Global warming boosts toxic mercury in lakes and oceans: study

Levels of toxic mercury could increase up to seven-fold in plankton as a result of climate change, a new study warns.

Sofi Jonsson at Umeå University in Sweden and colleagues calculated levels of methylmercury could increase significantly in lakes and coastal areas as global warming increases runoff of organic matter, such as leaves and soil.

Already, they write in their paper published in Science Advances, “the input of mercury (Hg) to ecosystems is estimated to have increased two- to five-fold during the industrial era”. 

Mercury is considered one of the top 10 chemicals of public health concern by the World Health Organisation. A mercury compound called methylmercury is a strong neurotoxin that accumulates in food webs.

And a danger lies in the increase of runoff of natural organic matter – such as leaves, dirt and soil – into coastal waters. “Climate change has been projected to induce 10 to 50% runoff increases for large coastal regions globally,” the researchers write. 

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To find out what effect this runoff could have on ecosystems, the researchers made miniature estuaries and dosed them up with increased levels of nutrients and organic matter.

They found the methylmercury levels in species of zooplankton increased between two and seven times when subjected to a 20 to 30% increase in runoff, an increase predicted for some areas. 

The increase in methylmercury levels is partly due to a shift in the aquatic food web. Leaves and soil block sunlight from entering the water. This encourages more bacterial growth and stifles phytoplankton.

Eventually, this causes a shift in the ecosystem’s food web from autotrophic – which can feed using energy from, for instance, sunlight – to heterotrophic, which rely on organic materials for food. 

In a heterotrophic environment, higher organism diversity leads to higher levels of methylmercury concentration. And in the models, an increase in organic matter doubled mercury retention in the water column, which further increased its exposure to sea-life.

“The study has revealed a phenomenon that has not been described before,” says lead researcher Erik Björn from Umeå University. 

“The results are critical in the prediction of how global climate changes can affect the exposure of methylmercury to ecosystems and humans.” 

Although the reality will be more complex than the models suggest, the researchers urge scientists to consider methylmercury levels when modelling the impact climate change could have on marine ecosystems.