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Assessing nonpoint‐source nitrogen loading and nitrogen fixation in lakes using δ 15 N and nutrient stoichiometry
Author(s) -
Jankowski KathiJo,
Schindler Daniel E.,
Holtgrieve Gordon W.
Publication year - 2012
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.4319/lo.2012.57.3.0671
Subject(s) - nutrient , ecosystem , nitrogen , environmental science , stable isotope ratio , phosphorus , δ15n , nitrogen fixation , nonpoint source pollution , environmental chemistry , isotopes of nitrogen , watershed , nutrient cycle , stoichiometry , ecology , surface runoff , chemistry , δ13c , biology , physics , organic chemistry , quantum mechanics , machine learning , computer science
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 < 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.