New MPCA-funded research finds cleaner conditions in Lake of the Woods

February 18, 2026

Researchers collect samples of sediment from the bottom of Lake of the Woods in 2024. (Photo courtesy of St. Croix Watershed Research Station)

After years of study and monitoring, new data suggest that Lake of the Woods, along Minnesota’s Canadian border, is on a path to experiencing fewer and smaller harmful algal blooms.

“Both total phosphorus and chlorophyll are trending downward, and that’s strong evidence the lake is improving,” said Mark Edlund, a senior scientist at the St. Croix Watershed Research Station, part of the Science Museum of Minnesota.

Mark Edlund collects samples of sediment from Lake of the Woods to be tested for phosphorus. (Photo courtesy of St. Croix Watershed Research Station)

In 2010, the Minnesota Pollution Control Agency (MPCA) commissioned several studies to better understand the lake’s conditions. One study looked at core samples of sediment from the lake’s bottom to measure how much phosphorus was stored there. This created an inventory of legacy pollution, contaminants laid down over decades that still sit in sediment at the bottom of the lake. A second study focused on phosphorus “availability,” looking at how much of the sediment-bound phosphorus could be released into the water and absorbed by algae. Edlund’s team continued this work in 2023.

Together, the studies helped scientists understand how much phosphorus is in the lake, and how it moves through the ecosystem. Comparing data from 2010 to current data reveals a potential 23%-29% reduction in total phosphorus in the lake’s active sediment. It’s promising news in the wake of a century of pollution.

“For much of the 20th century, particularly the first 70 years, Lake of the Woods served as a dumping ground,” Edlund said. “Sewage and industrial waste were discharged into the Rainy River and carried directly into the lake. These inputs severely degraded water quality.”

By the late 1960s, people were raising concerns about the lake’s health. Many discharges were eventually stopped, but water quality didn’t immediately improve. Legacy pollution was still a source of phosphorus, which cycled between the sediment and the lake’s surface, acting as fertilizer and fueling algae growth.

Blue-green algae grow in abundance in Lake of the Woods during late summer and fall. (Photo courtesy of St. Croix Watershed Research Station)

People often associate northern lakes with clean, clear water. But for much of the summer, and into the fall, algae can cover nearly three-quarters of Lake of the Woods’ surface. These algal blooms are why in 2008, the MPCA placed the lake on the state’s list of impaired waters. The listing is for aquatic recreation, meaning the lake does not meet water-quality standards for activities that involve direct contact, such as swimming.

The designation triggered a requirement under the Clean Water Act to develop a total maximum daily load (TMDL), which determines how much pollution a waterbody can handle and still meet water quality standards. At the time, however, state scientists knew they didn’t have enough information to write a TMDL.

“We sat down in 2009 and figured out everything we needed to know to perform a TMDL study,” said Cary Hernandez, a project manager for the MPCA. “We realized we were nowhere near having it.”

Sediment core incubation determines how sediments release phosphorus under different lake conditions. (Photo courtesy of St. Croix Watershed Research Station)

That’s when Edlund’s team got involved. Between the lake being listed as impaired in 2008 and the TMDL’s approval in 2021, more than a decade of data collection and analysis was required. The Red Lake Department of Natural Resources was another partner in this work, teaming up to help monitor water quality in Lake of the Woods.

“Combining our sampling efforts with those of the MPCA allowed for biweekly sampling, resulting in a far more comprehensive dataset for the season,” said Shane Bowe, water resources director for the Red Lake DNR.

Bowe said the lower phosphorus concentrations in the upper layer of sediment is an indication that work being done in response to the TMDL is producing positive, albeit gradual results.

“We knew that the road to recovery would be long for this lake, so it’s no surprise that it’s happening slowly,” he said.

Lake of the Woods has only 19 permitted “point sources,” water discharge that flows in from a specific spot. This leaves limited opportunity to reduce phosphorus through regulation alone. Instead, the MPCA has focused on steadying the situation and allowing the lake’s natural processes to gradually reduce what was already there.

“If we could keep the inputs from increasing,” Hernandez said, “we believed we’d start to see reductions over time.”

When Lake of the Woods was listed as impaired in 2008, it held an average concentration of 36 micrograms of phosphorus per liter, above the state’s water quality standard of 30. By 2015, the number had climbed to 39. The most recent assessment, completed in 2025, showed encouraging improvement, with the phosphorus level dropping to 32 micrograms per liter.

“It’s trending in the right direction,” said Cary Hernandez, a project manager at MPCA. “To go from 39 down to 32 in about 10 years? That’s significant. That tells us the system is responding. The lake is slowly working through what’s there.”

Numerous factors have helped improve conditions, including wastewater treatment upgrades, better land-use practices, reductions in industrial waste, and long-term regulatory changes on both the U.S. and Canadian sides of the lake. Scientists use satellite imagery to measure algal blooms by looking at chlorophyll-a, a pigment found in algae. This allows researchers to track algal blooms across the lake’s surface.

22446: Lake of the Woods pull quote

It’s trending in the right direction. To go from 39 down to 32 in about 10 years? That’s significant. That tells us the system is responding. The lake is slowly working through what’s there.” — Cary Hernandez, MPCA project manager Despite the progress, algal blooms still emerge on the lake each summer and last into October, a season extended by climate change, as warmer water creates better conditions for algae. This makes continued monitoring essential.

“The indicators we are tracking all point in the same direction: Lake of the Woods is getting better,” Edlund said. “That is the best outcome we could hope for at this point.”

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