Ocean Origin of Seafood Contaminant Methylmercury Identified

This poisonous form of mercury bioaccumulates in fish, becoming more and more concentrated as it moves up the food chain from smaller to larger fish until people eating tuna sushi are exposed to levels of mercury high enough to warrant warning signs. Pregnant women who consume mercury can pass on life-long developmental effects to their children.

Previous studies have shown that 75 percent of human exposure worldwide to mercury is from the consumption of marine fish and shellfish. In the United States, about 40 percent of all human exposure to mercury is from tuna harvested in the Pacific Ocean, according to Elsie Sunderland, a coauthor of the study.

This study shed some light on how metal mercury emitted by burning coal and waste and from factories in Asia beomes methylmercury in the North Pacific Ocean.

"It appears the recent mercury enrichment of the sampled Pacific Ocean waters is caused by emissions originating from fallout near the Asian coasts," said U.S. Geological Survey scientist and coauthor David Krabbenhoft.

Power plant in China burns bituminous coal, emitting mercury and affecting Pacific Ocean fish. (Photo courtesy Skoda Export)

Then the methylmercury that originates in the western Pacific Ocean, off the coast of Asia is transported far and wide.

"The mercury-enriched waters then enter a long-range eastward transport by large ocean circulation currents," said Krabbenhoft.

Because much of the mercury that enters the North Pacific comes from the atmosphere, scientists have predicted an additional 50 percent increase in mercury in the Pacific by 2050 if mercury emission rates continue as projected.

“This unprecedented USGS study is critically important to the health and safety of the American people and our wildlife because it helps us understand the relationship between atmospheric emissions of mercury and concentrations of mercury in marine fish,” said Secretary of the Interior Ken Salazar. “We need to reduce the mercury emissions so that we can reduce the ocean mercury levels.”

Currently, ocean mercury levels are rising. Water sampling cited in the study shows that mercury levels in 2006 were approximately 30 percent higher than those measured in the mid-1990s.

This study documents for the first time that methylmercury is produced in mid-depth ocean waters by processes linked to what the authors call “ocean rain.”

Algae, which are produced in sunlit waters near the surface, die quickly and “rain” downward to greater water depths.

At depth, the settling algae are decomposed by bacteria and the interaction of this decomposition process in the presence of mercury results in the formation of methylmercury.

Many steps up the food chain later, predators like tuna receive methylmercury from the smaller fish they consume.

"This study gives us a better understanding of how dangerous levels of mercury move into our air, our water, and the food we eat, and shines new light on a major health threat to Americans and people all across the world,” said EPA Administrator Lisa Jackson.

“With this information in hand, plus our own mercury efforts," said Jackson, "we have an even greater opportunity to continue working with our international partners to significantly cut mercury pollution in the years ahead and protect the health of millions of people.”

The study appeared today in Global Biogeochemical Cycles, which is published by the American Geophysical Union. In addition to USGS mercury expert David Krabbenhoft, the authors include Elsie Sunderland, Harvard University; John Moreau, University of Melbourne, Australia (until recently a USGS, NRC Post Doctoral Candidate); William Landing, Florida State University; and Sarah Strode, Harvard University.

Click here for the paper, "Mercury sources, distribution and bioavailability in the North Pacific Ocean - Insights from data and models and information on other USGS mercury research."

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