assessment of the trophic state of te aka aka (ashley

Assessment of the trophic

state of Te Aka Aka

(Ashley River/Rakahuri –

Saltwater Creek Estuary)

Report No. R19/01 ISBN 978-1-98-859301-2 (print) 978-1-98-859302-9 (web)

Lesley Bolton-Ritchie

May 2019

Name Date

Prepared by : Lesley Bolton-Ritchie Scientist

December 2018

Reviewed by : Helen Shaw Manager Surface Water Science

May 2019

Approved by: Tim Davie Chief Scientist

May 2019

Report No. R19/01 ISBN 978-1-98-859301-2 (print) 978-1-98-859302-9 (web) 200 Tuam Street PO Box 345 Christchurch 8140 Phone (03) 365 3828 Fax (03) 365 3194 75 Church Street PO Box 550 Timaru 7940 Phone (03) 687 7800 Fax (03) 687 7808 Website: www.ecan.govt.nz Customer Services Phone 0800 324 636

Assessment of the trophic state of Te Aka Aka (Ashley River/Rakahuri – Saltwater Creek Estuary)

Environment Canterbury Technical Report i

Executive summary

Background:

Te Aka Aka is a shallow intertidal dominated estuary at the mouth of the Ashley River/Rakahuri, Saltwater Creek, Taranaki Creek and a number of small lowland creeks and mad-made drains. Eutrophication, as expressed by the excessive growth of macroalgae, is a potential ecological issue for Te Aka Aka. The macroalgae that occur in this estuary are Ulva spp. and Gracilaria chilensis.

What we did:

The current trophic state of Te Aka Aka was determined by using the extent of macroalgae through the estuary as the primary indicator. As well as measuring macroalgae extent, the sediment was analysed, and the secondary indicators of total organic carbon, total nitrogen and total recoverable phosphorus concentrations, sediment redox potential and sediment grain size distribution measured.

What we found:

The results indicate that Te Aka Aka is in a state of moderate eutrophication (Eutrophication band B). However, an assessment of the nutrient loads to, and the physical susceptibility of, this estuary (Dudley and Plew, 2018) determined that Te Aka Aka has the potential to be in a state of high or very high eutrophication (Eutrophication bands C and D). The possible reasons for the discrepancy are:

• A considerable amount of the annual nitrogen load to the estuary from the Ashley River/Rakahuri likely occurs during high flows and the volume and speed of water flow could result in the fast flow of nitrogen through the estuary and out to the open coast.

• There are two areas of intertidal flat where environmental conditions are such that there is little or no macroalgae.

• There is reduction of nitrogen in estuary water through denitrification processes within estuary sediments and the saltmarsh vegetation.

• It is possible that there is a discrepancy between what the modelling suggests and the actual hydrodynamics within Te Aka Aka.

What it means:

Te Aka Aka is in a state of moderate eutrophication. However, the discrepancy between the current state and the potential state based on nutrient loads and physical susceptibility means there is:

• uncertainty on whether the trophic state of the estuary will decline if there is no increase in nitrogen loads.

• uncertainty on whether the trophic state of the estuary will decline if there is an increase in nitrogen loads.


ii Environment Canterbury Technical Report


Environment Canterbury Technical Report iii

Table of contents

Executive summary ............................................................................................... i

1 Introduction ................................................................................................ 1

2 Assessment of the trophic state of Te Aka Aka ...................................... 3

2.1 Eutrophication susceptibility ..................................................................................... 3

2.2 Determining the actual trophic state ......................................................................... 4

3 Methods ...................................................................................................... 4

3.1 Mapping the macroalgae .......................................................................................... 4

3.2 Measuring supporting indicators .............................................................................. 6 3.2.1 Sampling at each site .................................................................................. 6

4 Results ........................................................................................................ 7

4.1 Macroalgae ............................................................................................................... 7 4.1.1 Area extent of macroalgae .......................................................................... 7 4.1.2 Area of Gracilaria, Ulva and both species together in 2018 and 2019 ........ 8 4.1.3 Area for each percent cover category in 2018 and 2019 ............................ 8

4.2 Supporting indicators .............................................................................................. 12 4.2.1 Percent mud .............................................................................................. 12 4.2.2 Redox potential .......................................................................................... 12 4.2.3 Total organic carbon .................................................................................. 12 4.2.4 Total nitrogen ............................................................................................. 12 4.2.5 Total recoverable phosphorus ................................................................... 12

4.3 Overall eutrophication band ................................................................................... 24

5 Discussion ............................................................................................... 24

6 Recommendations ................................................................................... 25

7 Acknowledgements ................................................................................. 26

8 References ............................................................................................... 26

Appendix 1: Details on the supporting indicators ........................................... 27

Appendix 2: Estuary Trophic Index (ETI) comparison values ........................ 28

Appendix 3: Laboratory analyses ..................................................................... 29

Appendix 4: ETI data for supporting indicators .............................................. 30


iv Environment Canterbury Technical Report

List of Figures Figure 1-1: Ulva sp. within Te Aka Aka. 100% cover and a thick layer ......................................... 2

Figure 1-2: Gracilaria chilensis within Te Aka Aka ........................................................................ 2

Figure 3-1: Te Aka Aka with details on the areas of macroalgae mapping ................................... 5

Figure 3-2: Measuring redox potential ........................................................................................... 6

Figure 3-3: An example of one of the sites .................................................................................... 7

Figure 4-1: Location and percentage cover of macroalgae within Te Aka Aka, December 2014........................................................................................................... 9

Figure 4-2: Location and percentage cover of macroalgae within Te Aka Aka, January-March 2018 ................................................................................................. 10

Figure 4-3: Location and percentage cover of macroalgae within Te Aka Aka, 2019................. 11

Figure 4-4: Percent mud at each site, 2016 and 2018 ................................................................ 13

Figure 4-5: Eutrophication band at each site based on percent mud ......................................... 14

Figure 4-6: Average redox potential (mV) at each site, 2016 and 2018 ..................................... 15

Figure 4-7: Eutrophication band at each site based on redox potential ...................................... 16

Figure 4-8: Total organic carbon (%) at each site, 2016 and 2018 ............................................. 17

Figure 4-9: Eutrophication band at each site based on percent total organic carbon ................. 18

Figure 4-10: Total nitrogen concentration (mg/kg dry wt) at each site, 2016 and 2018 ................ 19

Figure 4-11: Eutrophication band at each site based on total nitrogen concentration .................. 20

Figure 4-12: Total recoverable phosphorus concentration (mg/kg dry wt) at each site, 2016 and 2018 .......................................................................................................................... 21

Figure 4-13: Status of each site based on total recoverable phosphorus concentration .............. 22

Figure 4-14: An example of a muddy site ...................................................................................... 23

Figure 4-15: An example of a firm sediment site ........................................................................... 23

List of Tables

Table 2-1: Description of the four eutrophication bands .............................................................. 3

Table 2-2: The potential eutrophication bands (susceptibility) of Te Aka Aka based on physical and nitrogen loads (ETI - Tool 1) and CLUES modelling of current management practice ........................................................................................................................ 3

Table 4-1: Macroalgae extent within Te Aka Aka ........................................................................ 7

Table 4-2: Area (ha) of the estuary with Gracilaria, Ulva and both species together in 2018 and 2019 ............................................................................................................................ 8

Table 4-3: Area (ha) of each percent cover category in 2018 ...................................................... 8

Table 4-4: Area (ha) of each percent cover category in 2019 ...................................................... 8


Environment Canterbury Technical Report 1

1 Introduction This report assesses the current trophic state of Te Aka Aka. Te Aka Aka is a shallow intertidal dominated estuary at the mouth of the Ashley River/Rakahuri, Saltwater Creek, Taranaki Creek and a number of small lowland creeks and mad-made drains. It is a semi-enclosed embayment, with a free connection to the sea such that fresh and salty waters mix within the estuary. A current state report (Bolton-Ritchie, 2019) identified eutrophication as a potential ecological issue for Te Aka Aka. This prompted investigations and reporting on the trophic state of this estuary. This report sits alongside the 2019 report and has provided information to the Waimakariri Water Zone Committee for the Waimakariri Land and Water Solutions Programme and in limit setting for the Waimakariri zone. Eutrophication is the enrichment of nutrients in a water body, often resulting in excessive growth of aquatic plants. Within this estuary elevated nitrogen concentrations in water cause the growth of fast-growing macroalgae but there is limited potential for excessive phytoplankton growth (Dudley and Plew, 2018). There is the fast-growing macroalgae Ulva spp. (Figure 1-1) and Gracilaria chilensis (Figure 1-2) within Te Aka Aka. The macroalgae Ulva spp. and Gracilaria provide habitat for a diversity and abundance of estuarine species such as topshells, hoppers and worms (Bressington, 2003). In turn this is food for the birds and fish that feed on these species. I have seen many birds including godwits, oyster catchers and spoonbills feeding in and around the edges of a dense bed of Ulva sp. within Te Aka Aka. However, the excessive growth and hence occurrence of extensive areas of macroalgae within an estuary can have the following detrimental ecological effects:

1. The macroalgae cover intertidal sediments. This cover can result in the sediments becoming anoxic. As a consequence, there is no oxygen to support the worms and other animals that live within the sediment and keep the sediment healthy. Anoxic sediment is black and smells of sulphur.

2. Where there is seagrass, the macroalgae smothers the seagrass. The seagrass does not survive. There is no seagrass in Te Aka Aka.

3. The macroalgae traps fine sediment particles so over time the estuary could become muddier. 4. The respiration of an abundance of macroalgae can lower/deplete the water of oxygen at night

when there is no oxygen production through photosynthesis. Depleted oxygen levels can result in the death of the animals that live in the water including fish.

5. When the macroalgae die they can dislodge and be carried either out of the estuary or to elsewhere within the estuary such as washed up on the shore or carried into backwaters. The breakdown of the algae by micro-organisms can deplete the water of oxygen which in turn can result in the death of the animals that live in the water, such as fish. The decaying macroalgae is very smelly and unsightly.

With potential for intensification of land use in the estuary catchments in the future, there is potential for an increase in the nitrogen loads to Te Aka Aka. Given that this estuary has high ecological and cultural value there is a need to understand its current trophic state. By assessing the current nitrogen loads and the resulting trophic state of the estuary, it may be possible to predict how the trophic state could change with an increase in nitrogen loads to the estuary in the future. This report provides an assessment of the current trophic state of Te Aka Aka.


2 Environment Canterbury Technical Report

Figure 1-1: Ulva sp. within Te Aka Aka. 100% cover and a thick layer

Figure 1-2: Gracilaria chilensis within Te Aka Aka



2 Assessment of the trophic state of Te Aka Aka In 2016 tools that can be used to assess the trophic state of a NZ estuary (ETI – Estuary Trophic Index) were released for use (Robertson et al, 2016a, 2016b). There are four possible eutrophication states (bands) of an estuary (Table 2-1).

Table 2-1: Description of the four eutrophication bands

From Robertson et al., (2016b)

The tools include:

• determining the eutrophication susceptibility using estuary physical characteristics and nitrogen load data (ETI – Tool 1), and

• using monitoring indicators to assess the actual trophic state (ETI – Tool 2).

2.1 Eutrophication susceptibility

Environment Canterbury contracted NIWA staff to determine the eutrophication susceptibility of Te Aka Aka using estuary physical characteristics and nitrogen load data (ETI-Tool 1), and the CLUES (Catchment Land Use Environmental Sustainability) model. The nitrogen load data were provided by Environment Canterbury and included nitrogen loads for current management practice (CMP). The calculated potential eutrophication bands for current management practice (Dudley and Plew, 2018) are provided in Table 2-2.

Table 2-2: The potential eutrophication bands (susceptibility) of Te Aka Aka based on physical and nitrogen loads (ETI - Tool 1) and CLUES modelling of current management practice

5% likelihood

percentile N load

95% likelihood

percentile N load

5% likelihood

percentile N load

95% likelihood

percentile N load

CMP D D C D

Scenario

ETI Tool1 - Eutrophication susceptibility CLUES - Eutrophication susceptibility



2.2 Determining the actual trophic state

The primary indicator1 is the extent of the estuary covered by macroalgae. This indicator can be used as a standalone measure of estuary trophic state. The supporting (secondary) indicators2 include the sediment indicators of total organic carbon concentration, total nitrogen concentration, total recoverable phosphorus concentration, sediment redox potential and percent mud. Details on these indicators are provided in Appendix 1. The guideline values for these indicators, against which the measured values are compared, are provided in Appendix 2. The secondary indicator values for total organic carbon concentration, total nitrogen concentration, sediment redox potential and percent mud can be used to provide a eutrophication band for a site for each parameter (Appendix 2). The overall eutrophication band for the estuary was calculated using the tools available at https://shiny.niwa.co.nz/Estuaries-Screening-Tool-2/. This tool uses the primary indicator result along with the average redox potential, total organic carbon and total nitrogen values. This tool does not include the percent mud values.

3 Methods

3.1 Mapping the macroalgae

Broadscale mapping of the intertidal sediments of Te Aka Aka was undertaken in December 2014. At the same time the extent of the estuary covered by macroalgae was also mapped. This mapping was undertaken on foot using a hand-held GPS and all areas with >5 % macroalgae cover were mapped. The macroalgae species present in the mapped areas was not recorded in 2014. In 2014 the percent cover categories used were:

Medium: >5 - <30 % cover; High: >30 - <70 % cover; Very high: >70 - 100 % cover The extent of the intertidal sediments covered by macroalgae was again mapped in January-March 2018 and January 2019. This mapping was undertaken on foot using a hand-held GPS. In 2018 a large area north of Saltwater Creek along Ashworths Spit was not mapped; this area was mapped in 2014 (Figure 3-1). A larger area was surveyed in 2019 than 2018 including an area with a fence across the intertidal area (marked by a blue oval in Figure 3-1) and a larger area on the northern side of Saltwater Creek. In 2018 and 2019 the macroalgae areas were more precisely mapped than in 2014. This was achieved by mapping more points when walking around each area of macroalgae and recording the macroalgae species present in each mapped area. The species present were Gracilaria chilensis and Ulva spp. Both Ulva and Gracilaria were present together in some areas. In 2018 and 2019 all areas with >5% cover of macroalgae were mapped. The percent cover categories used in 2018 were:

Medium: >5 - <25 % cover; High: >25 - <75 % cover; Very high: >75-100 % cover

The percent cover categories used in 2019 were:

> 5 - < 15; >15 - <25; > 25 - <50; >50 - <75;

The increase in the number of percent cover categories in 2019 aligns the mapping with the percent cover categories listed in Robertson et al. (2016) and provides more accurate data on the percent cover of macroalgae. Having more accurate data will allow for a better assessment of changes in macroalgae abundance in Te Aka Aka over time.

1 A primary indicator: a variable that exhibits an unambiguous response to eutrophication. 2 Secondary indicators: variables that have variable or ambiguous relationships with eutrophication but are useful

in its measurement.

https://shiny.niwa.co.nz/Estuaries-Screening-Tool-2/



Figure 3-1: Te Aka Aka with details on the areas of macroalgae mapping

Solid yellow line - The intertidal area along the northern margin of Saltwater Creek was not mapped for macroalgae Blue oval – mapped in 2019 but not 2018 Yellow dotted lines – northern extent of macroalgae mapping in the given years



3.2 Measuring supporting indicators

The supporting indicators of total organic carbon (TOC), total nitrogen (TN) and total recoverable phosphorus (TRP) concentrations, sediment redox potential (Redox) and sediment grain size distribution were measured at selected sites. In November/December 2016, sixteen sites were sampled. Most of these sites were sampled again in February/March 2018. In 2018, eighteen sites were sampled. Supporting indicators were not measured in 2019. The locations of the sampling sites are shown in the maps of the results.

3.2.1 Sampling at each site

At each site a 10 m long tape was set out on the sediment surface. The redox potential at 1 cm depth in the sediment was measured using an ECoSense pH 100A m with a probe (Figure 3-2). Five redox measurements were taken at a site. These were at 0, 2, 4, 6 and 8 m along the tape. One composite sediment sample was collected from each site. One 2 cm deep, 7 cm diameter sediment core was collected at each of 0, 2, 4, 6, 8 and 10 m along the tape and placed in a labelled plastic bag. This sediment was stored in a chilly bin and then frozen before being taken to Hill Laboratories for analysis. This sediment was analysed for TOC, TN, TRP and sediment grain size distribution. The analytical methods are described in Appendix 3.

Figure 3-2: Measuring redox potential



Figure 3-3: An example of one of the sites

4 Results

4.1 Macroalgae

4.1.1 Area extent of macroalgae

The extent of macroalgae within Te Aka Aka in 2014, 2018 and 2019 is summarised in Table 4-1 and shown in Figure 4-1 to 4-3.

Table 4-1: Macroalgae extent within Te Aka Aka

YearArea surveyed

(ha)

Area of

macroalgae (ha)

Macroalgae area

as % total area

2014 146.1 8.2 5.6

2018 91.8 5.8 6.3

2019 91.8 7.1 7.7

2019 117.3 8.6 7.4 NOTE: There are two 2019 results. The results for the total area surveyed in 2019 (117.8 ha) and the results for the same area as surveyed in 2018 (91.8 ha). The area of the estuary covered by macroalgae in 2014, 2018 and 2019, indicate that in each year Te Aka Aka was in a state of moderate eutrophication (Band B).



4.1.2 Area of Gracilaria, Ulva and both species together in 2018 and 2019

The total area of Gracilaria, Ulva and both species together in 2018 and 2019 was calculated. The results are presented in Table 4-2. In both years there was a greater area of Gracilaria than Ulva within the estuary.

Table 4-2: Area (ha) of the estuary with Gracilaria, Ulva and both species together in 2018 and 2019

Year Area surveyedArea of Gracilaria

(ha)Area of Ulva (ha) Area of both (ha)

2018 91.8 3.69 1.80 0.32

2019 91.8 4.67 0.92 1.46

2019 117.3 6.25 0.92 1.46 Note: the macroalgae species present in the mapped areas was not recorded in 2014.

4.1.3 Area for each percent cover category in 2018 and 2019

The total area of macroalgae in each percent cover category was calculated, the results for 2018 and 2019 are in Table 4-3 and 4-4 respectively.

Table 4-3: Area (ha) of each percent cover category in 2018

Table 4-4: Area (ha) of each percent cover category in 2019

YearArea surveyed

(ha)5 - <15 % 15 - <25 % 25 - <50 % 50 - <75 % >75 - 100 %

2019 91.8 0.80 0.67 2.97 1.91 0.71

2019 117.3 0.80 0.69 2.98 2.84 1.32

YearArea surveyed

(ha)

Medium:

>5 - <25 %

High: >25 -

<75 %

Very High:

>75 -100 %

2018 91.8 1.64 2.04 2.13



Figure 4-1: Location and percentage cover of macroalgae within Te Aka Aka, December 2014



Figure 4-2: Location and percentage cover of macroalgae within Te Aka Aka, January-March 2018

The orange areas have both species

KEY



Figure 4-3: Location and percentage cover of macroalgae within Te Aka Aka, 2019

Red – Gracilaria Green – Ulva Orange - both

KEY



4.2 Supporting indicators

The results for the supporting indicators of percent mud, redox potential, total organic carbon, total nitrogen and total recoverable phosphorus are presented in Figures 4-4 to 4-13 and tabulated in Appendix 4. For each supporting indicator the measured value is provided in one figure and, except for TRP, the eutrophication band is provided in another figure. The eutrophication band for each indicator, except TRP, has been determined by comparing the measured values to the relevant guideline values from Robertson et al. (2016b) (Appendix 2). For TRP the measured values are compared to condition ratings (Appendix 2) used by Wriggle consultants (Robertson and Stevens, 2013) to assess the state of many New Zealand estuaries.

4.2.1 Percent mud

The percent mud ranged from 2.2 to 95.4 % (Figure 4-4). At the fourteen sites sampled in both 2016 and 2018 the percent mud was:

• similar between years at five of the sites.

• higher in 2016 than 2018 at five sites.

• higher in 2018 than 2016 at four sites. Based on the percent mud, the eutrophication band at most sites was a D (Figure 4-5). At three of the sites, the eutrophication band differed between 2016 and 2018.

4.2.2 Redox potential

The redox potential ranged from -170 to 250 mV (Figure 4-6). At all sites sampled in both 2016 and 2018 the redox potential differed between years. Based in the redox potential the eutrophication band at sites ranged from band A to band D (Figure 4-7). At five of the sites the eutrophication band differed, while at the other nine sites there was no change in the band, between 2016 and 2018.

4.2.3 Total organic carbon

The total organic carbon ranged from 0.07 to 1.15 % (Figure 4-8). At the fourteen sites sampled in both 2016 and 2018 the total organic carbon was:

• similar between years at three of the sites.

• higher in 2016 than 2018 at six sites.

• higher in 2018 than 2016 at five sites. Based on the total organic carbon the eutrophication band at sites ranged from band A to band C (Figure 4-9). At four of the sites the eutrophication band differed, while at the other nine sites there was no change in the band, between 2016 and 2018.

4.2.4 Total nitrogen

The total nitrogen concentration ranged from <500 – 1300 mg/kg dry weight (Figure 4-10). At the fourteen sites sampled in both 2016 and 2018 the total nitrogen concentration was:

• the same between years at seven of the sites.

• higher in 2016 than 2018 at four sites.

• higher in 2018 than 2016 at three sites. Based on the total nitrogen concentration the eutrophication band at sites ranged from band A to band C (Figure 4-11). At two of the sites the eutrophication band differed, while at the other twelve sites there was no change in the band, between 2016 and 2018.

4.2.5 Total recoverable phosphorus

The total recoverable phosphorus concentration ranged from 280 - 700 mg/kg dry weight (Figure 4-12). At the fourteen sites sampled in both 2016 and 2018 the total recoverable phosphorus concentration was:

• similar between years at five of the sites.

• higher in 2016 than 2018 at five sites.

• higher in 2018 than 2016 at four sites. Based on the total recoverable phosphorus concentration the status at sites was good or fair (Figure 4-13). At three of the sites the status differed, while at the other twelve sites there was no change in status, between 2016 and 2018.



Figure 4-4: Percent mud at each site, 2016 and 2018

Symbols: Red circle – sampled 2016; blue triangle – sampled 2018 Values: In Orange – 2016 result; In Yellow – 2018 result



Figure 4-5: Eutrophication band at each site based on percent mud




Figure 4-6: Average redox potential (mV) at each site, 2016 and 2018




Figure 4-7: Eutrophication band at each site based on redox potential




Figure 4-8: Total organic carbon (%) at each site, 2016 and 2018




Figure 4-9: Eutrophication band at each site based on percent total organic carbon




Figure 4-10: Total nitrogen concentration (mg/kg dry wt) at each site, 2016 and 2018




Figure 4-11: Eutrophication band at each site based on total nitrogen concentration




Figure 4-12: Total recoverable phosphorus concentration (mg/kg dry wt) at each site, 2016 and 2018




Figure 4-13: Status of each site based on total recoverable phosphorus concentration




Figure 4-14: An example of a muddy site

Figure 4-15: An example of a firm sediment site



4.3 Overall eutrophication band

The overall eutrophication band was calculated using the tools available at https://shiny.niwa.co.nz/Estuaries-Screening-Tool-2/. Using the macroalgae data from 2014 in combination with the 2016 values for redox potential, total organic carbon and total nitrogen the ETI value is 0.3 which equates to eutrophication Band B. Using the macroalgae data from 2018 in combination with the 2018 values for redox potential, total organic carbon and total nitrogen the ETI value is 0.31 which equates to eutrophication Band B.

5 Discussion The susceptibility to eutrophication calculations indicate that Te Aka Aka has sufficient nitrogen coming from the catchments to cause water quality degradation to the extent that this estuary could be in eutrophication band D (Dudley and Plew, 2018). The field investigation results show that at present this is not the case and the estuary is in eutrophication band B, i.e. in a state of moderate eutrophication. Possible reasons for the discrepancy between these results are described in the following four paragraphs. The hill fed Ashley River/Rakahuri is the source of the highest volume of water to the estuary. Flows in this river are affected by rainfall and it may be that a considerable amount of the annual nitrogen load to the estuary occurs during high flow events, rather than in base flows. During high flow events, the volume and hence speed of water flow likely results in the fast flow of nitrogen through the estuary and out to the open coast of Pegasus Bay. There are two extensive areas of intertidal flat, i.e. along Ashworths Spit and north of the Ashley River/Rakahuri between the river and the estuary mouth, where there is little or no macroalgae. In the Ashworths Spit area there are no localised freshwater inputs, rather the water that flows in and out of this area is a mix of seawater and the freshwater from inputs elsewhere into the estuary. In the intertidal area north of the Ashley River/Rakahuri between the river and the estuary mouth the sediments are variable over time (Bolton-Ritchie, 2019). This variability which is likely driven by sediment inputs, river flows/floods and the hydrodynamics within the estuary indicates this is a dynamic area of the estuary. It is possible that the dynamic nature of the sediments in this area limits the occurrence and survival of macroalgae here. Within an estuary denitrification occurs in estuary sediments and the saltmarsh vegetation (Douglas, 2018, Plew et al., 2018). Denitrification removes bioavailable nitrogen by the action of heterotrophic bacteria converting it to nitrogen gas. The wetlands of the saltmarsh vegetation can remove significant quantities of nitrogen, however the denitrification rates are influences by the type of vegetation that is present, climate and the hydraulic and nutrient loading (Plew et al., 2018). The ability of estuary sediments to carry out denitrification is influenced by sediment grain size distribution, temperature, chlorophyll-a content of the sediment, water column pH and the biomass of worms and crustacea present (Gongol, 2010). There are no data on denitrification rates for the saltmarsh vegetation and the intertidal soft sediment flats of Te Aka Aka. Te Aka Aka has a high flushing potential and using two different methods it has been determined that the estuary flushes out every 0.72 – 0.75 days (Dudley and Plew, 2018). At high tide approximately 55% of the volume in the estuary is freshwater from river inflows (Dudley and Plew, 2018), and therefore 45% is coastal water mixed with returning estuary water. These flushing and volume values, which have been calculated using modelling approaches (ASSETS and CLUES), were used in the calculation of the susceptibility of Te Aka Aka to eutrophication. It is possible that there is a discrepancy between what the modelling suggests and the actual hydrodynamics within Te Aka Aka. That is, more research on the hydrodynamics of this estuary is required.

https://shiny.niwa.co.nz/Estuaries-Screening-Tool-2/



Most of the macroalgae growth occurs in localised areas, notably along the Saltwater Creek channel, along the narrow channels of small creeks and drainage channels, in proximity to a stormwater discharge from Waikuku (Figure 4-2 southern-most area of macroalgae) and in sheltered areas fed by small seeps from the Ashley River/Rakahuri (Figure 4-2 the green and orange areas in the southern area of the estuary). It is likely that the channels of the small creeks and drainage channels in the upper most estuarine areas are not well flushed during a tidal cycle. In addition, there is obviously enough nitrogen within these freshwater inflows to promote macroalgae growth. The abundance of macroalgae along the Saltwater Creek channel indicates the nitrogen concentrations in Saltwater Creek water, along with any localised nitrogen inputs into the man-made drainage channels, promote macroalgae growth in this area. The macroalgae growths that occur in proximity to the freshwater seeps from the Ashley River/Rakahuri indicate that this water provides plenty of nitrogen for macroalgae growth. In 2018 the largest and densest area of Ulva sp, (Figure 1-1) occurred in this area. In the future the nitrogen inputs could increase as the ZIPA (Waimakariri Water Zone Committee, 2018) does allow for an increase in nitrate concentrations in Saltwater Creek. This increase could increase the density and extent of macroalgae along the Saltwater Creek channel and in the lower parts of the estuary influenced by Saltwater Creek water. This could degrade the estuary to the point it changes to band C or even worse band D. However, the disparity between the current eutrophication state (Band B) and the susceptibility calculations (Band C or D) creates uncertainty as to whether the allowable increase in nitrate concentrations and hence nitrogen loads could cause this change. In the future the ideal is that the trophic state of this estuary should not reach band C.

6 Recommendations

1. Long-term annual monitoring to assess the trophic state of Te Aka Aka is recommended so changes can be picked up quickly. This monitoring should include:

• Annual broadscale mapping of the macroalgae within Te Aka Aka –species, area of cover, % cover

• biennial sampling of the sediments at ~ 20 sites within Te Aka Aka to measure the secondary indicators of redox potential, total nitrogen, total organic carbon, total recoverable phosphorus and sediment grain size distribution, algae biomass.

NOTE: Recommendation 2.13 in the ZIPA states ‘Development and implementation of a programme to assess trophic state and to monitor trophic state over time (important considerations are location of sites, parameters to be measured, frequency of sampling, seasonality of sampling).’ (Waimakariri Zone Committee, 2018).

2. Research, that needs to be considered for future funding, to understand why there is a difference between the potential trophic state (based on nitrogen loads and physical susceptibility) and the actual trophic state, could include:

• assessing whether there is a discrepancy between the modelling and the actual hydrodynamics within Te Aka Aka.

• quantifying denitrification processes in the saltmarsh vegetation and intertidal sediments.

• more accurately quantifying N loads in each of the freshwater inflows into the estuary. This includes the Ashley River/Rakahuri, Saltwater Creek, Taranaki Creek and the other small creeks and mad-made drains.

NOTE: Recommendation 3.19 in the ZIPA does state that one of the key areas for an improvement of understanding includes nitrate discharges to the estuary (Waimakariri Zone Committee, 2018).

The completion of this research would allow for an understanding of the potential impact of future nitrogen input increases on the trophic state of this estuary.



7 Acknowledgements Emma Woods carried out the broad scale and macroalgae mapping within the estuary in 2014. In 2016 the fieldwork involved with collecting all the supporting indicator data was undertaken by Sophie Maloney. Thanks go to Dr. Tina Bayer of Environment Canterbury for editing and to Helen Shaw from Environment Canterbury for reviewing this report.

8 References Bressington, M. 2003. The effects of macroalgal mats on the marine benthic fauna in the Avon-

Heathcote Estuary. University of Canterbury M.Sc, thesis. Bolton-Ritchie, L. 2019. Ecological and water quality assessment Ashley River/Rakahuri-Saltwater

Creek Estuary (Te Aka Aka). Environment Canterbury Report R19/21. Douglas, E.J. 2018.Denitrification response to nutrient enrichment in New Zealand estuaries. PhD

Thesis, University of Waikato. Dudley, B. and Plew, D. 2018. Te Aka Aka eutrophication susceptibility assessment. NIWA Client

Report for Environment Canterbury. 2017041CH. Gongol, C. L. 2010. Denitrification and oxygen consumption in the sediments of four New Zealand

estuaries. PhD Thesis, University of Otago. Plew, D., Dudley, B., Shankar, U. and Zeldis, J. 2018. Assessment of the eutrophication susceptibility

of New Zealand estuaries. NIWA Client Report 2018206CH prepared for the Ministry for the Environment.

Robertson, B. and Stevens, L. 2013. Waikawa Estuary: Fine Scale Monitoring 2012/13. Report

prepared for Environment Southland. Robertson, B.M, Stevens, L., Robertson, B., Zeldis, J., Green, M., Madarasz-Smith, A., Plew, D., Storey,

R., Hume, T., Oliver, M. 2016a. NZ Estuary Trophic Index Screening Tool 1. Determining eutrophication susceptibility using physical and nutrient load data. Prepared for Envirolink Tools Project: Estuarine Trophic Index, MBIE/NIWA Contract No: C01X1420. 47p.

Robertson, B.M, Stevens, L., Robertson, B., Zeldis, J., Green, M., Madarasz-Smith, A., Plew, D., Storey,

R., Oliver, M. 2016b. NZ Estuary Trophic Index Screening Tool 2. Determining Monitoring Indicators and Assessing Estuary Trophic State. Prepared for Envirolink Tools Project: Estuarine Trophic Index, MBIE/NIWA Contract No: C01X1420. 68p.

Waimakariri Water Zone Committee, 2018. Zone Implementation Programme Addendum (ZIPA).



Appendix 1: Details on the supporting indicators From Robertson et al., 2016b Redox potential Redox potential is used as a measure of sediment oxygenation. Reduced sediment oxygenation is related to reduced sediment quality and volume available for benthic infauna and alterations in benthic community structure. These effects have been linked to reduced availability of food for fish and birds and other invertebrates as well as to undesirable changes in biogeochemical cycling. Values below 0 indicate reducing conditions and the lower the number the more reducing the condition, Values above zero indicate oxidising conditions and the higher the number the more well oxygenated the sediment. Sediment mud content Macroalgae can trap fine sediment making the sediment muddy in areas where the macroalgae is prolific. The input of fine sediment to estuaries is often accompanied by elevated nutrient loads, resulting in significant mud deposition zones in upper estuary tidal flats that can become eutrophic. Sediment mud content is a strong predictor of estuarine benthic fauna. Total organic carbon Total organic carbon (TOC) is a measure of organic matter in sediment. The rate of TOC production and decomposition, and the resulting microbiological biomass, are at the heart of the eutrophication problem. The larger the TOC content, the greater the growth of microorganisms that can contribute to the depletion of oxygen. Total nitrogen The sources of the total nitrogen in estuarine sediment are organic matter, nitrogen cycling processes in the sediment and nitrogen in sediment pore water. Nitrogen is considered the limiting nutrient for marine primary production. Nitrogen in the sediment supports primary production including microphytobenthos and macroalgae. Total recoverable phosphorus The sources of the total recoverable phosphorus in estuarine sediment are organic and inorganic matter, phosphorus cycling processes in the sediment and phosphorus in sediment pore water. While phosphorus is not considered the limiting nutrient for marine primary production, it is required for primary production.



Appendix 2: Estuary Trophic Index (ETI)

comparison values (from Robertson 2016b)

(from Robertson and Stevens, 2013)

Band A B C D

Minimal Eutrophication Moderate Eutrophication High Eutrophication Very High Eutrophication

Ecological qualityEcological communitied are

healthy and resilient

Ecological communties are

slightly impacted by additional

macroalgae growth.

Ecological communities are

moderately to strongly impacted

by macroalgae

Ecological communities are

strongly impacted by

macroalgae.

% of available intertidal

habitat covered with

macroalgae

0 - < 5 >5 - <15 >15 -<25 >25-100

Band A B C D

Minimal Eutrophication Moderate Eutrophication High Eutrophication Very High Eutrophication

Ecological quality

No stress caused by the

indicator on any aquatic

organism

A minor stress on sensitive

organisms caused by the

indicator

Moderate stress on a number of

aquatic organisms caused by

the indicator exceeding

preference levels for some

species and a risk of sensitive

macroinvertebrate species being

lost

Significant, persistent stress on

a range of aquatic organisms

caused by the indicator

exceeding tolerance levels. A

likelihood of local extinctions of

keystone species and loss of

ecological integrity.

Mud (%) < 5 5 to 15 15 - 25 > 25

Total organic carbon

(g/100g dry wt. = %)< 0.5 0.5 - 1 1 - 2 > 2

Total nitrogen (mg/kg) < 250 250 - 1000 1000 - 2000 > 2000

Redox potential (mV) > 100 100 to -50 -50 to -150 < -150

For individual sites

Condition rating Very good Good Fair Poor

Total recoverable

phosphorus (mg/kg) < 200 200 - 500 500 - 1000 >1000

For individual sites



Appendix 3: Laboratory analyses

Parameter Laboratory analysis method

Total organic carbon

Total nitrogen

Total recoverable phosphorusDried and sieved sample. Nitric/Hydrochloric acid digestion. ICP-MS, screen

level. USEPA 200.2.

Dry matter for grainsize samples Drying for 16 hours at 103 °C, gravimetry (free water removed before analysis)

Fraction>/= 2 mm Wet sieving with dispersant, 2.00 mm sieve, gravimetry

Fraction < 2 mm, >/= 1 mmWet sieving with dispersant, 2.00 mm and 1 mm sieves, gravimetry (calculation

by difference).

Fraction < 1 mm, >/= 500 µmWet sieving with dispersant, 1.00 mm and 500 µm sieves, gravimetry (calculation

by difference).

Fraction < 500 µm, >/=250 µmWet sieving with dispersant, 500 µm and 250 µm sieves, gravimetry (calculation

by difference).

Fraction <250 µm, >/= 125 µm Wet sieving with dispersant, 250 µm and 125 µm sieves, gravimetry (calculation

by difference).

Fraction < 125 µm, >/=63 µmWet sieving with dispersant, 125 µm and 63 µm sieves, gravimetry (calculation by

difference).

Fraction < 63 µm Wet sieving with dispersant, 63 µm sieve, gravimetry (calculation by difference).

Catalytic combustion (900 °C, O2) separation. Thermal conductivity detector

(Elemental Analyser)

7 Grain size profile



Appendix 4: ETI data for supporting indicators Figure A1: Location of the sampling sites



2017 Results

2018 Results

Site Easting Northing

TRP

mg/kg dry

weight

TRP TN mg/kg

dry weightETI TN

TOC g/100

g dry

weight

ETI TOCRedox

value mV ETI Redox % mud ETI mud

1 1577197 5207763 390 Good <500 B 0.32 A 30 B 37 D

2 1577246 5207835 650 Fair 800 B 0.65 B 32 B 76 D

3 1577300 5207869 560 Fair 800 B 0.61 B -44 B 59 D

4 1577411 5208052 580 Fair 1300 C 1.14 C -59 B 85 D

6 1577460 5208189 330 Good <500 B 0.22 A 236 A 15 C

7 1577256 5208276 280 Good <500 B 0.13 A 236 A 9 B

9 1577419 5208768 360 Good <500 B 0.35 A 133 A 37 D

10 1577463 5208873 390 Good <500 B 0.34 A 52 B 41 D

11 1577328 5209045 380 Good <500 B 0.32 A 158 A 50 D

12 1577414 5209261 360 Good <500 B 0.35 A 60 B 39 D

13 1577122 5209258 450 Good <500 B 0.24 A 186 A 66 D

14 1577350 5209471 420 Good 700 B 0.44 A -91 C 48 D

15 1577099 5209481 380 Good <500 B 0.36 A 11 B 56 D

16 1576970 5209402 470 Good <500 B 0.37 A 130 A 55 D

17 1576866 5209911 510 Fair 700 B 0.6 B -98 C 76 D

18 1576450 5209896 560 Fair 600 B 0.57 B -115 C 84 D

Site Easting Northing

TRP

mg/kg dry

weight

TRP TN mg/kg

dry weightETI TN

TOC g/100

g dry

weight

ETI TOCRedox

value mV ETI Redox % mud ETI mud

1 1577197 5207763 650 Fair 900 B 0.82 B -17 B 55.9 D

2 1577246 5207835 590 Fair 800 B 0.87 B -140 C 77.7 D

3 1577303 5207886 700 Fair 1200 C 1.15 C -170.4 D 85.8 D

4 1577411 5208052 400 Good < 500 B 0.33 A 42 B 31.3 D

5 1577344 5208130 450 Good 500 B 0.75 B 40.6 B 64.4 D

6 1577460 5208189 430 Good 700 B 0.51 B 49.8 B 41.4 D

7A 1577394 5208348 320 Good < 500 B 0.23 A 76 B 24 C

9 1577419 5208770 340 Good < 500 B 0.23 A 258.6 A 58.6 D

10 1577463 5208872 260 Good < 500 B 0.07 A 257.4 A 2.2 A

11 1577327 5209046 360 Good < 500 B 0.23 A 237.4 A 45.5 D

12 1577416 5209268 320 Good < 500 B 0.21 A 209.6 A 23.5 C

13 1577120 5209259 380 Good < 500 B 0.2 A -74.8 C 58.5 D

14 1577350 5209471 400 Good < 500 B 0.34 A -53.2 C 38.8 D

15 1577099 5209481 460 Good < 500 B 0.48 A 7.4 B 63.4 D

17 1576866 5209911 440 Good 500 B 0.62 B -81.6 C 61 D

18 1576450 5209896 550 Fair 700 B 0.6 B -143 C 80 D

19 1576809 5209826 530 Fair 700 B 0.66 B -106.8 C 87.1 D

20 1576756 5209456 600 Fair 700 B 0.59 B 80.8 B 95.4 D

ecan.govt.nz

Facilitating sustainable developmentin the Canterbury region

assessment of the trophic state of te aka aka (ashley

Documents