Ecology

Hypereutrophic

A lake classification indicating excessive nutrient levels, very poor clarity, and persistent algae blooms, the most impaired category.

What It Means for Your Lake

Hypereutrophic lakes represent the most nutrient-enriched and impaired category, with a Carlson Trophic State Index exceeding 70. These lakes have total phosphorus above 60 ug/L, chlorophyll-a above 30 ug/L, and Secchi depth typically below 3 feet, often less than 1 foot during peak summer conditions. The water is intensely green, and thick algae scums frequently cover portions of the lake surface. Cyanobacteria (blue-green algae) dominate the phytoplankton community and may produce toxins (microcystins and anatoxins) that pose health risks to humans and animals. Approximately 10% of monitored lakes in Minnesota and Wisconsin are classified as hypereutrophic. These lakes are concentrated in the heavily agricultural regions of southern and western Minnesota, the Minnesota River basin, and parts of south-central Wisconsin. Many are shallow prairie lakes that have always been naturally productive but have been pushed to extreme levels by agricultural nutrient loading and urban stormwater. Swimming and body-contact recreation in hypereutrophic lakes is strongly discouraged and may be prohibited during bloom events. Fish communities are typically dominated by rough fish species like common carp and bullheads, though some hypereutrophic lakes maintain channel catfish and black crappie populations. Restoring hypereutrophic lakes is extremely challenging because internal phosphorus loading from nutrient-rich bottom sediments can sustain poor conditions for decades even after external inputs are reduced.

Frequently Asked Questions