How Mercury Emissions Reach Tuna And Other Seafood, And Why Mercury Contamination Is Likely To Worsen

“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 have always known that mercury can pose a risk, now we need to reduce the mercury emissions so that we can reduce the ocean mercury levels.”

"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 P. Jackson. “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 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 the formation of methylmercury in the North Pacific Ocean. It shows that methylmercury is produced in mid-depth ocean waters by processes linked to the “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 fish they consume.

One unexpected finding from this study is the significance of long-range transport of mercury within the ocean that originates in the western Pacific Ocean, off the coast of Asia.

“Mercury researchers typically look skyward to find a mercury source from the atmosphere due to emissions from land-based combustion facilities. 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,” said USGS scientist and coauthor David Krabbenhoft.

Scientists sampled Pacific Ocean water from 16 different sites between Honolulu, Hawaii and Kodiak, Alaska. In addition, the scientists constructed a computer simulation that links atmospheric emissions, transport and deposition of mercury, and 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, a coauthor of the study. Methylmercury is a highly toxic form of mercury that rapidly accumulates in the food chain to levels that can cause serious health concerns for those who consume the seafood. Pregnant women who consume mercury can pass on life-long developmental effects to their children. That is why in 2004 EPA and FDA issued the landmark Joint Guidance on the Consumption of Fish specifically targeted towards pregnant women and nursing mothers. Previous studies show that 75 percent of human exposure worldwide to mercury is from the consumption of marine fish and shell fish.

Scientists have known for some time that mercury deposited from the atmosphere to freshwater ecosystems can be transformed (methylated) into methylmercury, but identifying the analogous cycles in marine systems has remained elusive. As a result of this study we now know more about how the process which leads to the transformation of mercury into methylmercury.

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.