Assessing nonpoint-source nitrogen loading and nitrogen fixation in lakes using δ15N and nutrient stoichiometry
Limnol. Oceanogr., 57(3), 2012, 671-683 | DOI: 10.4319/lo.2012.57.3.0671
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 δ15N of particulate organic matter (POM) and primary consumers (R2 = 0.60) as well as a model that included a third N source (N fixation; R2 = 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 δ15N among lakes extremely well. In lakes with P concentrations > 20 µg L−1 (N : P mass ratio < 15.3), N fixation became an increasingly important component of the N cycle, accounting for > 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.