Mercury's Harmful Effects

This dangerous pollutant is accumulating in more habitats and wildlife than previously thought

  • Paul Tolmé
  • Conservation
  • Nov 07, 2012
LOCATED IN THE MIDDLE OF SCENIC LAKE GEORGE in upstate New York, tiny Dome Island would seem an unlikely location for mercury contamination. The 16-acre preserve is a conservation gem of the Adirondack region, where songbirds flitter among old-growth cedars, hemlocks, beech, hickory and oak.

“Dome is one of the last untouched islands on Lake George,” says Henry Caldwell, chairman of the Dome Island Committee, which oversees The Nature Conservancy preserve. “It has never been inhabited, and there is no indication it has ever been logged.”

In 2006, however, researchers discovered that some of the island’s songbirds—including red-eyed vireos, black-capped chickadees and song sparrows—have among the highest mercury levels of any upland forest songbirds in the Northeast. That startling news was followed in 2011 by research showing the island’s spiders also had elevated mercury levels. “We were very surprised,” says Caldwell, whose family has lived in the area for generations.

Dome Island highlights a continuing and growing threat to wildlife: airborne mercury pollution. Coal-fired power plants are the leading sources of mercury in the United States, belching more than 50 tons of the neurotoxin annually. Much of the mercury falls near the source, but some flows into the atmosphere and drifts long distances, falling back to Earth in rain, snow and as dry particulates. This problem of “atmospheric deposition” of mercury is a global dilemma but is particularly acute in the Northeast, which lies downwind of the coal-burning epicenters of Ohio, Pennsylvania, Indiana and Illinois.

Now, as the United States prepares to implement long-overdue regulations on coal-fired power plants, a flurry of new science shows that atmospheric deposition of mercury is affecting more lands and waters, and harming more wildlife, than previously known. “It has become clear in recent years that no corner of the food web is untouched by mercury,” says Joe Mendelson, NWF’s director of climate and energy policy.

Mercury: A Hidden Risk to Wildlife

Mercury, which can adversely alter the neurological and reproductive systems of humans and wildlife, has long been known to contaminate fresh-water lakes, fish and fish-eating birds, including loons and eagles. A recent study by New York State scientists and other researchers in the Adirondack region reiterated this problem by showing that common loons (below) with elevated mercury levels produced significantly fewer chicks. Until recently, however, terrestrial species that do not eat fish were thought to be safe. “Mercury used to be considered just a loon and lakes problem,” says Tim Tear, director of science for The Nature Conservancy in New York.

In 2005, that perception changed when researchers documented mercury in Bicknell’s thrushes, terrestrial birds that inhabit mountaintops in the Northeast. The groundbreaking discovery jump-started inquiry into mercury levels in other songbird species. The resulting data are compiled in Hidden Risk: Mercury in Terrestrial Ecosystems of the Northeast, a report published in 2011 by the Biodiversity Research Institute in Maine and The Nature Conservancy. Combining data from 2,680 animals across 92 species collected in 11 states, from Virginia to Maine, the report sheds new light on the threat of atmospheric mercury deposition.

“We now have a much better grasp on which species are in trouble,” says David Evers, executive director and chief scientist for the Biodiversity Research Institute, a leading research center for mercury in wildlife. Wetland-obligate birds in mercury hot spots are highly prone to contamination, particularly in the Northeast. That’s because water, especially water acidified by air pollution, speeds up the process through which bacteria convert inorganic mercury from coal into its organic, poisonous cousin, methylmercury.

Wetlands, bogs, beaver ponds, coastal marshes, even foggy mountaintops and forest floors are all incubators for methylmercury. Insects and invertebrates in these moist habitats feed on plant debris containing the substance, allowing it to enter the terrestrial food web. Larger insects and spiders eat the smaller insects, and songbirds eat the toxic insects and spiders. At each step up the food chain, the mercury becomes more concentrated, a process known as biomagnification.

Saltmarsh sparrows and rusty blackbirds, species of high conservation concern, show the highest contamination levels, according to Hidden Risk. The sparrows breed in coastal wetlands, feeding on spiders, flies and amphipods. Researchers worry that methylmercury could hamper the birds’ ability to choose safe nesting sites.

Even trace amounts of methylmercury not previously considered dangerous can harm reproduction. Carolina wrens with 1.2 parts per million of methylmercury in their blood showed a reduced nesting success of 20 percent, according to a 2011 study along two mercury-contaminated rivers in Virginia. The wrens are considered “gleaners” that scavenge the forest floor for spiders, which have some of the invertebrate world’s highest mercury levels.

Researchers believe songbirds with feeding habits similar to Carolina wrens will have comparable rates of reproductive failure. “We are using Carolina wrens as surrogates,” Evers says. In addition to the saltmarsh sparrow, rusty blackbird and Bicknell’s thrush, examples of other northeastern songbirds with mercury levels above the 0.7 parts per million threshold (the level at which reproduction is reduced by 10 percent) include the seaside sparrow, red-winged blackbird, eastern wood peewee, palm warbler, indigo bunting and Nelson’s sparrow.

Songbirds aren’t alone in facing the threat of mercury poisoning. Bats, already stressed by white-nose syndrome and facing population declines, consume vast quantities of insects during their long lifespans, allowing them to accumulate lots of mercury. The big brown bat, little brown bat, northern long-eared bat, endangered Indiana bat (right) and eastern small-footed bat showed mercury levels that could affect reproduction, according to Hidden Risk.

Based on these findings, scientists now have a better understanding of which localities and ecosystems have mercury buildups. “We are using wildlife as bioindicators to determine the potential effects of mercury in different habitats,” Evers says.

Back on Dome Island, researchers are doing follow-up testing on crayfish and other species around Lake George. No one can say for sure where the island’s mercury originated, but Tear and others say atmospheric deposition is the prime suspect. Measuring such deposition is expensive, but conservationists hope the new findings will help push forward a comprehensive national mercury policy. The United States has slashed mercury emissions from municipal and medical waste incinerators by more than 95 percent during the past two decades. But coal power plants, due to the political power of the coal industry, have avoided direct mercury regulations. That is likely to change.

New Rules for Coal Plants

Enacted by the U.S. Environmental Protection Agency in late 2011, the Mercury and Air Toxics Standards Rule requires power plants to reduce their mercury output by 90 percent through 2016. “This is the biggest step the country has ever taken to control our mercury problems,” Mendelson says. “We are finally bringing our power plants into the modern age.”

As this article went to press, opponents were trying to stall the new regulations. “Critics will say these regulations will kill jobs and harm the economy,” Mendelson says. “But history shows that just the opposite is true.”

“Regulations work,” adds Rutgers professor Joanna Burger. Since 1971, Burger has been studying heavy metal contamination in common terns in Barneget Bay, New Jersey. Levels of lead and cadmium have declined dramatically in the birds due to federal rules on those toxics, but “mercury levels in the birds have remained the same.”

U.S. regulations alone, however, will not solve the problem. Coal is the cheap fuel of choice for much of the developing world. China is erecting new coal power plants at a rapid pace. Some of the mercury emitted in Asia rises into the atmosphere and feeds a growing global mercury cloud, depositing the neurotoxin in locations where it never was found in the past. Hoping to stem this problem, the United Nations Environment Programme will attempt to ratify an international mercury treaty in 2013.

Mercury, alas, is a coal problem. Burning it fuels a host of environmental ills, from climate change to acid rain, smog and habitat loss, at great cost to human health and wildlife. “The big question is: How do we transition from coal to a power system that utilizes cleaner, renewable sources and technologies?” Mendelson asks. “We have the technology. We need to implement it.” Only then will the Dome Island songbirds be safe.

NWF Priority: Controlling Mercury Pollution

Reducing the serious threats posed by mercury pollution to people and wildlife has been an NWF priority for more than two decades. Among its efforts, the Federation launched the “Clean the Rain” campaign 12 years ago in conjunction with local and state organizations, catapulting the problem into national consciousness and starting the process for placing controls on coal-burning power plants. Late last year, that process helped produce the nation’s first national limits on mercury levels released from those power plants.

“Progress on mercury reduction would not have occurred without the many NWF members who took action to help move this issue through some very stormy political waters and who mobilized last year against congressional attacks designed to stop the new rules,” says Jeremy Symons, NWF’s senior vice president for conservation and education.

Recently, the Federation also released a report examining the potential harm to communities and wildlife in the Pacific Northwest as a result of an industry plan to export coal to markets abroad using ports in the region. To learn more and read the full reports, visit 

Paul Tolmé is based in California.

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Related Resources

Mercury Pollution and Wildlife
Historic Limits on Mercury Emissions
3 Million Urge Action on Air Pollution
NWF at Work: Global Warming

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