In September, Edina officials alerted the residents surrounding Lake Cornelia: Don’t go in the water.

The city and Nine Mile Creek Watershed District had detected high levels of the toxin microcystin in the lake, a result of an increase in blue-green algae. Not exactly a swamp monster sighting, but all the more frightening — exposure to the toxin can lead to liver and kidney damage, and in extreme cases, death.

So the watershed district and the city joined forces to develop a treatment plan for the algae. The residents, 31 of whom own properties along the lake, also banded together to voice their concerns.

Lake Cornelia, just off Crosstown Hwy. 62 and west of Southdale Center, is technically a wetland and only 7 feet at its deepest point. Both the city and neighbors agree the lake’s water quality is less than favorable, which may be putting it lightly; one resident called it “crappy looking” and a “mess of garbage” during a city meeting in mid-November.

Cornelia’s issues have even grabbed the attention of the federal government. The lake’s north basin is on the Environmental Protection Agency’s “impaired waters” list for failing to meet water quality standards. Jessica Wilson, Edina’s water resources coordinator, said the south basin likely will be added to the list in 2018.

“It’s not really comparable to other lakes” in the city, Wilson said. “Lake Cornelia has a lot of challenges with it.”

At the November meeting, Wilson said most of the initial algae bloom can be traced to a high concentration of phosphorus. Add a little sunlight, and the blue-green algae grows and expands.

The major source of the phosphorus is runoff from the greater watershed emptying into the lake’s south basin, Wilson said. But other sources — shifting sediment, past agricultural activity, even goose droppings — also contributed to its overabundance.

In addition, Lake Cornelia was the site of a curly-leaf pondweed invasion in 2015 and 2016. The plant unleashes significant amounts of phosphorus when it dies off in the summer, Wilson said.

During a research period, city staffers found that Lake Restoration, the company hired to treat Edina’s lakes, did not do so as required by its contract.

“Typically, algae is treated twice a year in Lake Cornelia, and in 2016 it only happened one time,” Wilson said. “We’re really disappointed that they did not fulfill their contract obligations.”

Residents call this either an act of fraud or negligence. But Wilson said the curly-leaf pondweed would have grown either way.

The city hopes to organize one treatment for curly-leaf pondweed and two for algae next year. It is also thinking about getting a new lake treatment provider, Wilson said.

The treatments should address the most visible issues in Lake Cornelia. But many lakeside residents still feel the city is not delivering a comprehensive plan to improve the lake’s appearance and water purity.

Edina doesn’t have a policy to prioritize which lakes get water improvements, Wilson said. Its Lake and Pond Management Policy addresses vegetation issues but not outside pollutants like watershed runoff.

“I think you need a plan specific for the lake,” resident Susan Lee said at the meeting. “Right now there is no plan.”

The lake management policy will be updated next year as part of the city’s 10-year Comprehensive Water Resource Management Plan. The Edina City Council is scheduled to get an update on the lake Dec. 20.