Climate change slows Arctic methylmercury detox, new study shows

Global warming appears to be slowing down the reduction of methylmercury in Arctic waters, delaying the effects of a global movement to reduce industrial mercury emissions.

“Mercury pollution has gone down in the atmosphere. We’re doing things to tackle it, but climate change is throwing things for a loop,” says Igor Lehnherr, an assistant professor of geography at U of T Mississauga. “It can actually undo some of the benefits from emission reductions.”

In a recent paper in the journal Environmental Science & Technology Lehnherr reports on his team’s latest fieldwork, which assesses the build-up of methylmercury, a dangerous neurotoxin, in Lake Hazen, one of Canada’s northernmost lakes. The area where they collected samples is not close to any northern communities, but Lehnherr says their work is relevant for Indigenous people who hunt and fish for food.

“What we’re learning is not constrained to that location,” he says. “We put a lot of import on understanding mechanisms that affect methylmercury, so we can apply what we learn in one place somewhere else.”

Methylmercury is a “persistent organic pollutant,” which means it becomes more concentrated as it moves up through the food chain from bacteria to fish to predators and people. It affects the nervous system and can also cause cardiovascular damage. The toxin is especially dangerous for pregnant women, and for fetuses, babies, and young children whose nervous systems are still developing.

Methylmercury levels rise only indirectly from human activity. Burning fossil fuels, mining, and many industrial processes release unmethylated mercury into the atmosphere. As the mercury settles into aquatic ecosystems, certain types of microbes metabolize it to form the much more dangerous methylmercury.

Arctic methylmercury levels depend on a complex mix of factors, including industrial emissions, precipitation patterns, microbial numbers and activity, changes in seasonal sea ice, and other interrelated influences. The complexity, along with the remoteness of northern ecosystems, make Lehnherr’s work challenging.

“The field work spanned a few seasons,” he says of his latest study. “Some years we were there in the spring when it’s all snow and ice cover, some years in the summer, some years for both. By combining efforts with other teams, we expanded the sampling. Arctic research by nature is fairly collaborative — we share costs, time, and ideas.”

In general, methylmercury-producing microbes are more active in warmer environments, implying a direct correlation between global warming and increased toxicity. But climate change also has many other effects that can exacerbate, mitigate, and further complicate the situation.

“Temperature in the Arctic also controls permafrost thaw. It also affects the amount of precipitation by controlling cloud cover, sea ice cover, rates of evaporation and these kinds of things,” he says.

Changing weather patterns also affect how much methylmercury builds up in specific isolated areas, or how efficiently it flows from one lake to the next creating more widespread problems. In the short term, it looks as though reduced emissions have not fully translated into cleaner Arctic ecosystems. While that news is not great, Lehnherr says it should not be interpreted as a sign that reducing mercury isn’t worth it.

“I mostly think it validates the ongoing efforts to reduce anthropogenic mercury emissions,” he says. “Countries have shown this is something they’re willing to take on. These results allow us to have reasonable expectations on how long it will take for mercury levels to go down and stabilize.”

He also wants to reassure people in northern communities who may be concerned for the safety of their food supply.

“Whenever I talk about the risks of mercury and negative health impacts, I always stress that the benefits of consuming traditional foods vastly outweigh the risks of contaminants. Locally caught Arctic char has better nutritional value than dried goods and flown-in goods,” he says.

Lehnherr plans to continue his study of methylmercury in the Arctic region to get a better sense of the long-term impacts of climate change.