Accounting for nitrogen fixation in simple models of lake nitrogen loading/export

Area of Science:

• Environmental Science
• Limnology
• Ecosystem Modeling

Background:

• Coastal eutrophication is a major global environmental issue driven by excess nitrogen.
• Current management strategies focus on reducing watershed nitrogen (N) export, often through decreased fertilizer use.
• Existing watershed models assume a constant fraction of N loading is exported, neglecting potential biological feedback mechanisms.

Purpose of the Study:

• To develop and evaluate a model (Fixation and Export of Nitrogen from Lakes, FENL) that incorporates nitrogen fixation by cyanobacteria.
• To assess how changes in nitrogen loading affect the effective fraction of N exported from lakes.
• To provide a more realistic tool for evaluating watershed management scenarios for coastal eutrophication.

Main Methods:

• Developed the FENL model based on a steady-state mass balance including loading, output, retention, and N fixation.
• Parametrized and evaluated the model using microcosm laboratory experiments.
• Validated the model with lake field observations, N budgets, and lake ecosystem model applications.

Main Results:

• The model demonstrates that nitrogen fixation by cyanobacteria can increase when nitrogen loading is reduced, particularly when the nitrogen to phosphorus (N/P) ratio falls below a threshold.
• This feedback mechanism means that N export is not reduced proportionally with N loading.
• The effective fraction of N exported downstream can change significantly under reduced N loading scenarios.

Conclusions:

• Watershed management aimed at reducing coastal eutrophication must account for the dynamic process of nitrogen fixation.
• The FENL model provides a valuable tool for predicting the impact of management interventions on nitrogen export.
• Ignoring nitrogen fixation can lead to overestimation of the effectiveness of nitrogen reduction strategies.