The amount of mercury that falls in rain and snow and builds up in fish has declined substantially in two remote lakes in Voyageurs National Park, an 11-year study shows, probably thanks to pollution controls in the United States and Canada.
But not all lakes in the study showed a decline in mercury, and it appears that water chemistry and flow rates can play a large factor in how much mercury is in each lake.
The study, published in the current edition of the journal Environmental Science and Technology, was conducted by the U.S. Geological Survey from 2001-12 and found that mercury in small perch from Ryan and Peary lakes declined 34.5 percent.
Mercury in the water declined 46.5 percent during the same period.
“It appears that decline can be directly attributed to a decline in the amount of mercury deposition for airborne mercury pollution,” Mark Brigham, a Minnesota-based USGS mercury expert and chief author of the study, said Wednesday.
The study found that the amount of mercury in the air that falls into the lakes in rain and snow declined 32 percent between 1998 and 2012, Brigham said, according to data from regional precipitation monitoring sites.
It’s one of the first studies to document long-term trends of mercury in rainfall, lake water and Minnesota fish all at the same time.
Mercury is released when many things are heated or burned, such as coal in power plants and iron ore in taconite plants, as well as from natural sources such as volcanoes.
Airborne mercury can move around the globe with prevailing winds before falling to Earth, where it can be converted to toxic methyl mercury in wetlands and other sediments.
That toxic mercury can build up in fish and animals and people who eat fish, sometimes to dangerous levels that affect human health.
Many lakes and rivers in the region have advisories for people, especially women and children, to limit the amount and size of fish they eat because of mercury.
A recent study found that 10 percent of children born in the Lake Superior region have unsafe levels of mercury in their bloodstream.
Experts say airborne mercury can come both from local and faraway sources. But the new evidence appears to show that reducing local and regional mercury sources already has made a difference in reducing mercury deposition in northern Minnesota, even if distant mercury sources haven’t declined.
“That’s exactly one of the implications of this study. … There’s been a pretty clear decline in wet mercury deposition which correlates with efforts over the last 30 years in the U.S. and Canada to reduce mercury pollution,’’ Brigham said. “So it would seem that if China and other nations were taking the same steps, we’d see mercury decline even more.”
Small lakes in Voyageurs National Park were picked because of their remote location and pristine conditions, scientists said. There is almost no natural mercury in local rocks, and there is no point source of mercury such as a factory.
While two of four Voyageurs lakes studied showed substantial declines, another lake, Shoepack, showed almost no change in mercury levels. A fourth, Brown Lake, saw a major increase in mercury - more than 80 percent - both in fish and in the water itself.
Brigham said specific circumstances clearly are affecting those two lakes, noting no more mercury fell into them from the air and that no local source of mercury exists. Shoepack Lake saw major water fluctuations because of a beaver dam that blew out in a flood during the study period but then was rebuilt by the beavers. That high-low-high water cycle probably allowed more mercury to cycle out of the shoreline sediment and into the water, Brigham said.
In Brown Lake, mercury probably flowed in from upstream lakes that have higher mercury levels as water levels have increased in recent years. It’s believed that those upstream lakes pull more mercury out of the ecosystem because of an abundance of wetlands and a chemical reaction with high sulfate levels, he said.
“We think (Shoepack and Brown) were the exceptions due to specific circumstances and that the results we saw in (Ryan and Peary) were likely more indicative of what’s going on regionally,’’ Brigham said. “But it’s clear that, even when we see major declines in mercury deposition levels, it’s not always going to mean lower mercury in all lakes. … They’re going to react differently.”
The USGS and the University of Wisconsin-La Crosse collaborated with the National Park Service and the National Atmospheric Deposition Program on the report.
Mark Sandheinrich, an aquatic toxicologist at UW-La Crosse, said 1-year-old yellow perch accurately reflect the amount of recent mercury in the food chain because they haven’t been around long enough to “bioaccumulate’’ mercury.
“We expected the water and fish mercury levels to correlate pretty closely with these perch,’’ he said. “With fish and mercury, you are what you eat and then some.”
If older, larger fish were studied, they probably would show higher concentrations of mercury that built up over time. But, eventually, if less mercury is falling into the lakes, even big fish should start to show lower mercury levels, making them safer for people to eat.
“The mercury goes from the water to the algae to the zoo plankton to the little perch, and then on to the game fish,’’ Sandheinrich said. “It’s going to take longer to show up, but if we continue with the decline in atmospheric deposition, we’re going to see less mercury in the fish that eagles and people eat.
“What we’re seeing,’’ he added, “shows us that (mercury pollution) regulations do make a difference.”
