Western land, wildlife contaminated with mercury

An extensive analysis led by the U.S. Geological Survey has found widespread mercury contamination in western habitats and wildlife. Mercury levels varied from place to place, fluctuating with factors such as hydrology and climate. Moreover, it turned out that levels of inorganic mercury — the relatively benign form of mercury common in air and soil — was a poor indicator of the threat to people and animals. Areas with low inorganic mercury tended to have high levels of methylmercury, the toxic form of the element that insinuates itself into food webs.

“One of the most surprising and compelling patterns that we found was a very large disconnect between the distribution of inorganic mercury in the environment in comparison to the distribution of methylmercury in biological communities,” said Collin Eagles-Smith, a research ecologist with the USGS and an author of several studies published in a special issue of Science of the Total Environment. “Where you have high inorganic mercury does not necessarily equate to elevated risk to fish, wildlife and humans.”

Most of Earth’s mercury is in inorganic forms. These compounds can be released into the environment through natural processes such as volcanic eruptions, as well as by human actions such as mining, agriculture and coal burning. The released mercury travels through the landscape in air and water, and when it collects in water bodies such as wetlands, bacteria convert some it into methylmercury.

Past studies in eastern North America have assessed landscape-scale patterns of contamination with both types of mercury. To get a similar big-picture look at the West, researchers gathered long-term monitoring data from multiple government and academic databases. The data included mercury levels in soil, water, plants, fish and birds.

Methylmercury contamination was common in wildlife, with eight percent of bird samples showing levels high enough to pose major health risks. Birds tended to be most contaminated in ocean and salt marsh habitats, with the highest mercury levels found in species that eat fish. The most contaminated bird species of all was the Forster’s tern (Sterna forsteri).

While many animal samples had mercury levels known to cause health problems in some species, the actual danger to wildlife populations is hard to pin down, says Eagles-Smith. Different species vary widely in their sensitivity, and more research is needed to understand the impacts of mercury exposure on each species.

“We looked at mercury concentrations in a lot of taxa, and we can get an understanding of their exposure,” said Eagles-Smith. “The really difficult next step is identifying risk.”

In general, fish and birds tended to have high levels of methylmercury in arid habitats, while in wet forest ecosystems, methylmercury was relatively scarce. This pattern was reversed for inorganic mercury. The researchers found that rain deposited large amounts of inorganic mercury on northwestern forests, where it was absorbed by the soil.

Currently, most efforts to curb mercury pollution focus on inorganic mercury, with strategies such as cleaning up mine tailings and developing cleaner-burning fossil fuel technologies. These efforts are valuable, since all methylmercury ultimately comes from inorganic forms of the element, says Eagles-Smith. But the disconnect between organic and methylmercury at the local scale suggests that targeting inorganic mercury may not be the best strategy everywhere. To reduce mercury exposure in living things, says Eagles-Smith, we should also try to prevent methylmercury from forming and spreading.

“Addressing the mercury at the source is important, but it’s not going to completely solve the problem,” he said. “Once the inorganic mercury enters a water body, it can be methylated. And once it’s methylated, then it enters the food web.”