In December 2011, the Environmental Protection Agency released new standards sharply limiting future emissions of mercury and other toxic pollutants from coal- and oil-burning power plants in the United States. Earlier this year, the United Nations Environment Programme negotiated the Minamata Convention on Mercury, an international treaty aimed at curbing future mercury emissions; it is still unclear what level of mercury-emission reductions will result.
It has been known for some time that large predatory marine fish contain high levels of methylmercury in part because they eat lots of smaller, mercury-containing fish. The toxin builds up in the tissues of the top-of-the-food-chain predators through a process called bioaccumulation.
In 2009, researchers at the University of Hawaii determined that the depth at which a species of fish feeds is nearly as important as its position in the food chain in determining how much methylmercury it contains.
"We found that predatory fish that feed at deeper depths in the open ocean, like opah and swordfish, have higher mercury concentrations than those that feed in waters near the surface, like mahi-mahi and yellowfin tuna," said Brian Popp, a professor of geology and geophysics at the University of Hawaii at Manoa and co-author of both the 2009 paper and the new Nature Geoscience paper. "We knew this was true, but we didn't know why."
That observation was difficult to explain because researchers had presumed that if methylmercury production occurs in the open ocean, it most likely takes place in the biologically active surface layer, carried out by microbes that convert inorganic mercury into the toxic organic form through a process called methylation.
But in the latest study, the Michigan and Hawaii researchers showed that perhaps as much as 80 percent of the methylmercury found at depth in the central North Pacific is produced below what is known as the surface mix
|Contact: Jim Erickson|
University of Michigan