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A New Source of Methylmercury Entering the Pacific Ocean

Scientists prepare to lower a rosette of 12 Niskin bottles
Scientists prepare to lower a "rosette" of 12 Niskin bottles on the vessel R/V Thomas G. Thompson. The device enables the collection of samples in the ocean via remote triggering of each bottle at different depths. Extreme care was taken to ensure that the rosette does not contaminate the samples. Photo courtesy of William Landing, Florida State University.

A U.S. Geological Survey (USGS) scientist and his university colleagues have discovered a new source of methylmercury entering the waters of the eastern North Pacific Ocean. Consumption of ocean fish and shellfish account for over 90 percent of human methylmercury exposure in the United States, and tuna harvested in the Pacific Ocean account for 40 percent of this total exposure (Sunderland, 2007). Given the obvious importance of marine food webs to human methylmercury exposure, scientists were still trying to answer the question - where do fish, such as Pacific Ocean tuna, acquire their methylmercury? The findings of these scientists published in the journal Global Biogeochemical Cycles, might be a major step forward toward solving this mystery.

Scientists have known for some time that mercury deposited from the atmosphere to freshwater ecosystems can be transformed (methylated) into a highly toxic form of mercury called methylmercury, given the right conditions. Methylmercury rapidly accumulates up the food chain to levels that can cause serious health concerns for people and wildlife that frequently eat fish. In contrast to the growing knowledge base on the processes leading to the production of methylmercury in freshwater ecosystems, very little is known about the sources of methylmercury in marine systems. Filling this information gap has been challenging scientists for years. This new paper provides a significant step forward in understanding oceanic methylmercury sources. The authors:

  • Present the first published data on methylmercury in the water column in the Pacific Ocean. Total mercury and methylated mercury were quantified at 16 hydrographic stations along a North-South trending transect in the eastern North Pacific Ocean, including six stations sampled in profile to depths of 1,000 meters.
  • Propose the existence of a mercury methylation cycle in the Pacific Ocean using corroborating observations of methylated mercury concentrations and geochemical modeling results.
  • Indicate that total mercury levels in the North Pacific Ocean water have risen about 30 percent over the last 20 years. The authors attribute the rise to increases in global mercury atmospheric emission rates, particularly from Asia.
  • Project a 50 percent increase in Pacific Ocean mercury levels by the year 2050 based on published projections of increases in mercury emissions over the same timeframe.

Before this work, some scientists hypothesized that methylmercury in the open ocean was geologic in origin and associated with deep-sea spreading centers. Data and modeling results from this paper support the notion of a Mercury Methylation Cycle, in which much of the methylmercury in the open ocean is the result of biologically mediated transformation of mercury into methylmercury. Most of the mercury originates from atmospheric fallout to the ocean surface and the subsequent transport of the mercury to greater ocean depths (200 to 700 meters) where the methylmercury production process occurs. At these depths, naturally occurring bacteria decompose organic matter, which is largely comprised of settling algae (commonly referred to as ocean rain) that are produced in the sunlit waters near the surface (the photic zone). However, the decomposition of organic matter also results in unintentional conversion of mercury to methylmercury, which is then passed up the food web through the bioaccumulation process, eventually to top predator fish like tuna.

Graph of the location of the maximum methylmercury concentration at depth in the Pacific Ocean
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. Graph created by David P. Krabbenhoft with data from Sunderland and others, 2009.

Currently, national and international groups are seeking the most effective ways to minimize human methylmercury exposure, and this paper presents the first evidence linking current atmospheric mercury deposition to methylmercury in Pacific Ocean fish. The scientists constructed a computer simulation model that links atmospheric emissions, transport and deposition of mercury to an ocean circulation model. The combined model allows the scientists to estimate future ocean mercury concentrations given anticipated future mercury emission rates from human-related sources. The model predicts that Pacific Ocean mercury levels will rise another 50 percent by the year 2050 if mercury emission rates continue to rise as projected. Such increases could have implications for resulting methylmercury levels in Pacific Ocean fish. The overall findings from this study suggest that measurable changes in mercury levels of very large systems (such as the Pacific Ocean) are plausible over reasonably short time frames (decadal), and it is equally plausible that reductions in ocean mercury levels would follow if mercury emissions were decreased. Environmental professionals, regulators, resource managers, and other decision makers can use these results to help make informed decisions about atmospheric mercury emissions and potential human exposure to methylmercury from fish consumption.


Sunderland, E.M., Krabbenhoft, D.P., Moreau, J.W., Strode, S.A., and Landing, W.M., 2009, Mercury sources, distribution, and bioavailability in the North Pacific Ocean--Insights from data and models: Global Biogeochemical Cycles, v. 23, no. 2, p. 1-14, GB2010, doi:10.1029/2008GB003425.

Sunderland, E. M., 2007, Mercury exposure from domestic and imported estuarine and marine fish in the United States seafood market: Environmental Health Perspectives, v. 115, no. 2, p. 235-242, doi:10.1289/ehp.9377.

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