Climate change, other pollutants boost mercury’s Great Lakes health threat


Randy Kolka, a researcher with the U.S. Forest Service Northern Research Station, holds a northern pike caught in Voyageurs National Park in northern Minnesota. Large fish have higher levels of toxic mercury as concentrations increase up the food chain. Image: Randy Kolka.

By Kayla Nelsen

Mercury levels in the Great Lakes are increasing as temperatures warm and extreme weather becomes more frequent, according to research in Minnesota’s Marcell Experimental Forest.

Mercury is a global pollutant released into the atmosphere through fossil fuel burning. It is then deposited onto land and water. Once it seeps into waterways, bacteria convert it into methylmercury – a form that is ingested by and accumulated in fish.

“The overall big picture is that as temperatures increase, we are seeing an increase in the amount of methylmercury concentrations in the water that is available to enter systems where people could be exposed to it,” said Caroline Pierce, a soil scientist and climate researcher at the University of Minnesota.

People are exposed to methylmercury when they eat these fish. The neurotoxin can cause loss of senses, lack of coordination and nervous system damage to infants in the womb, according to the U.S. Environmental Protection Agency.

“Fish that accumulate these methylmercury levels become toxic,” said Robert Haight, emeritus research forester for the U.S. Forest Service’s Northern Research Station. “There’s been quite a bit of research to determine appropriate levels of fish consumption. If those thresholds are exceeded, they cause potential injuries, especially in young people, pregnant women and nursing moms.”

Fish consumption advisories are issued at both federal and state levels. Children, pregnant and nursing women are advised to be especially more restrictive in their fish intake.

And climate change may increase people’s mercury exposure threat, Pierce said.

Since the 1950s, average temperatures in the Great Lakes region have increased by 2.6 degrees Fahrenheit, according to the National Centers for Environmental Information.

In soil and water, warmer temperatures increase the bacterial activity that produces methylmercury in Great Lakes ecosystems.

“What we tend to see is as seasons warm up, microbe activity becomes stronger or they multiply their populations and so you have more hungry microbes chewing up more mercury,” said Carl Mitchell, a professor in the department of physical environmental sciences at the University of Toronto.

Temperature-control chambers within Minnesota’s Marcell Experimental Forest allow researchers to test soil responses to different climate change scenarios. Researchers found that increased temperatures increased the amount of mercury that gets into the Great Lakes watershed. Image: Caroline Pierce.

In addition to warmer temperatures, other effects of climate change like forest fires and flooding can bring large amounts of mercury into different landscapes, Mitchell said.

There are two ways to intervene in mercury pollution: regulate polluters or advise the public, Haight said.

“The cleanup is expensive, especially for large water bodies, like the Great Lakes,” Haight said. “Putting out fishing advisories is the preferred alternate intervention. The hope is that warning people not to consume too much fish from polluted bodies would reduce the health costs associated with consuming mercury.”

The question is how communities might respond to stricter advisories.

Instead of changing their fish consumption, anglers tend to change the location they fish at, Haight said. He is studying the costs to anglers of forgoing their preferred fishing spots.

“If my first preference would be to fish in Lake Superior but the fish advisories are telling me that I can’t consume those fish, then I’m going to have to fish somewhere else,” Haight said.

Other pollutants like sulfur also exacerbate the production of methylmercury in soils.

“The thing that most helps prevent mercury increases associated with climate change is reducing other pollution happening at the same time,” said Keith Nislow, a U.S Forest Service Northern Research Station fish biologist.

Nislow and Haight are part of a U.S. Forest Service program that studies the effects of sulfur and mercury pollution on the Great Lakes watershed. Research began in 2021. The goal of the five-year project is to inform environmental policy, like fish consumption advisories.

“As we’re thinking about adapting to climate change, we’re going to be increasingly focused on not just whether species and systems can persist but also their ability to provide key ecosystem services, like unpolluted fish that people can eat,” Nislow said.

One of the project’s aims is to implement a tree planting method, called phytoremediation, on a wide scale within the Great Lakes, Nislow said. Along with mercury reduction, the technology would capture sulfur.

Starting in 2016, researchers at the U.S. Forest Service planted over 20,000 poplar and willow trees in Rhinelander, Wisconsin. The trees act as a buffer between atmospheric mercury and the Great Lakes watershed, capturing the chemical before it deposits into aquatic soils.

Atmospheric mercury deposits in soils and is converted to a toxic form of mercury. That mercury then runs off into lakes and rivers. Image: Caroline Pierce.

“The idea behind this is to grow plants that can tolerate really tough conditions,” Nislow said. “Those plants then take up whatever pollutants are at those sites and immobilize it in their tissues before it gets into the aquatic flow.”

Reducing sulfur pollution in the air reduces sulfur that gets into soils and water. This decreases mercury methylation in soils, which will decrease methylmercury in the lakes.

“I don’t know that we have many options, other than to stop climate change and to stop putting mercury in the environment,” Mitchell said. “And of course, weaning ourselves off of fossil fuels.

“Those are the tough answers, but those are the likely ones that need to be done.”

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