title: Accounting for nitrogen fixation in simple models of lake nitrogen loading/export.
abstract: Coastal eutrophication, an important global environmental problem, is primarily caused by excess nitrogen and management efforts consequently focus on lowering watershed N export (e.g., by reducing fertilizer use). Simple quantitative models are needed to evaluate alternative scenarios at the watershed scale. Existing models generally assume that, for a specific lake/reservoir, a constant fraction of N loading is exported downstream. However, N fixation by cyanobacteria may increase when the N loading is reduced, which may change the (effective) fraction of N exported. Here we present a model that incorporates this process. The model (Fixation and Export of Nitrogen from Lakes, FENL) is based on a steady-state mass balance with loading, output, loss/retention, and N fixation, where the amount fixed is a function of the N/P ratio of the loading (i.e., when N/P is less than a threshold value, N is fixed). Three approaches are used to parametrize and evaluate the model, including microcosm lab experiments, lake field observations/budgets and lake ecosystem model applications. Our results suggest that N export will not be reduced proportionally with N loading, which needs to be considered when evaluating management scenarios.
authors.affiliation: Civil and Environmental Engineering, Northeastern University , Boston, Massachusetts 02115, United States.
authors.forename: Xiaodan
authors.initials: X
authors.lastname: Ruan
authors.forename: Frank
authors.initials: F
authors.lastname: Schellenger
authors.forename: Ferdi L
authors.initials: FL
authors.lastname: Hellweger
mesh_terms: D000458:Cyanobacteria / Q000378:metabolism; D017753:Ecosystem; D005068:Eutrophication; D060106:Lakes / Q000737:chemistry*; D008962:Models, Theoretical; D009584:Nitrogen / Q000032:analysis*; D009586:Nitrogen Fixation; D045483:Rivers / Q000737:chemistry
chemical_list: D009584:Nitrogen