Decreased Pollution Has Mixed Effects on Minnesota Lakes

Methylmercury contamination is decreasing in some lakes in northern Minnesota as a result of reduced mercury pollution, according to a U.S. Geological Survey study published today. 

Mercury from man-made pollution is converted in lakes and wetlands to methylmercury, a toxic form of mercury that accumulates in fish. USGS and University of Wisconsin – La Crosse scientists recently found that methylmercury levels in water and year-old yellow perch, a type of fish, in Ryan and Peary lakes of Voyageurs National Park decreased significantly from 2001 to 2012. However, methylmercury levels increased in Brown Lake water and yellow perch during this time, likely as a result of higher inflow from a contaminated lake upstream. No significant trend in methylmercury levels was observed in Shoepack Lake. 

The study also found that levels of mercury, sulfate and hydrogen ion in precipitation decreased from 1998 to 2012 in northern Minnesota. These decreases likely resulted from reduced atmospheric pollution in the U.S. and Canada, and may have contributed to the reduced methylmercury contamination in the park’s lakes.

“Not all northern Minnesota lakes appear to be responding similarly,” said Mark Brigham, USGS scientist and lead author of the study. “These mixed results demonstrate the complexity and variability of lake ecosystems, and can be used to guide future water resource management plans.” 

The lakes examined are small and remote with minimal human disturbance within Voyageurs National Park. Most of the mercury that enters these lakes comes from distant sources and is delivered by precipitation and settling of particles.

The regional decreases in wet deposition of mercury, sulfate and hydrogen ion pollutants are likely related to improved pollution controls on coal-fired utility boilers and other sources implemented in the 1980s, and to regulations that removed mercury from numerous consumer products such as batteries in 1990. 

“Although scientists expect methylmercury levels to decrease in response to decreased emissions of mercury and acid rain pollutants, not all lakes will respond in similar time scales,” Brigham said.

Methylmercury is rarely measured in lake-monitoring programs for fish consumption advisories. The new study documents long-time trends in methylmercury in water and in the young fish that inhabit the lakes. 

“In lakes where methylmercury in water decreased, methylmercury in fish also decreased,” Brigham said. “And in Brown Lake, where methylmercury in water increased, methylmercury in fish also increased.” 

Although fish-consumption advisories due to methylmercury contamination of game fish are prevalent in northern Minnesota, the current study focused on parts of the ecosystem that are not a direct concern for human health. 

The USGS and University of Wisconsin – La Crosse collaborated with the National Park Service and the National Atmospheric Deposition Program on the report. 

The new study is available online in the journal Environmental Science and Technology. 

For more information on water resources in Minnesota, please visit the USGS Minnesota Water Science Center website.