But a team of scientists from the U.S., Russia, and Canada has discovered that assumption is wrong in much of the continental Arctic.
In addition to differences in mercury processes as a result of diverse atmospheric, geological, and biological conditions, "It turns out that the economic decline of the former Soviet Union, which collapsed in 1991, appears to have been good for the Arctic environment in that part of the world," said Leandro Castello, an assistant professor of fish and wildlife conservation in the College of Natural Resources and Environment at Virginia Tech.
Atmospheric mercury comes largely from mining and ore processing, such as smeltering, according to a United Nation's environmental program study. Under certain water conditions, mercury is converted to a special form that can be absorbed by living organisms, through a process called methylation.
"Methylmercury is highly toxic," said Castello.
But the research team determined that burbot fish in two Russian rivers, the Lena and the Mezen, are safe to eat.
The fish from these rivers were compared to burbot from 20 locations along the Pasvik River on the Norwegian-Russian border and along the Mackenzie River in Canada, where decades of studies have found high levels of mercury that make the fish unsafe.
"The burbot fish was chosen because they are top predators that integrate many bio-geo-chemical processes in the river watersheds," said Castello. "The fish were collected downstream of the watersheds, so that they would present everything that happened upstream."
Sampling was done using an ice-fishing method in the peak burbot season, November and December, by co-author Alexander V. Zhulidov of the South Russian Centre for Preparation and Implementation of International Projects.
"We developed and led an initiative of biological monitoring of the water quality of major rivers of Russia in 1980 and continued to do it until 2001, because we knew it could provide useful information one day. In 2002 the funding was cut and the program was closed. Unfortunately we have no funding to continue collecting such interesting data," said Zhulidov.
"Good news since the Lena River is one of the largest watersheds in the world," said Castello.
Why the differences? The researchers admit in their paper, "There are no ancillary environmental data from the time period of the study in Russia," but they suggest the differences across the Arctic "may be explained by differences in water quality, geological bedrock formations, and proximity to polluting sources."
Until the 1970s, atmospheric mercury were on the rise as a result of industry in Europe and in North America, but began to decline from those sources due to emission controls, with Asia coming on line as a source, the paper explains.
In Russia, metallurgic industries in Murmansk region and smelter companies in the Pasvik watershed explain high levels of atmospheric mercury in the Pasvik River. The economic decline near the watersheds of the Lena and Mezen lowered polluting activity there.
A confounding factor has been Climate-Change'>climate change, said Robert Spencer, an associate scientist at the Woods Hole Research Center. In burbot in the Canadian Arctic, mercury concentrations in fish tissue have increased despite declining atmospheric concentrations because rising temperatures appear to increase availability of mercury to fish populations.
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