Bass Lake in Plymouth has a problem — a green one.

"It can look like pea soup in July," said Julie Olsen, who has lived on Bass Lake for 19 years. When her kids were younger, Olsen's family would waterski, kayak and "look out over the water and enjoy all the nature."

But Bass Lake is threatened by an explosion of tiny floating plants called algae. These algae feast on phosphorus, a nutrient that bubbles up from the lakebed sediment and causes the algae to reproduce like crazy. Algae blooms can turn the whole lake green. They can also stink, scaring away would-be swimmers or anglers.

When lake temperatures rise, algae thrives. Dan Kraker | MPR News 2017

And Bass Lake is not alone in its water woes. Harmful algae blooms are growing more common across the state, according to research from the University of Minnesota.

To combat the aquatic scourge, lake managers are increasingly turning to aluminum sulfate, or "alum" for short. The chemical blankets the sediment, keeping the lake's phosphorus levels in check. And without phosphorus, the algae starve. That can turn the lake from grimy green to crystal clear.

That's why lake scientist John Holz came to Bass Lake. After 16 years researching lake chemistry, Holz founded HAB Aquatic Solutions. The company has injected alum into more than 80 lakes since 2013, including 16 in Minnesota. Holz's crew applies the chemical to Bass Lake from their high-tech tanker barge.

"The barge is automated. It has a computer on board," Holz said. "There are also automated valves that open and close as needed to keep the flow constant to achieve that target dose rate."

A tanker barge dispenses aluminum sulfate into Bass Lake. Daniel Ackerman for MPR News

The barge looks like a multirow planter that might be found on a farm field, rather than on a lake. It sports two wings covered in dozens of vertical tubes that shoot alum down into the water.

While the alum smothers phosphorus in the lake bed, it cannot stop phosphorus carried into lakes by streams. So Holz emphasized that alum treatments work best only after lake managers have reduced phosphorus in a lake's in-flowing streams.

Holz added that the process is benign. "As long as it's applied safely, there are really no negative components to an alum application," said Holz.

Most outside experts agreed that harmful side effects are rare. However, if the lake turns too acidic during an alum treatment, it could kill fish. Holz is monitoring the acidity throughout the day, so the lake should stay safe — not just for fish but also for human swimmers. In fact, Holz uses the same food-grade alum that water treatment plants use to purify drinking water.

But stopping algae blooms doesn't always mean a clean bill of aquatic health. The clearer water can give a boost to invasive aquatic plants.

After an alum treatment, "you get a little more light penetration that goes through that water column," said Department of Natural Resources scientist Kylie Cattoor. "That's going to promote more plant growth."

One invasive plant is particularly concerning to Cattoor — curly-leaf pondweed. She said the plant can grow "all the way to the top of the water and obviously interfere with things like boating, recreating and fishing."

Dogs as sentinels: Blue-green algae brings toxic mystery to Minnesota waters

The DNR has issued a permit for the Shingle Creek Watershed Management Commission, which manages Bass Lake, to apply herbicide that will prevent curly-leaf pondweed from taking over after the alum treatment. The DNR will also monitor plant growth in the lake in the months and years following the alum treatment.

In addition to Bass Lake, at least half a dozen other Minnesota lakes are slated for alum treatments this year.

Invasive plants aside, local residents like Julie Olsen are happy that pea soup in Bass Lake will soon be a thing of the past.

"I expect it to be more clear, probably the most clear that I've ever seen it," said Olsen. "We'll have to have a celebration."