Submitted to fulfill the academic requirements for the degree of Master of Science in the School of Biological & Conservation Sciences, University of KwaZulu-Natal, Durban. The overall aim of this project is therefore to provide an insight into the sources of nutrients that drive phytoplankton productivity in the bay. Synoptic studies were conducted to provide an indication of the distribution of Total Suspended Solids (TSS), Particulate Organic Matter (POM) and phytoplankton in the bay, while focused experiments used stable isotopes to investigate the rate processes involving C and N collection, as well as sources of N available in the surface water.
The concentration of particulate organic phosphorus and nitrogen was found to be higher in the wet season compared to the dry season. During the wet season, a large variation in chlorophyll-a fluorescence was observed across the bay, while natural abundance isotopic data indicated a seasonal change in the available nutrient source. The results indicate that terrestrial nutrient sources play a major role in influencing nutrient concentrations in the bay and thus affect the coastal ecosystem in the region.
The experimental work described in this thesis was carried out in the School of Biological and. Where the work of others has been used, this is duly acknowledged in the text.
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Publications
INTRODUCTION
- General overview
- Physical environment
- Regional climate
- The coastline
- Hydrography
- Phytoplankton ecology
- Phytoplankton ecology of the KZN Bight
- The current study
- ACEP aims
- Aims of this study
- Structure of this thesis
METHODS
- Synoptic study field work
- Site selection
- Quantitative analysis for the synoptic cruise
- Focussed study field work
- Site
- Quantitative analysis for the focussed cruise
- Laboratory analysis
- POM
- Data analysis
- Aim 1
- Aim 2
- Aim 3
RESULTS AND DISCUSSION
- Particulate organic matter (POM) distribution
- A synoptic view
- Focussed studies
- Nutrient uptake and associated environmental parameters
- Environmental variables
- Nitrate uptake
- The influence of environmental variables on nitrate uptake
- Daily comparison of ammonium and nitrate uptake at the focus sites
Water in the wet season showed higher amounts of POP in the Tugela River estuary, up to approx. In the dry season, water showed a concentration of approximately 1 – 2 µg.l-1 in the Tugela River estuary and north of there along the coast (Figure 3.3b). The surface water of the KZN Bight shows lower salinity in the wet season at the mouth of the Tugela River, up to the mid-shelf (Figure 3.4a).
Salinity graphs, presented for the wet season, show the extent of fresh water along most of the KZN Bight area, which is not seen at all during the dry season (Figure 3.4). Phytoplankton in the Fmax depth showed a bloom in the middle of the shelf area as well as south of Durban (Figure 3.5c). Water in the wet season showed significant differences between sites for both TSS and PON (Figure 3.6a: psite.
Runoff with a high particulate organic nutrient content would explain the TSS as well as the difference in PON during the rainy season. The differences observed in the rainy season indicate that the southern sites are different from the Richards Bay areas. During the dry season, there was a shift in nutrient concentration in the KZN Bight (Figure 3.9).
The nutrients found in the KZN Bight system may be added naturally or anthropogenically. In the wet season, the Tugela River had a high outflow rate, releasing nutrients into the KZN Bight area (Begg, 1978; Whitfield, 2000). In the dry season, the mouth of the Tugela River separated from the others, as did the middle shelf.
This could mean that the KZN Bight was more homogeneous in the wet than in the dry season. The difference observed in surface water during the dry season is due to the high rate of uptake in the northern area of Richards Bay compared to the eddy and mid-shelf areas of Durban. Nitrate uptake rates (µg N.l-1.h-1) for dark and light surface incubations in a) wet and e) dry season and depth Fmax in b) wet and f) dry season.
Another possible explanation for this is the homogenized waters illustrated by the environmental parameters found during the rainy season (Figure 3.13). However, in the context of the wider KZN Bight area, its role is likely to be minor.
CONCLUSION
A study in the Swartkops Estuary concluded that it played a role in outflow and could support productivity in coastal areas of the coast (Baird, 1987). Later studies by Winter et al. 1996) concluded that 4755 tonnes of carbon were exported annually from the Swartkops estuary, which would then be available for coastal primary production. Subsequent literature found that inputs from nutrient-rich estuaries are likely to promote primary phytoplankton production (Whitfield and Bates, 2007).
However, Whitfield and Bates (2007) said that further work needs to be done in the surf zone to determine the true influence of estuaries. Fisheries studies in the KZN Bight area have also hypothesized that the Tugela River has a major influence on the productivity of the system (Lambert et al., 2009; Hutchings et al., 2010). The terrestrial sources of nutrients can either be natural, such as the Tugela and the many other rivers in the KZN Bight area, or man-made, from one of the many outfall pipes discharging into the KZN Bight area, as algal blooms can often be seen along plumes at these tube (Lutjeharms et al., 2000).
Further research needs to be done to confirm this conclusion as the volume flowing from these pipes can be minuscule when the grid bend area is taken into account. The natural abundance of isotopes should be collected from the river mouth, the surf zone and further offshore, as well as from the effluent pipes, and everything should be compared. This will provide a clear indication of the influence of terrestrial sources and further, whether it is river outflow or effluent that plays an important role in influencing phytoplankton biomass in the region.
In addition, studies should be conducted when the Durban eddy and Richards Bay upwelling are present. These results could be compared with those from this study and would help clarify the true impact of these oceanic processes on production within the KZN Bight. The KZN Bight is an oligotrophic environment that receives bursts of nutrients from oceanographic processes throughout the year and a large input of nutrients from land-based inflow, especially during the rainy season (Lutjeharms et al., 2000, Lutjeharms, 2006).
However, to confirm this and determine the magnitude of the impact, further work needs to be carried out in the KZN Bight area.
Distribution and composition of organic particles in the Tikehau Lagoon (Tuamotu Archipelago, French Polynesia). Ammonium uptake by seagrass epiphytes: isolation of the effects of water velocity using an isotope label. River input controls the feeding frenzy on the nearby Thukela Bank (KwaZulu-Natal, South Africa).
A review of the commercial and shallow source Penaeid Prawn of South Africa: Status, Fisheries, Aquaculture and Management. Activity of heterotrophic bacteria and their coupling to primary production during a spring phytoplankton bloom in the southern gulf of the North Sea. Distribution, partitioning and fluxes of dissolved and particulate organic C, N and P in the eastern North Pacific and Southern Oceans.
15N measurements of ammonium and nitrate uptake by Ulva enestra (Chlorophyta) and Gracila riapacifica (Rhodophyta): Comparison of net nutrient. Downbrush trends in the KwaZulu-Natal Drakensberg region of South Africa during the twentieth century. Primary productivity and nitrogen uptake in the subsurface chlorophyll maximum on the eastern Agulhas Bank.
Primary organic production in relation to the chemistry and hydrography of the western Indian Ocean. Natural abundance level measurement of nitrogen isotopic composition of oceanic nitrate: an adaptation of the ammonia diffusion method. Transport of total suspended matter, particulate organic carbon, organic nitrogen, and phosphorus in the inner part of Osaka Bay.
Nitrite metabolism in the euphotic layer of the central North Pacific Limnology and oceanography. A review of information on temporarily open/closed estuaries in the warm and cool temperate biogeographical regions of South Africa, with particular emphasis on the influence of river currents on these systems. Interim report to the Water Research Commission on the project “The freshwater requirements of intermittently open Cape estuaries” WRC report no. 1581/1/07.
APPENDIX A
APPENDIX B
APPENDIX C
APPENDIX D
APPENDIX E
