Study: Wildfire led to poorer water quality in northern Minnesota lakes

The aftermath of a wildfire in northern Minnesota
Areas burned by the Greenwood Fire which was first spotted near Greenwood Lake north of Two Harbors in 2021 — are seen one month after the fire passed through.
Photo by Michael Furtman

Researchers at University of Minnesota, Duluth have discovered that wildfire can have a negative impact on the health and water quality of lakes.

The study, published last month, was the largest examination in North American history of how lakes responded to a single wildfire. 

Scientists from the university's Natural Resources Research Institute in Duluth collected water samples from 30 lakes between April and October last year.

About half were in watersheds burned by the Greenwood fire, which consumed more than 26,000 acres in the Superior National Forest in 2021.

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Chris Filstrup is a lake and stream scientist at NRRI and the study's co-author. He said experiencing the Greenwood fire sparked his curiosity about how it was affecting lakes.

“I started seeing all these images coming out in the media — flames shooting into the sky near the shorelines of lakes,” he said. “And then seeing the aftermath of that, where basically all the vegetation had been burned away.”

Smoke plumes from the fire drifted across northern Minnesota even as far south as Duluth, Filstrup said.

“That just really solidified in my mind that this was such a big disturbance within the watershed,” he said. 

Filstrup said the research team was able to study lakes in a wide range of how severely their watersheds had been burned. Previous studies had only looked at how one or two lakes responded to a single fire.

The researchers found lakes in burned areas had higher levels of nutrients, such as phosphorus and nitrogen, compared to control lakes. An overabundance of nutrients can cause lower levels of oxygen in the water, which can harm aquatic life and cause algal blooms.

Lakes affected by the fire also had more sediment and dissolved organic carbon, which can turn the water brown.

The reason: when wildfire burns trees and other vegetation near lakeshore, it increases runoff from more rain or melting snow.

“There's nothing really there to stabilize the soil anymore,” Filstrup said. “So you start seeing the soil move with the runoff into the lake. And with that you have nutrients attached to it, potentially other contaminants.”

Researchers did find a couple of surprises: Despite the higher nutrient levels, lakes within the burned areas didn’t have more algae growth. That could be because the murkier water didn’t let as much light through, Filstrup said.

They also learned that it mattered not just how much of the watershed burned, but how hot the fire was and how close it came to the water’s edge.

“What we learned is the placement and the severity of the fire was much more important than just the amount of the watershed burned, or the size of the fire,” Filstrup said. “So if a fire burned lakes near the shoreline, that had a bigger impact on overall water quality in the system."

Lakes fed by streams flowing through burned areas also were more affected than isolated lakes, the study found.

It remains to be seen whether the impacts are enough to cause lakes to undergo what’s called a regime shift, or a rapid change in conditions, Filstrup said. He is awaiting funding to study the same lakes again in 2024.

“The question is are these going to be long lasting impacts, or just kind of short-term impacts that will go away within a year or two?” he asked.

Since 2021, groups such as the Nature Conservancy have undertaken efforts to replant the forests scorched by the Greenwood fire.

The study was published in the journal Geophysical Research Letters. It’s especially timely as temperature and precipitation changes driven by climate change lead to more frequent and intense wildfires across the U.S. 

While fire-prone western states differ significantly from lake-rich and heavily forested northern Minnesota, Filstrup does think the study’s findings could be applied to other lakes in the Midwest.

They also could help inform scientists as they try to identify susceptible lakes, he said.

“So if we have like these really big, hot fires in (close) location to the shoreline, that's when we likely want to go out to those lakes and start monitoring for potential impacts — as opposed to smaller fires that aren't as hot and maybe are further away from the lake,” Filstrup said.