Scientists examine lake mud to learn why toxic algae is in protected lakes

MARINE ON ST. CROIX — Why are toxic algae blooming in Northern Minnesota’s most pristine lakes?

The environmental research division of the Science Museum of Minnesota aims to answer that question over the next three years as it researches the causes of toxic algal blooms in protected northern lakes.

The Minnesota Legislature awarded more than $1.3 million to the research station to study what leads to toxic algae growing in lakes in the Superior National Forest and the Boundary Waters Canoe Area Wilderness. These lakes are in protected areas where toxic algae are not expected, said Lienne Sethna, an assistant scientist at the research station and the principal investigator of the project.

The research station specializes in paleolimnology, or the study of lake mud.

“You expect that these toxin-producing cyanobacterial blooms occur in more impacted lakes, like urban lakes or agricultural systems where you get a lot of farm runoff and fertilizer,” Sethna said.

Cyanobacteria — a species of bacteria that functions like algae — make up some of these blooms that produce toxins like microcystin, anatoxin and saxitoxin, making the waters unsafe to drink from and swim in, Sethna said. Certain evolutionary advantages allow them to thrive even when there is less sunlight and outcompete beneficial algae for nutrients in warm waters.

The museum’s St. Croix Watershed Research Station will be working with researchers from the Red Lake Department of National Resources, the 1854 Treaty Authority, the U.S. Forest Service and the Minnesota Pollution Control Agency. The 1854 Treaty Authority will be researching how these blooms impact wild rice abundance and density, which it has been monitoring since 1998.

The research, called the Wilderness Lakes Project, started three years ago when the research station was under the impression that foreign dust from places as far away as the Dakotas was increasing nutrients and causing these blooms, Sethna said. The researchers deployed buoys to monitor the lake temperature from top to bottom as part of researching this initial hypothesis.

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St. Croix Watershed Research Station

The temperature data showed that the same lakes that had toxic blooms were switching between stratified and unstratified, Sethna said. A lake is stratified when the water is warmer toward the top and colder toward the bottom, and it is unstratified when the water is the same temperature throughout.

When a lake is stratified, the bottom layer has little to no oxygen, or is anoxic, causing the sediments to release phosphorus, Sethna said. In a lake that stays stratified, the phosphorus does not move up past the higher oxygenated layers.

“But when you have mixing, that phosphorus that’s in the bottom waters is distributed through the water columns, and you get blooms,” Sethna said. The research station calls this process anoximixus.

The next phase of the research will directly measure the potential for lake sediments to load phosphorus and test the hypothesis of anoximixus. Sethna and her fellow researchers will collect samples of lake mud to analyze in the lab as well as deploy buoys to continue to monitor temperatures.

The project aims to develop a profile of which lakes are more susceptible to anoximixus and thus more vulnerable to toxic blooms so agencies can notify the public of which lakes are unsafe and when, Sethna said. The project’s ultimate goal is to inform strategies to manage and limit the growth of these blooms.

This summer, the research station is preparing equipment and coordinating with its research network, and it plans to deploy its buoys this fall and begin monitoring and sampling in May 2026, Sethna said. The researchers will monitor the lakes in the summer and collect sediment cores in the winter for two full years.

“I think it’s a really exciting project in terms of protecting some of our most prized and pristine lakes,” Sethna said.