Total phosphorus loading to Lake Erie was calculated from 1965 through 2013 by using available data and statistical models to determine a whole lake phosphorus load and estimate the phosphorus contribution from a variety of sources (defined below). A detailed description of the methods is 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, click above label (e.g. Load, all sources) to turn off and on the different contributions from different sources.

  • Load, all sources: Data from Fraser (1987), which reported annual total phosphorus (TP) loadings from 1967 through 1973.

  • 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.

  • Indirect Point Sources: Point sources that discharge directly to Lake Erie, a connecting channel, downstream of a tributary monitoring location, or an unmonitored watershed.

  • Direct Point Sources: Point sources that discharge to a monitored watershed upstream of the monitoring location.