of lake characteristics that are geographically-distinct have led to
the ecoregion approach of lake classification
developed by the U.S. Environmental Protection Agency (EPA) Research
Laboratory at Corvallis, Oregon. Ecoregions are areas of relative homogeneity
based on landuse, soils, land and surface form, and potential natural
vegetation. Minnesota is divided into seven ecoregions (Figure 23),
but most of its lakes are found in four of these. Minnesota Pollution
Control Agency (MPCA) researchers found regional patterns in numbers
of lakes, lake water quality, morphometry, and watershed characteristics
(Table 5) among these ecoregions. For example, lakes of the Northern
Lakes and Forests ecoregion have significantly lower total phosphorus
and chlorophyll than lakes in the Western Corn Belt Plains ecoregion.
Furthermore, the MPCA discovered through lake-user surveys that user
perception of water quality varied by ecoregions. This has led to ecoregion-specific
criteria for phosphorus and, in general, helped to clarify expectations
and goals for protecting lakes in Minnesota.