A new landmark study documents for the first time the process in which increased mercury emissions from human sources across the globe, and in particular from Asia, make their way into the North Pacific Ocean and as a result contaminate tuna and other seafood. 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.
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.
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.
– from United States Geological Survey | May 3, 2009