Vertical methylmercury distribution in the subtropical North Pacific Ocean

Humans are exposed to toxic and bioaccumulative monomethylmercury (MMHg) principally by consuming seafood. However, knowledge of the sources of MMHg to surface-dwelling marine organisms has been hampered by a paucity of information on its vertical distribution in the open ocean. Here, we report the first complete high-resolution profile of MMHg, from sea surface to bottom water, in the Pacific Ocean. Filtered water and suspended particles were sampled at the SAFe station (140°W, 30°N) during the U.S. GEOTRACES Intercalibration. Distributions of MMHg and dimethylmercury suggest that both are synthesized in low-oxygen and oxic strata of the water column and that deep-sea sediments are not an important source. Scaling estimates imply that a majority of MMHg in phytoplankton and, by extension, the pelagic food web at this location results from production in the mixed layer, which is impacted by anthropogenic mercury inputs and thus may be affected by future changes in emissions to the atmosphere.