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Mercury pollution is a global problem. Emissions from gold mining, coal burning, and other industrial processes travel through the atmosphere, eventually falling to Earth as rain or snow. The poison can make its way to fish and the humans who eat them, where it can damage the developing nervous system, causing problems with memory and language in children exposed in the womb.
Now, a 15-year study of an experimental lake shows reducing mercury pollution that entered the ecosystem caused concentrations of the toxin in fish populations to drop substantially within only a few years. The results send “a clear message that reducing mercury has huge benefits for our aquatic ecosystems,” says Karen Kidd, an ecotoxicologist at McMaster University who was not involved with the research.
When mercury lands in wetlands and lake sediment, microbes change the metallic element into a dangerous compound called methylmercury that builds up in food webs. Concentrations are highest in larger, predatory fish. Public health agencies regularly test such fish in many lakes, sometimes leading to warnings to limit consumption.
Since the 1980s, regulations to control air pollution have gradually lowered emissions of mercury in North America and Europe, but sources elsewhere are still increasing, particularly small-scale gold mining in Latin America and coal burning power plants in Asia. In 2013, nations agreed to an international treaty, called the Minamata Convention on Mercury, that requires signers to ban mercury in products such as light bulbs and batteries, as well as reducing industrial emissions.
But how quickly do these measures have an effect? One hurdle to answering that question has been the complicated behavior of mercury in ecosystems, which makes it hard to figure out how much of a given decline in mercury concentrations in fish is due to reductions in air pollution rather than to factors such as excessive nutrients, invasive species, and other ecological changes.
To get a clear understanding, a large research project began an experiment in 2001 using a kind of chemical tracer: enriched stable isotopes of mercury. These forms of the element behave the same way chemically, but they can be distinguished from typical mercury in the environment. For 7 years, the researchers added one isotope of mercury to the water of Lake 658, part of a remote Canadian research station called the Experimental Lakes Area where 58 lakes and their watershed have been reserved for science. They also sprayed different isotopes from an airplane onto the surrounding wetland and upland to study how it moves into the lake.
Soon after the experiment began, isotopically labeled methylmercury began to accumulate in invertebrates living in the lake, such as zooplankton. It also rose in yellow perch and other small fish that eat the zooplankton, and increased by about 40% in larger fish such as pike, which eat smaller fish.
After the first 7 years of the experiment, the researchers stopped adding the isotopic mercury and continued to check the concentrations in the animals living in the lake. During the next 8 years of the study, concentrations of isotopic mercury dropped by up to 91% in the small fish. Concentrations also fell in populations of the larger fish, the team reports online today in Nature.
“It’s a very good news story,” says Paul Blanchfield, an ecologist with Fisheries & Oceans Canada, a principal investigator of the study. Only a small amount of the mercury that was added to the surrounding land ended up in fish, and these levels also fell quickly.
This kind of controlled experimental study is very rare in pollution studies, says Celia Chen, an aquatic ecologist at Dartmouth College who was not involved in the research. She says the work provides valuable information for gauging the likely impact of the Minamata convention.
The exact benefits to particular lakes will be difficult to predict, researchers say, because local conditions, such as the size of the surrounding watershed and rates of methylation, influence how much mercury ends up in fish. And even if all atmospheric emissions cease, some mercury—the legacy of past air pollution—will continue to enter lakes from the surrounding watershed. But Blanchfield says the rapid drop in concentrations of newly added mercury in fish populations is encouraging news.
The researchers working at Lake 658 plan to continue to measure mercury concentrations in fish and other parts of the food web. This may reveal more about how the mercury that was sprayed on the surrounding watershed eventually makes its way into the lake. They’re also thinking about new experiments, such as burning some of the forest around the lake or flooding an adjacent wetland, both of which could trigger the release of more isotopic mercury. The worry is that, despite the best efforts to reduce pollution from human activity, fires and floods linked to climate change may exacerbate mercury contamination.