stable isotopes dynamics of macrophytes along … · mzamo sanele caleb a dissertation submitted in...
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STABLE ISOTOPES DYNAMICS OF MACROPHYTES ALONG
UMTATA RIVER IN THE EASTERN CAPE OF SOUTH AFRICA
by
MZAMO SANELE CALEB
A dissertation submitted in fulfillment of the requirements for the degree of
MASTER OF SCIENCE (MSc) (Zoology)
at
WALTER SISULU UNIVERSITY
SUPERVISOR: DR KURIAH, F.K.
SEPTEMBER 2013
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Abstract
The decline of freshwater ecosystems, generally result from land use activities in
the river catchment and is of great concern worldwide. This study was conducted
along Umtata River in the Eastern Cape province of South Africa between May
2010 and April 2011. The study was aimed at identifying macrophytes families
(to species level) and determining the stable isotope signatures (C:N ratios, δ13C
and δ15N) and to relate their isotopic signatures to the land use activities along
the river catchment. Analysis of variance was performed to test the effect of sites
and sampling period on the C:N ratios, δ13C and δ15N signatures. There were 16
macrophyte families represented by 26 species recorded along the river. There
was only a significant difference in sites and sampling period in δ15N (p< 0.05).
The highest C:N ratios value (30.75±9.65‰) was recorded in the upper reaches
while the lowest value (6.10±2.35‰) occurred in the lower reaches. The δ13C
values varied throughout the length of the river with highest values (-
19.63±5.44‰) obtained in the middle reaches. Spatial variation was evident in
δ15N throughout the length of the river and showed increase from the upper
reaches to middle reaches and decreased towards lower reaches. The δ15N
ranged from 3.92±2.43‰ in the upper reaches to 10.02±4.56‰ in the middle
reaches. Temporal variation in δ15N was also evident throughout the sampling
period with highest peak in May (9.77± 4.49‰) and lowest in February
(0.50±2.49‰) respectively. The highest values of isotope signatures at spatial
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level demonstrated the true reflection of urban development, sewage discharge
and agricultural activities taking place along the river system.
Continued monitoring is recommended that may ultimately come up with a
better management options for the communities living within the study area, and
also to better enhanced land utilization.
Key words: Macrophytes, anthropogenic activities, carbon:nitrogen ratios,
stable isotopes of carbon, nitrogen and Umtata River.
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Declaration
I ‘Mzamo Sanele Caleb’ Student Number 205623204, hereby declare that the
information contained in this thesis is my original work.
Signature………………………………………………………………
Date………………………………………………………………………
Supervisor: Dr F.K. Kuriah
Signature:
Date………………………………………………………………………
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Declaration on Plagiarism
I “Mzamo Sanele Caleb” is aware that plagiarism is regarded as a theft in
a whole or piece of information recklessly taken from the source without
acknowledging the original author of the information or the whole paper is
reproduced and presented as someone’s original work.
All the authors cited in this thesis are acknowledged and confirmed to
avoid plagiarism as defined by WSU.
I have followed the citation and referencing style stipulated by Walter
Sisulu University.
This submitted thesis is my original work.
Any unauthorized use of this document is prohibited, no duplication of this
document for academic or any other practice is allowed.
This document was submitted for academic purposes (Master of Science
in Zoology).
__________________ __________________
Signature Date
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Acknowledgements
I would particularly thank Dr Kuriah, F.K for his commitment and
dedication in assisting and guiding me during the course of my study.
I acknowledge the help of Mr. Nasila offered in statistical data analysis.
I would also like to thank the department of Zoology for giving me an
opportunity to do my masters degree.
I cannot forget my colleagues (Zoology masters students) and my friends
for their support during my study.
Finally, I would like to thank my family for their unwavering support they
have given me throughout my time at the university.
This project was funded by National Research Foundation (NRF) to whom
I am grateful.
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Dedication
I’d like to dedicate the work that I have put into this thesis to my parents (Mrs.
Mzamo F.N.N and Mr. Mzamo G.M) and my sister (Ms. Mzamo Vuyelwa Mavuyi),
for their dedication and encouragement during my study period at Walter Sisulu
University.
Mzamo Sanele Caleb
Lusikisiki
Republic of South Africa
JUNE 2013.
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TABLE OF CONTENTS
Chapter 1
Introduction 1
1.1 Study background 1
1.2 Problem statement 2
1.3 Agricultural practices and urban development as sources of nutrients to
aquatic ecosystems 3
1.4 Stable isotopes 3
1.5 Rationale of the study 5
Chapter 2
Literature review 8
2 Macrophytes 8
2.1 The C:N ratios 10
2.2 The δ13C 12
2.3 The δ15N 14
2.4 The use of δ15N in studying aquatic ecosystem functioning 15
2.5 Justification of the study 15
2.6 Aims and objectives 16
2.7 The research hypothesis 17
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Chapter 3
Materials and methods 18
3.1 Description of the study area 18
3.2 Selection of sampling sites 19
3.3 Macrophytes 20
3.3 Isotopic analysis 23
3.5 Statistical data analysis 24
Chapter 4
Results 25
4.1 The macrophyte species composition of Umtata River 25
4.2 The C:N ratios at family level 26
4.3 The δ13C values at family level 28
4.4 The δ15N at family level 28
4.5 The effect of sampling sites on C:N ratios of macrophytes 31
4.6 The effect of sampling period on C:N ratios of macrophytes collected
along Umtata River 32
4.7 The effect of sampling sites on δ13C signatures of macrophytes 34
4.8 The effect of sampling period on δ13C signatures 36
4.9 The effect of sites on δ15N signatures of macrophytes 38
4.10 The effect of sampling season on δ15N signatures of macrophytes 40
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Chapter 5
Discussion 43
Conclusion 52
References 53
Appendix 64
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LIST OF TABLES
Table 1: Showing sampling locations, coordinates and activities along the
sampling sites 20
Table 2: Showing the number of macrophytes families and species from
the sites along Umtata River 22
Table 3: Showing the stable isotope composition of macrophytes families
from the sites along Umtata River 28
Table 4: Showing the effect of sampling sites and seasons on the C:N
ratios, δ13C and δ15N signatures of macrophytes along Umtata
River 29
Table 5: Showing the mean spatial values and standard deviations of
C:N ratios of macrophytes recorded along Umtata River 30
Table 6: Showing the mean seasonal values and standard deviations of C:N
ratios of macrophytes recorded along Umtata River 32
Table 7: Showing the mean spatial values and standard deviations of δ13C
of macrophytes recorded along Umtata River 34
Table 8: Showing the mean seasonal values and standard deviations of
δ13C of macrophytes recorded along Umtata River 36
Table 9: Showing the mean spatial values and standard deviations of
δ15N signatures of macrophytes recorded along Umtata River 38
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Table 10: Showing the seasonal mean values and standard deviations of δ15N
signatures of macrophytes recorded along Umtata River 40
Table 11: The list of all macrophyte families and species collected
along Umtata River 63
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LIST OF FIGURES
Figure1: Map showing study area and sampling sites (Source: Department
of Water Affairs, King Sabata Dalindyebo Municipality) 19
Figure 2: Graph showing mean spatial C:N values of macrophytes recorded
along Umtata River 31
Figure 3: Graph showing mean seasonal C:N values of macrophytes recorded
along Umtata River 33
Figure 4: Graph showing mean spatial δ13C values of macrophytes recorded
along Umtata River 35
Figure 5: Graph showing mean seasonal δ13C values of macrophytes recorded
along Umtata River 37
Figure 6: Graph showing mean spatial δ15N values of macrophytes recorded
along Umtata River 39
Figure 7: Graph showing mean spatial δ15N values of macrophytes recorded
along Umtata River 42
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List of abbreviations
Carbon: Nitrogen ratio (C:N ratio).
Carbon stable isotope (δ13C).
Nitrogen stable isotope (δ15N).
Stable isotope signatures (SIS).
C3 plants: plants that use ribulose-1.5biphosphate carboxylase/ oxgenase
to fix carbon.
C4 plants: plants that use phosphoenol pyruvate carboxylase to fix CO2..
CAM plants: plants that use Crassulacean Acid Metabolism to fix carbon.
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CHAPTER 1
Introduction
1.1 Study background
This study was aimed at investigating the effect of different land uses occurring
from the catchment of the Umtata River system. The Umtata River is under
anthropogenic pressure due to different land uses which include agricultural and
urban development (Table 1).
Rivers and their runoff, including groundwater, both of which are impacted by
anthropogenic activities provide freshwater to estuarine systems. Most rivers
receive nitrogen-enriched runoff loaded with agricultural fertilizers, animal and
human waste from many diverse sources (Tappin, 2002). Various forms of land
uses influence runoff quality and quantity during and following rainfall. The
middle reaches of the Umtata River are severely affected by urban development.
This part of the river receives waste water from waste treatment plants and
leaking drainage pipes. This waste water is rich in nitrogen and phosphorus both
of which are required in large quantities by living organisms. This has led to the
growth of water hyacinth and deterioration of water quality in this area and the
river downstream.
The Umtata River system is mainly used for agricultural, domestic and
recreational purposes which include drinking, swimming and laundry in the lower
reaches. The poor water quality from middle to lower reaches becomes a limiting
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factor to the importance of this river to human population along this section. This
raises a concern to carry out extensive research to provide suggestions for water
quality improvement of the river.
1.2 Problem statement
Land use within river catchments are widely known to be major donors of
nutrients to aquatic environments where they cause eutrophication (Kohzu, et
al., 2008; Nyenje, et al., 2010). Eutrophication is a complex process in which
fresh and marine waters become enriched by nitrogen, phosphorus etc from both
internal and external sources (Xu, et al., 2007; Nyenje, et al., 2010). It is one of
the most prevalent global problems (Nyenje, et al., 2010; Wozniak, et al., 2012),
and is the most pressing environmental problem in both developing and
developed countries (Xu, et al., 2007; Kohzu, et al 2008). It alters ecological
integrity, cause fish extinction and toxic algal blooms (Xu, et al., 2007). The
major external sources of nutrients to rivers are agricultural practices (Forsberg,
1993; Lake, et al., 2001; Martinelli, et al., 2002; Birgand, et al., 2007; Kohzu, et
al., 2008) and urban development (Kohzu, et al., 2008; Miller, et al., 2010;
Kendall, et al., 2010; Nyenje, et al., 2010). The Umtata River experiences both
industrial and agricultural nutrient inputs. As a result the river has been found to
accumulate alarming levels of heavier metals (Fatoki, et al., 2001).
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1.3 Agricultural practices and urban development as sources of
nutrients to aquatic ecosystems
In areas with extensive agricultural practices, aerosols from biomass burning are
a cause of change in chemical composition of the atmosphere, altering rain water
leading to changes in aquatic systems (Martinelli, et al., 2002). Agriculture has
been found to be the only single largest nutrient source to aquatic systems
(Birgand, et al., 2007). The nutrients from agricultural activities include nitrogen
(N), phosphorus (P) and carbon (C) (Nyenje, et al., 2010). Sewage and industrial
waste in both developed and developing countries add nutrients in elevated
quantities to river systems (Kendall, et al., 2010; Miller, et al., 2010; Nyenje, et
al., 2010). Human waste contains both N and P in the form of ammonium,
ammonia, nitrates, urea and phosphates respectively (Nyenje, et al., 2010).
These inputs make nutrients available to macrophytes (Miller, et al., 2010), and
become limiting factors for primary producers (Forsberg, 1993; Flair and
Heikoop, 2006; Birgand, et al., 2007).
1.4 Stable isotopes
Isotopes are elements that have same numbers of protons but different numbers
of neutrons (Ehleringer, et al., 1993; Madder, 1996). They are classified into two
categories based on the physical chemistry properties or reactive characteristics
namely stable isotopes and radio-active isotopes (Ehleringer, et al., 1993;
Madder, 1996; Southwood, et al., 2000). Stable isotopes are those that do not
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decay over time, while radio-active isotopes are subjected to decay and have
shorter life span (Southwood, et al., 2000). Stable isotope composition is
expressed in terms of the delta (δ) notation which are parts per thousands (‰)
or “mils” (δ=relative, ∆= absolute) (Ehleringer, et al., 1993). Stable isotopes of
biological interest include carbon, hydrogen, nitrogen, oxygen, and sulfur and
exist in two or more forms e.g. carbon isotopes include δ12C, δ13C and δ14C &
nitrogen isotopes include δ14N and δ15N (Ehleringer, et al., 1993). The use of
stable isotopes in studying aquatic systems is complex and involves complex
processes (Otero, et al., 2000), and as a result the measurement of stable
isotopes in ecology has emerged as an approach that integrates ecophysiological
processes over time (Smedley, et al., 1991). For this study the δ13C, δ15N and
their C:N ratios have been chosen for a number of reasons which include:
Their ability to trace sources and sinks of nutrients to aquatic systems and
pinpointing areas that need immediate attention (Kendall, et al., 2010).
Their ability to differentiate between autochthonous (within a system) and
allochthonous (from outside system) nutrient inputs (Otero, et al., 2000).
Their ability to mark plants of different photosynthetic pathways
(Ehleringer, et al., 1997).
Their ability to indicate the effect of environmental change and
contamination in aquatic ecosystem functioning (Ehleringer, et al., 1991;
Wozniak, et al., 2012).
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Their ability to act as useful tracers of changes in nutrients loads to
aquatic ecosystems, and to detect spatial and seasonal variations in C:N,
δ13C and δ15N signatures of macrophytes (Smedley, et al., 1991; Baeta,
et al., 2009).
They serve as powerful tools to study biogeochemical cycles (Xu, et al.,
2007).
1.5 Rationale of the study
Human activities such as urban sewage, industrial waste disposal and runoff
from agricultural fields negatively affect water bodies (Kohzu, et al., 2008;
Chang, et al., 2009; Kendall, et al., 2010; Miller, et al., 2010; Nyenje, et al.,
2010). These activities are associated with high nitrogen values while agricultural
activities are associated with both high carbon and low nitrogen loads to water
bodies (Flair & Heikoop, 2006; Chang, et al., 2009; Kohzu et al., 2008; Miller, et
al., 2010). Enriched carbon and nitrogen nutrient loads cause algal blooms,
faunal extinction and loss of socio-economic importance of aquatic systems (Flair
and Heikoop, 2006; Chang, et al., 2009; Miller, et al., 2010; Nyenje, et al.,
2010).
The effect of human activities in aquatic environments is mainly caused by
transformation of lateral flows of water, air, soil, or organisms from the
catchment, and they exact pressure in offsite areas (Van Noordwirjk, et al.,
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2004). Pollution problems which are caused by sewage disposal and agricultural
activities have caused a detrimental change in sub-tropical wetlands (Chang, et
al., 2009; Nyenje, et al., 2010). Studies conducted in the 21st century on the
effect of land use have shown interdependency on the physical and biological
concepts of aquatic systems and terrestrial landscapes (Cole, et al., 2006; Kohzu,
et al., 2008; Van Noordwijk, et al., 2004). Furthermore, it has been noted that
species composition and richness change in response to different land use
activities in the river and the drainage system (Maltais-Landry, et al., 2009).
The studying of spatial and temporal patterns of nutrient loads is important as it
can identify areas or locations that are severely affected by anthropogenic
activities (Kendall, et al., 2010). Samples taken in one sampling station are
representative of what is happening around and above the catchment area,
because nutrients are mainly transported by ground water, wind, and runoff and
by the river itself (Cole, et al., 2006; Kendall, et al., 2010).
The human population growth, urban development and human settlements
closer to river systems cause disturbances and water quality deterioration (Xu, et
al., 2007; Kohzu, et al 2008; Kendall, et al., 2010). The occupation of river flood
plains is associated with rapid economic growth and organic pollution (Miller, et
al., 2010). Elevated nutrient loads to aquatic ecosystems stimulate production of
phytopkton and macrophytes, leading to loss of other organisms and aesthetical
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value of the surrounding environment (Flair and Heikoop, 2006; Miller, et al.,
2010; Nyenje, et al., 2010). To estimate the amount of nutrient loads, it is vital
to examine the linkage between land uses and land derived nutrients loads so as
to identify point sources and their contributions (Kendall, et al., 2010).
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Chapter 2
Literature review
2.0 Macrophytes
In studying aquatic systems, it is important to look at those factors that influence
energy sources at the base of the food web (Estevez & Suzuki, 2010). These
factors include the importance of macrophytes, seasonality, anthropogenic
nutrient loading (Kendall, et al., 2010). Macrophytes have wide but restricted
distribution in river networks (Michener & Lajtha, 2007), and this spatial
distribution is the indication of species dispersal and sensitivity function.
Reduction and deterioration of macrophytes result from enhanced anthropogenic
nitrogen (Chang, et al., 2009; Kohzu, et al., 2008) and phosphorus nutrient
loading (Estevez and Suzuki, 2010; Cohen and Bradman, 2010).
The difference in nutrient loads among sites, forces macrophytes to be isotopically
distinct, variable and less predictable than terrestrial plants (Michener & Lajtha,
2007). This is due to the large variation in the amount and stable isotope ratios of
dissolved inorganic nutrients in the water column (Kendall, et al., 2010). The plant
isotopic composition is influenced largely by environmental processes (temperature
and humidity), making their isotopic composition fluctuate with environmental
changes (Smedley, et al., 1991). For an example, the seasonal increase in
temperature and evaporative demands leads to a decline in soil moisture which in
turn lead to nutrient concentration and availability to plants (Smedley, et al., 1991;
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Nordt, et al., 2002). The other factors that influence the isotopic composition of
macrophytes include stream geomorphology, water current and plant form and part
being analyzed (Kendall, et al., 2010). The isotopic signatures of plants reflect the
available elemental sources in the atmosphere, soil, aquatic and the whole host of
environmental conditions (Kendall, et al., 2010). Consequently, submerged
macrophytes differ in the sensitivity of their isotopic signatures to environmental
change (Chang, et al., 2009). Research in isotopic composition and variation on
aquatic ecosystems over spatial and temporal scales provides information on
natural ecosystem function and the effects of land use (Kohzu, et al., 2008; Chang,
et al., 2009; Kendall, et al., 2010). As a result, macrophytes growing in areas with
elevated levels of nitrogen and phosphorus are expected to be distinctively higher
in N & P levels compared to those in lower levels (Kohzu, et al., 2008; Cohen and
Bradman, 2010).
Macrophytes provide habitat and food for different species of invertebrates and
fish (Esteves & Suzuki, 2010). The type of macrophytes and their nutritive quality
is important in fishery industry, as the contribution of different macrophytes
groups to fauna production variation depends on their relative importance as
primary producers (Forsberg, et al., 1993).
Plants exhibit different life forms which include herbaceous macrophytes, trees,
periphyton and phytoplankton (Ehleringer, et al., 1991; Forsberg, et al., 1993).
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They also exhibit different modes of photosynthesis and they are grouped into
C3, C4 and CAM plants (Ehleringer, et al., 1991; Smedley, et al., 1991; Forsberg,
et al., 1993). The C4 plants represent 15% of all species found on the planet and
are only found in Angiosperms (Smedley, et al., 1991). They are common in hot,
low CO2 areas and in open environments (Ehleringer, et al., 1991). C4 plants use
phosphoenol pyruvate carboxylase to fix CO2 (Ehleringer, et al., 1991; Smedley,
et al., 1991). The second group (C3 plants), accounts for 75% of the planet's
plant species. They are found in cold to warm areas with high CO2 levels
(Ehleringer, et al., 1991; Smedley, et al., 1991; Forsberg, et al., 1993). The C3
Plants use Ribulose-1,5 bisphosphate carboxylase/oxygenase (Rubisco)
(Ehleringer, et al., 1991; Smedley, et al., 1991). The last group (CAM plants) is
mostly found in hot to dry areas and account for only 10% of the planet's flora
(Ehleringer, et al 1991).
2.1 The C:N ratios
The C:N ratios are the measure of the dry weight of total organic carbon to total
nitrogen of the sample analyzed (Durga, et al., 2011). The C:N ratios of
macrophytes are influenced by both natural factors and anthropogenic nutrient
loads (Durga, et al., 2003; Baeta, et al., 2009). The isotopic ratios of carbon and
nitrogen in freshwater plants act as indicators of ecosystem functioning and the
effect of land use change (Durga, et al., 2003; Cohen and Bradman, 2010). The
use of C:N ratios in ecological studies is based on the assumption that they
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remain relatively constant as they are passed from producer to consumer
(Forsberg, et al., 1993), and that their suitability as bio-indicators depends on
the nature of nutrient supply (e.g. Inorganic C & N supply) (Baeta, et al., 2009).
It has been noted that seasonal nitrogen variations reflect the variation in C:N
ratios of submerged macrophytes (Herzschuh, et al., 2010). The change in C:N
ratios are also believed to be influenced by the selectivity in biochemical
processes of the organisms (Lake, et al., 2001). The C:N ratios in detritus based
systems are higher compared to macrophytes because of the decrease in carbon
as a result of the function of microbial organisms (Greenwood & Rosemond,
2007). it has been noted that in estuarine systems, C:N ratio signatures of
aquatic plants decline in relation to the higher proportion of phytoplankton rich
detritus in the sediment’s organic pool (Otero, et al., 2000). This is because soil
carbonate layers take on the δ13C composition of the material that was buried on
the site during carbon formation (Ehleringer, et al., 1991).
The isotopic composition of plants differ based on their bio-physiological
adaptations and environmental conditions (Zhao, et al., 2010), their biochemical
processes (Ehleringer, et al., 1991; Forsberg, et al., 1993), plant life forms and
life cycles (LaZerte & Szalados, 1982; Smedley, et al., 1991; Forsberg, et al.,
1993). The total organic carbon to total nitrogen (C:N ratio) of C3, C4 and CAM
plants differ considerably due to plant’s habitat requirements and bio-
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physiological adaptations (Ehleringer, et al 1991; Durga, et al., 2003). The C:N
ratios of phytoplankton range from (5‰ to 6‰), planktonic detritus (6‰ to
9‰), and phytogenic material (4‰ to 10‰) (Durga, et al., 2003). Further, C3
plants (trees, shrubs and some grasses) and (macrophytes and grasses) range
from 15‰ to 20‰ and 20‰ to 39.4‰ respectively (Durga, et al., 2003;
Fellerhoff, et al., 2003; Wu, et al., 2007).
2.2 The δ13C
The δ13C signatures have been used extensively in tracing different carbon
sources and in studying food relations (Peterson, and Whitfield, 1997; Baeta, et
al., 2009) and quantifying the off-site effects of biomass burning (van Noordwijk,
et al., 2004). The plant biochemical processes and anthropogenic activities are
the major factors affecting the δ13C signatures of macrophytes (Chang, et al.,
2009; Cohen, et al., 2010). Therefore, the δ13C of macrophytes depend on the
tissue being analyzed and nature of land - use changes (Chang, et al., 2009).
The δ13C signatures are sensitive to land-use changes in the river catchment
because of the different amounts of nutrients added in the agricultural field close
to the river systems (Lake, et al., 2001). The δ13C signatures in urban areas are
lower compared to agricultural areas (Kendall, et al., 2010). These nutrient loads
cause spatial and temporal variation in δ13C signatures of macrophyte (Troxler
and Richards, 2009). The aquatic environments that receive large inflows of
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allochthonous (nutrients derived from outside the river system) inputs, are
expected to have depleted δ13C signatures due to the abundance of trees in their
upper reaches (Otero, et al., 2000).
The physiological processes of C3 plants increase the ratio of carbon fixation
during growing season leading to the decline of δ13C of these plants (Smedley, et
al., 1991). The δ13C of plants also differ due to stomatal opening and because of
variation in the chlorophyll-a carbon demand (Otero, et al., 2000). The δ13C
values in C3 plants have known ranges of -32 to -20‰ in freshwater systems, -
36‰ to -21.5‰ in marine systems (Boutton, 1991), and -37‰ to -22‰ in
terrestrial environments (Zhao et al 2010). The global δ13C mean value of C3
plants is -27‰ (Otero, et al., 2000; Zhao, et al., 2010). The δ13C in C4 plants
have been found to range from -17 to -9‰ with a mean of -13‰ in aquatic
systems (Boutton, 1991) and -15 to -9‰ in terrestrial environments (Zhao, et
al., 2010). The known δ13C signatures of macro-algae are higher than those of
C3 and C4 as macro-algae assimilate detritus from both plant forms (Boutton,
1991). The δ13C in CAM plants have also been found to range from -28 to -10‰
but their values are variable as these plants can change from CAM to C3 plants
under stressful conditions (Boutton, 1991).
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2.3 The δ15N
Nitrogen (N) has two stable isotopes, lighter 14N, which is the most abundant
element of all naturally occurring nitrogen, and heavier 15N (Ehleringer, et al.,
1993). The least abundant δ15N is often used in ecological studies to investigate
the effect of fertilizers, urbanization etc on aquatic and terrestrial ecosystems
(Lake, et al., 2001; Lepointa, et al., 2004; Kohzu, et al., 2008; Chang, et al.,
2009). Its behaviour, form, abundance and rate processes have been studied in
deeper details in aquatic systems (Wada and Hattori, 1991).
Nitrogen stable isotopes provide information on sources and sinks of nutrients
(Kohzu, et al., 2008; Kendall, et al., 2010). The atmosphere has a value of ~
0‰ of δ15N. Consequently any value above that has a different origin (Forsberg,
et al., 1993). The δ15N is known to increase with increase in land use activities
and the values of δ15N range from –3‰ to +3 ‰ in agricultural areas and
+10‰ to + 20‰ in human dominated areas with human waste (Forsberg, et
al., 1993). The δ15N in all plants is variable and depends on the nutrient supply
and environmental conditions (i.e. soil type) and is reported to range from 0‰
to 8‰ in undisturbed areas (Chang, et al., 2009). The δ15N of allochthonous
inputs to river is known to range from -5‰ to + 18‰ with an average value of
~ 3‰ (Wada and Hattori, 1991). This is in support of Cohen and Bradman
(2009) suggestions that, δ15N has high values in anthropogenic impacted areas
than those thought to be pristine. This difference is the result of the δ15N inputs
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of nutrient sources, the rate of isotopic fractionation and the type of nitrogen
transformation (Wozniak, et al., 2012). As a result the analysis in macro-algae
and macrophytes has helped identify sources of anthropogenic nitrogen loads at
sites directly impaired by land use changes (Wozniak, et al., 2012).
2.4 The use of δ15N in studying aquatic ecosystem functioning
Nitrogen is a limiting factor in highly nitrogen enriched ecosystems (Wozniak, et
al., 2012). Nutrients from human induced activities can severely change aquatic
food webs and ecosystem functioning (Greenwood, et al., 2007; Kohzu et al.,
2008; Miller, et al., 2010; Nyenje, et al., 2010). The δ15N is the most preferred
nitrogen stable isotope by plants and has been found to be high in macrophytes
raised or naturally growing in waste water receiving sites of river systems
(Kohzu, et al., 2008; Cohen and Bradman, 2010). The δ15N from anthropogenic
activities is important in understanding nitrogen dynamics and aquatic
contamination (Wozniak, et al., 2012). It is widely used to trace nitrogen sources
which include precipitation, fertilizers, animal and human wastes (Cohen and
Bradhman, 2009) and to study lake paleo-productivity (Herzschuh et al., 2010).
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2.5 Justification of the study
Rivers and coastal oceans are active sites for carbon cycling because of larger
autochthonous production relative to the open oceans and closed lakes (Kuzyk,
et al., 2010). Human activities such as agricultural and sewage discharge put
pressure on aquatic systems and cause algal blooms (Xu, et al., 2007; Kohzu et
al., 2008; Chang, et al., 2009; Nyenje, et al., 2010). The effects of land use in
the study area are important in view of the fact that Umtata River runs through
forested plantation in its upper reaches, is densely populated along Umtata town
and subsistence farming in its middle/ lower reaches (Fatoki, et al., 2001). These
activities are the major causes of eutrophication to this river.
2.6 Aims
To identify macrophytes species composition along Umtata River system.
To determine SIS of C:N ratios, δ13C and δ15N composition of
macrophytes along Umtata River.
To determine effect of selected sites and sampling period on the SIS of
C:N ratios, δ13C and δ15N of macrophytes along Umtata River.
To relate the C:N ratios, δ13C and δ15N composition of macrophytes to
land use activities occurring along Umtata River system.
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2.7 The research hypothesis
Ho: The δ13C, δ15N and C:N ratio signatures of submerged macrophytes
are the same in all sites in all seasons.
Ha: The δ13C, δ15N and C:N ratio signatures of submerged macrophytes
are not the same in all sites in all seasons.
The overall hypothesis was to test whether the δ13C, δ15N and C:N ratio
signatures of submerged macrophytes reflect the land-uses occurring in
the Umtata River catchment.
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CHAPTER 3
Materials and methods
3.1 Study area
The study was conducted along the Umtata River which originates in the plateau
region of the former Transkei region of the Eastern Cape Province, South Africa,
approximately midway between the Drakensberg escarpment and the Indian
Ocean. The catchment of the river is about 100km long and up to 50km wide
(Fatoki, et al., 2001). The terrain is generally undulating and in Mthatha town
vicinity it flows through a wider flat plain with a shallow gradient (Mafereka, et
al., 2009). Soils in the catchment are moderate to deep, and vary between
sandy-loam in the upper half to clay-loam in the middle reaches to sandy soils in
the lower reaches and the mouth (Fatoki, et al., 2001). The river flows through a
steep slope in its upper reaches which are extensively covered with both natural
and artificial forests, and has clean water in this area owing to the vegetation
cover (Fatoki, et al., 2001). In the middle to lower reaches water hyacinth
(Eichhornia crassipes) covers much of the surface water.
For simplicity of data interpretation, the sampling period was divided into winter
(May, June, July), spring (August, September, October) summer (November,
December, January) autumn (February, March, April). The study area was
divided into seven sampling points (S1 to S7, Table 1). Sites (S1 to S3) were
at (upper reaches), sites (S4 to S6) (middle reaches) and site (S7) (lower
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reaches). All the land use activities taking place in the study area were recorded
(Table 1).
3.2 Selection of sampling sites
Seven sampling sites were recorded using Global Positioning System (GPS)
device along Umtata River. The sampling sites were chosen to reflect different
land use activities in the catchment - upper stream, midstream and downstream,
Figure1 & Table 1.
3.3 Macrophytes
Submerged macrophytes were handpicked in the river banks along the seven
selected sites, placed in plastic Ziploc bags, transferred to a portable refrigerator
and taken to the laboratory. In the laboratory all macrophyte samples were
identified into species level according to guides to aquatic plants (Gerber, et al.,
2004). All samples were rinsed severally with distilled water to remove sediment
particles and thereafter oven dried at 60oC for 48 hours. The dried samples were
later crushed in a mortar using a pestle. Samples were then sealed in plastic
eppendorf tubes for later stable isotope analysis.
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Figure1: Map showing study area and sampling sites (Source: Department of
Water Affairs, King Sabata Dalindyebo Municipality).
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Table 1: Showing sampling locations, coordinates and activities along the sites
Sampling sites Coordinates Anthropogenic activities
Langeni forest1
(S1)
31º 29, 58’’S:
28º 28, 61’’E
Indigenous, artificial forests,
deforestation, grazing land and road
construction.
Langeni forest 2
(S2)
31º 29, 92’’S:
0 28º 29, 61’’E
Artificial forest, subsistence farming,
deforestation and grazing land.
Kambi
(S3)
31º 29, 92’’S:
0 28º 36, 1’’E
Human settlement, mixed forest,
deforestation and grazing land.
Mthatha dam
(S4)
31º 33, 02’’S:
0 28º 44, 5’’E
Human settlement, domestic water use,
wood processing industry, subsistence
farming and grazing land.
Tipin
(S5)
31º 31, 57’’S:
0 28º 48,10’’E
Informal human settlement, poor
sanitation, subsistence farming,
wastewater discharge point, brick making
and dumping site.
Ntsaka
(S6)
31º 4, 13’’S:
028º 49,25’’E
Human settlement, domestic water use,
subsistence farming and grazing land.
Mdumbi
(S7)
31º55,52’’S:
0 29º 08, 19’’E
Human settlement, domestic water use,
road construction, subsistence farming
and grazing land
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3.4 Isotopic analysis
The stable isotope signatures (SIS) analysis were done at the Archaeometry
laboratory, University of Cape Town using Finnegan-MAT 252 stable light mass
spectrometer. Stable isotope signatures (SIS) were expressed in the following
standard notion:
δX (%)= (Rsample/ Rstandard)-1* 100
where X is 13C or 15N and Rsample is the 13C/12C or 15N/14N respectively. R standard
for 13C is Pee Dee Belemnite (PDB). All resultant values were expressed in mils
(o/oo) (Lepointa, et al., 2004).
3.5 Statistical data analysis
The data was analyzed using two-way Analysis of Variance (ANOVA). The two-
way ANOVA(s) were followed by post hoc test. The Sites and seasons were
independent variables while stable isotopes of C:N ratios, δ13C and δ15N of
macrophytes were dependent variables. The statistical data analysis was
performed using Statistical 7.
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Chapter 4
Results
4.1 The macrophyte species composition of Umtata River
A total number of 126 macrophyte samples consisting of 16 families and 26
species were recorded. The highest number of macrophytes family was 7 at site
(S5), 6 families at sites S6 and 4 families at S7 compared to sites (S1, S2, S3
& S4 which had 3 families at each) (Table 2). The upper reaches (S1, S2 & S3)
showed a least number of families (5 in total) compared to middle reaches (S4,
S5, S6) (10 families in total) and lower reaches (S7) (4) (Table 2).
Most macrophyte families showed site specific occurrences while others occurred
in more than two sites (Table 2). The family Cyperaceae was the most
abundant with 8 species and occurred throughout the length of the river (Table
2). The only site specific families which did not occur at any other site were;
freshwater algae at site (S4), Brassicaceae, Onagraceae (S5), Typhaceae (S6)
and Commelinaceae at site (S7). At the species level, the species composition
decreased from upper to lower reaches. A large number of exotic species (Alisma
plantago-aquatica, Eichhornia crassipes, Phragmites lapathifolia, Nasturtium
officinale, Myriophyllum aquaticum) was recorded in the middle reaches (Table
2) The Juncus effusus occurred only at the upper (S1) and Isolepis fluitans at
lower reaches (S7) respectively (Table 2). The following two species: Berula
erecta, Typha capensis and were recorded only in the middle reaches (S5 & S6)
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(Table 2). The following species were recorded only at S7: Cyperus
sexangularis, Floscopa glomerata and Phragmites australis (Table 2). The
Eichhornia crassipes species covered the river in the middle reaches (S5 & S6)
and was also collected at the lower reaches (S7) after heavy rain falls (Table 2).
4.2 The C:N ratios at family level
For ease of data interpretation, the macrophytes species were grouped into
families and the average values of C:N ratios were taken for each family at each
site (Table 3).
The isotopic composition of macrophytes and C:N ratios showed variation in all
families collected from the upper to lower reaches of the river. There were site
specific differences in C:N ratios values of macrophytes observed along Umtata
river (Table 3). The most depleted C:N ratios signatures (6.01‰) were
recorded in family Poaceae at site 7 while the most enriched were recorded in
family Cyperaceae (30.75‰) at site (S2, Table 3).
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Table 2: Showing the number of macrophytes families and species from the
sites along Umtata River. *Alien species.
Sites Family Species No.
Families
No. Species
S1 Cyperaceae Cladium mariscus 3 6
Cyperaceae Schoenoplectus brachyceras
Polygonaceae Persicaria senegalensis Junacaceae Juncus effusus Junacaceae Juncus lomatophyllus
S2 Cyperaceae C. mariscus 3 6
Potamogetonaceae Potamogeton pectinatus
Cyperaceae S. brachyceras Potamogetonaceae P. corex-austra-africana
Poaceae Phragmites mauritianus * Persicaria lapathifolia S3 Cyperaceae S. brachyceras 3 4
Cyperaceae C. margmatus Polygonaceae P. lomatophyllus Potamogetonaceae P. schweinfurthii S4 Potamogetonaceae P. pectinatus 3 3
Freshwater algae Spirogyra Alismataceae *Alisma plantago-
aquatica S5 Apiaceae Berula erecta 7 6
Brassicaeae *Nasturtium officinale
Cyperaceae Cyperaceae sexangularis
Pontederiaceae *Eichhornia crassipes Lemnaceaeae Lemna gibba Polygonaceae * Persicaria lapathifolia Polygonaceae Persicaria decipiens Onagraceae Ludwigia adsendens S6 Apiaceae Berula erecta 6 6
Haloragidaceae *Myriophyllum aquaticum
Poaceae P. mauritiatus Polygonaceae P. decipiens Pontederiaceae *E. crassipes Typhaceae Typha capensis S7 Cyperaceae Isolepis fluitans 4 8
Cyperaceae C. marginatus Cyperaceae C. sexangularis Commelinaceae Floscopa glomerata Haloragaceae P. australis Polygonaceae P. senegalensis
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4.3 The δ13C values at family level
The δ13C values in macrophytes showed site-specific variations and longitudinal
variation in all sites sampled. The Juncaceae at site (S7) had the most depleted
δ13C value of less than -31‰, while the most enriched family, the
Potamogetonaceae at site (S6) was -15.14‰ (Table 3). The family Cyperaceae
occurred in all river stages sampled except sites (S4 and S6) and its δ13C values
fell between -19.50‰ to -29.25‰. The family Polygonaceae showed
longitudinal variation in δ13C signatures and followed family Cyperaceae with its
values ranging between -18.0‰ to -29.02‰.
4.4. The δ15N at family level
The families that showed longitudinal distribution along the river system also had
varying values of δ15N signatures (Table 3). The most depleted δ15N values
were recorded in family Cyperaceae at site (S1) (3.42‰), while the most
enriched δ15N value of about 24.32‰ was recorded in Pontederiaceae at site
(S6). All the families in the middle reaches (Apiaceae, Brassicaceae, Cyparaceae,
Holaragidaceae, Onagraceae, Poaceae, Polygonaceae, Pontederiaceae,
Potamogetonaceae and Typhaceae had highest values compared to other
families in the upper and lower reaches (Table 3).
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Table 3: Showing the stable isotope composition of macrophytes families from
the sites along Umtata River. Each family represents average SIS of species
collected.
Sites Family δ15N (‰) δ13C (‰) C:N Ratio (‰)
S1 Cyperaceae 3.42 -20.75 19.75
Juncaceae 3.35 -21.51 24.51
Polygonaceae 7.4 -21.50 17.50
S2 Cyperaceae 6.5 -29.25 30.75
Poaceae 5.0 -28.38 23.25
Polygonaceae 6.13 -26.75 13.26
S3 Cyperaceae 8.26 -20.23 23.25
Polygonaceae 4.75 -24.75 13.76
Potamogetonaceae 5.27 -28.0 17.69
S4 Potamogetonaceae 7.83 -21.67 17.83
Poaceae 6.01 -17.5 19.33
Potamogetonaceae 6.81 -15.14 16.05
S5 Apiaceae 7.64 -25.24 18.08
Brassicaceae 8.99 -25.01 11.98
Cyperaceae 8.47 -19.50 6.47
Onagraceae 21.05 -21.64 10.01
Poaceae 7.46 -22.23 20.15
Polygonaceae 16.27 -29.02 9.50
Pontederiaceae 8.91 -29.14 11.45
S6 Apiaceae 14.98 -15.40 15.7
Haloragidaceae 20.43 -20.35 10.45
Poaceae 12.04 -20.34 6.52
Polygonaceae 18.61 -18.04 21.0
Pontederiaceae 24.32 -24.03 8.78
Typhaceae 17.11 -22.15 19.86
S7 Cyperaceae 9.88 -22.61 14.79
Commelinaceae 7.0 -22.43 27.35
Cyperaceae 6.33 -19.99 17.33
Cyperaceae 7.50 -23.81 22.0
Haloragidaceae 8.52 -24.52 13.41
Juncaceae 8.99 -31.21 17.22
Poaceae 8.67 -22.50 6.01
Polygonaceae 9.45 -23.54 7.65
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The ANOVA(s) showed no statistical significances (p>0.05) between sampling
sites and seasons on C:N ratios and δ13C signatures of macrophytes collected
along Umtata River (Table 4). The effect of both sampling sites and seasons
was only observed in δ15N signatures (p<0.05) (Table 4).
Table 4: Showing the effect of sampling sites and seasons on the C:N ratios,
δ13C and δ15N signatures of macrophytes along Umtata River.
Sources of
variation
SS DF MS F P
C:N ratios Months 163.33 11 52.43 3.19 0.07
Sites 117.18 6 52.53 2.24 0.16
Interaction 37.56 11 30.58 2.37 0.79
δ13C Months 37.56 11 30.58 1.22 0.40
Site 72.34 6 30.58 2.36 0.14
Interaction 26.77 66 30.58 0.87 0.65
δ5N Months 39.07 11 6.35 6.15 0.01
Sites 45.22 6 20.42 7.12 0.01
Interaction 6.84 66 57.58 1.08 0.51
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4.5 The effect of sampling sites on C:N ratios of macrophytes
The C:N ratio signatures varied throughout the length of the river (Table 5).
Although there was no statistical significance (P>0.05), the trends showed a
decrease in C:N ratios from upper to lower reaches (Figure 2). The highest C:N
ratios value (21.95±9.65‰) was recorded at site (S2) while the lowest value
(12.14±5.0‰) occurred at site S6 (Table 5, Figure 2).
Table 5: Showing the mean spatial values and standard deviation of C:N ratios
of macrophytes recorded along Umtata River, N= 84 .
SAMPLING SITES MEAN VALUES (‰) STANDARD DEVIATION
S1 19.68 9.05
S2 21.29 9.65
S3 17.02 5.92
S4 18.90 8.03
S5 14.47 7.21
S6 12.14 5.0
S7 13.73 3.92
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0
5
10
15
20
25
S1 S2 S3 S4 S5 S6 S7
Sampling sites
C:N
ratios (
‰)
Figure 2: Graph showing mean spatial C:N values of macrophytes recorded
along Umtata River.
4.6 The effect of sampling period on C:N ratios of macrophytes
collected along Umtata river
The mean C:N ratio values varied throughout the sampling period with February
recording the lowest value of 5.8‰ (Table 6, Figure 3). The monthly C:N ratio
mean values that exceeded 20‰ were recorded in May, November and January
(Table 6). The trends of seasonal C:N mean values decreased from May to
October and then increased variably between November to April with highest and
lowest peaks recorded in January and February respectively (Table 6, Figure
3).
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Table 6: Showing the mean seasonal values and standard deviation of C:N
ratios of macrophytes recorded along Umtata River, N= 84.
SAMPLING PERIOD MEAN VALUES (‰) STANDARD DEVIATION
May 21.06 10.45
June 17.02 9.01
July 15.50 7.95
August 17.72 7.45
September 15.10 4.52
October 11.67 1.97
November 20.31 8.34
December 17.30 3.51
January 24.0 7.44
February 5.80 1.53
March 19.01 6.98
April 16.31 2.54
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0
5
10
15
20
25
30
May
June
July
August
Septe
mber
Octo
ber
Novem
ber
Decem
ber
January
Febru
ary
Marc
h
April
Sampling period (Months: 2010 - 2011)
C:N
ratios (
‰)
Figure 3: Graph mean showing seasonal C:N values of macrophytes recorded
along Umtata River.
4.7 The effect of sampling sites on δ13C signatures of macrophytes
The mean δ13C signature values varied across the length of the river, with the
highest enriched value recorded at site S4 and lowest depleted values at sites
S5, S6, and S7 ( Table 7, Figure 4). The upper sites (S1, S2, S3 and S4)
were marginally enriched compared to sites (S5, S6 and S7) (Table 7, Figure
4).
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Table 7: Showing the mean spatial values and standard deviation of δ13C of
macrophytes recorded along Umtata River, N= 84.
SAMPLING SITES MEAN VALUES (‰) STANDARD DEVIATION
S1 -22.64 4.08
S2 -24.23 5.19
S3 -24.65 5.31
S4 -19.63 6.12
S5 -26.33 5.77
S6 -25.65 4.70
S7 -26.37 4.14
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-30
-25
-20
-15
-10
-5
0
S1 S2 S3 S4 S5 S6 S7
Sampling sites
Carb
on s
table
isoto
pe (
‰)
Figure 4: Graph showing mean spatial δ13C values of macrophytes recorded
along Umtata River.
4.8 The effect of sampling period on δ13C signatures
The mean monthly δ13C signatures varied throughout the sampling period
(Table 8, Figure 5). The most depleted δ13C signatures were recorded in the
months of July (-27.05±3.27‰), September (-26.52±3.05‰) and April (-
26.10±6.84‰) (Table 8). The mean δ13C signatures values appeared to
variably increase and decrease between the sampling period of May to April
(Table 8, Figure 5).
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Table 8: Showing the mean seasonal values and standard deviation of δ13C of
macrophytes recorded along Umtata River, N=84.
SAMPLING PERIOD MEAN VALUES (‰) STANDARD DEVIATION
May -21.45 6.83
June -24.47 5.33
July -27.05 3.27
August -24.85 3.96
September -26.52 3.05
October -24.89 3.35
November -25.13 4.41
December -20.92 5.65
January -24.61 7.72
February -21.6 1.14
March -25.20 6.55
April -26.10 6.84
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-30
-25
-20
-15
-10
-5
0
May
June
July
August
Septe
mber
Octo
ber
Novem
ber
Decem
ber
January
Febru
ary
Marc
h
April
Sampling period (Months: 2010 - 2011)
Carb
on s
table
isoto
pe (
‰)
Figure 5: Graph showing mean seasonal δ13C values of macrophytes recorded
along Umtata River.
4.9 The effect of sites on δ15N signatures of macrophytes
The mean δ15N signatures of macrophytes collected along Umtata river revealed
statistical significance (p<0.05) on sampling sites. The trends of δ15N signatures
and δ15N site mean values increased variably from upper to middle sites (S1 to
S6) and decreased towards lower reaches site (S7) respectively (Figure 6,
Table 9). The middle sites (S5, S6) and lower site (S7) had highest mean δ15N
values compared to lower values at the other sites respectively (Figure 6 &
Table 9).
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Table 9: Showing the mean spatial values and standard deviation of δ15N
signatures of macrophytes recorded along Umtata River, N=84.
SAMPLING SITES MEAN VALUES (‰) STANDARD DEVIATION
S1 3.92 2.43
S2 6.61 2.12
S3 6.22 3.35
S4 6.98 2.11
S5 9.12 4.81
S6 10.02 4.56
S7 8.12 3.10
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0
2
4
6
8
10
12
S1 S2 S3 S4 S5 S6 S7
Sampling sites
Nitro
gen s
table
isoto
pe (
‰)
Figure 6: Showing mean spatial δ15N values of macrophytes recorded along
Umtata River.
4.10 The effect of sampling season on δ15N signatures of macrophytes
The mean δ15N signatures of macrophytes had statistical significance (p<0.05,
N=84) for sampling period. The δ15N signatures mean values between the
sampling period ranged from approximately 7‰ to 10‰ but dropped sharply to
less than 1‰ in February (Figure 7 & Table 10).
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39
Table 10: Showing the mean seasonal values and standard deviation of δ15N
signatures of macrophytes recorded along Umtata River, N=84.
SAMPLING PERIOD MEAN VALUES (‰) STANDARD
DEVIATION
May 9.77 4.35
June 9.25 5.81
July 9.67 2.97
August 7.19 2.55
September 8.33 2.94
October 7.01 3.35
November 7.01 1.45
December 6.70 2.42
January 7.50 3.88
February 0.50 2.49
March 7.21 1.95
April 7.50 2.10
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40
02
46
810
May
June
July
August
Septe
mber
Octo
ber
Novem
ber
Decem
ber
January
Febru
ary
Marc
h
April
Sampling period (Months: 2010 - 2011)
Nitro
gen s
table
isoto
pe(‰
)
Figure 7: Graph showing mean spatial δ15N values of macrophytes recorded
along Umtata River.
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41
CHAPTER 5
Discussion
The occurrence of macrophytes in rivers is mainly determined by their
biogeographic distribution, nutrient availability, and anthropogenic activities
(Cole, at al., 2006; Benoit, et al., 2009; Esteves & Suzuki, 2010). The dominant
agricultural activities along Umtata River catchment may account for the
macrophytes family composition along this river system. The addition of sewage
and industrial waste discharged in the middle reaches sites (S5) to (S6),
favoured the presence of the highest macrophyte groups collected with alien
plants dominating over the local plants (Table 2, Eastern Cape River Health
Programme, 2004-2006).
The lowest number of species was recorded at the impounded site, Umtata dam
(S4), and it’s vegetation along its banks is mostly grass and algae (Table 2).
The development of macrophytes in lentic and lotic systems is complex and
involves the interaction between physiological, physic-chemical and ecological
factors (Esteves & Suzuki, 2010). The longitudinal distribution of macrophytes in
Umtata River may be attributed to habitat transformations due to different land
uses along its catchment (Benoit, et al., 2009; Esteves and Suzuki, 2010) and
the difference in the number of families in sampling sites which results from
availability of different nutrient sources (Michener & Lajtha, 2007).
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The isotopic composition recorded along Umtata River indicated high values in
middle reaches of the river and this may have been the case, for macrophytes as
bio-indicators of eutrophication depends on the nature of nutrient supply to river
systems (Troxler and Richards, 2009), and they differ from one macrophyte to
another (Cohen & Bradham, 2010). These results (Table 3) indicated nutrient
accumulation and availability at sites sampled and the response of submerged
macrophytes to environmental changes (Cole, et al., 2006; Chang, et. al., 2009).
The C:N ratios and δ15N signatures of the river were lower in the upper reaches
compared to middle reaches (Table 3). This may be due to contribution of
depleted δ15N forest derived values compared to urban inputs (Cohen &
Bradham, 2010, Kendall, et. al., 2010; Ogrinc, et al., 2008). The anthropogenic
activities taking place along Umtata River system may have altered sediment and
water chemistry leading to modified habitat for macrophytes growing in this river
and also great variation in isotopic values measured in the study site (Michener
and Lajtha, 2007; Ogrinc, et. al., 2008; Baeta, et al., 2009; Chang, et al., 2009;
Troxler and Richards, 2009) (Table 3).
The signatures of the three variables (C:N ratios, δ13C, δ15N) investigated in this
study had higher values both in dry and wet periods (Tables 6, 8 & 10,
Figures 3, 5 & 7). These high values maybe attributed to biomass burning
(Martinelli et. al., 2002), less effluents received by water systems (Kendall, et al.,
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2010), nature of nutrient supply (Ehleringer, et al., 1991; Baeta, et al., 2009;
Otero, et al., 2000), plant biochemical processes (Forsberg, et al., 1993; Lake et
al., 2001) and plant life forms and life cycles (LaZerte & Szalados, 1982;
Smedley, et al., 1991; Forsberg, et al., 1993). Other factors reported in the
literature include temporal variation in the nitrogen source (Cohen & Bradham,
2010), the transport and accumulation of nutrients from one site to another
(Cole, et al., 2006) and the response of aquatic organisms to seasonal changes
(Esteves and Suzuki, 2010).
The C:N ratios trends indicated areas with elevated nutrient inputs with the
maximum values of 37.05‰ (Figure 2, Table 5). This value represents the
total dry weight of organic carbon and total nitrogen present in water column
signaling high nutrient content in the river (Durga, et al., 2011). However, the
findings of Troxler and Richards, (2009) suggest that the nutrient retention and
seasonal nutrient limitations by submerged macrophytes and productivity
change, have minor effect on their C:N ratios. These may also account for the
findings of the varying mean C:N ratio trends in this study (Tables 2 & 3,
Figures 3 & 4) Moreover, the C:N ratios recorded along Umtata river fell
between 3.57‰ to 37.05‰ with an average value of 16.03‰. This range was
lower compared to those reported in the literature on macrophytes (20‰ to
39.4‰ and C4, 15‰ to 20‰ respectively) (Fellerhoff, et al., 2003; Durga, et
al., 2011), and even lower than that reported in planktonic detritus (6‰ to
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44
9‰) (Fellerhoff, et al 2003; Kuriah, 2008, Durga, et al., 2011). This C:N ratio
range recorded in this study can be attributed to different anthropogenic
activities in the catchment. The forested upper reaches of Umtata River seemed
to derive its nutrient from leaf litter and detritus from the mixed forest around it
as they had higher values. In this area decomposing plant material was
abundant and blocked water current in some parts forming temporal
impediments (Personal observations). Similar observations have been reported in
USA freshwater systems (Kendall, et. al., 2010) and in freshwater dominated
estuaries of southeastern USA (Ogrinc, et. al., 2008).
The variation of seasonal mean values of C:N ratios throughout the length of the
river and sampling period (Figures 3, Table 6), is thought to be a result of
stream geology and the river physics (Maltias-Landry et al., 2009), agricultural
and sewage nutrient inputs (Boullion, et. al., 2002; Ogrinc, et. al., 2008). This
variation is mainly related to contribution of different nutrient inputs which are
driven by both seasonality and spatiality to these systems (Kennedy, et al.,
2005). For this study the values of C:N ratios may be the result of dominant
agricultural practices in upper reaches (Doucett, et. al., 1996), waste water
discharge in the urban dominated middle reaches (Baeta, et al., 2009, Durga, et
al., 2011), the mixture of both land-use activities and within river nutrient inputs
towards the lower reaches (Doucett, et. al,. 1996).
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The high winter C:N ratios mean values found in this study are not different from
those reported in the literature in terms of seasonality. This is probably due to
the fact that the dry winter months of May and June promoted detritus and
reduced water level in rivers leading to higher carbonates and nitrates
concentrations in the water column (Salas and Dudgeon, 2001; Chang, et al.,
2009; Esteves and Suzuki, 2010; Kendal, et al., 2010), and terrestrial
environment (Zhao, et al., 2010) (Figure 3, Table 6). The rainfall may account
for the difference in C:N ratios between spring and autumn. The transport and
transformation of nutrients along the river in undeveloped and developed sites
account for isotopic differences (Cole, et al., 2006).
The δ13C signatures fell between -26.37‰ to-19.63‰ with an average of -
24.41‰. The δ13C signatures recorded in this study were within the known
range for freshwater macrophytes (C3 plants, -32 to -20‰ and C4, -17 to -9‰)
respectively (Boullion, 2002), but their mean value was lower than that reported
in the literature (-27‰ for C3 and -13‰ for C4 plants) (Paterson and Whitfield,
1997; Otero, et al., 2000; Zhao, et al., 2010). These C:N ratios and δ13C
signatures in macrophytes closely compared to the range observed for C3 plants
(-24‰ to -34‰) for two rivers in the Eastern Cape Province of South Africa
(Kuriah and Pakhomov, 2008). This may probably be due to the nature of land
uses occurring along the sites selected for this study and to δ13C sensitivity to
land use activities (Lake, et al., 2001). Slow river current flow is closely
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associated with high δ13C values in impoundments (Kendall, et. al., 2010),
leading to high nutrient retention by macrophytes (Maltias-landry, et al., 2009).
This may well explain the higher δ13C values trends at the dammed sites (S1 &
S4, Figure 4). The varying δ13C values recorded in agricultural dominated upper
and lower sites (Figure 4, Table 7) may also be attributed to vegetation cover
and soil type (Martinelli, et al., 1999). The sites S4 & S6 were in open areas
and showed unequal δ13C values signaling the contribution of nutrient sources
with distinctive isotopic composition (Kohzu, et al., 2008). The effect of different
land uses was evident with distinct isotopic signatures in all sites sampled. The
study revealed high values in agricultural areas in upper and lower reaches
compared to urban and below urban sites in middle reaches (Tables 7, Figure
4), probably due to the fact that forested areas add carbon rich detritus material
to river system than carbon depleted urban sewage (Meskumpumn, et al., 2005;
Salas and Dugeon, 2008). Furthermore, carbon is reported to be higher in
agricultural areas than in human dominated areas (Kohzu, et al., 2008).
However, these results contradict the findings of (Otero, et al., 2000) who
reported lower δ13C values in C3 dominated upper reaches of freshwater
dominated estuaries of South Eastern USA.
The δ13C values varied greatly throughout the sampling period. The high δ13C
values in months December and January (Figure 5, Table 8) may be attributed
to runoff which introduces nutrients to the river (Salas and Dudgeon, 2001; Van
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Noordwirjk, et al., 2004; Ogrinc., et. al., 2008), whereas high δ13C values in dry
season are associated with low rainfall and windblown terrestrial material to river
systems (Martinelli, et al., 2002; Kennedy, et al., 2005). Other reported driving
forces behind season differences in δ13C values include plant taxonomic
differences in photosynthetic pathways (Ehleringer, et al., 1997; Fellerhoff, et al
2003; Durga, et al., 2011), stomatal aperture and the chlorophyll-a carbon
demand (Otero, et al., 2000) and anthropogenic impacts (Kennedy, et al., 2005).
The seasonal and spatial differences in δ15N signatures of macrophytes depend
on the tissue being analyzed and the rate of exposure to contaminated water
systems (Cohen & Bradham, 2010). The significant results (p<0.05) in δ15N in
both sampling sites and period (Table 4), showed low values in the forested
upper reaches compared to human dominated urban middle reaches and in sites
below urban areas (Figure 6, Table 9). The δ15N is known to increase sharply
in urban dominated areas due to industrial and human waste (Cabana and
Rasmussen, 1996; Kennedy, et al., 2005; Kohzu et al., 2008; Kendall, et al.,
2010). The low δ15N signatures are also reported to result from primary
production in the catchment (Kohzu, et al., 2008; Cohen and Bradman, 2010).
The δ15N values of macrophytes of Umtata River increased from upper to middle
and decreased towards lower reaches indicating the contribution of different
nitrogen inputs with different δ15N composition (Figure 6, Table 9). These
finding are explained by the fact that the unidirectional flow of river water
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favours mixing and accumulation of nutrients of different origin, leading to
longitudinal differences in δ15N of the river systems (Kendall, et al., 2010).
Agricultural derived nutrient inputs have lower δ15N values than industrial and
human waste δ15N values (Cabana and Rasmussen, 1996, Meksumpun, et al.,
2005, Kendall, et. al., 2010) and this coincided with δ15N values from this study.
The δ15N trends showed enriched values in wastewater receiving middle reaches
of Umtata River (Figure 7, Table 10). Studies in freshwater systems have
found that δ15N values are sensitive to differences in land use (Kennedy, et al.,
2005; Kohzu, et al., 2008; Kendall, et al., 2010). The δ15N is mostly driven by
external factors for example difference in land use (Kennedy, et al., 2005), thus
δ15N values vary along sites which were chosen based on different activities
happening around the selected sites. These external factors are the major
influence of differences in the stable isotopes of δ15N (Kennedy, et al., 2005).
The δ15N mean seasonal values recorded in macrophytes along Umtata river fell
between 6‰ to 10‰ from May to April (Table 10), possibly due to the fact
that runoff is affected by seasonality in Umtata area (Eastern Cape river health
programme, 2004 – 2006). These mean seasonal δ15N values decreased variably
from winter to autumn with a highest peak in May and lowest in February
(9.77‰ and 0.5‰) respectively (Table 10, Figure 7). This variation in δ15N is
known to result from variation of the nutrient flow from pollution points (Cabana
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& Rasmussen, 1996) which is the function of runoff. The observed high δ15N
macrophytes signatures during the dry period (May to July 2010) coincided with
the low river flow and the fragmentation of the habitat. This likely resulted in
concentrated nutrients within the impoundments to be severely depleted. The
flushing of the terrestrial organic matter runoffs and subsequent decline after the
rainfall event corresponded to the high and low SIS variations. Further, it is
reported that runoff from agricultural sites tends to contain high concentration of
δ15N (Forsberg, et al., 1993; Kohzu, et al., 2008) as is the case with site 2
(Figure 6). Application of nitrogenous fertilizers to agricultural farmlands and
denitrification from human residential areas to drainage system enhance nitrate
and increase of δ15N (Chang, et al., 2008; Kohzu, et al., 2008). The δ15N of
macrophytes vary in watershed including both pristine and agricultural
catchment, with agricultural and urban sites having high values of δ15N
(Forsberg, et al., 1993; Chang, et al., 2009; Cohen and Bradman, 2009;
Wozniak, et al., 2012).
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Conclusion/recommendations
The macrophyte families and species composition showed both longitudinal and
site specific occurrences along Umtata River. The high abundance of exotic
plants along Umtata River system is an indication of high pollution along the river
system. Although only the stable isotope of nitrogen was statistically significant,
the mean values and trends of all SIS recorded in the river system indicated high
nutrient loading from the land uses from the river catchment. The longitudinal
increase of isotopic composition of macrophytes along the river indicates
accumulation of nutrients from one site to another. The upper reaches of the
river seemed to derive its nutrients from agricultural fields. The middle reaches
received waste water from Mthatha waste treatment facility hence the high δ15N
signatures around this area.
The seasonal variability of the variables tested in this study pointed out rainfall
as major factor controlling the nutrient concentrations along Umtata River. The
dry season seemed to be the most threatening season in terms of nutrient
concentration along the river as δ15N had high values during this season. Based
on the results obtained from this study, there is a need for continued monitoring
of land use of the catchment especially around its middle reaches as it happened
to be an affected area. Further studies on isotopic composition of Umtata River
need to be done as there is no published work on stable isotope ecology of this
system.
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Table 11: The list of all macrophyte families and species collected along Umtata River.
Macrophyte families
Alismataceae
Apiaceae
Brassicaeae
Commelinaceae
Cyperaceae
Freshwater algae (Unidentified)
Haloragidaceae
Haloragraceae
Junacaceae
Lemnaceaeae
Onagraceae
Poaceae
Polygonaceae
Pontederiaceae
Potamogetonaceae
Typhaceae
Macrophyte species
Alisma plantago-aquatica
Berula erecta
Cladium marginatus
Cladium mariscus
Cladium sexangularis
Eichhornia crassipes
Floscopa glomerata
Isolepis fluitans
Juncus effusus
Juncus lomatophyllus
Lemna gibba
Ludwigia adsendens
Myriophyllum aquaticum
Nasturtium officinale
Persicaria senegalensis
Persicaria decipiens
Persicaria lomatophyllus
Phragmites lapathifolia
Phragmites mauritianus
Potamogeton pectinatus
Potamogeton australis
Potamogeton corex-austra-africana
Potamogeton schweinfurthii
Schoenoplectus brachyceras
Spirogyra Spp
Typha capensis
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Appendix Study area photographs showing different anthropogenic land use activities along Umtata River Catchment.
S1. The water from this site is used for irrigation by Langeni forest Farm.
S2. The site is reported to be used for fishing and irrigation.
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S3. Kambi forest- high turbidity levels of the water after heavy rainfall.
S4. The Umtata Dam is mainly used for domestic and recreational purposes.
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A. Sewage discharged from Mthatha town waste treatment plant. B. leaking sewage
stream from a pipe at Fort Gale area in middle reaches (above Site 5).
A
B
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Site 5. C. Informal settlements. D. Water hyacinth (Eichhornia crassipes) on the banks.
S6. The site is mainly used by locals for swimming, fishing, domestic and laundry.
C D
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S7. River system in this area is used by locals for domestic and recreation.
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