GRAND FORKS — Algae blooms in Lake of the Woods in Minnesota remain an issue, the result of phosphorus levels that exceed federal water quality standards, but cleaner water flowing out of tributaries that feed the lake should continue to reduce the nutrient problem and bring it back in line with federal guidelines over the next two decades.

That’s a key finding in a new 180-page draft report from the Minnesota Pollution Control Agency addressing nutrient loads on the Minnesota side of Lake of the Woods and prospects for improving water quality to meet federal standards.

The report, which highlights a “total maximum daily load” — or TMDL — study on Lake of the Woods, indicates phosphorus levels need to decrease by 17.3% to reach federal standards. A TMDL is a federally set measure of the maximum amount of a particular pollutant that can be present and still meet water quality standards.

On Minnesota’s portion of Lake of the Woods, the federal TMDL standard for phosphorus is 30 parts per billion, according to Cary Hernandez, project manager for the MPCA in Detroit Lakes, Minn. The level on Lake of the Woods is closer to 36 parts per billion, he said.

“The concept is, (when) you have a body of water that isn’t meeting the standards, you have to come up with a plan or goals to reduce whatever pollutant you’re dealing with down to the standard,” Hernandez said.

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The draft report is based on studies from the MPCA, the University of Minnesota, the University of Wisconsin-Stout, the Science Museum of Minnesota’s St. Croix Research Survey and the U.S. Geological Survey.

The report only applies to the Minnesota portion of Lake of the Woods and not to Canada, which has its own set of standards, Hernandez said. Lake of the Woods covers nearly 1 million acres, according to Wikipedia, including about 320,000 acres in Minnesota.

Significant sources

Tributary streams contribute 39% of the phosphorus load on Minnesota’s portion of Lake of the Woods, Hernandez said; deposits that collect on the bottom of the lake over time also play a significant role.

That buildup of deposits, known as “internal loading,” results from sources such as industrial, municipal and agricultural runoff, Hernandez said.

“Basically, for decades, everybody was just dumping their wastes in the rivers and it was going into the lake,” he said. “The phosphorus would fall out and settle with the sediment at the bottom of the lake. Well, that lake is so wide and so shallow that if you get a sustained wind of over 10 miles per hour, it tends to stir up the bottom,” in turn reintroducing the phosphorus into the system.

The good news is that water quality in the Rainy River, the main source of water flowing into Lake of the Woods, has seen significant improvements since the federal Clean Water Act was passed in the 1970s, Hernandez said. The reduction in nutrients from municipal and industrial wastewater sources means the Rainy River now meets federal water quality standards.

Based on results from a 2015 study, phosphorus levels from internal loading on Lake of the Woods are decreasing by 1% annually as cleaner water flowing into the lake flushes out the stored nutrients, Hernandez said.

The draft MPCA report doesn’t call for additional nutrient reductions from most sources, Hernandez said, although the Little Fork River continues to exceed maximum phosphorus standards.

The Little Fork River flows into the Rainy River west of International Falls, Minn.

The MPCA is looking at options for reducing phosphorus levels in the Little Fork River, but further reductions in Lake of the Woods are limited to flushing out the phosphorus already in the system, Hernandez said.

“There really isn’t a lot of opportunity there, nor is there a lot of agriculture in that area, so it’s mainly internal loading,” he said. “So, where we’re at now is if you don’t do anything to make that situation worse ... eventually, we’ll be sending cleaner waters to the lake, kind of flushing out that phosphorus.

“It’s a big, complex lake and a big, complex system. The answers don’t come easy or overnight.”

Some algae blooms on Lake of the Woods are large enough that they have been photographed from space, as seen in this undated satellite imagery photo. (Photo courtesy of U.S. Geological Survey)
Some algae blooms on Lake of the Woods are large enough that they have been photographed from space, as seen in this undated satellite imagery photo. (Photo courtesy of U.S. Geological Survey)

Algae blooms are most prevalent later in the summer, but the phosphorus levels aren’t harmful to the big lake’s fish community, Hernandez said. The toxins produced by blue-green algae can be fatal to pets and livestock and pose health risks to humans.

“I don’t think anybody has a really good idea of how much harm there is to people or at what level you have an algae bloom that harms people,” Hernandez said. “But we don’t want to get there.”

The Lake of the Woods report is part of the MPCA’s approach to gauging the health of Minnesota’s 80 major watersheds, each of which will have an approved comprehensive watershed management plan by 2025, according to an agency news release. After intensive water monitoring, the agency and partners evaluate water quality and biological conditions in lakes and streams. The MPCA places waters that fail to meet standards on the Impaired Waters List and develops information and strategies to restore impaired waters and protect healthy ones.

The MPCA is taking public comment on the draft Lake of the Woods nutrient report until 4:30 p.m. March 24 and has scheduled two virtual public meetings: 1-3 p.m. and 6-8 p.m. Thursday, March 4.

For more information about the report, how to comment or to join the virtual meetings, check out the MPCA website at pca.state.mn.us/water/tmdl/lake-of-the-woods-excess-nutrients-tmdl-project or contact Hernandez at 218-846-8124, 800-657-3864 or cary.hernandez@state.mn.us.