Various forms of nitrogen in fresh water may stimulate algal growth and contribute to eutrophic conditions. Nitrogen input to the Great Lakes is represented by nitrate (+nitrite), which typically comprises to the majority of total inorganic nitrogen.
Like phosphorus, nitrogen loading has a tributary and an atmospheric component.
Tributary nitrogen loading
- Tributary nitrate loads were averaged for 1994-2008, except for Lake Erie, where tributary data were available only for 20051.
- The estimated inputs include the most important tributary sources and comprise nearly 60-80% of the total tributary load to each lake.
- Due to data availability, inclusion of small watersheds was not practical.
Estimated tributary nitrate loads to the Laurentian Great Lakes
Atmospheric nitrogen loading
Estimates of atmospheric deposition of nitrogen came from the National Atmospheric Deposition Program (NADP, wet nitrate deposition) and the Clean Air Status and Trends Network (CASTNET, dry nitrate deposition).
Mapping nitrogen loading as a Great Lakes stressor
Tributary nitrogen loads were propagated spatially from river mouths based on the assumption that the nutrient declines to 10% of its initial level at 15 km and to 1% at 30 km. Propagated nitrate was then combined with atmospheric deposition over the lake surface.
Spatial distribution of nitrogen loading as a stressor in the Laurentian Great Lakes. (Inset: Western Lake Erie).
1. Dolan, D.M. and S.C. Chapra. 2011. Great Lakes Total Phosphorus Loads and Models: A Fifteen Year Update. U.S. EPA, Final Report, Grant no. GL 00E58501.