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 (delta15N) in aquatic biota are thought to reflect anthropogenic nutrient inputs, and they may be a promising tool fortracing nutrient sources in watersheds. We measured delta15N 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 delta15N values (PCdelta15N) varied more than 14% among lakes. Models of PCdelta15N 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 delta15N 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 delta15N.