Ecology

Eutrophic

A lake classification indicating high nutrient levels, reduced clarity, and frequent algae blooms, a nutrient-rich and highly productive lake.

What It Means for Your Lake

Eutrophic lakes have high nutrient concentrations and biological productivity, with a Carlson Trophic State Index between 50 and 70. These lakes have total phosphorus between 25 and 60 ug/L, chlorophyll-a between 10 and 30 ug/L, and Secchi depth typically between 3 and 6 feet. The water appears distinctly green during summer months, and algae blooms, including potentially harmful cyanobacteria, are common during warm periods. Eutrophic lakes represent approximately 40% of monitored lakes across Minnesota and Wisconsin, making them the most common trophic state in the region. Many eutrophic lakes are in agricultural watersheds of southern Minnesota, the prairie pothole region, and central Wisconsin where fertile soils and intensive farming contribute substantial phosphorus to waterways. Despite their reduced clarity, eutrophic lakes often support productive warm water fisheries with abundant bass, walleye, crappie, and bluegill. The high biological productivity means more food for fish at every level of the food chain. However, eutrophic conditions create trade-offs: swimming may be unpleasant due to algae, and late-summer dissolved oxygen depletion in deep water (hypolimnetic anoxia) can cause fish kills in shallow lakes. The process of eutrophication, a lake becoming more eutrophic over time, is the most common form of lake degradation in the Upper Midwest, driven primarily by phosphorus loading from human activities.

Frequently Asked Questions