Freshwater lakes continue to get warmer across the globe, and an almost unseen problem has been growing far beneath the surface: The deepest, coldest reaches of lakes are rapidly losing oxygen.

The world's deep waters have lost an average 19% of their oxygen supply since 1980, according to research published this month from the University of Minnesota and Rensselaer Polytechnic Institute in New York. That loss is creating a host of problems, including fish kills, water quality issues and bacteria blooms.

"It creates this squeeze for fish," said Gretchen Hansen, co-author and researcher at the U. "If the colder temperatures they need are in the bottom of the lake but the oxygen is at the top where it's too warm, they're squeezed into smaller and smaller areas until there is nowhere left."

The problem has been affecting lakes not only in Minnesota, Hansen said, but also in just about every temperate climate in the world.

The loss of dissolved oxygen in freshwater lakes is about nine times worse than the more widely publicized loss of oxygen in the world's oceans, according to the researchers, who analyzed tens of thousands of dissolved oxygen and temperature readings collected since 1941 from about 400 lakes around the world, including 84 in Minnesota.

The depletion is primarily caused by two factors — warmer surface water and more phosphorus and nitrogen, Hansen said.

Surface water temperatures have risen by more than 7 degrees Fahrenheit on average over the past 40 years. Couple that with the excess nutrients that have crept into lakes from fertilizer runoff and urban development, and algae blooms can quickly take over the surface of a body of water.

That algae produces oxygen near the top of the lake, but the plants sink to the bottom when they die and decompose, burning through the oxygen at the bottom of the lake, Hansen said.

In the summer, warmer surface water is also a problem because deep lakes essentially split into two ecosystems. The warmer, oxygen-rich water stays on the top while the colder water with less oxygen sits below. Because the two waters don't mix very well, there are few ways for oxygen to make it to the depths of a lake until winter, when the water becomes a more uniform temperature and can circulate more freely.

Climate change is causing warmer water earlier in the season, and the period when lakes are stratified into two layers starts sooner now than it did a decade ago, Hansen said. That means whatever oxygen is at the bottom of a lake when the warm season starts is used up more quickly, she said.

Adding to the problem, the warm waters and excess phosphorus at the top of the lakes make ideal conditions for cyanobacteria — the organism that causes toxic blue-green algae blooms that have killed pet dogs and closed lakes throughout the state every summer for years.

Meanwhile, other bacteria thrive in the low-oxygen environments of the lake bottoms. Those bacteria happen to release methane, a powerful greenhouse gas.

The increasing temperatures show no signs of abating, so the best way to keep oxygen within lakes is to keep out excess nutrients, Hansen said.

"We know what to do to improve water quality," she said. "And we have had some success in Minnesota, at least, in protecting some of our watersheds."

Oxygen loss in Minnesota lakes has been a concern of the Department of Natural Resources (DNR) for years. Cold-water species such as cisco and lake trout have been eliminated from a number of their historic lakes because they couldn't breathe in many of the deep waters still cold enough to harbor them.

The new study's findings line up with what biologists have seen over the years in a number of the state's lakes, said Peter Jacobson, retired DNR research supervisor.

"It's interesting to see that it's been happening worldwide," he said.

The state's saving grace is that it still has many lakes that are deep enough to remain cold at their bottom and that are surrounded by enough intact forests to keep excess nutrients out. The state started a triage program, of sorts, over the past decade to focus on protecting forested lands around 175 "refuge lakes" that weren't already too far gone to save.

"What we found is that if you can keep 75% of a lake's watershed with some kind of permanent vegetation, it's going to maintain good water quality," Jacobson said.

The global study backs up the DNR's findings.

It found that the waters of Lake Itasca, surrounded by acres of ancient pines protected in a state park, didn't lose any oxygen. The deep waters of Leech Lake, whose watershed is primarily made up of forests and wetlands, actually gained oxygen. The two Minnesota lakes that lost the most oxygen have no such protections — Clearwater Lake, surrounded by mostly farmland west of Monticello, and Square Lake, which is surrounded by farmland north of Stillwater.

It's far easier to protect lakes than to try to restore them, Jacobson said.

The hope is to learn from the lakes that aren't changing as quickly, such as Itasca, and use them to make decisions on how to protect the clear waters that are left, said Lesley Knoll, one of the authors of the study and associate director at the U's Itasca Biological Station and Laboratories.

"A well-forested system will help buffer a lake against these sorts of changes," Knoll said. "But that said, at some point increasing air temperatures still can affect even those protected systems. There's going to be a tipping point."

Greg Stanley • 612-673-4882