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Reducing Mercury Entering Lakes Could Reduce Mercury in the Fish We Eat

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This is according to a new paper, published today in Nature. During the study, conducted over 15 years, scientists intentionally added a traceable form of mercury to a lake and its watershed.  They discovered that the new mercury they added quickly built up in fish populations, and then declined almost as quickly once they stopped additions.

Notably, the fish populations were able to recover from mercury much quicker than previously understood, which suggests that curbing mercury pollution through policy initiatives now will have a rapid and tangible benefit regarding the quality of fish we consume.

The findings provide indisputable, science-based support for necessary regulations on mercury emissions that have been undermined in recent years, especially in the USA. They also support the efficacy of existing and new policies around that globe that seek to curb how much mercury ends up in our environment.

“Showing that reducing mercury inputs to a lake will lower mercury concentrations in fish sounds simple,” said Dr. Paul Blanchfield of Fisheries and Oceans Canada and Queens University and a lead investigator of the Mercury Experiment to Assess Atmospheric Loading in Canada and the United States (METAALICUS). 

“But it required a dedicated team effort, including academic, government and NGO researchers from across North America, during the 15-year whole-ecosystem study to arrive at this conclusion.”

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.

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".

 “The near-term value of reducing mercury inputs to freshwater lakes was not a sure thing, because large masses of old mercury always exist in lakes from decades past,” said Mr. Reed Harris, of Reed Harris Environmental, one of the founders of the study. 

“So, it was critical for the experiment that isotopic form of mercury we added could be distinguished from older mercury in the ecosystem.” 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. 

 “While mercury exported to lakes from their watersheds may not decline exactly in step with lowering atmospheric deposition rates, this experiment clearly demonstrates that any reduction in the amount of mercury entering lakes will have immediate benefits to fish consumers,” said Dr. John Rudd, former Chief Scientist at the Experimental Lakes Area and a principal investigator on the study. 

“Fish is a high-quality protein that is beneficial to many people, providing that it is low in methylmercury.”

Reference: Blanchfield PJ, Rudd JWM, Hrenchuk LE, et al. Experimental evidence for recovery of mercury-contaminated fish populations. Nature. 2021:1-5. doi: 10.1038/s41586-021-04222-7

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