A STABLE CARBON-ISOTOPE STUDY OF THE FOOD-WEB IN A FRESH-WATER-DEPRIVED SOUTH-AFRICAN ESTUARY, WITH PARTICULAR EMPHASIS ON THE ICHTHYOFAUNA

The importance of macrophytes as food sources for estuarine nekton is unclear. Previous carbon isotope investigations in the macrophyte-domi nated, freshwater-deprived Kariega Estuary showed that the bivalve Sol en cylindraceus did not utilize the dominant estuarine macrophytes fou nd within the estuary as a primary food source. This finding prompted questions as to what the nekton of this estuary utilize as primary ene rgy sources. delta(13)C analyses of the principal autochthonous and al lochthonous primary carbon sources, as well as the dominant invertebra te and fish species, indicate that there are two main carbon pathways within the Kariega Estuary. The littoral community, which incorporates the majority of crustaceans, gobies, muller: and a sparid, utilizes d elta(13)C enriched primary food sources namely Spartina maritima, Zost era capensis and epiphytes. The channel fauna, which includes the zoop lankton, zooplanktivorous and piscivorous fish, utilizes a primary foo d source depleted in delta(13)C, which is most likely a mixture of phy toplankton, terrestrial plant debris and C, macrophyte detritus. The C , saltmarsh macrophytes Sarcocormia perennis and Chenolea diffusa, as well as benthic microalgae, appear to be less important as primary foo d sources to the nekton of the Kariega Estuary. (C) 1997 Academic Pres s Limited.