Wet spring, warm temps in Minnesota could spur toxic algae blooms

When lake temperatures warm, blue-green algae thrives.
A blue-green algae bloom on Peltier Lake, near Lino Lakes, Minn., in July 2017. When lake temperatures warm, blue-green algae thrives, often forming in thick, pea-soup colored blooms that spread out across the surface of lakes.
Dan Kraker | MPR News file

If the steamy weather forecast has you thinking about cooling off, pollution experts advise being on alert for harmful algae in Minnesota’s lakes and streams.

Those slick blooms that turn lakes bright green and scummy are more commonly spotted later in the summer.

But this spring’s heavy rains combined with recent hot weather has created ideal conditions for blue-green algae, which can contain toxic bacteria that can make people sick and kill pets.

Heavy rainfalls this spring carried runoff loaded with nutrients such as phosphorus into some lakes, said Lee Engel, water quality monitoring supervisor for the Minnesota Pollution Control Agency. 

Coupled with the forecast for very warm weather, that provides the perfect ingredients for algae blooms, he said.

“Like a garden or any plant, when you have good nutrients or available nutrients and warm temperatures, things grow like crazy,” Engel said.

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Lakes that already have high levels of nutrients are more likely to get algae blooms after a rainfall, he said.

Algae blooms on Little Rock Lake.
An algae bloom on Little Rock Lake near Rice, Minn., in June 2018.
Paul Middlestaedt for MPR News file

Swimming advisories were recently issued for beaches on two Minneapolis lakes, Hiawatha and Harriet. They have since been lifted, said Rachael Crabb, water resources supervisor for the Minneapolis Park & Recreation Board. 

“These algae when they become prevalent in the water column, the wind can actually push them into the shore and accumulate in kind of a thick layer,” Crabb said. “That’s what we call the scum or bloom, and those blooms just happened to blow right into two of our beaches.”

The water quality of a lake — how much sediment, nutrients or other pollutants it contains — can affect the likelihood that algae blooms will form.

Lake Hiawatha is considered impaired because of high nutrient levels. But Lake Harriet actually has about 20 feet of clarity, Crabb said, which makes the bloom somewhat surprising.

Crabb advises people to scan the water before they swim and stay out if there are areas that look like pea soup or spilled paint. 

Most algae blooms tend to be transient, Crabb said, so they form and dissipate quickly. The city of Minneapolis has a map of the water quality of all of its swimming beaches that is updated weekly.

Generally, Engel said people should follow the mantra of “when in doubt, stay out.”

“Whether it’s algae or sediment or something like that, you probably just don’t want to go in that lake if it does not look appealing,” he said. “You’re better off being on the safe side and staying out.”

There are also things people can do to help prevent polluted runoff from getting into storm sewers, which ultimately empties into lakes and streams, Crabb said. 

She advises using less fertilizer on lawns, keeping leaves and grass clippings out of the storm sewer, and picking up after your pet.

“Everybody plays a part in reducing nutrients,” Crabb said.