Leif Olmanson has spent most of his career tracking Minnesota's lakes from space, poring over decades of satellite images and crunching data on water clarity.

Now the University of Minnesota researcher is puzzling over a new question: What is driving the declining water clarity in Minnesota's northern lakes, some of the jewels of the state?

"My big concern is that the areas that are more pristine are where things are changing quickly," Olmanson said. "Why would these lakes be changing in northern Minnesota where there's not a lot of land use changes going on?"

Olmanson quickly mapped the state's late summer temperatures — the dog days when algae blooms — and saw they have risen fastest in Minnesota's north-central regions where lakes have been warming the most. This is the home of deep, cold lakes. Bit by bit, the change in a few degrees could alter the state's prized cabin country and angler havens.

"That's some of the best walleye fishing in the country," said retired DNR fisheries research biologist Peter Jacobson. "It's a part of the state we're very concerned about."

Other scientists at the U, the Minnesota Pollution Control Agency and the Department of Natural Resources (DNR) are monitoring the trend, too.

Casey Schoenebeck, a research scientist who runs the DNR's sentinel lakes program, said Olmanson's heat map is supported by what his team has found in the water. Lake water temperatures are rising statewide, but particularly in the state's transition zone from the plains to forest and in the northern forest area.

"It's all changing," Schoenebeck said, "but the changes are happening the fastest in those two central eco-regions."

Warmer water encourages the algae growth, including the toxin-producing cyanobacteria commonly called blue-green algae. It can clog fish gills, and when it dies and sinks to the bottom of a lake it consumes oxygen, starving fish and other aquatic life.

The murkiness can actually amplify the warming temperatures, said Gretchen Hansen, another U scientist studying the decline in water clarity. Murky surfaces absorb more of the sun's radiation, warming surface waters even faster.

The most ominous sign of the impact is the plunge in cisco populations across the Midwest as lakes warm. Also called tullibee, the small silvery fish are a main source of food for prized game fish such as walleye. They thrive in bands of deep cold water, and are highly sensitive to temperature changes. The DNR has been working to try find "refuge" lakes for them.

Changes on land

In addition to temperature changes, there are multiple factors that can make Minnesota lakes murkier that Olmanson, Hansen and others are trying to untangle, such as changes in precipitation and, perhaps more important, in land use.

Minnesota is losing forests to farmland as row crops spread north, for example, as timber is harvested and as communities grow with new homes, businesses and roads. Then there are cabin owners tinkering with shorelines.

Plus, more intense rainstorms wash more nutrients, sediments and solids, such as leaves, into lakes with tannins that turn water brown.

As Peterson, the retired DNR biologist, sees it, the solution to protecting water quality in the state's deep clear lakes is to protect the intact forests around them. If 75% of a lake's watershed is forested, you can protect it, he said.

"It's critical that it does not get converted to agriculture or homes, and shopping centers and roads," Jacobson said.

That's what the Northern Waters Land Trust has been working on. Based in Walker, Minn., the nonprofit conserves private land on strategic tullibee refuge lakes in Cass, Crow Wing, Hubbard and Aitkin counties. It uses grants from the state's sales-tax funded Outdoor Heritage Fund to arrange conservation easements for landowners and has protected nearly 2,500 acres that way since 2014. The trust also buys land outright.

Olmanson said the approach makes perfect sense: "It's cheaper to protect the lake before it gets impacted than to try to restore it."

To explore the effects of land-use changes on water clarity, Olmanson is analyzing new satellite-derived data that show changes in land cover. His goal is to build an automated data set to show which factors are most important in driving declining water clarity in different lakes.

"Different things are happening in different parts of the state," he said. In the near term, he's racing to finish a major update of the U's interactive LakeBrowser tool in time for this year's fishing opener May 15. It's popular with anglers and real estate agents.

The tool, which Olmanson helped create, displays information about the clarity of all Minnesota lakes down to 10 acres in size. It shows a lake's current and historic clarity measures and comparisons to other lakes in the watershed, for example, how much algae it has and the nature of the land around it, such as forest or fields. It complements the DNR's LakeFinder tool.

Warmth running north

The map Olmanson generated of late-summer temperature changes in Minnesota's center north reflect a broader pattern, climatologists say.

Northern Minnesota is warming faster than southern Minnesota, with north-central and northeast Minnesota warming a little more than west-central Minnesota, said Kenneth Blumenfeld, senior climatologist in the state Climatology Office.

If you zoomed out from Olmanson's map, Blumenfeld said, it would show that high readings in north-central Minnesota are part of a larger continuous belt extending north into Canada. In general, the farther north you go around the world, the faster warming is occurring. There are variations on our continent, he said, where the interior is warming faster than near the coasts.

"Northern Minnesota has some of the fastest warming rates in the contiguous U.S., including during the late summer," he said. "The variations we see to the east and west are based on topography, elevation, land cover, proximity to water, and other factors climate scientists do not fully understand."

Jennifer Bjorhus • 612-673-4683