A federal study released today explains for the first time the link between global mercury emissions and the contamination of tuna and other marine life in the North Pacific Ocean.
The U.S. Geological Survey study documents the formation in the North Pacific of methylmercury, a highly toxic form of mercury that rapidly accumulates in the food chain to levels that can cause serious health concerns for people who consume seafood. Scientists have known for some time that mercury deposited from the atmosphere can be transformed into methylmercury, but the study focuses on how that transformation occurs.
USGS showed that methylmercury is produced in mid-depth ocean waters by processes linked to "ocean rain." Algae, which are produced in sunlit waters near the surface, die quickly and "rain" downward to greater water depths. 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 fish they consume, the study shows.
The study unexpectedly reveals the significance of long-range movement of mercury within the ocean that originates in the western Pacific Ocean, off the coast of Asia, USGS scientist and co-author David Krabbenhoft said.
"Mercury researchers typically look skyward to find a mercury source from the atmosphere due to emissions from land-based combustion facilities," Krabbenhoft said in a statement.
"In this study, however, the pathway of the mercury was a little different. Instead, it appears the recent mercury enrichment of the sampled Pacific Ocean waters is caused by emissions originating from fallout near the Asian coasts. The mercury-enriched waters then enter a long-range eastward transport by large ocean circulation currents."
The Obama administration said the study demonstrates the need to curb global mercury emissions.
"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," Interior Secretary Ken Salazar said in a statement.
Scientists have predicted an additional 50 percent increase in mercury in the Pacific Ocean by 2050 if mercury emission rates continue as projected. USGS water sampling shows mercury levels in 2006 were approximately 30 percent higher than those measured in the mid-1990s.
"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," EPA Administrator Lisa Jackson added. "With this information in hand, plus our own mercury efforts, 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."
Water sampling, modeling
Scientists sampled Pacific water from 16 different sites between Honolulu, Hawaii and Kodiak, Alaska. They also constructed a computer simulation that links atmospheric emissions, transport and deposition of mercury with an ocean circulation model.
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 of Harvard University, a co-author of the study. Because pregnant women who consume mercury can pass on lifelong developmental effects to their children, EPA and the U.S. Food and Drug Administration issued the guidance on consumption of fish for pregnant women and nursing mothers.
The study appeared today in Global Biogeochemical Cycles, which is published by the American Geophysical Union. Its authors included Krabbenhoft, Sunderland and John Moreau of the University of Melbourne, Australia; William Landing of Florida State University; and Sarah Strode of Harvard University.
Click here to read the study.
Reporter Sara Goodman contributed.