Phosphorus Loading Trends

The following provides a quick snapshot of key findings.  A more in-depth discussion and references to the scientific literature can be found here.

Total Phosphorus Loading Total P loading into Lake Erie changed dramatically largely due to implementing P abatement programs beginning in 1972 under the Great Lakes Water Quality Agreement.  Loads reached the GLWQA target level of 11,000 metric tons per year during the 1980s.  Initial declines were due primarily to programs that reduced point sources of P (e.g., P restrictions in commercial detergents, enhancements of sewage treatment plants), leaving nonpoint sources as dominant.

Figure 1

 Dissolved reactive phosphorus – DRP loads from several Lake Erie tributaries (e.g., Maumee River, Sandusky River, Honey Creek, and Rock Creek) have increased dramatically since the mid-1990s in contrast to the relatively constant total phosphorus loads from those same watersheds.  As a result, the fraction as DRP more than doubled from a mean of 11% in the 1990s to 24% in the 2000s.

Figure 2

Results from the Soil and Water Assessment Tool (SWAT) watershed model suggest this increased DRP load was driven by increasing storm events, coupled with changes in fertilizer application timing and rate, and management practices that increase P-stratification of the soil surface like no-till farming.  However, while the current more extreme storms appeared to stimulate large fluxes of DRP, those same weather patterns modeled using agricultural landscapes of the 1970s did not.