The Annex 4 of the U.S.-Canada Great Lakes Water Quality Agreement (GLWQA) established binational phosphorus load reduction targets for the Western and Central basins of Lake Erie, with an emphasis on reducing phosphorus contributions from the Maumee River Basin (MRB), that are associated with the excessive algal blooms.  The U. S. Environmental Protection Agency (EPA) Region 5 in partnership with Indiana Department of Environmental Management (IDEM), Michigan Department of Environmental Quality (MDEQ), Ohio Environmental Protection Agency (Ohio EPA), and with contractor assistance from Tetra Tech collaboratively developed a strategy for translating Annex 4 nutrient targets to Total Maximum Daily Load (TMDL) endpoints in two watersheds in the Maumee River Basin (MRB).  The Annex 4 TMDL Methodology was devised to link Annex 4 nutrient targets at Waterville, Ohio to upstream watersheds in the MRB at hydrologic unit code (HUC) HUC-8 and HUC-12 scales and to understand whether Maumee watershed TMDLs are sufficient to prevent harmful algal blooms in Lake Erie.

The Annex 4 TMDL Methodology set baseline total phosphorus (TP) and soluble reactive phosphorus (SRP) loading targets at the outlet for each HUC-8 watershed in the MRB and at HUC-12 subwatersheds within the St. Joseph River Watershed and the Tiffin River Watershed. These baseline targets were extrapolated based on flow to best align with Annex 4 goals (i.e., flow weighted means).  Under certain conditions the Annex 4 loading targets were met by TMDLs but the success of TMDLs to address Annex 4 goals was dependent on state nutrient targets applied in the context of TMDL development.  HUC-12 analyses highlighted some HUC-12 subwatersheds which contribute excessive nutrients above Annex 4 goals and in some circumstances these HUC-12 subwatersheds do not have listed nutrient impaired segments and no future plans for nutrient TMDL development.

This exercise demonstrated that TP TMDLs in some watersheds attain the Annex 4 TP goals, but we cannot assume that watershed TP TMDLs, where they exist, will always meet Annex 4 targets.  Annex 4 SRP targets cannot be applied within TMDLs at this time because no state has SRP targets to protect biocriteria, therefore, there are no SRP impairments in Indiana, Michigan and Ohio and no SRP TMDLs.

• The Annex 4 TMDL Methodology, October 2018 (PDF)(127 pp, 4 MB, October 2018)
  Methodology for Connecting Annex 4 Water Quality Targets with TMDLs in the Maumee River Basin