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A total of 26,307 lakes in the Upper Midwest ecoregion are represented in the NLA. Of these lakes, about 97% are natural and 3% are man-made.  

Trophic State and Key Stressors: The NLA uses trophic state as an important indicator of lake condition and assesses the extent of key stressors in the nation’s lakes.

• In the Upper Midwest, 6% of lakes are rated as most disturbed based on trophic state, 34% are eutrophic, 55% are mesotrophic, and 5% are oligotrophic.
• The most widespread stressors assessed are nitrogen and lake habitat complexity (both 27% of lakes), and riparian vegetation cover (25%).

• Trophic State

  

• Indicator with the Highest Percentage of Lakes in the Most Disturbed Condition: Nitrogen

  

Human Use Condition: Cyanobacteria is one of three algal toxin related indicators presented in the NLA (see also chlorophyll a and microcystins). It serves as a proxy for the presence of algal toxins.

Big Gibson Lake in Wisconsin. Photo: Sarah Lehmann

• Based on cyanobacteria cell counts, 6% of lakes in the Upper Midwest are in the most disturbed condition (i.e., pose a high risk of exposure to algal toxins).

Change from 2007 (for lakes >4 hectares)*:

• For the Upper Midwest, the NLA reports a 12% decrease in the proportion of lakes in most disturbed condition between 2007 and 2012 based on the lake drawdown exposure indicator.