It's time to pull the plug on Normandale Lake.

The vast and shallow lake in Bloomington, frequented by joggers and workers on their lunch break, has become overrun with curly-leaf pondweed. The invasive plant has reduced the lake's water quality, spurring algae blooms that appear as a layer of green gunk on the surface.

To remove the plant, the city and the Nine Mile Creek Watershed District this week began to drain the lake. During the winter, the freezing temperatures will kill the pondweed's reproductive buds found at the bottom.

A substantial portion of the 112-acre lake will be empty by mid-September, said Erica Sniegowski, a project manager for the watershed district.

"Curly-leaf pondweed is really, really hard to get rid of, and one of the best ways to do it is to empty the water out of the lake," Sniegowski said.

Normandale Lake, which has an average depth of about 4 feet, was built in the 1970s to control flooding from Nine Mile Creek. City officials asked the watershed district to improve the lake's water quality more than a decade ago.

Although curly-leaf pondweed is found across the state, it can be especially problematic in shallow lakes and can dominate native plants, said Keegan Lund, an invasive species biologist for the state Department of Natural Resources. When it dies in the summer, it releases phosphorus that algae feeds on, reducing oxygen levels and emitting a foul smell.

"In Normandale Lake the population has gotten large," Sniegowski said. "It's outcompeting the native plants."

Draining the lake will cost more than $370,000 and will be paid for mostly by the watershed district, Sniegowski said. It is a rare and massive undertaking that will expose the lake and force the animals that call it home to find temporary places to live.

Crews began draining the lake by pumping water out and opening a bypass pipe. A larger pipe will be installed this fall, leaving only a small pool of water from the creek flow.

Animals such as birds, geese and fish are expected to move to other habitats during the winter, Sniegowski said.

A protective fence installed around the lake will keep turtles from coming in and hopefully guide them downstream to other wetlands.

Once spring arrives and most of the pondweed is dead, workers will begin treating remaining plants with herbicide. They will then release aluminum on the lake to bind with phosphorus in the sediment, keeping it from rising into the water. Both the herbicide and the aluminum treatment will be repeated in coming years.

The bypass pipes will be closed in March, allowing the creek to naturally refill the lake. Sniegowski said it could reach its regular water level by early April.

"What we hope people will see when this project is done is less algae growth and a healthier water body," she said.

On Thursday afternoon, dozens were at the lake to walk and jog the trails, or eat lunch at the picnic tables.

Charlie Cree, a 67-year-old Bloomington resident, has gone to the lake a couple of times a week this summer. He said he hopes killing the pondweed will improve the lake's ecosystem.

"The algae pretty much just strangles the lake," he said. "It seems to me that you have to do something to get rid of the invasive species, if just for the wildlife."

"It's not going to inconvenience us any," he added. "Won't be as pretty, I suppose."

Jim Bullington, who works as a software architect at a nearby business plaza, usually walks around the lake during the lunch hour. Draining it, he said, is a good idea.

"I'm just afraid of what it's going to smell like before it's done," Bullington said.

Sniegowski said she hopes visitors don't step over the protective fence and attempt to walk on the lake bottom.

"We don't want people out there," she said. "It's going to be very mucky and dangerous."

Miguel Otárola • 612-673-4753