Mercury biogeochemical cycling in the ocean and policy implications

Abstract

Anthropogenic activities have enriched mercury in the biosphere by at least a factor of three, leading to increases in total mercury (Hg) in the surface ocean. However, the impacts on ocean fish and associated trends in human exposure as a result of such changes are less clear. Here we review our understanding of global mass budgets for both inorganic and methylated Hg species in ocean seawater. We consider external inputs from atmospheric deposition and rivers as well as internal production of monomethylmercury (CH₃Hg) and dimethylmercury ((CH₃)₂Hg). Impacts of large-scale ocean circulation and vertical transport processes on Hg distribution throughout the water column and how this influences bioaccumulation into ocean food chains are also discussed. Our analysis suggests that while atmospheric deposition is the main source of inorganic Hg to open ocean systems, most of the CH₃Hg accumulating in ocean fish is derived from in situ production within the upper waters (<1000 m). An analysis of the available data suggests that concentrations in the various ocean basins are changing at different rates due to differences in atmospheric loading and that the deeper waters of the oceans are responding slowly to changes in atmospheric Hg inputs. Most biological exposures occur in the upper ocean and therefore should respond over years to decades to changes in atmospheric mercury inputs achieved by regulatory control strategies. Migratory pelagic fish such as tuna and swordfish are an important component of CH₃Hg exposure for many human populations and therefore any reduction in anthropogenic releases of Hg and associated deposition to the ocean will result in a decline in human exposure and risk.

Figure 1

a) A recent estimate of the fluxes of mercury at the Earth's surface based on simulations using the GEOS-Chem global mercury model, and building on previous studies (Soerensen et al., 2010; Selin et al., 2008 x; Sunderlansd and Mason, 2007; Holmes et al, 2010; Smith-Downey et al, 2010). The percentage values in brackets are the estimated increases in concentration and fluxes in the last century due to anthropogenic activities. (unpublished data); b) Overall budget for the sources of methylated mercury to the upper ocean (defined as waters above the permanent thermocline) using data and information discussed throughout the paper. In both figures fluxes are in Mmol yr-1 and reservoirs are given in Mmol.

Figure 2

Distributions of methylated mercury (methylmercury and dimethylmercury (Me2Hg)) in various ocean basins. Data compiled from the literature: equatorial Pacific (Mason and Fitzgerald, 1993); North Pacific (Sunderland et al., 2009); Southern Ocean (Cossa et al., 2011); North Atlantic (Mason et al,. 1998); South and equatorial Atlantic (Mason and Sullivan, 1999). In the plot for the Equatorial Pacific surface water concentrations were set to DL (50 fM) when measurements were below DL. Note that the scale for the bottom plots of dimethylmercury is different. All figures created using the data in the referenced manuscripts.

Figure 3

Consensus Value full depth profiles for BATS in the North Atlantic (31° 40′ N, 64° 10′W) (open triangles; UConn and WHOI data) and the SAFe site (30° N 140° W) in the North Pacific (WSU and WHOI data). Taken from Lamborg et al. (in press).

Figure 4

Comparison of concentrations of total mercury in waters of the North Atlantic and the North Pacific. a) data for a number of sites in the North Pacific Ocean. Figure taken from Sunderland et al. (2009); b) data for a site near Bermuda (BATS) in the North Atlantic Ocean. Figure taken from Mason and Gill (2005).

Figure 5

a) A compilation of data from a number of sources by Lamborg showing the historical trends in concentration for New Zealand sediment cores (Lamborg et al., 2002b; Alaskan lake sediments (Engstrom, pers. comm.; Santa Barbara basin marine sediments (Young et al.,1973; North Pacific tuna (Kraepiel et al., 003); and elemental mercury trapped in firn (Fain et al., 2009); b) historical trend in mercury concentration as recorded in bird eggs from the South China Sea Figure taken from Xu et al. (2011).

Figure 6

Seasonal distributions of total mercury in the upper ocean of the North Pacific collected during the VERTEX Program (33oN, 139oW). Taken from Laurier et al. (2004).

Figure 7

Reported mercury concentrations (μg/g wet weight) in fish sold in the U.S. commercial market as available on the US FDA website, accessed in 2012. Figure drawn using the data (averages and standard deviation) from the website. N/A indicates that no standard deviation was listed.

Figure 8

Map showing tracks of the various cruises in the ocean where detailed Hg analysis (water column profiles speciation measurements) has been made. Triangles indicate cruises where measurements were restricted to only one or a few profiles.