Landmark Study Demonstrates How Methylmercury, Known to Contaminate Seafood, Forms in the Ocean


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Landmark Study Demonstrates How Methylmercury, Known to Contaminate Seafood, Forms in the Ocean

Mercury found in large marine fish, such as tuna, may enter the food chain via an ocean mercury cycle proposed by a USGS scientist and his colleagues.

By David Krabbenhoft
August 2009

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A team of scientists from the U.S. Geological Survey (USGS) and several universities has documented for the first time the process by which increased mercury emissions from human sources across the globe, and in particular from Asia, make their way into the North Pacific Ocean and lead to the formation of methylmercury, the form of mercury found in tuna and other seafood. Mercury levels measured in 2006 were approximately 30 percent higher than those measured in the mid-1990s. Because much of the mercury that enters the North Pacific comes from the atmosphere, the scientists predict an additional 50-percent increase in mercury in the Pacific by 2050 if mercury emission rates continue as projected.

Above left: Scientists Cliff Buck (left) and Lauren Kaupp prepare to lower a "rosette" of 12 Niskin bottles from the research vessel Thomas G. Thompson. The device collects samples in the ocean by remote triggering of each bottle at different depths. Extreme care was taken to ensure that the rosette did not contaminate the samples. Photograph by William Landing, Florida State University. [larger version]

Above right: Locations of hydrographic stations sampled for mercury along the P16N Leg-2 cruise track from Honolulu, Hawaii, to Kodiak, Alaska, March 10-30, 2006. [larger version]

This study documents for the first time the formation of methylmercury, a highly toxic form of mercury, in the waters of the North Pacific Ocean. Previously, scientists had hypothesized that methylmercury in the open ocean was geologic in origin and associated with deep-sea spreading centers. The recent study, however, supports methylmercury formation from atmospheric mercury that is deposited on the ocean surface and absorbed by algae living in sunlit waters near the surface. After the algae die, they "rain" downward to greater water depths. At depths of about 200 to 700 m, the settling algae are decomposed by bacteria, and this decomposition process in the presence of mercury results in the formation of methylmercury. Methylmercury rapidly accumulates in the food chain to levels that can cause serious health concerns for those who consume the seafood.

The team collected samples of Pacific Ocean water during a hydrographic survey of the eastern North Pacific Ocean in March 2006 on the research vessel Thomas G. Thompson (P16N Leg-2). The cruise followed a north-south transect at approximately 152°W between Honolulu, Hawaii, and Kodiak, Alaska. A total of 41 stations were occupied, and samples for mercury analysis were collected at 16 stations. Vertical profiles (sets of water samples from various depths) down to 1,000 m were obtained at 6 stations and surface samples (from less than 20-m depth) were collected at 10 stations. All mercury and methylmercury analyses were performed at the USGS Mercury Research Laboratory in Middleton, Wisconsin. Additionally, the scientists constructed a computer simulation that links atmospheric emissions, transport and deposition of mercury, and an ocean-circulation model. Their results were reported last May in the journal Global Biogeochemical Cycles.

Above: The location of the maximum methylmercury concentration at depth in the Pacific Ocean was the first evidence noted by the researchers pointing to the newly documented methylation cycle. Graph shows sampling depth on the left (in meters), and oxygen concentration by color (scale on right in micromoles per kilogram of seawater [µmol/kg]) along a north-south transect in the eastern North Pacific Ocean. The maximum methylmercury concentration (black dots) 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"—settling algae produced near the surface of the ocean. The decomposition process consumes oxygen from the water and leads to methylmercury production. [larger version]

One unexpected finding from this study is the significance of long-range transport of mercury in the ocean. USGS scientist and coauthor David Krabbenhoft stated: "Mercury researchers typically look skyward to find a mercury source from the atmosphere due to emissions from land-based combustion facilities. In this study, however, the pathway of the mercury was a little different. It appears that the recent mercury enrichment of the sampled Pacific Ocean waters originated from fallout of atmospheric mercury near the Asian coasts. The mercury-enriched waters then enter a long-range eastward transport by large ocean-circulation currents."

Above: Mercury concentrations in surface water (less than 20-m depth) interpolated from data collected during cruises in 2006 (P16N Leg-2), 2002 (Intergovernmental Oceanographic Commission), and 1987 (Vertical Transport and Exchange Survey) on the North Pacific Ocean. Note high mercury concentrations near the coast of Asia. Surface circulation adapted from Pickard and Emery’s Descriptive Physical Oceanography (1990, 5th ed., Woburn, Mass., Elsevier Sci., 320 p.). White dots, sites of observational data; Hg, mercury; pM, picomolar; NPIW, North Pacific Intermediate Water. [larger version]

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 the process that leads to the transformation of mercury into methylmercury.

Krabbenhoft said, "National and international groups are seeking the most effective ways to minimize human exposure to methylmercury, and this paper presents the first evidence likely linking modern atmospheric mercury deposition to methylmercury in Pacific Ocean fish."

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. Pregnant women who consume mercury can pass on life-long developmental effects to their children. That's why in 2004 the U.S. Environmental Protection Agency (EPA) and the U.S. Food and Drug Administration (FDA) issued the landmark Joint Guidance on the Consumption of Fish specifically targeted toward 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 shellfish.

The paper, "Mercury Sources, Distribution and Bioavailability in the North Pacific Ocean: Insights from Data and Models," appeared on May 1, 2009, in volume 23 of Global Biogeochemical Cycles, which is published by the American Geophysical Union. In addition to USGS mercury expert David Krabbenhoft, the authors include Elsie Sunderland, Harvard University; John Moreau, University of Melbourne, Australia (until recently a USGS, National Research Council Postdoctoral Fellow); William Landing, Florida State University; and Sarah Strode, Harvard University.

The paper is posted online at http://dx.doi.org/10.1029/2008GB003425. For additional information about USGS mercury research: see Mercury in Aquatic Ecosystems.

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