Mercury pollution: fish recover quickly when water is no longer contaminated

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Mercury pollution is a major global environmental problem, with small-scale gold mining and charcoal burning being the two main sources, but fish can recover quickly when the pollution stops.

Environment


December 15, 2021


Northern pike recover quickly when mercury pollution stops

Imagebroker / Alamy

Fish populations appear to recover quickly from mercury pollution once humans stop adding it to their environment.

A 15-year study of a lake in Canada found that eight years after the metal supply ended, concentrations of methylmercury – a highly toxic substance made from mercury by bacteria in aquatic ecosystems decreased 76 percent in northern pike (Esox lucius) and 38 percent in lake whitefish (Coregonus clupeaformis).

“I can’t imagine a much faster recovery,” says Paul Blancfield to the government agency Fisheries and Oceans Canada, which led the research. The team is not suggesting that fish excrete mercury quickly – experience actually shows that they cling to it for a long time – but that rapid generational turnover sees concentrations drop rapidly when new pollution stops.

Mercury pollution remains a major global environmental problem, with small-scale gold mining and coal combustion being the two main sources. Carried into the atmosphere and dumped into lakes and oceans, metal build-up in freshwater and marine species has raised concerns about the impact on human health of consuming fish.

Yet, not much is known about observations about how quickly mercury levels decline after pollution is stopped. To find out, the team conducted a study in the Experimental Lakes Region in Ontario, Canada, a remote set of lakes reserved for science. Researchers added mercury to a lake there for seven years, on par with the average amounts found in mercury-polluted rivers in North America. They used mercury with a distinctive isotope so that the pollutant could be distinguished from anything that fell in the rain.

After the team stopped adding mercury, the lake’s main predator, the pike, had the greatest amount of methylmercury. But concentrations in the species have fallen about twice as fast as in the other large, tall species, whitefish.. The faster recovery appears to be due to a faster return of new fish among pike, which, on average, were much younger than whitefish.

The rapid drops in mercury are good news for communities dependent on the sale of fish and countries that consume a lot of fish, Blanchfield says. But he warns that only a “tiny fraction” of the mercury that the team also added to the forest and wetlands around the lake got into the water and this could reach it later, underscoring the persistent nature of the pollutant. .

John munthe at the Swedish Institute for Environmental Research IVL, said the study was a “unique scientific experiment that will probably never be repeated”. He says the results are solid and demonstrate “what we have always assumed, but only partially or indirectly able to provide evidence.” They provide a strong argument to spur efforts to reduce mercury emissions, he adds.

Meanwhile, new research shows that mercury accumulates at “remarkably” high rates in the hadal zone, the deepest part of the ocean that stretches downward. up to 11 kilometers. The paper, published in the journal PNAS, shows that the average accumulation rate in submarine trench sediments since 1950 is 30% higher than it was between 1900 and 1950.

Journal reference: Nature, DOI: 10.1038 / s41586-021-04222-7

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