Primary Consumer Stable Nitrogen Isotopes as Indicators of Nutrient Source

Non-point source loading of nitrogen and phosphorus is a primary cause of eutrophication of inland waters, although the diffuse and variable nature of nutrient inputs makes it difficult to trace and identify nutrient pathways. Stable nitrogen isotope values (δ15N) in aquatic biota are thought to reflect anthropogenic nutrient inputs, and they may be a promising tool for tracing nutrient sources in watersheds. We measured δ15N of aquatic consumers from a suite of 27 Danish lakes spanning a range of trophic states (oligotrophic to eutrophic) and land uses (forest, urban, agriculture). Primary consumer δ15N values (PCδ 15 N) varied more than 14‰ among lakes. Models of PCδ 15 N were developed from limnological, nitrogen loading, and nitrogen source variables using an information-theoretic approach (Akaike's Information Criteria, AIC). Models based on land use/land cover performed best, indicating that elevated δ15N is not only associated with high nitrogen loading, but is also reflective of nitrogen source. Urban and agricultural land use in the watershed, and particularly within the riparian buffer areas, was quantitatively linked to elevated biotic δ15N.