Site Specific Eutrophication Criteria for Lake Pepin

The State of Minnesota promulgated lake eutrophication standards in 2008. These standards were developed for natural glacial lakes characterized by long hydraulic residence times and relatively small watersheds. Our lake standards specifically allow for site specific standards for reservoirs and riverine lakes. Lake Pepin is a unique natural lake on the Mississippi River formed by the alluvial fan of the Chippewa River. Lake Pepin has a surface area of about 40 square miles and a mean depth of 18 feet. Pepin is characterized by two somewhat distinct segments. The inflow segment accounts for about 40% of the lake by area (˜10,700 acres) but only about 28% by volume because it is very shallow (mean depth ˜12 feet) and is more “riverlike” in nature. The lower segment is somewhat deeper (mean depth ˜22 feet) and accounts for about 72% of the lake by volume and is more “lake-like” as compared to the upper segment. Lake Pepin's 48,634 square mile watershed contains five separate Level III Ecoregions. The watershed-to-lake ratio for Lake Pepin of about 1,225:1 promotes short water residence times that range from 6 to 47 days, with an average of 16 days.Eutrophication is most problematic on Lake Pepin during summer (June-September) low-tomedian flow conditions. We used multiple lines of evidence, including sediment diatom reconstruction of historical total phosphorus (TP) concentrations, to determine the appropriate TP criterion (100 μg/L) for Lake Pepin. The Upper Mississippi River-Lake Pepin linked hydrodynamic and water quality model (UMR-LP) was used to predict chlorophyll-a (32 μg/L) and Secchi criteria (0.8 m) that would be expected if our TP criterion was achieved. Model predictions of the frequency of nuisance blooms (chl-a >50 μg/L), percent composition of bluegreen algae and user perceptions factored into the decision as well. Even though the TP and chlorophyll-a criteria for Lake Pepin are slightly higher than our criteria (standards) for glacial lakes, we believe they are fully supportive of aquatic recreational use, as required by the Clean Water Act (CWA), because Lake Pepin produces less chlorophyll-a per unit TP, the frequency of nuisance blooms is low, and blue-green algae are a smaller proportion of the algal community in Lake Pepin as compared to glacial lakes.