The study by Gilbert Cabana and Joseph B. Rasmussen explores the use of δ15N isotopes to model trophic structure and contaminant bioaccumulation in aquatic food webs. They highlight that while δ15N is a useful tool, cross-system comparisons can be challenging due to baseline variations at the base of the food chain. These variations are particularly significant in plankton due to their fast nitrogen turnover, making it difficult to resolve these differences. However, large primary consumers, which have stable tissue isotopic signatures, show a marked increase in δ15N with human population density in the lake watershed, likely due to the high δ15N content of human sewage. The authors correct for this baseline variation in δ15N and find that the food chains leading up to fish vary by only about one trophic level among 40 lakes studied. This suggests that δ15N signatures at the base of the food chain can be a useful tool for assessing anthropogenic nutrient inputs, such as sewage, which is a significant contributor to the nitrogen budget of aquatic systems. The study also demonstrates that correcting the δ15N signatures of higher trophic level consumers for baseline variation can provide a more accurate reflection of their trophic position, enhancing the utility of this modeling approach for understanding contaminant biomagnification and testing hypotheses related to food-chain length and environmental variables.The study by Gilbert Cabana and Joseph B. Rasmussen explores the use of δ15N isotopes to model trophic structure and contaminant bioaccumulation in aquatic food webs. They highlight that while δ15N is a useful tool, cross-system comparisons can be challenging due to baseline variations at the base of the food chain. These variations are particularly significant in plankton due to their fast nitrogen turnover, making it difficult to resolve these differences. However, large primary consumers, which have stable tissue isotopic signatures, show a marked increase in δ15N with human population density in the lake watershed, likely due to the high δ15N content of human sewage. The authors correct for this baseline variation in δ15N and find that the food chains leading up to fish vary by only about one trophic level among 40 lakes studied. This suggests that δ15N signatures at the base of the food chain can be a useful tool for assessing anthropogenic nutrient inputs, such as sewage, which is a significant contributor to the nitrogen budget of aquatic systems. The study also demonstrates that correcting the δ15N signatures of higher trophic level consumers for baseline variation can provide a more accurate reflection of their trophic position, enhancing the utility of this modeling approach for understanding contaminant biomagnification and testing hypotheses related to food-chain length and environmental variables.