Phytoplankton studies in the KwaZulu-Natal Bight.
The KwaZulu-Natal Bight is an important area along the South African east coast, stretching 160 km north from Scottsburgh to St Lucia (Lutjeharms et al., 2000). The Bight is of interest to the region as the area contains some distinct physical features, which are presumed to drive the ecological functioning of the shelf ecosystem through their role in nutrient sources. These include the Tugela River, the second largest river in South Africa in terms of outflow, and the Agulhas Current that forms an outer border at the edge of the continental shelf. Phytoplankton interacts with the majority of essential ecological networks and therefore greatly influences marine ecosystems. To this end, it is necessary to understand their ecophysiological rate processes – particularly those that are influenced by the dominant nutrient inputs to the Bight. The overall aim of this project is therefore to provide an insight into the sources of nutrients driving phytoplankton productivity in the Bight. Synoptic surveys were conducted to provide an indication of the distribution of Total Suspended Solids (TSS), Particulate Organic Matter (POM) and phytoplankton in the Bight, while focussed experiments used stable isotopes to examine the rate processes involving C and N acquisition, as well as sources of N available in the surface water. Concentration of particulate organic phosphorus and nitrogen were found to be higher in the wet season when compared to the dry season. During the wet season a large variation in chlorophyll-a fluorescence was observed across the Bight, while natural abundance isotope data indicated a seasonal change in the nutrient source available. For the wet season nutrient concentration varied with site and depth, however uptake rates (μg N.1ˉ¹.hˉ¹) measured using ¹⁵N tracer additions were not significantly different with site and depth. Alternatively, the dry season showed a significant difference between site in surface waters. In the wet season the mid shelf area had the highest uptake rate and phytoplankton biomass while the Richards Bay north site dominated, with regard to the previously mentioned factors, in the dry season. At the time of the experiments, neither the Durban eddy nor the upwelling cell were present, and hypotheses regarding the importance of these physical features in driving phytoplankton nutrient acquisition could not be assessed. However, a notable difference in uptake rate between the wet and dry seasons was observed, and this difference is likely due to the fluvial sources of nutrients from the Tugela and many other rivers entering the KZN coast, which are absent during the dry season. The results indicate that terrestrial nutrient sources play a major role in influencing nutrient concentrations on the Bight, and hence influence the nearshore ecosystem of the region.