In a striking example of how harmful impacts of pollution can be reversed, mercury levels inside fish tissue at a handful of Minnesota lakes have fallen so far that scientists are rethinking how much can be accomplished through local pollution-reduction efforts.

The drops were so steep that 12 lakes around the state have been removed from the state's impaired waters list due to reductions in mercury pollution.

"We didn't expect to see a decline," said Bruce Monson, research scientist at the Minnesota Pollution Control Agency. "It could be these local reductions are having much more of an impact than we thought they were going to have."

Previously, scientists believed that global emissions, which have been steadily increasing for years, would minimize any gains from local cuts to mercury pollution. That's because the vast majority of the toxin that reaches Minnesota's waters was thought to come from as far away as China and India. But while global emissions have been creeping up, Minnesota and the rest of the United States have drastically cut the amount of mercury they are putting into the environment. Minnesota has halved its 2005 levels, while the rest of the Upper Midwest has slashed emissions by 75% over that time.

In some lakes, at least, the fish seem to be rebounding.

Mercury levels have steadily dropped in fish tissue at Lake Owasso in Arden Hills and three others in the Twin Cities, as well as in five lakes in northern Minnesota near the headwaters of the Mississippi River and in lakes in central and western parts of the state. When the 12 lakes were taken off the state's impaired waters list, it marked the first time in Minnesota's history that any water with a mercury impairment was removed.

Scientists can't say exactly what caused the improvements.

"But it could be that these local sources of mercury are actually more important as to what's getting into the fish than we thought," Monson said.

Mercury is a neurotoxin that builds up in the food chain. It becomes a risk to people primarily through eating certain kinds of fish and can be especially harmful to children.

Almost all the mercury reductions in the U.S. have come from energy production. Coal-powered plants were long the biggest source of mercury in the country. Many of those plants have been retired in favor of natural gas, while the rest have had to meet stricter federal emission standards since 2015.

Nine coal-powered generators in Rochester, Burnsville, Schroeder and Hoyt Lakes were retired or converted to burn natural gas over the last decade.

Garbage incinerators, another major source, have also made drastic cuts in their emissions because mercury was banned in a number of products that end up in landfills, such as alkaline batteries.

A puzzling improvement

It's difficult to say why Minnesota's lakes may be improving because the way mercury travels from the air, into water, into fish and ultimately into humans is extremely complicated, said Edward Swain, adjunct professor at the University of Minnesota.

Almost everything has a little bit of mercury in it, he said. Coal has just a tiny amount — about a tenth of a part per million. But people have burned such massive amounts of coal that the released mercury continues to add up. It gets into the air when the coal — or taconite, or a discarded battery or any number of items — is burned or heated. Some of the mercury, a soluble form, is carried for hundreds or thousands of miles in the air before it falls into wetlands, lakes and rivers. It dissolves in the water and becomes part of the food chain.

Another form of mercury — one that does not dissolve in water or travel as far — is also released, Swain said. That form of mercury, called elemental mercury, has never been the priority in pollution control because it was thought that it wouldn't as easily make its way into fish tissue.

But in recent years, scientists have discovered that elemental mercury is, in fact, becoming soluble. The culprit is likely fallen tree leaves, which have tiny openings that capture the neurotoxin, Swain said. As bacteria break down the dead leaves they pick up the captured mercury, too. It becomes soluble and finds its way into lakes.

"We may have underestimated the importance of local and regional emissions because of this phenomena of the leaves picking up mercury," Swain said.

Swain cautions that we still don't know if the local pollution reductions are the cause of the improving lakes.

"The environment is so complicated, it could be that at these [dozen] or so lakes something changed in the biomagnification, or water chemistry or bacteria," he said.

But it is cause for hope that actions taken over the last several decades have already helped, he said, adding that restoring Minnesota lakes may be more in the hands of Minnesotans than previously thought.

"We've seen declines in sediment cores, and in the Great Lakes," he said. "Collectively, we recognized mercury was a problem, and have taken action. Maybe not to the degree we'd like. But we are seeing that when mercury loads to lakes go down, the fish recover."