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By Kirsti Marohn

Lakes across the Upper Midwest are at risk of losing oxygen-rich coldwater habitat due to climate change and nutrient pollution, according to a new University of Minnesota study.

But the study, published last week in the journal Ecosphere, also offers some hope: It concluded that protecting or restoring forested lands within some lakes’ watersheds could help conserve that critical habitat, even amid warming temperatures.

Gretchen Hansen, an assistant U of M professor and the lead author, said the study built on work over the past decade by the Minnesota Department of Natural Resources and other agencies to protect forested lands around coldwater lakes.

“We were interested in extending that approach to multiple states, and also refining it to be a little bit more lake-specific,” she said.

The study covers more than 10,000 glacial lakes in eight Upper Midwest states, including Minnesota. Researchers used statistical models to calculate how resilient those lakes are to changes in climate and land use.

“We wanted to identify lake-specific targets for conservation, so we could say which lakes are more vulnerable, and which lakes are more able to maintain these critical habitats even as conditions change,” Hansen said.

Lakes with deep, cold water and plenty of oxygen provide important habitat for certain fish species, such as cisco, tullibee and lake trout. An oxygen-rich lake also tends to have better water quality than one with low dissolved oxygen levels.

But as temperatures heat up, those lakes tend to lose oxygen that fish depend on, due in part to an increase in a condition known as stratification. As the surface water gets warmer, it becomes less dense than the colder, deeper water, reducing the amount of mixing of water that normally happens in a lake.

“Climate change can make that go on for longer,” Hansen said. “And that’s important, because when that bottom cold water is cut off from contact with the air, that’s when it can lose a lot of oxygen.”

A lake’s oxygen levels also are influenced by how the surrounding land is used, Hansen said. When forests or prairies are converted into farms or urban development, runoff containing nutrients such as phosphorus and nitrogen can cause algae blooms that deplete a lake’s oxygen.

“So it’s really an interaction between climate and the land use that’s going on in the watershed, and how much nutrient pollution is coming into the lakes,” she said.

The study makes some dire predictions: It estimates the number of lakes with suitable coldwater habitat will decline by 67 percent by mid-century. Meanwhile, lakes with unsuitable coldwater habitat are predicted to increase by more than 200 percent.

But more optimistically, researchers also found that for about a quarter of the lakes, their response to climate changes was significantly affected by what happened in the surrounding watershed.

The study identifies which lakes could be protected as refuges for coldwater habitat, and which ones could benefit from some restoration of forested lands.

“As the climate gets warmer, we might need to protect more of the watershed in order to conserve that habitat,” Hansen said. “We want to keep it there. So we do have local actions that can influence how they respond to this global driver of climate change.”

Joe Nohner is coordinator of the Midwest Glacial Lakes Partnership, a group of state and federal agencies and other conservation organizations that helped fund the study, and is also a resource analyst for the Michigan Department of Natural Resources. He said the study will be valuable to help guide efforts to protect 40,000 lakes across the Upper Midwest.

The key finding, Nohner said, “is that there are ways that we can reduce the effects of climate change, even if we’re not able to affect the overall trajectory of the warming temperatures and changing climate.”

Working to reduce nutrient pollution into lakes can help reduce those climate effects, he said. Strategies already being used in Minnesota and other states include encouraging farmers to use no-till methods, plant buffer strips along waterways and reduce the amount of fertilizer they apply.

For lakeshore properties, it means leaving shorelines natural with native plants that slow runoff and prevent erosion, and making sure septic systems aren’t leaking, Nohner said.

“This paper then provides that roadmap for us, and it shows that it can be theoretically done.” he said. “And it also shows very specifically which lakes we ought to target to be most efficient and effective.”