Cutting mercury inputs to lakes quickly reduces mercury in the fish we eat, say scientists

Two Maryland scientists studied methylmercury production as part of the Mercury Experiment to Assess Atmospheric Loading in Canada and the United States (METAALICUS) study. Methylmercury is a neurotoxin that is produced from mercury pollution by natural microbes in the environment. Methylmercury is also the form of mercury that accumulates in food web; its production is critical step in evaluating risk. 

Notably, the fish populations were able to recover from mercury much quicker than previously understood, which suggests that curbing mercury pollution through policy initiatives will have a rapid and tangible benefit regarding the quality of fish we consume. These findings support the benefit of existing and new policies around that globe that seek to curb how much mercury ends up in our environment.

University of Maryland Center for Environmental Science geochemist Andrew Heyes has been studying mercury in the environment for 30 years with an emphasis on its movement within and through watersheds. “The study’s use of enriched mercury stable isotopes allowed us to accurately trace our additions and obtain a clear answer, which is often difficult to achieve at the ecosystem level studies,” said Heyes.    

Senior scientist Cynthia Gilmour’s team at the Smithsonian Environmental Research Center found that methylmercury production in the lake responded rapidly to increases lake methylmercury load, which led to the observed increased in fish methylmercury. “Fortunately, new methylmercury production also slowed down quickly once the study team stopped adding extra mercury to the lake,” said Gilmour.

Mercury is a serious threat to human health throughout the world. Many people, particularly pregnant women and their fetuses, and young children, are highly susceptible to the neurological effects. Important sources of mercury in the United States have been power plants and waste incinerators.

A study of air quality from the University of Maryland Center for Environmental Science in 2017 found that levels of mercury in the air from coal-fired power plant emissions in western Maryland dropped more than half over a 10-year period, with stricter pollution controls. The State of Maryland has aggressively moved to reduce internal mercury emissions.

“If we can continue to reduce emissions, mercury concentrations in fish of Maryland waters are expected to decrease, although the rate of response is not known because there is a time lag between emission reductions and reductions in mercury deposition,” said Heyes.

The study, “Experimental evidence for recovery of mercury-contaminated fish populations,” is published in Nature.

The team applied about one teaspoon of a special form of mercury to a lake and its watershed, at a cost of over one million CAD. They were able to measure this mercury as methylmercury in the ecosystem and to track its rapid decline in fish once they stopped adding it to the environment. Methylmercury is a much more toxic form of mercury that accumulates to high concentrations in many freshwater fishes leading to many adverse, and even life-threatening, symptoms in humans. In the U.S., most fish tissue consumption advisories for “recreationally-caught fish” are due to the presence of mercury contamination.

For the first seven years, the researchers supplied specific isotopes of mercury (to directly monitor the added mercury) to a remote, undisturbed lake and its watershed in Canada. During this period, the authors recorded an increase in the uptake of this isotopically labelled mercury as methylmercury in the fish within the aquatic ecosystem. Methylmercury concentrations increased by 45–57% in invertebrates (plankton, for example) and small fish (such as yellow perch), and by more than 40% in large fish, such as pike and whitefish. Thereafter, mercury additions ceased and the effects on the food chain were observed for eight years. Labelled methylmercury quickly diminished in the smaller fish, with concentrations dropping by at least 85% by the end of the study period. This triggered a subsequent reduction of labelled methylmercury in the larger fish, with concentrations declining by 76% in pike and 38% in whitefish

As new mercury inputs to the experimental lake were increased and then decreased in a controlled manner, the methylmercury in the lake water, surface sediments, invertebrates and fish both increased and decreased quickly. This was true whether the mercury ‘rained’ directly onto the lake surface or entered the lake from the surrounding watershed in streams.

The study was carried out at IISD Experimental Lakes Area (IISD-ELA) in Ontario, Canada, which is one of the only facilities in the world where lakes and their watersheds can be experimentally manipulated to determine the many ways in which humans are impacting lakes.

“Whole-ecosystem experiments are incredibly powerful because they examine the effects of a single factor at a time and provide solutions to globally-important issues in a real-world setting,” said Dr. Carol Kelly, who has spent decades conducting research on the experimental lakes. 

Part of that real-world setting was working with natural fish populations. “Studying fish only in laboratories was not revealing the full story,” said Lee Hrenchuk, a biologist with IISD-ELA. “Individual fish retain mercury they have previously accumulated for a long time, and so it could be assumed that decreasing mercury input to a lake might not be very beneficial. However, we discovered that the hatching of new fish into a lower mercury environment was sufficient to lower the mercury level of the population as a whole in a short period of time.”