Great Lakes Environmental Assessment and Mapping Project

Phosphorus loading

Lake Erie Harmful Algae Bloom, July 2011
Phosphorus loading contributes to harmful algal blooms.

Phosphorus runoff from watersheds is considered one of the most important drivers of eutrophication and the proliferation of nuisance algae. Phosphorus inputs to the Great Lakes are represented by total phosphorus (TP), and include both tributary and atmospheric inputs.


Tributary TP Loads

  • Our analysis includes Great Lakes tributaries with drainage areas greater than 250 square km.Because tributaries with drainage areas of this size encompass only 60-90% of the total drainage area for each Great Lake, our estimate of tributary TP loading is likewise reduced.
  • Tributary TP data for Lakes Superior, Michigan, Huron, and Ontario were averaged for 1994-2008. Lake Erie TP tributary data were averaged for 2003-2007.


Atmospheric TP Loads

  • Atmospheric deposition of phosphorus to the Great Lakes is a small amount that is not well quantified.
  • Estimates of deposition range from 5-25 kg/km2.
  • For our analysis, we used an average of 8 kg/km2 for Lakes Superior, Michigan and Huron, and 16 kg/km2 for Lakes Erie and Ontario.


Mapping phosphorus loading as a Great Lakes stressor

Our estimate of phosphorus loading combines inputs from major tributaries, propagated from river mouths spatially based on the assumption that loads declined to 10% of initial levels at 15 km and 1% of initial at 30km. Propagated TP was then combined with atmospheric deposition over the lake surface.

Spatial distribution of total phosphorus as a stressor in the Laurentian Great Lakes. (Inset: (Western Lake Erie).