Abstract
Runoff from human‐dominated watersheds has greatly altered nitrogen (N) and phosphorus (P) cycling in lakes. Nutrients from human sources are distinct from those from undisturbed ecosystems in several ways including lower N : P ratios, which can drive ecosystems to N‐limited conditions, and enriched stable N isotope ratios. In this study, we used these distinct characteristics to estimate shifts in N sources to 27 lakes across a human density gradient in western Washington. We compared an N stable isotope two‐source mixing model with a mixing model that coupled N stable isotopes to N : P stoichiometry and included N fixation. We found that a two‐source mixing model (human and watershed sources) did not explain observed variation in δ 15 N of particulate organic matter (POM) and primary consumers ( R 2 = 0.60) as well as a model that included a third N source (N fixation; R 2 = 0.72). When fixed N was facultatively added to the ecosystem below a critical N : P ratio, the more complex mixing model captured the observed patterns in POM and primary‐consumer δ 15 N among lakes extremely well. In lakes with P concentrations > 20 µg L −1 (N : P mass ratio 50% of lake N budgets. This model provides a novel way to estimate the contribution of nonpoint N sources and N fixation to lakes in watersheds subject to human nutrient inputs.