of diurnal stratification during the day and

complete mixing at night were apparent in Lake 1 and Lake 5, whereas stratification

persisted in Lake 12, with the exception on the morning of 22 April, where the mixed

layer did not reach 2-m. As the magnitude of the wind stress was similar throughout the

period, the lower solar radiation on the previous days (mean=252.8 W/m2) may have

contributed to the deepening of the mixed layer depth to the 2-m layer. A similar

phenomenon was described by (MacIntyre and Melack 1988).

Figure 5: Thermistor time series at the pelagic zone, at (a) Lake 1 (b) Lake 5, and (c)

Lake 12 between 20-24 April 2010.

In contrast to the thermal structure during low water level, a distinct pattern of

stratification was observed in both Lake 1 and Lake 12 during high water level (Fig. 6).

Stratification persisted in Lake 1 with heat been entrained at the upper layer above the

1.5 m depth causing strong metalimnion to develop between the 0.5 and 1.5-m depths

(Fig. 6a). Heat loss during the night induces cooler and denser surface temperature

which led to deepening of the surface mixed layer by morning as shown by the


48

reference contour (26oC) (Fig. 6b). Conversely, the temperature structure in Lake 12

indicates that the littoral area experienced isothermal mixing that induced differential

temperature between the littoral zone and the pelagic zone and the subsequent

intrusion of cold water into the stratified pelagic water (Fig. 6d). Shading by high

canopy forest along lake fringes could contribute to isothermal littoral temperature at

the littoral zone. Steedman et al. (1998) showed that riparian shading reduces the

littoral water temperature and this can cause unstable density gradient to develop as

dense cold water sinks to the bottom (Folkard 2008).

Differences in light penetration were apparent between the low- and high water periods

in both lakes (p<0.001). During low water periods, PAR reached ca. 0 µmols-1m-2 near

the bottom of the submerged stands (Station 3). As qualitatively observed in Lake 12,

wind-driven motion was attenuated over the dense submerged C. furcata stand where

its shoots and floating-leaves covering the surface water. The vertical attenuation

coefficient, Kd (PAR) in Lake 12 (Kd= 1.54 m-1) was not statistically higher (p>0.01) than

in Lake 1 (Kd= 1.47 m-1) during this period, which in contrast to high water period where

Kd (PAR) in Lake 1 (Kd= 2.92 m-1) was significantly higher (p<0.001) than in Lake 12

(Kd= 1.61 m-1). Higher Kd (PAR) in Lake 12 during low water level could be attributed to

humic substances generated by decomposition of plant matter [26]. This is consistent

with mean DOC concentration which were higher in Lake 12 (74.6 ± 5.7 mg/L)

compared to Lake 1 (33.8 ± 25.5 mg/L). The direct inflow of the turbid water of Pahang

River and Gumum River into Lake 1 (Sharip pers. observation) could contributed to

high Kd (PAR) in this lake during the high water period which reduce light availability for

submerged C. furcata. Mean turbidity in Lake 1 (43.1±24.0 NTU) was significantly

higher than Lake 12 (5.2±3.6 NTU) (p<0.001). Higher mean turbidities above 30 NTU in

this lake were associated to flooding by the Pahang River (Shuhaimi-Othman et al.

2007).


49

Figure 6: Longitudinal and vertical variations in water temperature in Lake 1 at around

1540 h on 29th March, at 0820 h on 30th March, and in Lake 12 at 1400 h on 29th March

and at 0910 h on 30th March 2011.

In both lakes, phosphate concentrations were high in the littoral areas as compared to

the pelagic zone (Fig. 7a and Fig. 7b). Phosphate concentration in both lakes

increased with water level (Fig. 7c and Fig. 7d). The pattern of phosphate

concentration in Lake 1 during high water level shows that phosphate concentrations

were higher at Station 3, which is along the deepest area of Lake 1 where the Gumum

River flows. Gumum River, which provides the main outflow from the heavily

agricultural catchment areas, could be the source for the high phosphate concentration


50

in this lake. Higher phosphate concentration near the substrate in Lake 12 during high

water period could be associated to nutrients release from sediments or mineralisation

of litters (Carpenter 1980; Shilla et al. 2006).

Figure 7: Longitudinal and vertical variations in phosphate concentration (mg/L) in Lake

1 on (a) 22nd April 2010, (b) 29th March 2011, and in Lake 12 on (c) 22nd April 2010, (d)

29th March 2011


51

3.4.2 Heat budget and water exchange

The boundary layer above the air-water interface was unstable (zL-1<0) over the period

of 20-23 April 2010 (Fig. 9a & Fig. 9b) causing a higher transfer coefficient than neutral

values (Fig. 9c & Fig. 9d). As transfer coefficients were a function of wind speed, drag

(CD) and heat exchange (CH and CE) coefficients increases as wind increases and z/L

decreases, and, according to MacIntyre et al. (2002), as exchange coefficient increase

buoyancy effects becomes larger and tend to dominates. The hourly averages of the

sensible heat fluxes were small compared with past studies in prairie wetlands [36] and

ranged between -41W/m2-4W/m2. Strong sensible heat fluxes (H) were found mostly in

the early morning. Evaporative heat losses (E) were high during noon and late

afternoon with a maximum value of -270W/m2 in Lake 1 and -208 W/m2 in Lake 12 (Fig.

9e & Fig. 9f). Net radiation reached 790 W/m2, and was an important heat flux that

influences the heating and cooling of the water.

Using the method in Verburg and Antenucci (2010), where roughness lengths of

momentum for smooth flow were used to parameterize the transfer coefficient for wind

speed less than 5 m/s (Smith 1988), mean E were found lower by 9% in Lake 1 and

7.3% in Lake 12 while H was higher by 1.2% in Lake 1 and 3.8% in Lake 12. As the

solar energy from the incoming solar radiation during the day was still larger than these

atmospheric surface fluxes, the stratification and horizontal density gradient were

therefore primarily controlled by differential heating induced by solar insolation.


52

21/04 22/04 23/04 24/04 25/04-20

-10

0

10

z/L

21/04 22/04 23/04 24/040.000

0.002

0.004

0.006

Tra

nsfe

r coef.

CH CD

21/04 22/04 23/04 24/04-300

-200

-100

0

Surf

ace f

lux (

W/m

2)

E

H

LW

21/04 22/04 23/04 24/04 25/04-20

-10

0

10

z/L

21/04 22/04 23/04 24/040.000

0.002

0.004

0.006

Tra

nsfe

r coef.

CH CD

21/04 22/04 23/04 24/04-300

-200

-100

0

Surf

ace f

lux (

W/m

2)

E

H

LW

21/04 22/04 23/04 24/04

0

500

1000

Net

flux (

W/m

2)

Net f lux total surf ace f lux

21/04 22/04 23/04 24/04

0

500

1000

Net

flux (

W/m

2)

Net f lux total surf ace f lux

Figure 9: Air column stability, z/L in (a) Lake 1 and (b) Lake 12; Heat (CE) and

momentum (CD) transfer coefficient corrected for atmospheric stability in (c) Lake 1 and

(d) Lake 12; surface heat fluxes for the open water in (e) Lake 1 and (f) Lake 12.

Surface heat flux components: sensible heat (H), latent heat (E) and net long wave

radiation (LW), and net heat fluxes in the open water in (g) Lake 1 and (h) Lake 12. Net

heat fluxes components: total surface flux (sum of E, H and LW) and net heat flux.

The water temperature distribution within the floating-leaved plant bed was based on

the average temperature from the three sensors that spread along the transect in the

littoral areas (Station 1). The horizontal temperature difference over the study period

was more than 1oC/m for 37% of the time during the day and 25% of the time during

the night. This means the two areas could be recurrently circulated by the convective

currents. We adopted similar approach as in Smith et al. (1995) and Stefan et al.

(1989) in estimating the lateral heat fluxes by simplifying that the exchange flow by

differential heating occurs in the upper half of the water column and a return flow at the


53

bottom. Correspondingly, the exchange flow by differential cooling was assumed to

occur in the lower half of the water column with surface return flow. The atmospheric

heat fluxes and lateral heat transfer between the pelagic and littoral zone in Lake 1 is

shown in Fig. 10a. The measured and calculated depth-averaged temperature (Fig.

10b) indicates that the calculated temperature was reasonably predicted by the heat

budget estimation especially during the differential heating period (p<0.001). Wind

speeds and atmospheric temperature were lowest during the night which led to a highly

unstable boundary layer that could enhance vertical transport and affect the estimation

of the latent and sensible heat fluxes.

The floating-leaved vegetation also affects the energy budget by altering the solar

radiation penetration into the water by surface absorption during the day and lowers the

evaporation heat loss from the surface water (Shaw 1994). Correspondingly, the

estimation of the latent heat fluxes has the largest uncertainties (Lovstedt and

Bengtsson 2008) as plant surfaces not only evaporate but also transpire depending on

the vapor pressure of the surrounding (Shaw 1994). The evapotranspiration over the

plant surfaces depend on the physiological aspect of the species (Lafleur 1990).

N. nucifera has through-flow system which provides efficient transport of convective

flow of gases (Konnerup et al. 2011), subsequently complicates the calculation of the

latent and sensible heat fluxes.

The water exchange between the littoral region covered by aquatic plants and the open

water were derived from the lateral heat flux estimation, which is the heat difference

between the surface heat fluxes and the measured temperature changes. Our results

show the presence of a weak convective current due to differential heating and cooling

between littoral and pelagic zones. The mean lateral heat fluxes were approximately -

155 W/m2 and 160 W/m2 during mid-afternoon and early morning respectively. This

corresponds to a mean current of 1.6 cm/s induced by differential heating between the

floating-leaved N. nucifera stand and open water (Lake 1), and 1.9 cm/s induced by

differential cooling. This result is consistent with findings by Coates and Folkard (2009),

who found weak thermally-driven motion within the littoral zone induced by shading by

the floating-leaved Nymphaea alba based on measurement of the turbulence field

within the vegetative littoral zone (VLZ). To confirm our estimation of the current flow,

the horizontal velocity scale for the vertically mixed water column as described by

Monismith et al. (1990) was calculated, and the velocity of the convective current by


54

differential heating was approximately 1.4 cm/s, which is close to the calculated

velocity based on the surface energy budgets.

14:24 19:12 00:00 04:48 09:36 14:2428

30

32

34

T (

oC

)

14:24 19:12 00:00 04:48 09:36 14:24-400

-200

0

200

400

Heat

Flu

x (

W/m

2)

(a)

14:24 19:12 00:00 04:48 09:36 14:2428

30

32

34

Time of the day

T (

oC

)

(b)

Figure 10: (a) Heat fluxes within the floating-leaved bed on 20-21 April 2010. Heat flux

components: net radiation (dashed line), sensible heat (dotted line), latent heat (thin

line) and lateral heat (solid line). (b) Measured temperature (solid line), and calculated

temperature (star) at open water and littoral temperature (dotted line)

By estimating the hydraulic residence time (t=V/Q) according to Stefan et al. (1989),

the results suggest that the mean areal convective flow induced by differential cooling

and heating could exchange the whole volume of water within the floating-leaved bed

in 1 h and 1.5 h respectively. In contrast to Lake 1, the convective current due to

differential heating was smaller in Lake 12, with a mean velocity of 0.3 cm/s.

3.5 Discussion

3.5.1 Temperature structure and water exchange

Steep temperature gradients were observed between the littoral and pelagic areas in

this floodplain wetland and contributed to the convective circulation between the two

zones. Surrounding topography and macrophyte abundance in the system influenced

the onset and pattern of differential heating and cooling between lakes. Shading by

riparian forest has been shown to reduce littoral temperature in boreal lakes (Steedman


55

et al. 1998), and the early onset of differential cooling at the littoral area of Lake 12

could be associated to the high-canopy forest surrounding the lakes causing early

movement of cooler water towards the pelagic zone. Depth variation and shape of the

lake contributed further to the pattern of differential heating and cooling, resulted to

overflows from shallow littoral region to the pelagic zone during the day and movement

of cooler littoral water as an underflow current into the deeper pelagic zone during the

night (Horsch and Stefan 1988; Monismith et al. 1990). Pattern of movement of cool

water in this lake appears to be shaped by the presence of thick monospecific stands

of a submerged macrophyte. Compared to findings by Herb and Stefan (2004), our

study shows the presence of dense submerged vegetation affects the circulation

pattern induced by the horizontal density gradient.

Stratifications were marked in both lakes during high water level. In contrast to Lake 1,

where stratification persisted at both littoral and pelagic zones by the morning period,

pattern of isothermy observed in Lake 12 at the littoral zone during high water level

could be induced by shading by riparian forest. Few studies have shown the role of

shape and surrounding topography in influencing lake stratification and mixing regime

(MacIntyre and Melack 1984; Melack 1984) which could explain the thermal pattern in

this wetland. Similar patterns of persistent thermal stratification have been documented

in other floodplain lakes in the Amazon River during high water level (Coates and

Folkard 2009; MacIntyre and Melack 1984; Melack 1984). Marked differences of the

light climate in Lake 1 during high water level could be associated with increased

turbidity in the water column, contributed largely from flooding by the turbid Pahang

River (Shuhaimi-Othman et al. 2007). Increased turbidity probably induces temperature

stratification, where penetration of solar radiation reduced to a smaller depth with

surface temperature rise as more heat absorbed at the upper water column and the

bottom temperature falls and becomes colder (Imberger and Patterson 1990).

The estimated flow at the littoral was different between sites due to the different

vegetation effects. Generally, floating-leaved species would alter the amount of

radiation entering the water column by physically covering the water surface with its

leaves (Folkard and Coates 2010). The interception of the radiation would reduce the

depth averaged temperature within the water column (Lovstedt and Bengtsson 2008).

Higher littoral temperature within the floating leaved bed could be attributed to the low

percent cover by N. nucifera floating leaves over the surface water. In addition, N.


56

nucifera stands have emergent leaves, with stems elongated above the water, and this

may affect the light climate. The presence of a dense monospecific stand of C. furcata

enhances daytime stratification in the water column as has been observed in past

studies (Coates and Ferris 1994; Dale and Gillespie 1977a). The plant leaves

underneath the water surface may have prevented light penetrating deeper into the

water column, causing cooler temperatures to develop relative to the pelagic zone

(Coates and Ferris 1994). Surface water above the macrophytes has higher

temperature as a result of attenuated wind-driven mixing and interception of solar

heating at the surface water (Coates and Ferris 1994; Monismith et al. 1990). The

water surface temperature between 0900 – 1800 h was on average 2.5oC warmer in

the littoral zone dominated by submerged species. Temperature gradients of more than

1oC/m would give rise to density differences that can significantly reduce both vertical

and horizontal mixing (Stefan et al. 1989).

The horizontal density flow found in this work was within the rates of convective

currents caused by differential heating and cooling as estimated in past studies; 1 cm/s

between open water and stands of Typha (Waters 1998), 1.5-2.8 cm/s between open

water and reed beds (Lovstedt 2008; Lovstedt and Bengtsson 2008), 3-5 cm/s between

the wetland and lake (Nepf and Oldham 1997), and 2-3.5 cm/s (Palmarsson and

Schladow 2008) and 0.4-11 cm/s (Monismith et al. 1990) between shallower and

deeper water. It should be emphasized that the estimation of the exchange flow in this

study were based on Monin-Obukhov similarity theory which breaks down as wind

speeds decrease to nearly wind-less free convection (Beljaars 1995; Deardorf 1972)

and this could have an effect on the night-time heat flux calculations. Some cautions

are necessary on the values of the estimated current induced by differential cooling as

heat flux calculation during the night rely on latent and sensible heat fluxes under low

wind conditions. Further study is necessary to estimate the heat flux under free

convection within the floating-leaved stand using reliable wind speed measurement

over the water-plant surface at the littoral zone and to determine the associated

exchange flow between vegetated and open water during cooling period.


57

3.5.2 Implications of physical-chemical coupling to macrophyte dynamics

The temperature structure implies a potential feedback mechanism, which could

support the occurrence of alternative states. Reduced floating leaf cover allows

increased heating of the water and the high temperatures and mortality would then

increase decomposition and nutrient release. This nutrient release could promote

submerged C. furcata to grow within the N. nucifera bed.

The induced horizontal density gradient could improve transport of phosphate from the

littoral area to other areas in the pelagic zones. The circulation pattern provides an

increase in dispersion of nutrient allowing for the growth of submerged macrophytes

within the area. As Cabomba populations can uptake nutrients through stems and

shoots (Wilson et al. 2007), the improved nutrient delivery to submerged macrophyte

bed in other areas by horizontal transport may promote the proliferation of submerged

C. furcata in the wetland. The undercurrent may transport nutrients to submerged C.

furcata that grow in deeper pelagic areas (Sharip et al. 2011a) during the night and

early morning, and the available resource could be used by the plant for

photosynthesis during the day for its growth, increasing its abundance.

Additionally, the increase in water column turbidity and thermal stratification in this

wetland, in particular, during the flooding by the main river could lead to anoxic

conditions and subsequently an increase in nutrients. Pattern of persistent thermal

stratification in other floodplain lakes during high water level caused anoxia (Melack

and Fisher 1983) and elevated concentrations of nutrients (MacIntyre and Melack

1988). Anoxic conditions near surfical sediment would likely enhance P-mobilization

(Boers and Zedler 2008) and have the potential to contribute to internal eutrophication

due to enhanced P release from sediments. High phosphate concentrations in the two

lakes during high water level were attributed by different sources of nutrient with run-off

from agriculture area (Shuhaimi-Othman et al. 2007) that flows into the nearby river

and mineralisation of the dominant submerged plant litters (Carpenter 1980; Shilla et

al. 2006) possibly explains the elevated phosphate level in Lake 1 and Lake 12

respectively. The availability of nutrients in the lake after flooding could contribute

further to elevated abundance of submerged macrophytes in the system (Sharip et al.

2011a). The thermal structure in floodplain lakes during low- and high water affects the

oxygen and nutrient dynamics in the water column (Melack and Fisher 1983; Monismith

et al. 1990), which could be an important factor shaping the productivity in the wetland


58

system. Further study should investigate the oxygen dynamic, which is influenced by

the temperature structure as shown by numerous studies (MacIntyre and Melack 1988;

Melack and Fisher 1983), and how it relates to production by the macrophyte in this

wetland.

3.6 Conclusion

This study documents the contribution of aquatic vegetation to thermal structure and

water exchange dynamics. The horizontal density gradient may also be affected by

rooted floating-leaved vegetation and submerged vegetation. Floating-leaved

vegetation creates shading and alters the light climate that could induce convective

motion while submerged vegetation affects the light and temperature pattern by

reducing light penetration, increasing stratification and altering the circulation pattern

which affects the water quality in the lakes. A rise in the water level altered the thermal

gradient and the exchange dynamics and mixing in the lake system and the variation in

physical characteristics of the lake affected the temperature and convective circulation.

This horizontal density gradient and water exchange has important implications for

nutrient cycling in this shallow lake system. A change of macrophyte dominance from

floating leaved to submerged macrophyte affects the physical circulation and exchange

flow and contributes towards improved delivery of constituents such as phosphate and

other nutrients to C. furcata beds. The increased availability of resources could further

promote growth of this submerged species and subsequently contributes to invasion in

the wetland system. The findings from this study have implications for the rehabilitation

and management of the wetland.

3.7 Acknowledgements

The first author gratefully acknowledges the support from National Hydraulic Research

Institute of Malaysia (NAHRIM) for permission to use their data and provision of

financial assistance for the field experiments. Thanks to Ahmad Taqiyuddin Ahmad

Zaki for his assistance and coordination during all field surveys and Justin Brookes for

provision of sensors and assistance during April field experiment.


59

Chapter 4 – Metabolism differences among habitat and

macrophyte communities in a tropical lake3

4.1 Abstract

Aquatic macrophytes differ in their primary productivity and their responses to

environmental changes which in turn modify their biological processes specifically their

eco-physiological dynamics. Different macrophyte species have different growth and

respiratory rates (i.e. metabolism) and this is reflected in the amount of dissolved

oxygen (DO) produced or removed and subsequent changes in DO dynamics in the

water column. This study assessed the dynamics of DO concentration fluctuation

between wetland habitats dominated either by Nelumbo nucifera or Cabomba furcata,

with the aim of estimating the primary production and respiratory rates of the two main

aquatic macrophyte communities in the system. The study adopted benthic chambers

to measure the process rates of benthic metabolism of the macrophyte communities,

including determination of the contribution of microflora and microphytobenthos (MPB)

to the total production of the C. furcata community bed. The free-water metabolism

estimation method was also to look at net ecosystem scale production and respiration

in the littoral and pelagic zones of sub-lakes dominated by the different vegetation

communities. The gross primary productivity (GPP) for the littoral area dominated by

N. nucifera and C. furcata communities were slightly different 3.07 ± 0.48 g O2 m-2d-1

and 3.55 ± 0.34 g O2m-2d-1. The MPB had lower mean GPP and higher community

respiration (R) rates (478.1 mg O2 m2 h-1 and 363.7 mg O2 m

2 h-1 respectively) than C.

furcata (562.7 mg O2 m2 h-1 and 100.3 mg O2 m2 h-1). The free water metabolism

estimation shows the littoral zone in Lake 1, dominated by the floating-leaved species,

was autotrophic (GPP/R>1) while the littoral zone in Lake 12, dominated by the

submerged vegetation, was heterotrophic (GPP/R<1). The pelagic zones in both

lakes, however, were heterotophic. An increase in abundance of the invasive

submergent C. furcata into the pelagic zone increased community production and

respiration and affected net ecosystem production.

3 To be submitted to Hydrobiologia Journal


60

Keywords

Aquatic plants, benthic-pelagic coupling, diel oxygen dynamic, community metabolism,

macrophyte productivity, microphytobenthos

4.2 Introduction

Macrophytes that grow along depth gradients are important ecological components of

productive wetland ecosystems (Wetzel 1992a; Wetzel 1992b), and can shape aquatic

habitat and strongly control ecosystem processes (Rasmussen et al. 2011). Past

studies have shown that emergent vegetation and epiphytic algae on submerged plants

contribute the largest contribution to the overall productivity in many wetlands (Barko et

al. 1977; Howardwilliams and Allanson 1981; Ikusima 1978; Ikusima and Furtado 1982;

Wetzel 1992a; Wetzel 1992b). Macrophyte production is defined as the rate of the

amount of organic carbon produced by plants per unit area (Westlake 1963). The

organic matter produced by aquatic macrophyte productivity subsequently affects the

dissolved oxygen (DO) dynamics in the water column and these changes are often

used to measure variability in the net metabolism of the wetland (Barko et al. 1977;

Howardwilliams and Allanson 1981; Ikusima and Furtado 1982; Penfound 1956; Wetzel

1992a; Wetzel 1979).

Macrophytes with different structure, morphology and physiology respond differently to

the surrounding environment which controls their gross primary production and

respiration. Emergent and floating-leaved species utilise solar energy for

photosynthesis and uptake atmospheric carbon dioxide and release oxygen into the

atmosphere through their internal stomata (Cooke et al. 2005; Longstreth 1989;

Schulthorpe 1967; Wetzel 1990), and subsequently persistent low DO levels were

found in the surface water under their canopies (Caraco and Cole 2002; Frodge et al.

1990). In contrast, submerged macrophytes, with their thin and dissected submerged

leaves, depend on the supply of carbon from the water, either in the form of dissolved

carbon dioxide or bicarbonate ions (Sand-Jensen 1987; Schulthorpe 1967), and they

can produce more oxygen in the water (Caraco et al. 2006; Goodwin et al. 2008;

Hamilton et al. 1995; Melack and Fisher 1983; Takamura et al. 2003).

According to Quinn et al. (2011), native or non-native species of the same life form also

respond differently to the environment due to their inherent differences in ability to

capitalize on anthropogenic disturbance. This functional trait causes variation in a


61

species response to environmental change, which can either contribute to ecosystem

stability or trigger further functional modification (Chapin et al. 1997). Ecosystem

processes, such as productivity, are therefore expected to change as a result of

alteration in species composition (Chapin et al. 1997).

Vegetation dynamics have been reported in Lake Chini, Malaysia between the two

dominant species, the native floating-leaved Nelumbo nucifera and the non-native

submergent Cabomba furcata with a possible shift in macrophyte dominance from

floating-leaved to submerged species (Sharip et al. 2011a). Further assessment of the

productivity of the dominant species (Sharip and Jusoh 2010; Sharip et al. 2011a) will

provide better understanding of the changes in the vegetation and the consequential

alteration of biological processes due to the presence of the non-native species in this

wetland.

To our knowledge no previous research has investigated the primary production of C.

furcata in introduced areas in Malaysia such as in Lake Chini. Populations of a closely

related species, Cabomba caroliniana, in introduced areas were reported to differ from

populations in native ranges (Schooler et al. 2009); they frequently develop thick

monospecific stands (Schooler et al. 2009; Wilson et al. 2007) like many other wetland

invaders (Zedler and Kercher 2004). Studies that have examined the impact of

ecosystem processes by non-native species within the introduced range are also

limited (Ehrenfeld 2010).

Diel changes in dissolved oxygen (DO) are influenced by not only physical and

chemical processes, but also the biological processes of the biotic communities (Kemp

and Boynton 1980; Melack and Fisher 1983). Caraco and Cole (2002) showed the

contrasting impacts of native and non-native species to DO dynamics in their habitat.

As invasive plants have been indicated to respond to the influx of resources by

increasing their growth rates (Zedler and Kercher 2004), the altered ecosystem

processes affect the DO signal in the water column. Therefore, understanding the DO

dynamics can give a better assessment of the macrophyte community metabolic rates,

and the ability for C. furcata to increase abundance and exert dominance in the

system.


62

In this study, we 1) describe the DO dynamics and the associated water quality in Lake

Chini, Malaysia; 2) estimate net productivity and respiratory rates of the communities

dominated by a native floating-leaved species (N. nucifera) and an introduced

submerged macrophyte (C. furcata); 3) estimate the microflora and microphytobenthos

(MPB) production within the C. furcata bed; and 4) estimate the net ecosystem

production in lakes dominated by N. nucifera (Lake 1) and C. furcata (Lake 12). The

study is focused on the productivity of the two macrophyte communities during their

early growth phase, which is after the monsoon season in particular, since C. furcata

seems to have increased its growth from the previous year‟s flood pulse (Sharip et al.

2011a).

4.3 Methods and materials

4.3.1 Experimental design and water quality measurements

Two sets of benthic metabolism experiments were performed at Lake Chini, Malaysia

on 19th-23rd April 2010 using benthic chambers and in-situ DO loggers. The first

experiment was set up to study the habitat and macrophyte community primary

production over a diurnal cycle in two differing lakes; Lake 1 is dominated by N.

nucifera (floating-leaved) and Lake 12 is dominated by C. furcata (submerged species).

The second experiment was set up to study C. furcata metabolism in particular, and to

separate the production and respiration signal of the plants from the production and

respiration of MPB and sediment microorganisms within the littoral zone of Lake 12.

The experiment took place on 22 April 2010 between the periods of 10 am to 6 pm.

Free water DO measurements were obtained by deploying a several oxygen loggers,

both within the littoral and pelagic zones of each lake, and this data was subsequently

used to estimate net ecosystem metabolism rate, described below. In addition, pH and

chlorophyll concentration were recorded by the multi-parameter probe at the centre of

each lake. The locations of the experiments and type of sensors used and parameters

measured are detailed in Chapter 3.

Water samples were collected at all sites and analysed for PO4, NO3, NO2, total

phosphorus, total nitrogen, DOC and chlorophyll-a concentration following APHA

(1992). At the end of 24 h cycle, additional water samples were only collected from the

benthic chambers dominated by N. nucifera and analysed for PO4, NO3, NO2, total

phosphorus, total nitrogen and DOC. Water samples were not collected from the


63

benthic chambers dominated by C. furcata due to failure of the battery connecting the

pumps to the chambers.

4.3.2 Benthic metabolism (specific processes, small scale)

4.3.2.1 Benthic chamber deployments

In the littoral areas, benthic chambers consisting of Perspec domes with an internal

base diameter of 28.5 cm and volume of approximately 10 L, were used in triplicates

(Aldridge et al. 2009). They were gently pushed into the substrates to avoid sediment

disturbance in each lake. A 12-V submersible pump was used to re-circulate the water

within the chambers. The TPS WP82 sensor was inserted into the top of each chamber

to measure DO and temperature at 20-minute intervals to capture a complete day-night

cycle. At the end of the 24 hrs, vegetation from each chamber dominated by C. furcata

was harvested and dried to constant weight, while benthic chambers from the littoral

areas dominated by N. nucifera were removed and re-deployed at another sites

dominated by C. furcata to estimate the effect of C. furcata plant biomass. At the end of

the second experiment, vegetation were also harvested from chambers containing C.

furcata plants and dried to constant weight.

4.3.2.2 Estimates of diurnal productivity

Net ecosystem productions (NEP) and community respiration (CR) of the littoral

macrophytes were estimated based on oxygen differences between the initial and final

concentrations (mmol O2 m-2 h-1) in chambers as described in Bott et al. (1978) and

Longphuirt et al. (2007): NEP (or R) O2 = ΔO2 V / s. Δt, where ΔO2 is the change in DO

(mg L-1), V is the volume of water enclosed by the benthic chamber (L), s is the surface

area (m2), and Δt is the incubation time (min). NEPday were calculated for each 20 min

interval from 60 min past dawn to 20 min before dusk. We calculated R for every 20

min interval from 20 min before dusk until 60 min past dawn and divided the results by

time length (h) to produce estimates for hourly rate of ecosystem respiration during

night-time. This study assumed that Rday is equal to Rnight, and the hourly rates were

multiplied by 24 hours to estimate Rdaily. GPP is the sum of Rday and NEPday and

NEP=GPP-Rdaily.

4.3.2.3 Estimates of MPB respiration and production

The DO changes in chambers at sites with bare sediments (ie. vegetation removed)

were compared with chambers at sites with sediment and C. furcata plants; together


64

the data was used estimate the difference between sediment microflora and MPB, and

C. furcata production. The chambers were left for 3 hours under light conditions and

then covered with black plastic for dark incubations for another 3 hours. Net production

was estimated based on the increase in oxygen under light condition and community

respiration was calculated based on the decline in oxygen under the dark condition.

NEP and R for MPB were estimated for each 20 min interval over the 3 h light- and

dark- incubation period respectively. GPP for C. furcata was calculated as the

difference between the GPP in chambers having plants, and chambers without plants.

4.3.3 Net ecosystem metabolism

4.3.3.1 Diel oxygen measurements

In the littoral water, a TPS WP82 DO-temperature sensor was deployed at about 20 cm

below the surface. In the pelagic waters, sondes were deployed at two sites at each

lake in pelagic waters (Sharip et al. 2011b) to measure the free water DO. A D-Opto

logger (Zebra-Tech Ltd, Nelson, New Zealand) and a multi-parameter profiler-hydrolab

model DS5X (Hach environmental, CO, USA), were deployed at approximately 0.5-m

depth and 1-m depth respectively. The sondes were first deployed at Lake 1 for two-

days for simultaneous oxygen and temperature measurements, and then redeployed to

Lake 12 to record similar data over a two day period.

4.3.3.2 Estimates of free water metabolism

We estimated metabolism for the pelagic zone from the oxygen signal using a similar

method as described in Hanson et al. (2003) and Lauster et al. (2006). This method

assumes NEP20min=ΔO2-D, where ∆O2 is the change in oxygen concentration over 20

min, and D is the gas exchange through diffusion in this period estimated as D=k (O2-

O2sat). NEPday were calculated for each 30 min interval from 30 min past dusk until 30

min before dawn to reduce the error. Estimation of D is based on the assumption of a

vertically well-mixed epilimnion over the survey period. The oxygen exchange

coefficient, k was calculated from a gas piston velocity K600 and Schmidt numbers (Sc)

according to Jahne et al. (1987) and Staehr et al. (2010b). As the wind speed during

the experiment was very low (Sharip et al. 2011b), K600 were estimated using the low

wind equation of Cole and Caraco (1998). We assume a neutrally stable boundary

layer and use the wind speed referenced to 10 m height above the surface.


65

We calculated R for every 30 min interval from 30 minute past dusk until 30 min before

dawn and divided the results by time length (h) to produce estimates for the hourly rate

of ecosystem respiration during the night-time. The half-hour buffers were considered

to reduce the estimation errors in R due to inflection in gas curve around dusk and

dawn (Hanson et al. 2003) and the interference from photosynthesis at low light

(Markager and Sand-Jensen 1989). During darkness, GPP=0 and the change in

oxygen is the sum of respiratory at night and air-water exchange through diffusion.

During the day, the change of oxygen is caused by the difference between GPP,

daytime R and D. Similarly, Rday is assumed to equal to Rnight, and the hourly rates were

multiplied by 24 hours to produce Rdaily (Cole et al. 2000; Hanson et al. 2003; Lauster et

al. 2006). The sum of NEPday and Rday provide the GPP for mixed layer metabolism.

4.3.4 Data analysis

Production and respiration rates were converted to carbon units following the method

described in Bott (2007) and using the photosynthetic quotient for tropical habitats of

1.35 (Westlake 1963) and respiratory quotient of 0.85 (Furtado and Mori 1982). GPP

for C. furcata was converted to dry weight unit by dividing with the average biomass

(DW/m2). All analyses of means and test of differences were performed in PASW

Statistic 18 (SPSS Inc.).

4.4 Results

4.4.1 Oxygen fluxes and water quality measurement

Diel changes of DO in benthic chambers were apparent in both lakes (Figure 1),

regardless of whether they were dominated either by the submerged species or

floating-leaved plants. Diel DO changes between the two sites showed that the rates

of oxygen consumed were increased in the littoral communities. The reduction in DO

concentrations from noon to evening were apparent in the littoral area dominated by

submerged species with depletion of oxygen in all chambers during the night and early

morning. Rapid increases in DO shortly after dawn were not detected even though

dissolved CO2, produced by respiratory activities during the night, would likely be large

in the chambers. DO concentrations during the day were correlated to solar radiation

due to plant photosynthetic requirements with sharp increases in DO concentrations in

all chambers from morning to noon. DO fluxes at littoral areas within the floating-leaved

stands changed more dramatically with solar radiation due to the littoral community‟s

direct exposure to sunlight. DO fluxes in the submerged bed tended to stabilise during


66

noon after it reached the DO level at the air-water interface. Mean DO concentration in

chambers at sites dominated by N. nucifera was 3.0±1.9 mg/L, significantly higher

(t=9.6, df=424, p<0.001) than the mean DO concentration in chambers at sites

dominated by C. furcata which was 1.4±1.6 mg/L.

-100

0

100

200

300

400

500

600

700

-50

0

50

100

150

200

250

14:52 19:40 00:28 05:16 10:04 14:52

So

lar

rad

iati

on

(W

/m2)

O2μ

mo

l li

ter

-1

Time of day (h)

Nn1 Nn2 Nn3 Cf1 Cf2 Cf3 FW Solar

Figure 1: Diel DO measurement; Nn1, Nn2, Nn3 (sondes deployed in benthic

chambers within N. nucifera bed), Cf1, Cf2, Cf3 (sondes deployed in benthic chambers

within C. furcata bed), FW (sonde deployed at free water in the littoral area).

DO values in chambers between Lake 12 with and without plants (C. furcata) showed

significant differences between light and dark incubations (p<0.001) (Figure 2).


67

0

20

40

60

80

100

120

140

10:33 12:21 14:09 15:57 17:45

02 µ

mo

l li

ter-

1

Time of day (h)

Cfwp1 Cfwp2 Cfwp3 Cfwo1 Cfwo2 Cfwo3 FW

Figure 2: Diel DO measurement; Cfwp1, Cfwp2,Cfwp3 (sondes deployed in benthic

chambers with plant), Cfwo1, Cfwo2, Cfwo3 (sondes deployed in benthic chambers

without plants), FW (sonde deployed at free water in the littoral area)

The amplitudes of the changes in DO concentrations in free water were significantly

differed between littoral and pelagic areas (p<0.001) (Figure 3a). Higher DO values

were found in Lake 1 at 0.5m depth. Conversely, the amplitudes of DO concentration in

pelagic water of Lake 12 showed a different trend between the two depths. A higher

DO amplitude at 0.5-m depth could be associated to the presence of dense submerged

macrophytes stand. Mean DO in pelagic zone in Lake 1 was 7.4±0.5 mg/L, also

significantly higher (t=35.9, df=672, p<0.001) than the mean DO in pelagic zone in

Lake 12 which was 5.7±0.8 mg/L. DO concentrations at all sites were correlated with

temperature (r = 0.598, p<0.001).

Significant differences between lakes were apparent in chlorophyll concentration and

pH values (p<0.001). Time series profile in the pelagic zone at 1-m depth also showed

higher variability in pH and chlorophyll concentration in Lake 1 compared to Lake 12

(Figure 3b and Figure 3c). Differences in mean chlorophyll concentrations and pH were

much lower in the pelagic zone of Lake 12 (t=75.8, df=224, p<0.001) and (t=51.3,

df=216, p<0.001). The mean chlorophyll-a concentration, however, was higher in Lake


68

12 (7.6 ± 5.1μg/L), compared to Lake 1 (5.4 ±1.2 μg/L) with highest chlorophyll-a

concentration found near the dense C. furcata beds.

Differences in PO4, NO3, NH4 and DOC concentration before and after diel cycle at

Lake 1 were not statistically significant (p>0.05). Mean above-ground dry weights for C.

furcata was 23.5 g DW/m2.

20/04 21/040

100

200

DO

(um

ol/L)

Lake 1

0.2m

0.5m

1.0m

22/04 23/040

100

200

DO

(um

ol/L)

Lake 12

0.2m

0.5m

1.0m

20/04 21/044

5

6

7

pH

22/04 23/044

5

6

7

pH

20/04 21/040

5

10

Chl (u

g/L

)

22/04 23/040

5

10

Chl (u

g/L

)

Figure 3: Sondes measurement in the pelagic zone on 19th - 21st April 2010 (Lake 1)

and on 21st – 23rd April 2010 (Lake 12), (a) diel DO concentration (μmol/L) (b) pH (c)

chlorophyll concentration (μg/L)

4.4.2 Diurnal productivity

Lowest values of NEP were found at the littoral area compared to the pelagic zone in

both lakes. NEP at littoral sites dominated by N. nucifera and C. furcata bed were

negative (Table 1). Lower GPP in sites dominated by the native N. nucifera (3.07 ±

0.48 g O2 m-2 d-1) was not statistically different (p>0.05) from GPP in sites dominated

(a)

(b)

(c)


69

by the non-native C. furcata (3.55 ± 0.34 g O2 m-2 d-1). We found NEP was much less

negative in sites dominated by the submerged species, which is comparable with the

study by Lauster et al. (2006). Both littoral zones dominated by either the floating-

leaved species or submerged vegetation were heterotrophic (GPP/R<1) while the

pelagic zones were autrotophic (GPP/R>1).

Table 1: Diurnal productivity estimated from the benthic chamber deployments. Error

bars are ± SD

Parameter GPP,

g O2 m-2 d-1

(g C m-2 d-1)

R,

g O2 m-2 d-1

(g C m-2 d-1)

NEP,

g O2 m-2 d-1

(g C m-2 d-1)

GPP/R Mean

T

- Lake 1

N. nucifera

3.07 ± 0.48

(0.85 ± 0.03)

4.02 ± 0.17

(1.28 ± 0.06)

-0.93

(-0.42)

0.77

31.2

- Lake 12

C. furcata

3.55 ± 0.34

(0.99 ±0.10)

3.73 ± 0.75

(1.19 ± 0.24)

-0.17

(-0.20)

0.97

31.7

4.4.3 MPB and microflora respiration and production

The mean GPP in sites having the combined C. furcata, microphytobenthos and

sediment community were higher (p<0.01) compared to the mean GPP measured from

sites with bare sediments and MPB (i.e., vegetation removed). Comparison between

sediments and C. furcata showed MPB and microflora had a lower mean NEP (p<0.01)

compared to C. furcata. In contrast, the MPB and microflora bed had a higher

respiration rate (p<0.05) compared to C. furcata. Photosynthetic activity by C. furcata

contributed to higher DO in chambers having macrophytes. Lower DO in chambers of

bare sediments (without plants), specifically during the night, indicates higher DO

consuming activities. The GPP and R of C. furcata were 562.7 mg O2 m-2 h-1 (23.9 mg

O2 g-1 DW h-1) and 100.3 mg O2 m2 h-1. The value of the GPP obtained using this

method is much higher than the simple technique method (Vollenweider 1969), which

calculate NEP and R based simply on changes in DO concentration between initial DO

values and final DO values (light- and dark- incubation). The GPP calculated using the

equation of Vollenweider (1969) was 62.5 mg O2 m-2 h-1.


70

Table 2: Comparison of MPB/sediment vs C. furcata productivity. Error bars are ± SD

Parameter GPP,

mg O2 m-2 h-1

(mg C m-2 h-1)

R,

mg O2 m-2 h-1

(mg C m-2 h-1)

NEP,

mg O2 m-2 h-1

(mg C m-2 h-1)

C. furcata + sediment

+ MPB

1040.8 ± 158.5

(331.8 ± 44.0)

464.0 ± 184.4

(147.9 ± 51.2)

576.9 ± 140.6

(160.2 ± 39.1)

Sediments + MPB 478.1 ± 208.7

(152.4 ± 58.0)

363.7 ± 148.9

(115.9 ± 41.4)

114.4 ± 129.3

(31.8 ± 35.9)

C. furcata 562.7 100.3 462.4

4.4.4 Free water productivity

The estimated k600 in this study was at the average of 0.55±0.06 (SE). We used a

constant value of 0.55 m d-1 as the basis for the diffusion calculation. This value is

about 10% larger than the values used in past studies (Lauster et al. 2006). Few

studies have shown the uncertainties in the estimation of k (Staehr et al. 2010a), an

over estimate in k resulted in an underestimate of R, which would affect NEP but this

error was more apparent in an unproductive lake. Tropical swamps, such as Lake

Chini, are a highly productive systems (Howard-williams and Gaudet 1985; Ikusima

and Furtado 1982; Sharip and Jusoh 2010; Westlake 1963), so this study does not

investigate variability in errors of k. Air-water exchanges were low in both lakes with

mean diffusion 0.15 g O2 m-2 h-1 in Lake 1 and 0.18 g O2 m

-2 h-1 in Lake 12.

The mean NEP for both the littoral and pelagic zone was positive for both lakes. In the

pelagic zones, GPP in Lake 12 was higher (p<0.05) than GPP in Lake 1. In the littoral

zones, GPP in Lake 12 was lower (p<0.05) than GPP in Lake 1. The ratios of GPP/R

were negative in Lake 12 in both littoral and pelagic zones which indicates

heterotrophy. While in Lake 1, the ratio of GPP/R was negative in the pelagic zone and

positive in the littoral zone.


71

Table 3: Gross primary production (GPP), respiration (R) and NEP for littoral and

pelagic sites in free water. Error bars are ± SD

Parameter GPP,

g O2 m-2 d-1

(g C m-2 d-1)

R,

g O2 m-2 d-1

(g C m-2 d-1)

NEP,

g O2 m-2 d-1

(g C m-2 d-1)

GPP/R Mean

T

Littoral

- Lake 1

- Lake 12

8.19

(2.28)

4.10

(1.14)

7.71

(2.46)

-2.37

(-0.76)

0.48

(-0.18)

6.48

(1.90)

1.06

-1.72

31.1

30.7

Pelagic

- Lake 1

- Lake 12

1.81

(0.50)

3.69

(1.02)

-1.91

(-0.61)

-0.94

(-0.30)

3.72

(1.11)

4.63

(1.32)

-0.95

-3.95

31.8

30.8

4.5 Discussion

4.5.1 Oxygen fluxes and water quality dynamic

Diel DO dynamics in the benthic chambers were evident in both lakes indicating the

metabolism of the benthic communities in producing and consuming DO. The decrease

in DO concentration in chambers having the submerged C. furcata communities was

more rapid due to the contribution of microphytobenthos within the communities that

depleted DO in the chambers. Formation of fluffy organic-rich sediments through plant

litter contributed to higher rates of decomposition of organic matter (Esteves et al.

1994) that consumes DO. Reduced DO at the sediment-water interface could caused

DO to be exhausted in chambers after periods longer than 12 h (Ibarra-Obando et al.

2004). The lags of DO after sunrise could be associated with a high consumption of DO

by decomposition of organic matter exceeding phosynthetic activities by the two plant

species under low light conditions.

Lower amplitude of diel DO in the pelagic waters were driven by wind-induced mixing.

However, wind speeds were mostly low (<2 ms-1) in this wetland (Sharip et al. 2011)

which could contribute to low air water exchanges and under-saturation of DO. Similar


72

under-saturation values have been reported in other floodplain lake, the Lake Calado

(Melack and Fisher 1983). Correspondingly influx of DO from the atmosphere could be

suggested as major source of DO (Melack and Fisher 1983) in this wetland.

Temperature is one of the main environmental factors regulating photosynthetic

process (Sand-Jensen 1989). Our study shows strong correlation between temperature

and DO concentration consistent with past studies (Staehr and Sand-Jensen 2007;

Staehr et al. 2010b). High temperature decreases the DO solubility and has a strong

effects on the metabolic rates including increases respiratory rates (Cooper 1975;

Howard-williams and Gaudet 1985; Rasmussen et al. 2011) and GPP (Rasmussen et

al. 2011). Higher surface temperature at the littoral areas (Sharip et al. 2011) could

partly explain the high community respiration at these sites.

Diel variability of pH in the pelagic water of Lake 1 suggested a productive system

where photosynthethic activities exceeded respiration during the day resulting in

increases in pH (Cole and Prairie 2010). Dominant respiratory activities during the night

decreases pH (Cole and Prairie 2010). Steady pH values ~5 were observed throughout

the day in Lake 12. Esteves et al. (1994) found a similar lack of diel pattern in pH

values in Lake Mussura, Amazon, and suggested the homogeneous pattern in pH

values was attributed to the lower thermal stability of the water column and smaller

phytoplankton biomass. This low pH value is within the range of pH preferred by other

Cabomba species (Schooler et al. 2009). Many studies have shown that pH and

dissolved CO2 were correlated with Cabomba abundance (Camargo et al. 2006;

Sanders 1979; Schooler et al. 2009; Tarver and Sanders 1977). Lower pH increases

dissolved CO2 availability and subsequently increases Cabomba biomass and

abundance (Schooler et al. 2009).

Lake 1 indicates diel variation of the chlorophyll concentration time series which could

be attributed to the phytoplankton photosynthetic activites (Esteves et al. 1994).

However, higher chlorophyll-a concentrations over the transect in Lake 12 also

suggests the contribution of the attached algae and utricularia-periphyton assemblages

in the total primary productivity.


73

4.5.2 Diurnal productivity

This study provides estimation of the photosynthetic and respiratory rates of the two

studied aquatic plant species communities during the early growth period. The findings

of this study shows that NEP were consistently negative in the littoral areas in both

lakes, which implies that the littoral zone consumes more organic matter and

subsequently tending toward a net heterotrophy (Lauster et al. 2006; Van de Bogert et

al. 2007). The positive NEP at the pelagic areas implied that this area tends toward net

autotrophy, and this compares with results by Barbosa et al. (1989), Lauster et al.

(2006) and Van De Bogert et al. (2007).

Our findings shows littoral communities dominated by submerged C. furcata vegetation

has higher mean GPP and lower mean R than littoral communities dominated by N.

nucifera, which could be due to higher abundance of the introduced species (Sharip et

al. 2011). The mean biomass of the submerged species were 10-fold than the mean

biomass of the floating-leaved vegetation. The dense monospecific stands of C. furcata

could explain this high above-ground biomass compared to floating-leaved species

where biomass accumulates in the rhizome. Dense communities of submerged

macrophytes and attached algae have high rates of photosynthesis in light and

respiration in the dark (Sand-Jensen 1989). Martin et al. (2007) also found higher

community respiration by invasive Crepidula fornicata species in European Bays in

sites having a high density stand (>1000 individuals/m2). Lower mean R in C. furcata

sites was associated to depletion of DO in the chambers by early morning period.

The GPP of C. furcata within its native sites, based on incubation of isolated shoot

method, ranged between 6.5-17.5 mg O2 g-1 DW h-1 (Camargo et al. 2006). In this

study, mean GPP for C. furcata communities, which was based on an in-situ oxygen

exchanges in light and dark enclosures, was much higher (23.9 mg O2 g-1DW h-1) than

the GPP within its native areas. The higher values could be associated to higher

production in the introduced areas, however, the DO fluxes measured was not

corrected for contribution by the attached algae or periphyton which could affect the

actual calculation.

4.5.3 MPB and microflora respiration and production

In the second experiments at the C. furcata sites, higher GPP in chambers having the

submerged macrophytes compared to chambers with bare sediment could be


74

contributed by the plant‟s photosynthetic activities. Conversely, higher respiration in

chambers having bare sediments indicated the strong contribution of the

decomposition of organic matter and respiration by microphytobenthos and microflora

community in the sediment. Sediments at C. furcata sites were fluffy, consisted

probably by clumps of masses containing detritus, algal mat and utricularia-periphyton

communities.

Utricularia spp. have been found to co-occur with C. furcata in many parts of this

wetland and were highly correlated with the submerged species (Sharip unpublished

data) and have been reported to form large clumps in tropical wetlands (Ikusima and

Furtado 1982; Lim and Furtado 1974; Schulthorpe 1967) and subtropical swamps

(Bosserman 1983; Greening and Gerritsen 1987). Utricularia mass reached maximum

density during the dry season in Tasek Bera, another shallow lake located about 50 km

away from Lake Chini (Lim and Furtado 1974). Heavily attached with rich epiphytic

algal community (Ikusima and Furtado 1982), this mass appeared brown and starts to

senesce in April (Lim and Furtado 1974) and could contribute to fluffy masses in this

system. This high organic matter decomposition consumes more oxygen which

contributes to higher respiratory activity. Algal mats have been shown to differ with

over-story vegetation with net primary productivity varying between 0.8 to 1.4 times of

the vascular plant (Zedler 1980). However, the NEP ratio of sediment communities,

including the algal mat, to C. furcata was only 0.25.

4.5.4 Free water productivity

This study confirms previous findings that differences in types of macrophytes can

contributed to metabolism differences among lakes (Caraco and Cole 2002; Lauster et

al. 2006). In addition, the results indicate that biotic communities affect the ecosystem

processes (Chapin et al. 1997). According to Ehrenfeld (2010), process rates would be

affected by the alteration in the abundance of species that differ in their structure and

traits. As C. furcata and N. nucifera have different structure, morphology and

physiology, the increasing abundance of C. furcata in the system (Sharip et al. 2011)

could contribute to modification of the plant‟s primary productivity and the overall rate of

carbon fixation in the system.

The results in this study shows both pelagic and littoral zones in Lake 12 which were

dominated by C. furcata, appeared to be heterotrophic (GPP/R<1) at the time of


75

sampling. Hanson et al. (2003) associated heterotrophy to high mean DOC exceeding

> 10 mg/L. Mean DOC concentration was found to be higher in Lake 12 (Sharip et al.

2011), which could contribute to this heterotrophy condition. However, limited data

caused insignificant correlation between NEP and DOC and other nutrient variables to

associate the heterotrophy to DOC. Higher GPP and R in the littoral zone of Lake 1

could be caused by higher advection associated to convective motion or movement of

water by a nearby river.

Our study assumed mixed layer metabolism in the pelagic zone. Recent studies have

shown that metabolism can be affected by physical processes such as internal waves

(Hanson et al. 2008; Kemp and Boynton 1980; Melack and Fisher 1983) and

microstratification (Coloso et al. 2010) at smaller scales of minutes to hours. These

physical processes were not quantified in this study and could affect the estimates of

metabolism in the pelagic zone.

The results of this study showed that the GPP and biomass accumulation were higher

in C. furcata bed than the N. nucifera stand. Increasing abundance and density of C.

furcata in many parts of the lake basins (Sharip et al. 2011) indicates that C. furcata is

increasing its production rates. Although our results support the assertion that the

wetland is shifting to submerged macrophyte dominance (Sharip et al. 2011) based on

the growth rate of the non-native species, quantifying the wetland response to shift of

macrophyte dominance and ecosystem succession requires long-term measurements

of the macrophyte growth (Staehr and Sand-Jensen 2007) spanning over decade

timescales (Carpenter and Lodge 1986).

4.6 Conclusion

This study is the first comparison of primary production between N. nucifera and C.

furcata communities in Lake Chini, Malaysia. This study also provides the first

estimation of production rates of the submerged C. furcata in its introduced range. The

results of this study indicate that the benthic communities are an important component

structuring the oxygen dynamics and subsequently the water quality in the system. The

high rate of carbon fixed by the introduced species, C. furcata into the sediment

indicates that the species is affecting further the ecological processes and may be a

contributing factor related to its increasing dominance in the system.


76

4.7 Acknowledgements

The first author gratefully acknowledges the support from National Hydraulic Research

Institute of Malaysia (NAHRIM) for provision of data and financial assistances for the

field experiments.


77

Chapter 5 – General discussion and conclusion

This thesis provides a better understanding of the current state of macrophyte

composition in Lake Chini, Malaysia, with a particular focus on the factors that

contribute to the relative abundance of the native floating-leaved macrophyte N.

nucifera and the invasive submerged macrophyte C. furcata. The impact of the physical

and biological processes that contribute to shifts in dominance of these macrophytes

when invasive species are present was also assessed in two studies. This chapter

summarises and discusses the results of these studies focusing on the characterisation

of macrophyte composition, physical and biological properties of the lake system, and

the associated management implications. Overall, this thesis contributes to an

improved understanding of the ecosystem processes that control the shift of

macrophyte dominance.

5.1 Summary of research findings

Chapter 2 placed the present issue of invasion in the context of the changes in Lake

Chini, Malaysia from 1975 to 2007. Conversion of lands to agricultural areas has

occurred and surface water phosphate concentrations have increased. In addition, in

1995 a weir was constructed in Chini River, downstream of the lake, and water level

fluctuations have subsequently reduced. This could cause a significant reduction of N.

nucifera communities as observed in the literature. Since a large flood of 2007, N.

nucifera has returned to the lake and C. furcata has spread widely to the lake sub-

basins. Currently, the plant communities were found to be highly variable in the lakes,

both spatially and temporally, potentially driven by flood and nutrient dynamics. The

main environmental variables that were found to affect their composition were depth,

nutrient concentration and sediment type.

The results presented in Chapter 2 also indicated that invasion by C. furcata strongly

associated to the overall plant community assemblages, with plant diversity decreasing

as the abundance of C. furcata increases. The observation that C. furcata biomass

increased after the monsoonal season suggests that C. furcata can establish quickly

after flooding events and may become increasingly abundant. Based on this data a

conceptual model of variation in ecological processes was developed. This model

indicated that possible alternate states can exist where the lake becomes dominated by


78

either the N. nucifera or C. furcata, and the changes in dominance of these species

and the associated feedback mechanism that shapes the stable states are influenced

by flooding regimes and nutrient concentrations. Recurrence of flooding creates a

turbid and low nutrient environment that could provide the feedback that gives N.

nucifera advantage over C. Furcata, while low water fluctuation and nutrient-rich waters

could provide the feedback for C. furcata to become dominant over N. nucifera.

The results from Chapter 3 indicate that both N. nucifera and C. furcata contribute to

the thermal structure and water exchange dynamics of the wetland system. The

floating-leaves of N. nucifera created shading that produced a differential temperature

gradient between littoral areas and open pelagic regions. This temperature differential

induced a density driven flow. The dense monospecific stands of C. furcata increased

stratification and appeared to alter the circulation pattern by causing changes in

movement of cool water intrusion, and improved delivery of constituents to other C.

furcata beds.

The water exchange dynamics of the wetland system was also influenced by patterns

of horizontal and vertical mixing dynamics. The horizontal mixing dynamics were

affected by the physical characteristics of the lake, including its shape, depth and

surrounding topography, which influenced the onset of differential cooling and heating.

The vertical mixing pattern changed during high-water periods with marked increases

in turbidity and thermal stratification. Also, phosphate was observed to increase

possibly due to run-off from agriculture areas, mineralisation of the dominant

submerged plant litters during high-water period and internal eutrophication.

Chapter 4 compared the gross primary productivity (GPP) of the littoral areas and the

overall lake production. The microphytobenthos associated with C. furcata communities

had lower mean rates for Net Ecosystem Production (NEP) and higher mean rates for

community respiration (R). The production rate of C. furcata in this system, as

measured using an in-situ oxygen exchange in light and dark enclosures, was higher

than the production rate of C. furcata reported in its native areas using an incubation of

isolated shoot method.

The overall lake production indicates higher NEP in Lake 12, which was dominated by

the C. furcata than in Lake 1, which was dominated by N. nucifera. In this study, Lake


79

12, was heterotrophic (GPP/R<1) in both littoral and pelagic zones while Lake 1 was

heterotrophic (GPP/R<1) in the littoral zone and autrotophic (GPP/R>1) in the pelagic

zone. An increase in abundance of C. furcata throughout the pelagic zone increased

community production, R and NEP.

5.2 Discussion

This study hypothesises that dynamic vegetation in the Lake Chini ecosystem has

possibly two alternate stable states that are dominated by either N. nucifera or C.

furcata. These alternative states are thought to be influenced by the flood regime and

resource availability in this floodplain wetland. According to the conceptual model of

ecological processes that was developed, if natural flow could be re-established in this

system, the stable state dominated by N. nucifera would be more likely to predominate

as this macrophyte can better tolerate flooding and fluctuating water levels than C.

furcata. The flushing out of available nutrients by high magnitute and high frequency

floods, and the increased duration of turbid environments may weaken invasion of this

submerged species by limiting both the light and nutrients availability for its growth.

This process which weakens C. furcata growth, provides an internal feedback

mechanism that can strengthen N. nucifera dominance. In contrast, the increase in

nutrient availability as a result of continuing landscape alterations and reduction in the

flushing out of available nutrients by a reduction in flood magnitude as a result of fixed

structures or weir, may strengthen the potential for invasion by C. furcata. These

processes which strengthen C. furcata growth provide an internal feedback

mechanisms that weaken N. nucifera dominance and sustain the invaded state. Other

studies have shown the importance of internal feedbacks mechanism in sustaining

ecosystem states (Scheffer and Carpenter 2003; Suding et al. 2004; Suding and Hobbs

2009; Zedler 2009). In Zedler (2009), for example, invasion of Phalaris arundinacea

could be reduced when feedbacks that maintain the natural state such as processes

that thicken sedge meadow canopy and diminish the potential for seedling

establishment is strengthened while invasion of Phalaris arundinacea could be

increased when feedbacks in the sedge meadow state is weakened. Further studies

should explore and test the potential feedback mechanisms that drive the alternate

stable states in this system.

C. furcata was more dominant and more widespread than N. nucifera in the sub-basins

of this wetland. C. furcata also appeared to be adapted to annual flooding events,


80

which implies that the invasive species could grow more rapidly than N. nucifera

following a flood event due to abundance of available nutrients. The ability of C. furcata

to adapt to annual flooding, quickly re-establish biomass and uptake nutrients from

water and sediments are attributes that may contribute to its invasive capacity. Studies

have shown that species with multiple attributes such as these have enhanced

invasiveness (Zedler and Kercher 2004). In contrast, N. nucifera dominated in lake

sub-basins that were closer to the weir and were therefore more directly affected by the

floods. The presence of the weir reduces the frequency of small to medium magnitude

floods into the wetland. Floods that exceed the weir height and are short in duration will

reach the nearby lakes where N. nucifera is abundant.

The results of this study shows the importance of water depth and sediment type in

influencing the macrophyte composition and structure of Lake Chini, which are

consistent with past findings. Water depth and substrate type have been identified as

some of the key environmental factors that affect Cabomba abundance in Australia

(Schooler and Julien 2006; Schooler et al. 2009). Fluctuating water levels have also

been found to sustain the dominance and seasonal variations in Nelumbonaceae in

temperate lakes and ponds such as Lake Kasumigaura (Nohara and Tsuchiya 1990),

Ojaka-ike Pond, Chiba Japan (Kunii and Maeda 1982), Old Woman Creek, Lake Erie

(Whyte 1996) and Volga Elta, Russia (Baldina, Leeuw et al. 1999).

From the results in Chapter 2, it is shown that invasion by introduced C. furcata affects

plant community composition and reduces diversity. This is consistent with numerous

studies that have shown the negative effect of invasion on diversity (Kercher and

Zedler 2004; Schooler et al. 2006). This decrease in diversity could lead to positive

feedbacks that ultimately lead to community collapse (Simberloff and Von Holle 1999),

enhanced invasion and facilitate further increases in abundance of invasive species

(Hobbs and Huenneke 1992; Schooler et al. 2006). The replacement of specific

species that differ in their environmental responses could further reduce ecosystem

resilience (Ehrenfeld 2010) and trigger irreversible shifts towards monospecific

communities (Folke et al. 2004).

This study also shows that eutrophication is a concern in Lake Chini since phosphate

concentrations were consistently high in these wetlands. This is consistent with past

studies (Sharip and Jusoh 2010; Sharip and Yusop 2007; Shuhaimi-Othman and Lim


81

2006). In other temperate wetlands, eutrophication pressures have also been shown to

make aquatic ecosystems more susceptible to invasion (Galatowitsch et al. 1999;

Zedler and Kercher 2004).

The results from Chapter 3 and Chapter 4 have important implications for the

management of wetland ecosystems. Macrophyte presence affected not only the

thermal structure of the ecosystem but also the pattern of physical circulation in the

system. The temperature structure affects the DO dynamic and contributes to internal

eutrophication and nutrient release from sediment. The physical circulation induced by

the horizontal density gradient can facilitate the transport of nutrients that promote the

growth and abundance of submerged invasive macrophytes. The increase in C. furcata

abundance as a result of rapid production rates further alters the thermal structure and

physical processes. Herb and Stefan (2004) have shown that submerged macrophytes

attenuate wind and increased stratification, which diminishes the wind-induced forcing

and increases thermal-induced forcing. This maybe is a possible positive feedback

mechanism for C. furcata that enables it to increase its dominance.

This study highlights the importance of understanding the historical context and system

dynamics of wetland ecosystems before recommending management or restoration

measures. Based on the vegetation dynamics found in this study, it can be

hypothesised that the Lake Chini wetlands have some resilience to alternate states of

dominance and may not reach the dynamic threshold limit causing an irreversible shift

or novel ecosystem. Further studies are required to determine the resilience of the

wetland to shifts in dominance and quantify the dynamic threshold limit of the

ecosystem. This study also contributes to an improved understanding of how physical-

chemical-biological processes interact in shallow floodplain wetlands. All of these

processes need to be considered in order to ensure successful management

measures.

Overall, the description of macrophyte dynamics presented here provides further

evidence of the complexity of the wetland ecosystem and its susceptibility to change as

a result of invasion by invasive species. Chapter 3 and 4 show that ecoystem

processes are influenced by changes in C. furcata dominance. The Cabomba

population has been naturalised in Malaysia (Chew and Siti-Munirah 2009) and have

been found in many parts of the country (Chew and Siti-Munirah 2009; Salim et al.


82

2009). Since its introduction C. furcata was likely to be a passenger in Lake Chini

where it probably proliferates under favourable conditions. However, the construction of

the weir and the resulting hydrological changes in this system, and ongoing landscape

changes and the resulting increase in resource availability has provided the ecological

niche for increased growth of C. furcata. Under these conditions, C. furcata is more

competitive than N. nucifera. The high production rates enable C. furcata to rapidly

increase its biomass with availability of nutrients and expand its coverage in the

wetland. In addition, the presence of dense C. furcata stands could affect the physical

circulation and provide better spread of nutrients for its advantage. Therefore, the

ecosystem processes identified here suggest that C. furcata in Lake Chini is possibly

changing its role from a passenger to a driver of ecological change.

This invasive species was also observed to drive changes in benthic communities by

forming fluffy organic-rich sediments through its litter. This attracts high

microphytobenthos activity which could benefit itself since it has been shown that the

formation of litter can drive plant community changes that promote invasive species

dominance (Farrer and Goldberg 2009). The driver model suggested by MacDougall

and Turkington (2005) indicates that removal of invasive species could cause a direct

increase in species richness and abundance of the native species. The return of the

native N. nucifera and disappearance of C. furcata in sites dominated by N. nucifera

since the flood 2007 seems to support the driver model and warrant further study.

5.3 Implications for wetland and lake management

The results of this study hypothesised that the resilience of the Lake Chini ecosystem

and its ability to resist change depends on the flood regimes and nutrient dynamics,

which are both influenced by climatic conditions and the surrounding landscape.

Despite the possible dominance of N. nucifera or C. furcata, continuous landscape

alteration, if not controlled, can lead to increases in nutrients availability. These

nutrients in the water column may contribute to internal feedback mechanisms that

favour C. furcata dominance due to its possible inherent ability to outcompete N.

nucifera for these nutrients.

Additionally, the expansion of C. furcata species based on biomass accumulation and

rapid production rates could have serious implications for the management, restoration

and conservation of this wetland. As C. furcata populations increase, they could reduce


83

plant diversity and reduce the integrity or eventually replace the native plant

community. It could also change the benthic community composition and affect oxygen

dynamics and water quality which would indirectly further alter biological diversity and

lead to a structural change of higher trophic levels. Expansion of C. caroliniana

population in Kasshabog Lake was found to affect the epiphytic algae, and promote a

new habitat for certain macroinvertebrates (Hogsden et al. 2007). Changes in benthic

macroinvertebrates associated with submerged macrophyte dominance have been

observed to cause an increase in waterfowl and certain type of fishes (Hargeby et al.

1994).

Because Lake Chini is an important eco-tourism site in Malaysia, the dense infestations

of monospecific stands of the submerged species could also act to disrupt navigation

and recreational activities, affect the ecosystem services such as transport services,

eco-tourism and fisheries activities (Sharip and Jusoh 2010), and disrupt the aesthetic

and cultural values.

A number of methods have been used to manage and control Cabomba population.

Previous studies have shown that physical method such as shading (Schooler 2008)

and benthic barriers (Wilson et al. 2007) can be successful in controlling Cabomba

populations in small areas. However, the abundance of invasive species observed in

this study extends to almost half of the wetland. Therefore, this method may not be

cost-effective in controlling C. furcata in the Lake Chini system. Mechanical removal

can also be used to manage invasive species but this method is highly disruptive, non-

selective (Wilson et al 2007), and short-term, as re-infestation has been observed

(Australian Water Technologies 1999). As Cabomba population propagate through

vegetative reproduction by means of stem fragmentation (Schooler et al. 2009), the re-

invasion of C. furcata in this area would lead to immediate expansion of the invasive

species due to its rapid growth.

Understanding the pattern of convective circulation could assist in development of

chemical methods to manage C. furcata invasion. Smith et al. (1993) suggests

circulation patterns produced by differential heating and cooling and the short

residence times should be considered when developing herbicide application

strategies. However, despite herbicides such as diquat and endothall being commonly

used for lake and reservoir management, they are relatively expensive (Cooke et al.


84

2005) and often have detrimental effects on wetland biodiversity. Given the sensitivity

and high biodiversity status of Lake Chini, this option is not considered to be ideal.

Application of lime may be more effective in managing this invasive species. Liming will

increase the pH level and shift inorganic carbon equilibrium to bicarbonate (HCO3-)

dominance (James 2011) which reduces the growth of submerged species such as

Cabomba population, which probably prefers free CO2 for their photosynthetic carbon

supply (James 2011; Sanders 1979). Liming has been suggested as a useful

management method for C. caroliniana (James 2011; Sanders 1979), but has not yet

been tried for C. furcata (Mackey 1996). Wilson et al. (2007) suggest liming may be a

practical control measure if size, isolation, and chemistry of the water body is suitable.

In experimental tanks, the addition of lime has been shown to effectively suppress

Cabomba (James 2011). In mesocosm experiments and in field trials, liming has also

been effective in reducing other submerged macrophyte abundance and biomass

(Chambers et al. 2001; James 2008; Prepas et al. 2001). Better understanding of the

circulation patterns in the system, as provided by this study, could improve C. furcata

control by helping determine liming contact times.

The conceptual model developed in this study showed that N. nucifera dominance

could be re-established if natural flow is also re-established. This natural flow could be

re-established by managing land-use alteration and catchment drainage practices to

control the overall catchment hydrology and subsequently flooding into the system. A

more proactive management approach to establish natural flow is to consider the water

level control structure and regulation of flood waters into the system to enable more

regular flow into the system to allow more frequent flooding. However, further studies

need to be carried out to better understand how managing water levels could be used

as means to reduce the dominance of Cabomba.

5.4 Original contributions

The original contributions of this study were the:

Provision of the first spatial and temporal quantitative analysis of the shift from a

native floating-leaved macrophyte (N. nucifera) to an invasive submerged

macrophyte (C. furcata) in a tropical lake system, and in particular its

relationship to human-induced environmental changes.


85

Examination of the role of monsoonal flooding in facilitating the invasion of

submerged aquatic plants.

Novel examination of the processes influencing invasion by C. furcata, and

measurement of its effects on the native plant community. C. furcata has not

previously been reported to be invasive in any other country.

Introduction of a conceptual model of the variation in ecological processes that

shape macrophyte distribution in floodplain systems. This model can be applied

to other pulsed floodplain wetlands and systems to assist in management of the

system towards conservation objectives.

Examination of the roles of different types of macrophytes in influencing

temperature gradients and horizontal density currents and the associated roles

of convective exchange in nutrient transport that promotes invasion.

Provision of the first estimation of exchange mechanisms induced by differential

temperatures between floating-leaved stands and open water.

Provision of the first estimation of primary production by Cabomba species,

specifically C. furcata in an introduced area.

Provision of the first estimation of microphytobenthos and microflora production

associated with C furcata bed in an introduced area.

Combination of high resolution biological and physico-chemical measurements.

The set of techniques used in the field comprised an original suite of water

quality measurements that provide a comprehensive biophysical study of the

system.

5.5 Study limitations and recommendations for future work

This thesis concentrated on understanding the ecosystem processes that control the

possible shift of macrophyte dominance in the shallow floodplain wetlands of Lake

Chini, Malaysia. Time-frame and logistical issues, however, limited the findings of this

research to some degree. The study was based on vegetation surveys that were only

carried out before and after monsoons, and therefore conclusions that have been

drawn represent only one seasonal cycle. The metabolism experiments were also only

performed for a limited time period after the monsoon and may not be representative of

conditions throughout the year. Data collection over a longer time period may provide a

more complete understanding of the pattern of vegetation dynamics and invasion by

the submerged species.


86

A greater understanding of the possible ecosystem processes driving the shift in

macrophyte dominance will assist in the formulation of successful management

strategies. Options for further work to build on the findings reported here could be

obtained by:

Continuing the spatial and temporal vegetation surveys and metabolism

experiments to better define the vegetation dynamics between the two possible

alternate states and the succession pattern of C. furcata invasion.

Continuing the vegetation surveys and metabolism analysis would allow

development of a better understanding of the progression of invasion and

successional patterns in this system under a more diverse range of

environmental conditions. Studies have shown that succession or shift between

alternate stable states will occur when ecosystems exceed a dynamic threshold

(Scheffer and Carpenter 2003; Suding and Hobbs 2009). Long-term monitoring,

together with controlled experiments, and modelling would allow further

exploration of the feedback mechanisms and alternate states (Scheffer and

Carpenter 2003) to determine the threshold(s) of ecological transition.

Extending the study to assess the influence of flood variability on the dynamics

of the plant communities.

This study did not examine the impact of flood frequency, magnitude or duration

on the plant species composition however the results did show a major

difference in the plant community after the 2009 flooding season and therefore

identified this as a significant driver of change. Flood variability, which is

exacerbated by changes in land-use that affects the hydrology, may exert

positive or negative feedback on the two plant species.

Applying multiple modelling approaches to identify options for restoring the

wetland ecosystem.

This study did not include simulation of mechanistic vegetation models. Further

modelling could be carried out to unravel the mix of environmental conditions


87

that lead to species dominance, and confirm the possible alternate stable

states. Such a model would not only lead to better insights into the control of

vegetation growth, but also would allow us to explore options to assist

restoration and management efforts in this system. In addition, these

mechanistic models could be coupled with structural dynamic modelling

(Jorgensen 2010) to (i) analyse changes in ecological structure, species

composition and spatial species dynamics (Jorgensen 1992; Mooij et al. 2010),

(ii) model biodiversity and ecological niches (Jorgensen 2010), (iii) discriminate

potential mechanism of species appearance and disappearance and species

adaptation with environmental changes (Wootton 1994). Increasingly,

uncertainties in ecological models have been noted in several ecological

research areas (Jorgensen 2007) such as habitat sustainability and

management and climate change. Therefore the application of multiple

modelling approaches to identify options for wetland restoration would be

beneficial (Mooij et al. 2010).

Extending the study to assess the extent of landscape changes on the

ecosystem for effective management and restoration aimed at conservation and

sustaining ecosystem services.

Landscape alteration can contribute to the intermittent influx of nutrient and

sediment (Hobbs 2000) into low lying catchment areas, such as wetlands.

Nutrient and sediment influx can also become a sink for material accumulation

and contribute to invasibility by invaders (Zedler and Kercher 2004). Historical

data analysis as shown in this study indicates the importance of landscape

changes in contributing to increased resource availability and the potential

relationship of this with the invasive species. Further assessment of continuing

landscape changes will provide a better understanding of hydrological dynamics

and the overall physical environment that affects the physical processes and

ecosystem function.

Extending the study to assess the overall food-web and potential top-down

ecosystem processes.

Changes in plant dominance and invasion may have an impact on the overall

food-web that could affect shifts in macrophyte dominance. Changes in water


88

quality have been shown to affect the distribution of benthic macroinvertebrates

distribution and reduce biological diversity (Idris and Abas 2005; Sharip and

Jusoh 2010). Further studies on macroinvertebrates, such as grazers and

predators, may provide a better understanding of grazer-plant interactions.

Further studies of biodiversity, including the biodiversity of fishes, birds, and

microbial communities, would provide a better understanding of the overall

ecosystem functioning and how increased pressure from C. furcata is impacting

on biodiversity values of the site.

5.6 Conclusion

This study has shown the dynamics of the macrophyte community in wetland

ecosystem of Lake Chini, Malaysia with possible alternate states dominated by floating-

leaved N. nucifera and submerged species C. furcata. The transition in plant

community dominance in this wetland ecosystem appears to be influenced by flood

regimes and nutrient dynamics. Invasion by C. furcata reduced the diversity of the plant

community and also appeared to affect the convective circulation of the ecosystems.

Increased physical circulation could improve nutrient transport to the advantage of C.

furcata. This invasive macrophyte also increased the net ecosystem production in the

ecosystem and appears to being changing from a passenger of ecological change to a

driver of ecological change. These changes not only have significant effects on the

overall plant community composition in Lake Chini but also affect the lakes both

ecologically and socio-economically. Overall, this study has significant implications for

understanding the wetland systems worldwide.


89

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108

Appendix 1: Supporting document for Chapter 2

Appendix 1a: List of methods for laboratory analysis

1. Phosphate (PO43-) – APHA 4500 P (F)

2. Nitrate (NO3-) – APHA 4500 NO3- (H)

3. Ammonium (NH4+) - APHA 4500 – NH3 (G)

4. Particle size distribution – BS 1377


109

Appendix 1b: Supporting information

Table S1: List of species in Lake Chini, Malaysia recorded in the literature Table S2: Mean values (± SE) of environmental variables


110

Table S1: List of species at Lake Chini recorded in the literature Plant species name 1987

a 1992

b 1998

c 2001

d 2009

e

Submerged Macrophyte

Cabomba furcata* + + +

Utricularia aurea + + +

Utricularia punctata + +

Utricularia gibba +

Utricularia sp. + +

Hydrilla verticillata + + + +

Cryptocoryne cordata +

Najas indica +

Chara sp. + +

Blyxa aubertii ssp. Echinosperma +

Hydrostemma kunstleri +

Aponogeton undulatus + + +

Floating macrophyte

Salvinia molesta + + + +

Ipomoea aquatica +

Azolla pinnata +

Rooted, Floating leaves

Barclaya kunstleri + + +

Nymphaea pubescens + + + + +

Nelumbo nucifera + + + + +

Emergent macrophyte

Lepironia articulata + + + + +

Eriocaulon sexangulare + + + +

Ericaulon longifolium + +

Thoracostachyum bancanum + + + +

Scirpus grossus + + + +

Pandanus helicopus + + + + +

Nepenthes gracilis + + + +

Flagellaria indica + + +

Melastoma malabatrichum + +

Eleocharis ochrostachys + + + +

Eleocharis acicularis + +

Bacopa monnieri +

Cerotopteristhalictroides + +

Fimbristylis globulosa + +

Fimbristylis acuminata + +

Fimbristylis miliacea + +

Cyperus sp. 1 +

Cyperus sp. 2 +

Cyperus sp. 3 +

Cyperus sp. 4 +

Cyperus pilosus +

Scleria sp. 1 +

Scleria sp. 2 +

Scleria terrestris + +

Polygonum sp. +

Echinochloa sp. +

Lycopodium cernuum L. + + +

Phragmites communis +

Lygodium microphyllum +

Panicum repens +

* was previously known as Cabomba piauhyensis

a Department of Wildlife & National Parks

b DARA

c Wetland International d

Gan & Bidin e

present survey

111

Table S2 Mean values (± SE) of environmental variables

Lake

Variable 1 2 3 4 5 6 7 8 9 10 11 12

Sep-09

Total depth (m) 1.5±1.0 1.3±0.7 1.1±0.7 1.4±1.1 0.9±0.7 1.0±0.5 1.1±0.5 0.9±0.3 1.2±0.6 1.3±0.8 1.4±0.8 1.8+1.0

Temperature(oC) 30.7+0.2 30.8±0.2 30.5±0.4 30.3±0.2 30.0±0.9 31.7±0.2 31.6±0.5 32.8±0.9 32.1±0.8 31.0±0.4 31.4±0.8 31.1±0.6

Conductivity (µS/cm) 23.3±0.5 24.0±0.4 23.0±1.1 22.0±0.0 23.4±0.5 22.9±0.4 18.7±1.9 26.7±2.1 30.7±2.8 26.0±0.0 26.4±0.8 21.7±0.8

pH 6.2±0.3 5.8±0.1 6.6±0.8 5.6±0.1 5.8±0.3 6.0±0.3 6.0±0.4 5.9±0.5 5.5±0.2 5.9±0.2 6.7±0.5 5.8±0.1

Turbidity (NTU) 19.3±10.0 35.3±65.4 23.7±12.2 20.1±19.1 14.4±4.6 19.5±15.9 17.3±20.5 13.4±4.2 16.0±9.3 12.3±6.4 10.8±1.9 10.7±2.5

DO (mg/l) 6.6±0.4 5.7±0.7 5.1±0.9 4.2±0.4 5.0±0.9 7.0±0.2 6.4±1.1 7.0±1.7 6.5±0.8 6.1±0.8 6.4±0.4 6.4±0.4

PO4 (mg/l) 0.089±0.047 0.054±0.045 0.057±0.029 0.054±0.033 0.042±0.043 0.042±0.028 0.020±0.014 0.025±0.011 0.025±0.011 0.042±0.028 0.024±0.013 0.046±0.028

NH4 (mg/l) 0.062±0.018 0.034±0.011 0.042±0.018 0.025±0.010 0.010±0.007 0.030±0.015 0.016±0.014 0.028±0.004 0.026±0.009 0.028±0.004 0.023±0.005 0.025±0.010

NO3 (mg/l) 1.094±0.277 0.896±0.249 0.976±0.098 0.618±0.085 0.765±0.110 0.283±0.339 0.432±0.537 0.790±0.179 0.786±0.080 0.858±1.457 0.410±0.640 0.673±0.025

Apr-10

Total depth (m) 1.1±0.9 0.8±0.7 0.9±0.7 1.0±0.8 0.8±0.6 0.7±0.4 0.8±0.4 0.7±0.3 0.9±0.5 0.9±0.7 1.2±0.6 0.9±0.6

Temperature(oC) 31.7±0.5 32.6±0.6 31.9±0.8 30.6±0.4 30.1±1.3 33.8±0.4 34.8±1.4 32.2±1.6 32.5±0.5 34.2±1.1 33.8±0.8 32.4±1.2

Conductivity (µS/cm) 29.0±0.0 27.6±0.5 26.3±3.1 27.1±1.1 28.6±0.9 28.1±1.0 29.5±1.1 29.3±2.2 31.7±3.1 35.8±3.1 38.8±6.4 25.1±1.9

pH 7.6±0.6 6.6±0.2 6.2±0.3 6.2±0.3 7.7±0.8 6.9±0.2 8.0±0.2 7.3±0.8 6.7±0.3 7.3±0.4 6.8±0.3 6.2±0.3

Turbidity (NTU) 23.9±13.9 66.6±134.6 168.2±593.6 17.8±6.5 592.9±867.9 24.1±8.7 21.2±10.5 25.6±15.2 32.8±27.7 391.7±773.7 21.8±21.2 414.6±781.6

DO (mg/l) 6.8±0.2 6.8±0.4 6.3±0.8 4.3±1.6 5.0±1.3 7.8±0.4 7.5±0.6 6.0±1.8 6.1±1.1 6.4±2.0 7.4±0.9 5.7±1.2

PO4 (mg/l) 0.008±0.003 0.121±0.053 0.033±0.016 0.028±0.035 0.153±0.170 0.132±0.063 0.033±0.045 0.009±0.010 0.052±0.105 0.033±0.038 0.075±0.030 0.024±0.038

NH4 (mg/l) 0.287±0.578 0.194±0.225 0.038±0.074 0.099±0.063 0.109±0.096 0.076±0.159 0.302±0.195 0.018±0.035 0.097±0.051 0.638±0.589 0.805±0.530 0.120±0.140

NO3 (mg/l) 0.007±0.003 0.006±0.002 0.006±0.002 0.010±0.000 0.010±0.000 0.009±0.002 0.010±0.000 0.009±0.002 0.006±0.002 0.018±0.021 0.008±0.015 0.005±0.000

%silt/clay 0.737 0.833 0.852 1.286 1.065 0.980 0.727 0.863 1.359 1.400 1.824 1.325 Rank distance to weir 8 5 3 1 2 4 6 7 11 9 10 12

112

Appendix 2: Conference Paper

113

Physical circulation and spatial exchange

dynamics in a shallow floodplain wetland

Z. Sharip1,2,3

, M. R. Hipsey1,4

, S. S. Schooler5

& R. J. Hobbs2

1Centre for Ecohydrology, School of Environmental Systems Engineering,

The University of Western Australia, Australia 2School of Plant Biology, The University of Western Australia, Australia 3National Hydraulic Research Institute of Malaysia, Malaysia

4School of Earth and Environment, The University of Western Australia,

Australia 5CSIRO Ecosystem Sciences, Australia

Abstract

This paper examines spatial patterns of water exchange based on water

temperature variations between littoral and pelagic zones; and compares the

patterns in lakes with different depths. The data were collected at Lake Chini,

Malaysia which comprises a series of shallow lakes connected by natural

channels. Our results indicate that the density-driven flow induced by the

differential temperature gradient between littoral areas, which are dominated by

either floating-leaved or submerged vegetation, and the open pelagic region

appeared to be an important transport mechanism for the exchange of dissolved

constituents between the zones. Persistent stratification in the narrower lakes

could be due to the presence of dense submerged vegetation that attenuates wind-

driven turbulence. Additionally, variation of thermal stratification and mixing

dynamics between these lakes has corresponding effects on the biological and

chemical regime. The circulation contributes to the spatial differences of the

water quality that could favour submerged species and subsequently induce

shifts of macrophyte community composition. The results of this study have

implications for the rehabilitation and management of lake ecosystems.

Keywords: convective circulation, density driven flow, floating-leaved plant,

lake Chini, shallow wetland, submerged macrophytes, thermal stratification,

water exchange.

114

1 Introduction

Circulation induced by horizontal density gradients resulting from depth

variation has been demonstrated to be an important exchange mechanism of

phosphorus between the littoral and pelagic zones, [1, 2], and nutrients between

side embayments and the main basins (Monismith et al [3]) of lakes. A

temperature gradient develops when shallow littoral areas and deeper pelagic

zones receive the same incoming solar radiation but distribute it differently over

their respective volumes [1, 3–5]. Differential heating created through shading

by aquatic plants can also induce convective motion [6–9]. Studies on the

convective exchange between a vegetated system and littoral zones, however,

have been focused on reed beds (Lovstedt and Bengtsson [7]), typha stands

(Waters [9]) and floating species such as Azolla sp. and Lemna sp. (Coates and

Ferris [6]). In all the above studies, a horizontal density gradient develops as the

surface water underneath or within the vegetation becomes colder than the

adjacent open water due to shading and subsequently induces overflow of

warmer pelagic water towards the plant bed at the surface during the day. Studies

on the effect of a horizontal thermal gradient and the associated convective

exchange induced by different types of macrophytes, specifically the rooted

floating-leaved plants and the submerged species, have been limited.

Differential nutrient gradients between the littoral and pelagic zones were

noted in Lake Chini by Sharip et al [10] and it is hypothesized that it may have

been controlled by the temperature differences between the two zones. In particular, the circulation induced by the thermal gradient may regulate transport

of nutrients to dense submerged Cabomba furcata beds (Sharip et al [10]) and

subsequently enable this species to proliferate in the system due to its ability to

uptake nutrients through stems and shoots (Wilson et al [11]).

The aim of this study is to examine the convective exchange pattern resulting

from differential cooling and heating between the littoral zone and the open

pelagic zone from two sites within a shallow floodplain wetland that are

dominated by different types of aquatic plants. The objective is to understand the

influence of the convective exchange pattern on water quality changes which

could contribute to the shifts of macrophyte community composition we

observed in the system (Sharip et al [10]). We analyzed temperature, light and

nutrient profiles and estimate the induced flow between the littoral zone and

main lake body at the two sites. The study was carried out in Lake Chini,

Malaysia, a shallow wetland located within the Pahang River floodplain, which

exhibits characteristics of a system having multiple ecological regime shifts

(Sharip et al [10]). The lake has a surface area of 1.69 km2

and an average depth

of 1.9 m (Sharip et al [12]) with its littoral areas extensively dominated either by

the floating-leaved plant (Nelumbo nucifera) or submerged vegetation (C.

furcata).

115

2 Methods

2.1 Field measurements

Surveys were carried out on 19–24 April 2010 to investigate the exchange flows

between the littoral and pelagic region. Transects were positioned in two lakes;

Lake 1 linking the open pelagic region to littoral area dominated by floating-

leaved vegetation, and Lake 12 linking pelagic zone to a littoral area dominated

by submerged species (Fig. 1). Thermistor chains were deployed at the centre of

three lakes; each anchored at the top on buoy and kept taut to the bottom by a

weight for long-term temperature measurements.

Figure 1: Lake Chini and location of temperature monitors along transect

(inset).

Thermistors recorded temperatures at intervals and depth as in Table 1.

Vertical profiles of Photosynthetically Active Radiation (PAR) and temperature

were made with a light sensor (LI-190 Quantum sensor) and a YSI 6600 multi-

parameter profiler, respectively. Water samples were collected at each station for

analysis of phosphate and nitrate. Linear interpolation was used to calculate

hourly interval values where necessary.

Hourly recorded meteorological data were obtained from an on-lake weather

station and the nearby Muadzam Shah meteorological station. Additional

temperature-depth profiles were carried out using YSI 6600 multi-parameter

probe on 28–30 March 2011 during a high water-level period.

116

Table 1: Temperature monitors in Lake Chini.

Sensor Parameter Recording

interval (min)

Depth (m) Lake

Hobo

RBR

D-Opto

TPS sensors

Temperature

Temperature

Temperature, DO

Temperature, DO

15

1

1

10–20

0.5, 1.0, 2.0

1.0

0.5

0.15–0.5

1,5,12

1,12

1,12

1,12

2.2 Analysis of physico-chemical structure and heat budget calculations

We assessed convective exchange patterns by generating contours (SURFER,

version 9.0; Golden Software, Inc) over the transects as in Smith et al [13] using

the temperature data recorded by thermistor probes and temperature-depth profile

data measured over 45-min intervals or less.

The atmospheric heat transfer was computed using the available

meteorological data and employed a standard bulk aerodynamic method as

describe by the Tennessee Valley Authority [14], with the neutral transfer

coefficients corrected for atmospheric stability and measurement height. The two

lakes were assumed to have similar meteorological parameters.

The overall heat budget calculation and exchange flow was based on eq. (1)

and (2) in Lovstedt and Bengtsson [7]:

.0

0 QsensibleLatentnet

h

P HHHRzt

TC

(1)

.. 0 QTTCLH plantopenPQ (2)

where ρ0 is the density (kg/m3), Cp is the specific heat capacity for water, 4.18 x

103J/(kg K), h is the water depth (m), ∂T is the average temperature change (

oC),

∂t is the time step (s), z is the vertical distance from surface (m), Rnet is the net

solar radiation (W/m2), Hlatent is the latent heat transfer (W/m

2), Hsensible is

the sensible heat transfer (W/m2), and HQ is the lateral heat flux between the

open water and the vegetation bed, and Q is the net exchange flow. The averaged- temperature as a function of time was calculated from eq. (1) using the initial temperature as in Lovstedt and Bengtsson [7] and compared with the measured temperature data (20-min interval) in the area. Heat fluxes were calculated based on temperature measurements between Station 1 and Station 2 in each lake. The estimation of the heat fluxes covered the whole one-day cycle for Lake 1 and only the differential heating (day) phase in Lake 12 due to sensor failure. The dissolved oxygen dynamic pattern will be described elsewhere.

117

3 Results and discussions

3.1 Meteorology and physico-chemical structure

Wind speed was generally low during the experiments in April 2010 with

average wind speed recorded around 0.8 m/s. Wind speeds were zero throughout

the night on 20–22 April 2010 (Fig. 2a). Thus, wind-driven exchanges were not

driving the transport mechanism during this period. The skies were occasionally

cloudy but no rain was recorded during the two days. Inflows and outflows of the

surface water and groundwater were not considered in the estimation.

Figure 2: Variations in (a) wind speed (b) air temperature (thin line) and

surface water temperature (dotted line) during surveys conducted in

April 2010.

Air temperature cooled from a maximum of 32.2oC at 1600h on 20 April to

24.6oC at 0600h on 21 April (Fig. 2b). A lower air temperature compared to

surface water temperature during the night can cause an increase in atmospheric

instability and subsequently an increase in sensible and latent heat loss (Verburg

and Antenucci [15]). This atmospheric condition was suitable for differential

cooling and differential heating to develop.

Mean surface water temperatures in the littoral were higher than the mean

water temperature in the pelagic zones in both lakes until late afternoon. High

temperature within the littoral zone vegetation dominated by the floating-leaved

plant, N. nucifera, was consistent with past studies (Coates and Folkard [16])

which could be due to sparse N. nucifera cover (13%) in the area. Surface water

temperature in the littoral cooled more rapidly at the littoral site compared to the

pelagic zone in both lakes during the night; causing movement of cooler littoral

water into the deeper pelagic zone by 0800h (Fig. 3c and 3g). However, intrusion

of cool water appeared to move away from the sediment in Lake 12 as shown in

the slanting contour at the surface (Fig. 3f) at 2200h, which could be caused by

the presence of a dense submerged macrophyte bed (Station 3) where the

temperature was slightly higher than the surrounding water (Fig. 3g) at 0800h.

118

Figure 3: Longitudinal and vertical variations in water temperature on 20–21 April in Lake 1 at (a) 1800 h, (b) 2200 h, (c) 0800 h,

(d) phosphate concentration (mg/L) and Lake 12 at (e) 1800 h, (f) 2200 h, (g) 0800 h (h) phosphate concentration

(mg/L).

119

The water at the surface and bottom layer also remained separated at Station 3 (Fig. 4c), which was in contrast to the pelagic zone (Station 4) (Fig. 4d) where the surface layer was completely mixed with the bottom layer. PAR at Station 3

reached ca. 0 µmols-1

m-2

within the submerged stands where wind-driven motion was attenuated (personal observation). The presence of dense C. furcata bed enhances daytime stratification in the water column matched with past studies

[17, 18]. In contrast to findings by Herbs and Stefan [18], our study shows the presence of dense submerged vegetation affects the circulation pattern induced by the horizontal density gradient.

Figure 4: Variation in water temperature in Lake 1 on 20–21 April 2010 (a)

Station 3, (b) Station 4, and in Lake 12 on 21–22 April 2010, (c)

Station 3, (d) Station 4.

120

In both lakes, phosphate concentrations were high in the littoral areas

compared to the pelagic zone (Fig. 3 d and Fig. 3h) and the induced horizontal

density gradient could improve transport of phosphate to other areas. The

circulation pattern provides a better spread of nutrients for the growth of

submerged macrophytes within the area.

Over the daily cycle, stratification and vertical mixing patterns were clearly

different between the lakes (Fig. 5). A pattern of diurnal stratification and

complete mixing at night were apparent in Lake 1 and Lake 5, whereas

stratification persisted in Lake 12, with the exception on the morning of 22 April,

where the mixed layer didn’t reach the 2-m layer. In contrast to thermal pattern

during low water level, a distinct pattern of stratification in both Lake1 and Lake

12 during high water level were driven by high turbidity which altered the

thermal gradient and the exchange dynamics and mixing in the lake system.

Stratification in these lakes could lead to anoxic conditions and subsequently an

increase in nutrients.

Figure 5: Thermistor time series at the pelagic zone, at (a) Lake 1, (b) Lake 5,

and (c) Lake 12 between 20–24 April 2010.

3.2 Water exchange

Solar radiation was high during the day, reaching 900 W/m2

and thus net radiation was an important heat flux that influenced the heating and cooling of the water. The predicted sensible heat fluxes were small and ranged between -3

to 12 W/m2. The latent heat fluxes were high during late afternoon. The

calculated temperature profile showed a similar trend with the measured temperature profiles (not shown). Significant differences (p<0.001) between the measured and calculated values could be due to the estimation of latent heat

121

fluxes which, as found by Lovstedt and Bengtsson [7], were difficult to predict.

Comparisons are being made with different methods as in Verburg and

Antenucci [15] and Lovstedt and Bengtsson [7] and using corrected

meteorological data to enhance the localized lake-effect to improve the

atmospheric heat budget estimation.

The results show the presence of convective currents due to differential

heating and cooling between littoral and pelagic zones. The estimated convective

current between floating-leaved vegetation stands and open water (Lake 1) was

2.6 cm/s. The estimated convective current due to differential heating by

submerged vegetation (Lake 12) was around 0.4 cm/s. The convective currents

were within the rates of differential heating between shallower and deeper water

estimated in Monismith et al [3] which was 0.4–11 cm/s.

4 Conclusions

This study documents the contribution of aquatic vegetation to thermal structure

and water exchange dynamics. The horizontal density gradient may also be

affected by rooted floating-leaved vegetation and submerged vegetation.

Submerged vegetation affects the temperature pattern by increasing stratification

and altering the circulation pattern which affects the water quality in the lakes.

The convective current due to differential heating and cooling between littoral

and pelagic is an important exchange mechanism that has important implications

for nutrient cycling, and subsequently shifts of macrophyte composition, in this

shallow lake system.

Acknowledgements

The first author gratefully acknowledges Ahmad Taqiyuddin Ahmad Zaki for his

assistance and coordination during all field experiments and Justin Brookes for

provision of sensors and assistance in the field. The National Hydraulic Research

Institute of Malaysia (NAHRIM) provided data, and financial assistance for the

field experiments.

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