Isotopic ecosystem studies in the KwaZulu-Natal Bight.

Abstract

The study area, the KwaZulu-Natal Bight, is an oceanographically important area, which, regardless of having two of the most important fisheries off the east coast of South Africa, has received little research attention regarding its biological functioning. Until now chiefly oceanographic processes have been considered the drivers of this generally oligotrophic system. This study seeks to understand which of three important processes, a topographically induced oceanic upwelling cell near Richards Bay, a cyclonic eddy near Durban, or fluvial fluxes centred around the Thukela River, forces ecological functioning through their nutrient or organic matter input. The overall aim of the thesis is to understand the pelagic and benthic ecosystems of the Bight in terms of these drivers through the use of stable isotope (δ¹³C and δ¹⁵N) analyses of a range of biotic and abiotic samples. These were collected on board of a number of research cruises – forming predominantly part of the larger African Coelacanth Ecosystem Programme suite of studies – in the wet and dry seasons of 2010. Isotopic analyses found distinctions between fluvial and oceanic particulate organic matter and indicate that upwelling was not occurring in either sampling season. Organic matter originating from the Thukela River did not play a significant role in the wet season, although it dominated the planktonic pelagic food web in the dry season. The organic matter of the most productive region in the Bight, the Middle Shelf, was of riverine origin in the dry season, but of indeterminate origin in the wet season when it may have been an artefact of an old upwelling event which had previously occurred to the north of the Bight. There is, however, some evidence suggesting that this organic matter may rather have been of riverine origin, with its δ¹³C signals subsequently having been modified by the diatom bloom occurring there. In the demersal ecosystem, sediment isotopic data show organic matter to be well-mixed throughout the Bight in both seasons, with riverine organic matter dominating most of the Bight except its northern and southern edges, where oceanic organic matter increases in importance. Sediment organic matter (most likely via the macrobenthic biota) was deemed an important food source for demersal animals and omnivory an important feeding strategy. Seasonal studies from 2008 to 2010 in the Thukela Bank area indicate that the demersal animals' stable isotope signatures responded to the seasonal isotopic changes in riverine organic matter, indicating the cross-seasonal importance of this food source to the demersal ecosystem. Parallel methodological studies examined how routine isotopic sample handling procedures could have affected the results of the ecological studies. These studies suggest that i) effects of preservation/fixation methods and the use of dyes are species-dependant; ii) acidification has no effect on zooplankton isotopic signatures, and that iii) drying methods alone and interactively with multiple thawing and refreezing of samples affect the stable isotope values offish muscle tissues. Recommendations are made for further improvements in methodology and considerations to be taken when processing samples. Overall, it is concluded that riverine input to the Bight has a more important biological role than previously thought, and that organic matter from this source is an important driver of ecosystems within the Bight throughout the year for the demersal and pelagic ecosystems.