Mercury in the Ocean
Tuesday, May 5, 2009 1:10:53 PM
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 “marine snow” (or “ocean rain”). Algae, which are produced in sunlit waters near the surface (the photic zone), die quickly and “rain” or “snow” downward to greater water depths (200 to 700 metres). 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.
Because methylmercury (CH3Hg+) is formed in aquatic systems and because it is not readily eliminated from organisms it is biomagnified in aquatic food chains from bacteria, to plankton, through crustaceans and mussels etc., to plant-eating fish and to fish-eating fish. At each step in the food chain, the concentration of methylmercury in the organism increases. The concentration of methylmercury in the top level aquatic predators can reach a level a million times higher than the level in the water. This is because methylmercury has a half-life of about 72 days in aquatic organisms, attributing to its bioaccumulation within these food chains. Organisms, including humans, fish-eating birds, and fish-eating mammals such as otters and whales that consume fish from the top of the aquatic food chain receive the methylmercury that has accumulated through this process. Fish and other aquatic species are the only significant source of human methylmercury exposure. Methylmercury is a neurotoxin and causes serious health concerns for people and wildlife that frequently eat fish.
Long-range transport of mercury within the ocean that originates in the western Pacific Ocean, off the coast of Asia, turns out to be significant. Mercury researchers typically look skyward to find a mercury source from the atmosphere due to emissions from land-based combustion facilities. But in this study 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.
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.
The graphic (from USGS) shows sampling depth on the left (in metres), 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.
Sunderland, E. M., D. P. Krabbenhoft, J. W. Moreau, S. A. Strode, and W. M. Landing (2009),
Mercury sources, distribution, and bioavailability in the North Pacific Ocean: Insights from data and models,
Global Biogeochem. Cycles, 23, GB2010, doi:10.1029/2008GB003425.
See also my post on Mercury Pollution.
PS: May I also recommend this post at Oceana Wavemakers.
PS of 6 May: Yesterday Scientific American brought this terrible story: Dangerous Mercury Spills Still Trouble Schoolchildren - it makes you sick!