Yellow-billed loons that migrate to nesting sites on Alaska's Arctic coastal plain are bearing an undesirable burden that comes from thousands of miles away -- mercury unleashed by industrial pollution, according to a new study.

The study, published in a special issue of the journal Waterbirds that is devoted to loon research, describes mercury levels found in feathers, blood and eggs of yellow-billed loons, birds that spend their summers nesting on the Arctic coastline.

While only a few individuals had levels of mercury that were considered to be at or above the threshold that would cause ill effects to the birds, the overall trend is worrisome, the study concluded.

Mercury levels in the tested samples were, in general, twice those found in birds collected prior to 1920, the study said. Birds that migrate from Alaska to Asia -- the wintering grounds for most of the Alaska-nesting yellow-billed loons -- are more likely to encounter mercury, the study said.

"The farther west they go in winter, on average, the more exposure they get," said Joel Schmutz, a U.S. Geological Survey and co-author of the study.

The proliferation of coal-burning power plants in China, India and other parts of Asia, the atmospheric currents that carry mercury emissions north and the climate trends that are causing mercury to be set loose from thawing permafrost are combining to increase mercury loads in the far north, said lead author David Evers. When that mercury becomes methylated -- a process triggered by bacteria that is often found in wetlands like peat bogs -- it becomes available to the food chain, said Evers, executive director and chief scientist for the Maine-based Biodiversity Research Institute.

Levels of mercury, including dangerous methyl mercury, are rising in the Arctic and elsewhere, as shown by numerous studies of other birds, Evers said. Projections are for Arctic ecosystems, including the seabirds that migrate to the Arctic, to have increasing levels of mercury, he said.

Enter the yellow-billed loon, a large, long-lived bird that, by avian standards, is high on the food chain and eats predatory fish that eat other fish. With each step up the food chain, the concentrations of methyl mercury increase, Evers said.

"Loons are a good indicator of what's happening," he said.

An increasing risk

In some ways, the findings are disheartening, Evers said. "You would like to think in a remote area like this that these birds are living in, they would be pretty clean," he said. But the findings about yellow-billed loons, which are new because that species has not been as widely studied as others, fit those about mercury levels in other, more plentiful birds, he said.

The study analyzes samples from 115 birds and some eggs collected from 2002 to 2012. Most of the sampling was done on the Alaska North Slope. Samples were also taken on the Seward Peninsula of northwestern Alaska and in an area of Canada's Northwest Territories; both are nesting grounds for a relatively small portion of the total yellow-billed loon population.

Evidence of direct exposure to Asian mercury is found in the birds' feathers. Yellow-billed loons drop their old feathers after they migrate south, and new feathers grow in over the winter, Schmutz said. "Once the feather stops growing, it really doesn't acquire any new mercury to be detected," he said. "So that gives us a nice signal of what's happening in the winter."

Even if they never fly to Asia, yellow-billed loons and other birds in the far north are at risk of mercury exposure, thanks to atmospheric processes and permafrost changes, Schmuts and Evers said

For yellow-billed loons, the prospect of mercury contamination could be a factor in a pending decision about listing under the Endangered Species Act. There are only about 3,000 yellow-billed loons in Alaska, and the U.S. Fish and Wildlife Service is mulling a petition to list the population as endangered or threatened.

Mercury concerns go beyond the loons

The Arctic Council's Arctic Monitoring and Assessment Program, for example, has devoted some of its research efforts to mercury and its increasing levels found in the far north.

A study by scientists from NASA and Nevada's Desert Research Institute, published in January in the journal Nature, described what appears to be a new dynamic in which accelerated sea ice loss is creating conditions that pull mercury from high in the atmosphere down to levels near the surface.

The DRI scientists, along with scientists from the University of Colorado, have launched a related study at the University of Alaska Fairbanks-operated Toolik Field Station to measure mercury coming down from the atmosphere and streaming up from the snow.

A new international treaty, so far signed by about 100 nations but needing ratifications to go into force, seeks to reduce mercury releases by phasing out some industrial practices and imposing new emissions controls. The Minamata Convention on Mercury, named after the Japanese bay that was the site of a notorious mercury-poisoning epidemic, was drafted in 2013. The treaty offers some hope, said Evers, who has been a scientific adviser in the treaty process.

Other distance-traveling contaminants found

Mercury is not the only contaminant that reaches Arctic-nesting birds from faraway places.

Red-throated loons also appear vulnerable to Asian PCB pollution, according to a 2009 study by Schmutz and colleagues from the U.S. Fish and Wildlife Service. They found elevated levels of PCBs in the eggs of red-throated loons that nest on the Arctic Alaska coastal plain and winter in southeast Asia. Other Alaska red-throated loons that spend their winters in North America did not have the same elevated PCB levels, the study pointed out.

Yellow-billed loons, it turns out, did not show the same signs of elevated PCB exposure, Schmutz said. That is possibly because, during their winters in Asia, yellow-billed loons stay much farther offshore than red-throated loons, he said.

Tundra swans that nest on the North Slope but spend their winters on the U.S. East Coast and Midwest have higher lead levels than those that stay throughout the year in Alaska, according to a study published in April in the journal Ecotoxicology.

"The good thing is the lead levels aren't too high," said co-author Craig Ely, a USGS biologist.

In general, adult swans had higher lead levels than cygnets, indicating exposure in wintering grounds away from Alaska, the study found. The tundra swans with relatively higher lead levels were those that nest in the Colville River Delta area and spend their winters on the East Coast, generally in the mid-Atlantic region from North Carolina to Maryland.

Several years ago, there was enough lead pollution in that part of the East Coast to kill thousands of swans, Ely said. The Colville-breeding swans might be picking up residual lead there, or possibly in the Great Lakes region, which is on the migratory path, he said.