Despite decades of urban sprawl, expanding cornfields and rising temperatures, a lot of lakes in Minnesota appear to be holding their own against nutrient pollutants such as phosphorus.
And never mind the recent appearance of potentially toxic algae in Edina's Lake Cornelia. A few dozen lakes in the metro area have less algae today than they did in 1990.
The trends emerge from a University of Wisconsin analysis of 3,000 lakes across 17 Midwest and Eastern states, including 742 in Minnesota.
Researchers concluded that pollution levels for phosphorus, nitrogen and chlorophyll — a proxy for algae — have remained largely unchanged since 1990. But it's hard to say whether it's good news or bad, because Minnesota has spent hundreds of millions to clean up lakes, and the federal government spends about $3.5 billion on the effort every year.
"It's sort of a balance between the money we are putting in to conserve them and what's happening on the landscape," said Samantha Oliver, a researcher at the University of Wisconsin who published the results last week in the journal Global Change Biology.
Oliver used detailed information for each lake from a vast database compiled by water scientists for the northeast United States. Called LAGOS-NE, the database includes lake water quality data provided primarily by state governments since the passage of the Clean Water Act in 1972.
The main goal, she said, was to see what kind of effect three decades of landscape changes across the upper corner of the U.S. have had on large lakes. That includes more land devoted to more intense agriculture, urban expansion, a warmer climate, and changes in what falls out of the atmosphere.
Nitrogen levels in the lakes declined a little — 1.1 percent per year overall — a trend that was apparent primarily in the East, where there's less farming, she said. That's most likely because nitrogen in the air has declined as a result of stricter standards that followed the Clean Air Act, she said.
In Minnesota and other farm states, "nitrogen deposition [from air] is so small relative to what comes off the landscape, we wouldn't expect to see improvements," she said. And she didn't.
As for phosphorus — more often a culprit in the formation of blue-green algae — 7 percent of the lakes saw increases and 9 percent decreased.
Chlorophyll showed a slightly different pattern — overall 10 percent of lakes saw an increase, compared to 5 percent that showed a decline.
Statewide, 94 percent of Minnesota's lakes saw no change in chlorophyll. But the 32 with less algae were primarily in the metro area, she said.
Her research, which examined lakes on a global scale, couldn't pinpoint what happened in the neighborhoods around those lakes that made a difference.
But Kelly Dooley, water-quality manager for the Minnehaha Creek Watershed District, which includes much of the western suburbs and Minneapolis, said her data show similar trends. Dooley said it's evidence that years of water-quality projects and landowner education about lawn fertilizers and rain gardens are paying off.
The creation of wetlands and ponds that hold and filter storm water reduces phosphorus, as does treating lakes with aluminum-based products that bind it to the sediment at the bottom.
She said the watershed has documented chlorophyll declines primarily in the Chain of Lakes in Minneapolis. Lake Minnetonka has also showed a decrease — but that probably results from the infestation of zebra mussels, which filter water and keep it clear, she said.
In another unexpected twist, the 12 Minnesota lakes that now get more algae are mostly in northern forests, where there is little or no agriculture or urban development to drive pollution higher.
But lakes across the region are warming as a result of climate change — and average temperatures in northern Minnesota are rising much faster than those in the southern part of the state.
"If it's warmer, algae can grow quicker, nutrients go farther," Oliver said. "So you would expect more algae growth in warmer lakes."
Josephine Marcotty • 612-673-7394
