Ecology

Mesotrophic

A lake classification indicating moderate nutrient levels, reasonable clarity, and balanced algae growth, the middle category between oligotrophic and eutrophic.

What It Means for Your Lake

Mesotrophic lakes occupy the middle range of the trophic classification system, with a Carlson Trophic State Index between 40 and 50. These lakes have total phosphorus between 15 and 25 ug/L, chlorophyll-a between 5 and 10 ug/L, and Secchi depth typically between 6 and 12 feet. The water appears green-tinged but generally clear enough for comfortable swimming throughout most of the summer. Mesotrophic lakes support the most diverse fish communities in Minnesota and Wisconsin, including walleye, northern pike, largemouth and smallmouth bass, and a variety of panfish. They produce enough phytoplankton to fuel a productive food web without creating nuisance algae conditions under normal circumstances, though brief algae blooms may occur during extended hot spells in late summer. Approximately 30% of monitored lakes in Minnesota and Wisconsin fall into the mesotrophic category. Many of the most popular fishing and recreation lakes in the region are mesotrophic, including several in the Brainerd Lakes area, the Chain of Lakes in Vilas County, and the Alexandria lakes chain. Mesotrophic lakes represent a tipping point, they can improve toward oligotrophic conditions with effective watershed management, or decline toward eutrophic conditions if nutrient inputs increase from development, agricultural intensification, or failing septic systems.

Frequently Asked Questions