Biomagnification of DDT through the benthic and pelagic food webs of Lake Malawi, East Africa: importance of trophic level and carbon source.

Lake Malawi, an East African Rift Valley lake, is internationally renowned for having the highest diversity of fish species in the world, and these cichlids are highly specialized in their dietary habits. In this lake, tissue stable carbon (delta13C) and nitrogen (delta15N) isotopes can be used over several trophic levels to distinguish those consumers relying upon carbon fixed by either benthic or pelagic primary producers. As such, it was possible to contrast the biomagnification of persistent organochlorines through the benthic and pelagic food webs. In 1996 and 1997, food-web organisms were collected from Lake Malawi and analyzed for organochlorines, delta13C and delta15N to determine the factors that affectthe biomagnification of contaminants in a tropical lake. The pesticide DDT was the most predominant pollutant in the biota from Lake Malawi and was found at the highest concentrations in the largest and fattiest fish species. As observed in temperate systems, log-transformed sigmaDDT concentrations in food-web organisms were significantly predicted by delta15N or log lipid (r2 = 0.32 and 0.40, respectively). In addition, the slope of the regression of log sigmaDDT versus delta15N was significantly higher in the pelagic than the benthic food web. These results indicate that pelagic organisms are at greater risk of accumulating these pollutants than biota relying upon benthic primary production.