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Claim: Climate change causes ‘roller coaster’ mercury levels in Wisconsin fish


“The ice is frozen on top and then there’s kind of a slush layer in there and then you start into hard ice again,” Rubsam said.

After decades of water sampling on Little Rock Lake and two other nearby lakes, Watras has concluded that climate change is causing fluctuations in the level of dangerous methylmercury in the environment. The finding adds to the list of negative effects of climate change, which include making storms more powerful, increasing the Earth’s temperature and causing polar ice to melt, which causes sea levels to rise.

Watras has discovered that accumulations of the toxin in fish fluctuate as water cycles driven by climate change raise and lower lake levels. When the water levels go up, Watras found, levels of mercury can increase to unsafe levels in walleye, one of the state’s most prized game fish. When levels go down, concentrations in walleye go down to levels considered safe by the Environmental Protection Agency, or 0.3 micrograms per gram.

He called the findings “a complete surprise.”

“We had no idea that water levels in lakes … could affect the production of methylmercury,” he said.

This dynamic was previously seen in reservoirs, where levels tend to fluctuate. But Watras said this phenomenon, and its implication for fish, was not understood for lakes until his study. He added that there is little similar data that spans three decades, allowing a glimpse at longer-term climate-related trends.

The state DNR already recommends limiting intake of fish from most of the state’s 15,000 lakes because of mercury.