Mercury levels in the Pacific Ocean are rising, a new study suggests. The increase may mean that more methylmercury, a human neurotoxin formed when mercury is methylated by microbes, accumulates in marine fish such as tuna.
Background: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.
The new study:A new landmark study published this month in the journal
Global Biogeochemical Cycles 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.
Pacific Ocean water samples were collected from 16 different sites between Honolulu, Hawaii and Kodiak, Alaska and both total mercury and methylmercury were quantified. In addition, the scientists constructed a computer simulation that links atmospheric emissions, transport and deposition of mercury, and an ocean circulation model. The combined approach provides a significant step forward in understanding oceanic methylmercury sources.
Water sampling cited in the study shows that mercury levels in 2006 were approximately 30 percent higher than those measured in the mid-1990s. The authors attribute the rise to increases in global mercury atmospheric emission rates, particularly from Asia. 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.
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.
Fig. : The location of the maximum methylmercury concentration at depth in the Pacific Ocean was the first evidence noted by the researchers pointing to the new methylation cycle. The graphic shows sampling depth on the left (in meters), and oxygen concentration on the right (in micromoles per kilogram of seawater [µmol/kg]) along a north-south latitudinal transect in the eastern North Pacific Ocean. The specific depth of maximal methylmercury concentration was consistently found at the ocean depth where the most rapid loss of oxygen was also observed. The process linking these two observations is microbial decomposition of "ocean rain", which is settling algae produced near the surface of the ocean. The decomposition process consumes oxygen from the water, but also leads to unintended methylmercury production. 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 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."
Source:
USGS
The new study:
E.M. Sunderland, D.P. Krabbenhoft, J.W. Moreau, S.A. Strode, W.M. Landing,
Mercury sources, distribution, and bioavailability in the North Pacific Ocean: Insights from data and models, Global Biogeochem. Cycles, 23 (2009) GB2010.
DOI: 10.1029/2008GB003425Related studies C. Chen, A. Amirbahaman, N. Fisher, G. Harding, C. Lamborg, D. Nacci, D. Taylor, Methylmercury in marine ecosystems: spatial patterns and processes of production, bioaccumulation and biomagnification, Ecohealth, 5/4 (2008) 399.
DOI: 10.1007/s10393-008-0201-1
Elsie M. Sunderland, Robert P. Mason,
Human impacts on open ocean mercury concentrations, Global Biogeochem. Cycles, 21/4 (2007) GB4022.
DOI: 10.1029/2006GB002876
F.J.G. Laurier, R.P. Mason, G.A. Gill, L. Whalin,
Mercury distributions in the North Pacific Ocean - 20 years of observations, Mar. Chem., 90/1-4 (2004) 3-19.
DOI: 10.1016/j.marchem.2004.02.025 R.P. Mason, G.-R. Sheu,
Role of ocean in the global mercury cycle, Global Biogeochem. Cycles, 16/4 (2002) 1093-1107.
DOI: 10.1029/2001GBC001440 R.P. Mason, W.F. Fitzgerald,
The distribution and biogeochemical cycling of mercury in the equatorial Pacific Ocean, Deep-Sea Res. Part I, 40/9 (1993) 1897-1924.
DOI: 10.1016/0967-0637(93)90037-4 Y. Miyake, Y. Suzuki,
The concentration and chemical forms of mercury in waters of the western North Pacific, Deep-Sea Res. Part A, 30/6 (1983) 615-627.
DOI: 10.1016/0198-0149(83)90039-0
EVISA Resources EVISA Link Database: Mercury cycling EVISA Link Database: Mercury pollution
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