Fish health improves when mercury release into lakes is interrupted

Mercury contamination in freshwater fish populations drops rapidly once new sources of the toxic chemical are cut off, according to new research.

Paul Blanchfield, lead author of the study published in the journal Nature, said the finding that lakes can recover quickly from mercury contamination is good news.

“I think it’s very good news,” said Blanchfield, a federal government aquatic ecologist. “The response to the reductions was very fast in fish stocks.”

Mercury is a powerful neurotoxin that is often released into the atmosphere by burning coal. Once it enters a lake and changes into a form that organisms can absorb, it accumulates in the tissues of fish and other animals.

As less coal is burned, Blanchfield and his colleagues wanted to know if that would affect the fish. Would the amounts of mercury in the ecosystem work to keep fish levels high or would the lack of fresh inputs reduce the population’s pollutant load?

It seems like a simple question. But it took me 15 years to answer it.

“Human activities have increased the amount of mercury reaching lakes over the years, so there is a lot of it stored in the lakes,” Blanchfield said. “It all comes down to the question of whether the new mercury or the old mercury is important.”

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The researchers used one of the watersheds in the Experimental Lakes Area, a unique series of Ontario lakes that have been used for decades in comprehensive real-world ecosystem studies.

Over seven years, from 2000 to 2007, they added carefully calibrated doses of mercury to the lake and the surrounding wetlands and highlands. Each type of environment has a different isotope of mercury, so the scientists were able to track where they all ended up.

Finally, mercury levels in the lake rose 60 percent, almost entirely from the mercury added directly to the lake. The levels of mercury added to the lake in insects and small fish increased between 45 and 57 percent and in large fish such as northern pike by 40 percent.

Then the team stopped adding mercury.

Not only did the levels in the lake drop, the fish stopped accumulating mercury in their tissues. In eight years, mercury concentrations in lakes decreased by 76% in the northern pike population and by 38% in the lake whitefish population.

The older fish still had high levels of mercury, but the levels in the younger fish were getting lower and lower, reducing overall concentrations.

“There was a possibility that that mercury that we had added to the food web for seven years could have continued to contribute for quite some time as well,” Blanchfield said. “But we saw that it went down very quickly, especially in the lower food chain.”

Where did the mercury go? Blanchfield suspects that it ended up in the lake’s sediments, morphed into forms that are not absorbed by plants or animals, and gradually buries itself away.

“It’s still all there. It’s less and less bioavailable all the time.”

Blanchfield said the study shows that environmental regulations can work to reduce pollutant loads, even for pollutants that have been around for many years.

“The positive message is that policies that reduce the amount of mercury entering lakes will be effectively effective,” he said.

“That’s a pretty clear demonstration from our study: These policies will work and they are effective.”

This Canadian Press report was first published on December 21, 2021.