Fish health improves when mercury releases into lakes cut off

Mercury contamination in freshwater fish populations falls quickly once new sources of the toxic chemical are cut off, says new research.

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

"I think it's a very good news story," said Blanchfield, an aquatic ecologist for the federal government. "Response to reductions was very quick in the fish populations."

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

As less coal gets burned, Blanchfield and his colleagues wanted to find out if that would affect fish. Would existing amounts of mercury in the ecosystem work to keep levels in fish high, or would the lack of fresh input reduce the population's contaminant load?

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

"Human activities have increased the amount of mercury coming into lakes for years, so there's a large amount that's stored in the lakes," Blanchfield said. "It comes down to the question of whether new mercury or old mercury is important."

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 real-world, whole-ecosystem studies.

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

Eventually, mercury levels in the lake were up 60 per cent, almost entirely from mercury added directly to the lake. Levels of the mercury added to the lake in insects and small fish increased between 45 and 57 per cent and in large fish such as northern pike by 40 per cent.

Then the team stopped adding mercury.

Not only did levels in the lake fall, fish stopped accumulating mercury in their tissues. Within eight years, lake mercury concentrations declined by 76 per cent in the northern pike population and by 38 per cent in the lake whitefish population.

Older fish still had high levels of mercury, but levels in younger fish were getting lower and lower, bringing overall concentrations down.

"There was the potential that that mercury that we'd added to the food web for seven years could also have continued to contribute for quite a while," Blanchfield said. "But we saw it reduced very quickly — especially in the lower food web."

Where did the mercury go? Blanchfield suspects it wound up in lake sediments, transformed into forms that aren't absorbed by plants or animals, and gradually getting buried.

"It's still all there. It's just getting less and less bioavailable all the time."

Blanchfield said the study shows that environmental regulations can work to reduce contaminant loads — even for pollutants that have been widespread for many years.

"The positive message in there is that policies that lower the amount of mercury coming into lakes will indeed be effective," he said.

"That's a pretty clear demonstration from our study — these policies will work and they are effective."

This report by The Canadian Press was first published Dec. 21, 2021.