I examined the distribution of stable isotope ratios of nitrogen (deltasp15N) and carbon (deltasp13C) in fish and aquatic invertebrates. Animals are enriched in deltasp15N compared to their diet and I hypothesized that among-lake variation in the deltasp15N of a top predator should reflect variation in the length of the food chain leading to it. A comparative study of a biomagnifying contaminant, mercury, confirmed that the presence or absence of certain key organisms such as pelagic forage fish and the crustacean Mysis relicta determined among-lake variation in mercury in lake trout, a top predator in the lakes. Mercury levels from the longest food chains where pelagic forage fish and Mysis were present were higher than those from the shortest food chains where these last two components were missing. This biogeographic variation in food chain length was correlated with variation in the deltasp15N of trout. The use of deltasp15N as a continuous, integrative measure of trophic position was further supported by its correlation to mercury in lake trout. However, such cross-system comparisons in deltasp15N can be complicated by differences in deltasp15N at the base of the food chain. Using large primary consumers (unionid mussels) as bio-indicators, I showed that deltasp15N increases markedly with the human population in the lake watershed, an effect of the high deltasp15N of human sewage. Correcting for this baseline variation in deltasp15N, I reported that food chains leading up to nearshore fish species varied by about only one trophic level among the 35 lakes studied. A study of the deltasp15N of coral reef and intertidal organisms collected along the coast of the the Carribean island of Barbados extended these patterns observed in lakes to coastal systems: baseline variation in deltasp15N was related to human density on the adjacent watershed and within-site variation deltasp15