Processes controlling the benthic food web of a mesotrophic bight (KwaZulu-Natal, South Africa) revealed by stable isotope analysis

Abstract

The KwaZulu-Natal Bight is considered oligotrophic/mesotrophic with distinct sour - ces of nutrients entering the system by a series of oceanographic processes, including an up - welling cell and several estuaries, of which the Thukela River is the most important; the upwelling cell is suggested to be the main factor controlling the biology of the Bight. Our aim is to describe the main nutrient/organic matter (OM) source driving the benthic system of the Bight and to produce a food web to aid in understanding the trophic interactions occurring in the demersal eco - system. For this, marine and riverine sediment samples, total suspended solids and marine de - mersal organisms were collected across the Bight, and δ13C and δ15N composition was analysed during 2 seasons. Our results, based on δ13C, δ15N, percent organic carbon, percent nitrogen and carbon:nitrogen ratios, suggest that the OM in the sediments is mainly dominated by riverine input and the benthic food web is controlled mainly by riverine total suspended solids input. Future studies should look at the role played by bacteria in the degradation of terrestrial OM and the overall effect in the food web of the Bight. Omnivory appears to be a widespread strategy for demersal organisms throughout the Bight, a finding supported by the lack of clear δ15N enrichment between prey and predators and the low variability of trophic positions across a wide array of organisms. © Inter-Research 2013.