Comparison of aquatic food chains using nitrogen isotopes.

TLDR

Delta‑15N is useful for modeling trophic structure and contaminant bioaccumulation in aquatic food webs, but cross‑system comparisons are hampered by baseline variation at the base of the food chain. Using large primary consumers, the study found that delta‑15N rises with watershed human population density, and after correcting for baseline variation the trophic levels of fish across 40 lakes differ by only about one level, indicating that baseline delta‑15N signatures can help assess anthropogenic nutrient inputs.

Abstract

Recent studies have shown the utility of delta(15)N to model trophic structure and contaminant bioaccumulation in aquatic food webs. However, cross-system comparisons in delta(15)N can be complicated by differences in delta(15)N at the base of the food chain. Such baseline variation in delta(15)N is difficult to resolve using plankton because of the large temporal variability in the delta(15)N of small organisms that have fast nitrogen turnover. Comparisons using large primary consumers, which have stable tissue isotopic signatures because of their slower 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 expectations based on previous food-web analysis, the food chains leading 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 nitrogen at the base of the food chain will provide a useful tool in the assessment of anthropogenic nutrient inputs.