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Oxygen levels are falling in freshwater lakes in Minnesota and across the globe — a trend likely driven by 

 and human development, a new study suggests.

The findings could be a sign that trouble is ahead for Minnesota lakes that provide important habitat for cold-water fish species, and could have more harmful algal blooms in their future.

published Wednesday in the scientific journal Nature

, was the first to look at temperature and oxygen changes across a large number of lakes across the globe. Its research team included scientists from the University of Minnesota and other institutions.

Researchers looked at nearly 400 lakes in temperate climate zones in the United States, Canada, Europe, Asia and South America. The scope included 84 lakes in Minnesota — many in the Twin Cities metro, plus Leech Lake and Lake Itasca in northern Minnesota.

The scientists looked at data on water temperature, clarity and oxygen levels from the lakes — some dating back to 1941, but mostly since the 1980s.

They found a widespread decline in oxygen in the surface water of most lakes and especially in deep water, a trend that could lead to a loss of biodiversity.

The deep waters of lakes provide important habitat for cold water-loving fish and other aquatic life, said Gretchen Hansen, assistant professor in the University of Minnesota’s Department of Fisheries, Wildlife and Conservation Biology and one of the study's authors.

“Lake trout, whitefish, tullibee — the things in Minnesota that we're pretty concerned about,” Hansen said. “As lakes get warmer, we need those bottom waters to contain oxygen for those fish to live there."

The study found that from 1980 to 2017, oxygen levels have dropped by 5.5 percent in the surface water of lakes and more than 18 percent in deep water, a much more rapid decline than in the world's oceans.

A few factors are likely driving the trend, including climate change, which is causing lake temperatures to rise. Scientists already know that as water warms, it cannot hold as much dissolved oxygen.

Not surprisingly, the researchers found that as the temperature of surface water in lakes has increased, the amount of oxygen in most of those lakes has dropped.

More interesting: The researchers found that deep water temperatures in lakes aren't rising much on average, and sometimes not at all. But the oxygen levels in those deep waters are still declining.

Kevin Rose, the study's coauthor and a biology professor at Rensselaer Polytechnic Institute in New York, said that's likely the result of 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.

Ocean water isn't mixing like it used to, and that could have grave consequences

“When spring hits earlier, then that stratification sets up sooner,” Rose said. “So your deep waters are cut off from the atmosphere earlier in the season.”

Another factor: When a lake loses oxygen, it can accelerate the release of nutrients such as phosphorus that accumulate in the lake's sediment, Rose said.

Excess phosphorus often enters lakes as runoff from farm fields or urban areas. It fuels the growth of harmful algal blooms that can turn a lake slimy and green in the summer, reduces water clarity and can sometimes be toxic.

Some algal blooms produce methane, a potent greenhouse gas that contributes to climate change — creating what scientists call a positive feedback loop.

In some lakes whose surrounding land has been heavily developed, researchers found that oxygen levels in the surface water had actually increased along with the temperature — a condition they attribute to increased algal blooms, which temporarily produce oxygen. But it doesn’t benefit the deep waters, Rose said.

Researchers say the study isn’t the final word on the subject. Hansen is part of a group that's planning to look more closely at what's driving the oxygen changes in lakes in Minnesota — including some where the surrounding land is heavily developed, and others that aren't developed as much, or at all.

Hansen said they want to look at what times of the year the lakes are losing oxygen and whether it's related to land use, big rain events or other causes.

“So we can make better recommendations for how to manage the watersheds around these lakes to conserve oxygen — even as they get warmer,” Hansen said.

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Kirsti Marohn

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Researchers found a widespread decline in oxygen in both the surface water of most lakes and especially in deep water, a trend that could lead to a loss of biodiversity.

Study: As temps warm, lakes' deep waters are losing oxygen

Study: As temps warm, lakes' deep waters are losing oxygen

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