COMPARISON OF AQUATIC FOOD-CHAINS USING NITROGEN ISOTOPES

Recent studies have shown the utility of delta(15)N to model trophic s tructure and contaminant bioaccumulation in aquatic food webs. However , cross-system comparisons in delta(15)N can be complicated by differe nces in delta(15)N at the base of the food chain. Such baseline variat ion in delta(15)N is difficult to resolve using plankton because of th e large temporal variability in the delta(15)N of small organisms that have fast nitrogen turnover. Comparisons using large primary consumer s, which have stable tissue isotopic signatures because of their slowe r nitrogen turnover, show that delta(15)N increases markedly with the human population density in the lake watershed. This shift in delta(15 )N likely reflects the high delta(15)N of human sewage. Correcting for this baseline variation in delta(15)N, we report that, contrary to ex pectations based on previous food-web analysis, the food chains leadin g up to fish varied by about only one trophic level among the 40 lakes studied, Our results also suggest that the delta(15)N signatures of n itrogen at the base of the food chain will provide a useful tool in th e assessment of anthropogenic nutrient inputs.