Swimmers are again being warned to stay away from Lake Cornelia in Edina because of elevated levels of blue-green algae that could irritate skin and cause stomach problems or liver damage.

Blue-green algae has made an appearance on the lake almost every summer for well over a decade, as Edina and the Nine Mile Creek Watershed District have struggled to maintain water quality in the shallow lake that sits just south of the Crosstown Highway 62 and a few streets west of the Southdale mall. Excessive levels of phosphorus, which is often used as a lawn fertilizer, have made it all too easy for the algae to grow in the lake.

This summer, both the north and south parts of the lake have levels that exceed the World Health Organization's standards for how much is safe, tests by the watershed district revealed last week.

Skin contact with blue-green algae can cause rashes, hives, swelling or blisters. Ingesting water contaminated with the algae's cyanotoxins can also cause liver or kidney damage, diarrhea and seizures, and can be deadly to people and animals.

The latest algae bloom comes despite the Nine Mile Creek Watershed District's seven-year efforts to clean up the lake. The work begun in 2017 has included herbicide to target invasive curly-leaf pondweed; a special solution that binds with phosphorus to make it harder for algae to absorb; and attempts to manage the lake's not-insubstantial population of abandoned pet goldfish and their descendants.

The city and Watershed District are also rebuilding stormwater basins in an effort to filter water that runs off of Edina's lawns and parking lots before it flows into the lakes.

Cornelia has been a problem for almost two decades. The Metropolitan Council listed it as one of the metro's "Worst Lakes" list in 2005. In 2008, the Minnesota Pollution Control Agency listed the section of the lake north of 66th Street as an impaired water, adding the south part of the lake to the "impaired" list in 2018.