Total phosphorus loading to the western and central basins of Lake Erie was calculated by using available data and statistical models to estimate the phosphorus contribution from a variety of sources (defined below). These calculations were based on methods that are published in Total and soluble reactive phosphorus loadings to Lake Erie A detailed accounting by year, basin, country, and tributary, Maccoux 2016.

In the above graphic, source types may be turned off and onto more closely examine sources within the stacked bar graph.

  • Adjustment for Unmonitored Area: The contribution of phosphorus from land downstream of a monitoring location is estimated by the unit area load (UAL). Briefly, the UAL (or yield, in loading per km2) is calculated from the upstream or an adjacent monitored watershed and is applied to the unmonitored area, excluding any indirect point sources from the UAL.
  • Tributary Monitored NPS: Constitutes the monitored tributary load minus the direct point sources.  
  • Lake Huron Input: Flow data for the St. Clair River were combined with observed Lake Huron TP and soluble reactive phosphorus (SRP) concentrations from spring Environment and Climate Change Canada open lake cruises to estimate the phosphorus load from Lake Huron.
  • Atmospheric deposition: Estimated from monthly precipitation depth and TP concentration data.
  • Point Sources: Both point sources that discharge directly to Lake Erie, a connecting channel, downstream of a tributary monitoring location, or an unmonitored watershed (indirect point sources), as well as point sources that discharge to a monitored watershed upstream of the monitoring location (direct point sources).