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. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2012.

St. Lucia estuarine lake system has a history of episodic droughts and floods leading to a high variability in physico-chemical characteristics which may influence the community structure of nano- and microplankton that are the main primary energy synthesisers in aquatic systems. Originally, the St. Lucia estuary and the Mfolozi River shared the same mouth with the Mfolozi River acting as the main source of fresh water to the system and as stabiliser of the common inlet. Due to prolonged mouth closure from 2001-2012 and high evaporation rates, severe extended droughts and high salinities were experienced in 2001-2012.This project aimed to characterise community composition and biomass of nano-and microplankton (autotrophic and heterotrophic) under the varying and extreme physico-chemical conditions in order to characterize the energy basis of the planktonic food web in St. Lucia. Its main objectives were to compare and add onto Johnson’s (1977) list of phytoplankton taxa in the system, to estimate the biomass (carbon) of nano- and microplankton from cell counts and biovolume measurements, and lastly to understand and establish trends in the change in community structure of these organisms with the varying physico-chemical characteristics. Nano- and microplankton samples were collected monthly from October 2010 to September 2011 at three different sites: Lister’s Point, Charters Creek and the mouth representing the lakes and estuary Channel. Chlorophyll a and physico-chemical parameters were also measured in situ during collection. In the lab, samples were settled using the Utermöhl method and species were identified to at least genus level, counted and cell measurements taken under an inverted microscope for biovolume calculations and biomass thereof. Abundance in cells per liter and biomass (carbon) in pg/L was then analyzed from the counts. The nano-and microplankton groups recorded in the system were cyanobacteria, chlorophytes, cryptophytes, dinoflagellates, ciliates and diatoms. Seventy eight phytoplankton taxa were identified composed of 56 diatoms, eight green algae, one cryptophyte, seven cyanobacteria and six dinoflagellate taxa. Nineteen ciliate taxa were also found. Only 12 of the diatom taxa identified in this study were listed by Johnson (1977), none of the taxa in the other phytoplankton groups was listed by Johnson (1977). The Johnson (1977) study conducted in the system from 1975-1977 listed the phytoplankton taxa occurring at that time. There was no significant difference in the community composition, biovolume and biomass between seasons hence no seasonal trend however, there were significant differences in the nano- and microplankton community composition, biovolume and biomass at the three different sites of the system. Cyanobacteria were the main taxa in the northern embayments dominating in abundance, biovolume and biomass (biological variables), green algae and cryptophytes dominated in abundance, biovolume and biomass in the Channels while in South Lake, green algae dominated in abundance but diatoms dominated in biovolume and biomass. Ciliate biological variables were higher in the northern regions than in the other parts of the estuary. The absence or limited grazing pressure of ciliate predators in the northern region due to their inability to cope with the extreme salinities compared to the other parts of the system explains why the northern embayments had the highest abundance, biovolume and biomass of ciliates. Ciliates and heterotrophic dinoflagellates were the heterotrophs in this study. Autotrophic:heterotrophic biomass ratio was lowest in the northern regions as heterotrophs had a higher biomass there. This ratio was higher in the South Lake and the Channel. The lack of stratification and generally high turbidity in the system made the system unfavourable for dinoflagellate growth. The higher presence of ciliate predators in the South Lake and Channel probably accounts for the low heterotrophic biomass hence high autotrophic:heterotrophic biomass ratio in South Lake and the Channel. Nutrients were not limiting during this study and salinity was the main physico-chemical characteristic accounting for the differences in nano- and microplankton biological variables. The Cyanothece bloom in the northern region was primarily due to high salinities (>150) which also indicated unfavourable conditions for other plankton types. The high diatom biomass in the southern lake was due to low salinities (<28) which favoured their growth, whereas chlorophytes and cryptophytes dominated in the Channel mainly due to low turbidity (median of 11.4NTU) and fresh water input from the Mfolozi lowering salinities (<5). The South Lake and Channel thus had the highest available energy for higher trophic level organism since 1) diatoms and green algae are the most favoured food source for phytoplankton grazers while the cyanobacteria though most abundant are the least favoured food source leaving the northern lake with smaller energy source for higher trophic level organisms and 2) The low autotrophic:heterotrophic biomass ratio in the northern region leaves the region with a lower net carbon biomass than the other parts of the system with a higher autotrophic: heterotrophic biomass. / Thesis (M.Sc.)-University of KwaZulu-Natal, Durban, 2013.