Mercury is known to cause serious health problems, especially for developing fetuses and young children. Most human exposure to mercury comes from eating fish and shellfish contaminated by industrial air pollution. But until now little was known about how quickly fish would respond to changes in mercury inputs from the atmosphere. A joint U.S.-Canadian study suggests that reducing air pollution will make a difference.

Coal-fired electric power plants are the largest source of mercury in U.S. air pollution, emitting about 40 percent of the toxin. Other major sources include industrial boilers, hazardous waste incineration, and chlorine production.

Some of this airborne mercury gets into lakes and other bodies of water, and makes its way up the food chain. Mercury concentrations increase as they move up the chain, so the top predators (fish that eat other fish) have the highest levels of contamination.

Most human exposure to mercury comes from eating this contaminated fish. But is that mercury from current air pollution, or from soil and sediment, where mercury has been deposited for more than a century, since the start of the industrial revolution?

Cynthia Gilmour is a microbial ecologist at the Smithsonian Environmental Research Center in Maryland and part of a team of about 50 researchers from the U.S. and Canada who have been studying how freshwater fish respond to changes in mercury deposition, the mercury that gets into freshwater from air pollution.

She says identifying whether the mercury in fish comes from the air or from the soil is important, if we want to predict how effective pollution control measures will be at reducing mercury levels in fish. "If this year's mercury and last year's mercury is mostly what's in the fish, then if we change deposition levels, we can reduce mercury levels in fish." But, she continues, "if this long history of mercury contamination is contributing to mercury in fish, then reducing mercury deposition now isn't going to have a whole lot of impact for a long time, because we're making a small change in this huge amount of mercury that's already in the system."

For their study, Gilmour says researchers added mercury to an entire lake and its surrounding watershed. Using an agricultural spray plane, they aerially applied the mercury to the forest around the lake and to an adjacent wetland, "just like you would spray a crop." Gilmour says that they were careful to make the applications during light rainstorms, in order to mimic the way mercury in air pollution is actually deposited.

The researchers also added mercury directly to the lake itself. But they needed a way to distinguish what they were adding, from the mercury that was already there. Mercury has seven natural isotopes, forms of the element that have different atomic weights, but that are otherwise chemically similar.

By using mercury enriched in a particular isotope, Gilmour explains, she and her colleagues were able to trace the "chemical signature" of the mercury they added to the lake, and distinguish it from mercury that was already there, prior to the experiment.

The researchers found that the mercury they added directly to the lake made its way up the food chain and into the fish within just a few years.

"That's the good news," says Gilmour, because it shows that changing the amount of mercury that is deposited on the water also changes the amount of mercury being taken up by the fish. "That means that if we can really reduce mercury deposition to ecosystems," Gilmour explains, "we can reduce mercury levels in fish, we can reduce risk to people, and we can reduce risk to the wildlife that eats fish."

The "bad news," as Gilmour puts it, is that in the seven years since the study began, the mercury added to the area around the lake never showed up in the fish at all ? it stayed trapped in the trees and shrubs, where it landed. That finding suggests that historical mercury contamination in soils and vegetation will be around for many years to come, and could eventually work its way up the food chain and into the fish.

Gilmour concludes that mercury levels in fish will exhibit a two-phase response: "a partial fast response, and a much longer complete response."

In 2005, the U.S. Environmental Protection Agency passed the Clean Air Mercury Rule, limiting mercury emissions from coal-fired power plants. If Gilmour and her colleagues are correct, less mercury in the air should mean less mercury in fish, at least in freshwater systems. Whether the same would be true for swordfish, sharks, and other marine fish is difficult to predict.

This year was the last that mercury was added to the experimental lake and its watershed. The next step in the research is to watch what happens as the lake recovers, and to see whether mercury concentrations in fish really do decline, as expected.