Results from a new study published in Nature offer promising insights into how to minimize the exposure to and impact of mercury pollution on human health.

Led by government, NGO groups and academia, including Dr. Holger Hintelmann, dean of Science at Trent University, the Mercury Experiment to Assess Atmospheric Loading in Canada and the US (METAALICUS) monitored the fate of tagged mercury isotopes added to a lake and watershed system part of the Experimental Lakes Area located in a remote region in northwestern Ontario.

“We set out to find out how quickly mercury emissions that are entering a lake or watershed are picked up in fish, and how quickly reductions in mercury emissions would lead to reductions in mercury in fish,” said Professor Hintelmann, who was a principal investigator on the study looking at the fate of mercury in the water.

Following the fate of mercury leads to novel discovery

What METAALICUS found were dramatic reductions of methylmercury contamination in fish quickly following significant reductions in mercury emissions and mercury depositions to the environment.

“Huge amounts of mercury are released in power generation. Burning fossil fuels increases the atmospheric content of mercury, which then finds its way into lakes and, ultimately fish, all around the world,” said Prof. Hintelmann. “Fish consumption —and marine mammal consumption in northern communities—is the main form of exposure to mercury for humans. Methylmercury is the organic form that bioaccumulates in fish and animals, which is highly toxic to humans, and can affect brain function.”

Prof. Hintelmann and the research team spent up to two years monitoring baseline mercury levels in the controlled study lake. Then for seven years (between 2000-2007), researchers added a tagged mercury isotope dissolved in water to the lake annually, roughly doubling natural mercury levels in the lake. After 2007, the mercury “emissions” stopped, simulating a reduction in mercury pollution.

The team collected water, fish, plankton and other samples throughout the entire timeframe of the project—from baseline levels, through periods of high emission levels, through reductions in mercury pollution. They found that between five and ten years after the start of reduced mercury emissions, there was up to a 76 percent average reduction in mercury levels in fish, depending on the species of fish and its life span. Some forage fish, which have short life cycles, showed up to 90 percent reductions after 10 years.

A massive coordinated effort

The 20-year, multimillion-dollar ecosystem study was a one-time chance to do this level of investigation, so METAALICUS augmented the large watershed study with smaller studies in various areas of the environment. The project is developing one of the most comprehensive pictures of how mercury contamination builds up and then fades from a natural environment, and how mercury is transformed into methylmercury, the organic form that can be consumed through seafood and is toxic to humans.

Tagged mercury was dissolved in water which was applied annually to the lake by boat, and to the surrounding forests and wetlands via a crop duster plane. It was also sprayed on the lake’s shorelines with firehoses.

“Once the mercury was in, the clock was ticking,” said Prof. Hintelmann. “We collected everything we could get our hands on: samples of water, sediment, soil, leaves, vegetation, fish, plankton, invertebrates, insects–essentially everything that is part of the mercury cycle we tried to cover and to find out exactly where the mercury went, how quickly it got there, and the pathway from the atmosphere and into the fish.”

The project was a huge logistical undertaking, with the research team comprising up to 60 scientists from the Experimental Lakes Area, which is now managed by the International Institute for Sustainable Development, the Department of Fisheries and Oceans, United States Geological Survey, and many universities, including post-docs and graduate students from Trent University's Environmental & Life Sciences program.

The Nature publication covers 15 years of the research, from the first experiments in 2000 to the recovery of mercury up until 2015. The research continues with mercury levels being monitored until the added mercury is no longer detectable. This is Professor Hintelmann’s second publication in Nature.