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.

We develop an improved treatment of the surface ocean in the GEOS-Chem global 3-D biogeochemical model for mercury (Hg). We replace the globally uniform subsurface ocean Hg concentrations used in the original model with basin-specific values based on measurements. Updated chemical mechanisms for Hg 0 /Hg II redox reactions in the surface ocean include both photochemical and biological processes, and we improved the parametrization of particle-associated Hg scavenging. Modeled aqueous Hg concentrations are consistent with limited surface water observations. Results more accurately reproduce high-observed MBL concentrations over the North Atlantic (NA) and the associated seasonal trends. High seasonal evasion in the NA is driven by inputs from Hg enriched subsurface waters through entrainment and Ekman pumping. Globally, subsurface waters account for 40% of Hg inputs to the ocean mixed layer, and 60% is from atmospheric deposition. Although globally the ocean is a net sink for 3.8 Mmol Hg y -1 , the NA is a net source to the atmosphere, potentially due to enrichment of subsurface waters with legacy Hg from historical anthropogenic sources. IntroductionAnthropogenic mercury (Hg) sources have enriched atmospheric Hg deposition globally by at least a factor of 3 (1).Atmospheric Hg is predominantly the gaseous elemental form (Hg 0 ), and is oxidized to Hg II , which is then rapidly deposited. It is estimated that more than 80% of the Hg deposited to oceans is reemitted to the atmosphere as Hg 0 , driving the cycle of Hg through biogeochemical reservoirs (2). Aqueous reduction of divalent inorganic mercury (Hg II ) and subsequent loss of Hg 0 reduces the potentially bioavailable Hg II pool that may be converted to monomethylmercury, the most toxic species that poses health risks to fish consuming populations and wildlife (3).Previous efforts to model Hg air-sea exchange (2) and atmospheric transport (4-6) have been unable to reproduce high atmospheric concentrations observed in the Northern Hemisphere marine boundary layer (MBL) during ocean cruises (7-9). We hypothesize that this results from subsurface seawater Hg enrichment, reflecting the legacy of past anthropogenic inputs and controlling Northern Hemisphere MBL concentrations. Previous research comparing preindustrial and contemporary Hg budgets for different ocean basins indicates that anthropogenic enrichment of Hg reservoirs in the Atlantic Ocean and Mediterranean Sea is >50% (3). Other regions such as deep waters of the Pacific Ocean have seen negligible anthropogenic impacts while Hg concentrations in intermediate waters of the North Pacific (NP) appear to be increasing (10). These gradients in subsurface Hg across ocean regions (11, 12) have not been represented in models simulating atmospheric Hg. Here we investigate the potential effects of legacy anthropogenic Hg accumulation on oceanic air-sea exchange in the GEOSChem global model (1) by including the effects of variability in subsurface ocean concentrations in our simulation.Most marine surface waters...

Methylmercury is a potent neurotoxin that accumulates in aquatic food webs. Human activities, including industry and mining, have increased inorganic mercury inputs to terrestrial and aquatic ecosystems. Methylation of this mercury generates methylmercury, and is thus a public health concern. Marine methylmercury is a particular concern in the Arctic, where indigenous peoples rely heavily on marinebased diets. In the summer, atmospheric inorganic mercury concentrations peak in the Arctic, whereas they reach a minimum in the northern mid-latitudes. Here, we use a global three-dimensional ocean-atmosphere model to examine the cause of this Arctic summertime maximum. According to our simulations, circumpolar rivers deliver large quantities of mercury to the Arctic Ocean during summer; the subsequent evasion of this riverine mercury to the atmosphere can explain the summertime peak in atmospheric mercury levels. We infer that rivers are the dominant source of mercury to the Arctic Ocean on an annual basis. Our simulations suggest that Arctic Ocean mercury concentrations could be highly sensitive to climate-induced changes in river flow, and to increases in the mobility of mercury in soils, for example as a result of permafrost thaw and forest fires.