This study compares aquatic food chains using nitrogen isotopes (δ¹⁵N) to assess trophic structure and contaminant bioaccumulation. δ¹⁵N values in aquatic food webs are influenced by the baseline δ¹⁵N at the base of the food chain, which can vary between ecosystems. Small organisms with fast nitrogen turnover show high temporal variability in δ¹⁵N, making it difficult to compare δ¹⁵N across systems. However, large primary consumers, with slower nitrogen turnover, show more stable δ¹⁵N signatures. The study found that δ¹⁵N increases with human population density in lake watersheds, likely due to high δ¹⁵N in human sewage. After correcting for this baseline variation, the study found that food chains leading to fish varied by only one trophic level among 40 lakes. This suggests that δ¹⁵N signatures at the base of the food chain can be a useful tool for assessing anthropogenic nutrient inputs. The study also shows that δ¹⁵N signatures of primary consumers are strongly correlated with human population density. The results indicate that the length of food chains leading to fish varies by only one trophic level among lakes, which contrasts with previous studies based on species presence and feeding relationships. The nitrogen isotope method provides a more accurate measure of food-chain length related to bottom-up mass transfer, which can be compared between lakes. This method is useful for assessing contaminant biomagnification and testing hypotheses about food-chain length in relation to environmental and demographic variables. The study highlights the importance of considering baseline δ¹⁵N variation when interpreting δ¹⁵N data in aquatic food webs.This study compares aquatic food chains using nitrogen isotopes (δ¹⁵N) to assess trophic structure and contaminant bioaccumulation. δ¹⁵N values in aquatic food webs are influenced by the baseline δ¹⁵N at the base of the food chain, which can vary between ecosystems. Small organisms with fast nitrogen turnover show high temporal variability in δ¹⁵N, making it difficult to compare δ¹⁵N across systems. However, large primary consumers, with slower nitrogen turnover, show more stable δ¹⁵N signatures. The study found that δ¹⁵N increases with human population density in lake watersheds, likely due to high δ¹⁵N in human sewage. After correcting for this baseline variation, the study found that food chains leading to fish varied by only one trophic level among 40 lakes. This suggests that δ¹⁵N signatures at the base of the food chain can be a useful tool for assessing anthropogenic nutrient inputs. The study also shows that δ¹⁵N signatures of primary consumers are strongly correlated with human population density. The results indicate that the length of food chains leading to fish varies by only one trophic level among lakes, which contrasts with previous studies based on species presence and feeding relationships. The nitrogen isotope method provides a more accurate measure of food-chain length related to bottom-up mass transfer, which can be compared between lakes. This method is useful for assessing contaminant biomagnification and testing hypotheses about food-chain length in relation to environmental and demographic variables. The study highlights the importance of considering baseline δ¹⁵N variation when interpreting δ¹⁵N data in aquatic food webs.