1b: Biogeochemical cycling of mercury in the ocean

Many studies have recognized abiotic photochemical degradation as an important sink of methylmercury (MeHg) in sunlight surface waters, but the rate-controlling factors remain poorly understood. The overall objective of this study was to improve our understanding of the relative importance of photochemical reactions in the degradation of MeHg across a variety of marine ecosystems by extending the range of water types studied. Experiments were conducted using surface water collected from coastal sites in Delaware, New Jersey, Connecticut, and Maine, as well as offshore sites on the New England continental shelf break, the equatorial Pacific, and the Arctic Ocean. Filtered water was amended with additional MeHg at environmentally relevant concentrations, which was allowed to equilibrate with natural ligands before being exposed to natural sunlight. Water quality parameters salinity, dissolved organic carbon, and nitrate were measured, and specific UV absorbance was calculated as a proxy for dissolved aromatic carbon content. Degradation rate constants were very similar (0.87-1.67 1/day) across all water types tested despite varying characteristics, and did not correlate with initial MeHg concentrations or other environmental parameters. The rate constants in terms of cumulative photon flux were comparable to, but at the high end of, the range of values reported in other studies. Further experiments observed little effect of nitrate, chloride, and bromide on the reaction. The HydroLight radiative transfer model was used to compute solar irradiance with depth in three hypothetical water bodies coastal wetland, estuary, and open ocean allowing for the determination of water column integrated rates. Methylmercury loss per year due to photodegradation was also modeled across a range of latitudes from the Arctic to the Equator in the three model water types. Despite the low concentrations found in the surface mixed layer of the open ocean, the loss of MeHg was greatest due to increased penetration of all wavelengths, especially the ultraviolet portion of the spectrum which has a greater ability to degrade MeHg. Overall, this study helps to better constrain the degree to which photochemical degradation impacts the cycling of MeHg in surface waters and its transport from coastal waters to the open ocean.

Elemental mercury (Hg0) evasion from surface seawater plays a principal role in the marine mercury cycle. Hg0 production in seawater is mainly controlled by abiotic (photochemical) and biotic (microbial) processes. In this study, we established a method using the gamma-emitting radionuclide Hg-203 as a tracer to evaluate the transformation of Hg2+ and methylmercury (MeHg) to Hg0 in seawater by coastal bacterial assemblages. The method, which used traps containing gold-coated beads to capture Hg0 released into air from seawater, can provide rapid and reliable Hg0 measurements that avoid potential contamination. Several natural bacterial assemblages in surface seawater collected 8 km off Southampton, New York, were used in our experiments. These bacterioplankton were contained in seawater collected 6 years prior to our study and stored in the dark at 4°C. Remarkably, these bacteria were still able to rapidly produce Hg0 following picomolar additions of 203Hg2+ or Me203Hg when brought up to 23°C in 2 days. Our results show that Hg0 production rates were independent of dissolved Hg2+ and MeHg concentrations, and were directly a function of bacterioplankton densities. Addition of antibiotics reduced Hg0 evasion to undetectable levels. These Hg evasion experiments showed that for 1 µm-filtered Long Island coastal waters from the Atlantic with natural bacterial assemblages and bacterial densities of about 1 x 106 ml-1, approximately 25% of Hg2+ and 18% of MeHg were transformed to Hg0 in 4 days at ~23°C. In Long Island Sound waters, with 5 x 106 bacterial cells ml-1, 60% of Hg2+ and 19% of MeHg were converted to Hg0 and trapped in the air within 4 days. When bacterial assemblages were exposed to Hg2+, the Hg0 production rate declined after one day, but the rate of Hg0 evasion from bacterial assemblages exposed to MeHg remained constant over 4 days, suggesting two distinct production pathways. Total Hg0 production for both Hg2+ and MeHg exposures at ~23°C were 6 times those at 4°C, indicating such transformations were mainly driven by metabolic processes.

Mercury (Hg) concentrations in Arctic wildlife and humans have increased during the Anthropocene at rates faster than those observed at temperate and tropical latitudes. One potential explanation for the anomalous increase of monomethylmercury (MMHg) in Arctic biota is that, superimposed on increasing atmospheric Hg deposition, climate change has exacerbated production and cycling of MMHg in the polar ocean. To better understand the distribution and cycling of Hg in the western Arctic Ocean, we measured dimethyl-Hg, monomethyl-Hg, and elemental and total Hg as part of the U.S. GEOTRACES Arctic cruise (GN01) in 2015 to the Bering Sea and Makarov and Canada Basins. Although Hg concentrations in the two basins were low relative to the Atlantic and Pacific Oceans, higher total Hg concentrations were observed in Arctic seawater that recently interacted with continental margins. Surface waters under ice contained elevated concentrations of total Hg and elemental Hg relative to ice-free surface waters. From our measurements, we are able to better understand sources, sinks, and cycling of Hg in the Arctic Ocean, and estimate that the Arctic Ocean receives about 20 kmol/yr of Hg from the Bering Strait, which is much less than the estimated atmospheric input (~400 kmol/yr). This comparison suggests that the atmosphere is the main source of Hg to the surface Arctic Ocean when compared to the Bering Strait, and we would expect concentrations in surface seawater and biota to respond to changes in atmospheric deposition.

Mercury (Hg) is a contaminant of major concern in the Arctic marine ecosystem for its high toxicity and biomagnification in the food web. In upper trophic level species, Hg concentrations are sufficiently high to pose heath risks to both animals and the Northern people who consume these animals as part of traditional diet. The dominant form of Hg that is transferred within the food web is monomethylmercury (MMHg), which is built up from their prey and ultimately from seawater. Although sedimentary production and release of MMHg can occur, recent studies indicate that the primary source of MMHg to seawater is in situ conversion of inorganic Hg(II) to MMHg in the water column. We have recently reported enriched methylmercury (MeHg, sum of MMHg and dimethylmercury) in subsurface seawater of the Beaufort Sea, and linked this subsurface peak to local and recent organic matter (OM) remineralization on the basis that MeHg has a short lifetime in seawater. Here we report depth profiles of total Hg (HgT) and MeHg from the ArcticNet/GEOTRACES 2015 cruise. In the Canada Basin (CB) and Baffin Bay (BB), HgT shows a transient-type distribution, with elevated concentrations in the surface and deep waters, and lower concentrations in the upper and middle ocean. Whereas in the shallower Canadian Arctic Archipelago (CAA), HgT concentrations are more uniform with depth, probably due to the enhanced terrestrial Hg input by river runoff and coastal erosion. Methylmercury in the Canadian Arctic exhibits surface minimum and subsurface peaks, similar to those observed in the Beaufort Sea and other world oceans. Positive correlations between MeHg and phosphate concentrations evidence the association of MeHg production in seawater with OM remineralization. Though with lower HgT and productivity than BB, MeHg in CB and West CAA peaks at higher concentrations. The fact that elevated MeHg are carried in the Pacific halocline suggests that high MeHg in these regions are more of an advected signal from OM remineralization in Chukchi Sea rather than local and recent ones, and the demethylation rates of MeHg might be slower than previously estimated. Another finding in this research is the high concentrations of MeHg in surface layer, including a second sub-surface peak close to the depth of subsurface chlorophyll maximum observed in many stations, and considerable MeHg found in surface water in a few stations. These results suggest that there might be another MeHg source in surface seawaters, which is most likely related to biological activities, and/or that photodemethylation in the surface ocean might not occur as fast as we previously thought.

Methylmercury (MeHg) accumulation in marine organisms poses serious ecosystem and human health risk, yet the sources of MeHg in the surface ocean remain uncertain. Here, we report the first MeHg mass budget for the Western Pacific Ocean estimated based on cruise observations (2012 and 2014 SHIPPO surveys). We found the major net source of MeHg in surface water to be vertical diffusion from the subsurface layer (1.8 to 12 nmol m-2 yr-1). A higher upward diffusion in the North Pacific (12 nmol m-2 yr-1) than in the Equatorial Pacific (1.85.7 nmol m-2 yr-1) caused elevated surface MeHg concentrations observed in the North Pacific. We furthermore found that the slope of the linear regression line for MeHg versus apparent oxygen utilization was about twofold higher in the Equatorial Pacific than the North Pacific. This could be explained by redistribution of surface water in the tropical convergence-divergence zone, supporting active organic carbon decomposition in the Equatorial Pacific Ocean. Base on this study, we predict oceanic regions with high organic carbon remineralization to have enhanced MeHg concentrations in surface as well assubsurface waters.

A number of numerical models have been developed to simulate the fate and transport of mercury in the global environment. Most of these models have been developed based on atmospheric general circulation models or air quality models. In these models, land/sea compartments are not explicitly considered, and inter-compartment transports are interpreted as secondary emissions. Integrated modeling of the atmosphere, ocean, terrestrial compartments, and biosphere is our prime focus. We developed a new global multimedia model called the Finely-advanced transboundary environmental model for mercury (FATE-Hg). In this contribution, we describe and evaluate the model, and discuss mercury biogeochemical cycles in the global oceans using the model. The outstanding features of our model are that 1) it is developed based on a coupled atmosphere-ocean transport sub-model and calculates transboundary transports both in the atmosphere and the ocean, including the deep ocean, with fine spatial and temporal resolutions (0.75°×0.75° horizontal resolution and 60 vertical layers in the atmosphere, 1.0°×1.0° horizontal resolution and 50 vertical layers in the ocean), 2) it implements a satellite-based ecosystem model, capable of estimating abundances and trophic structures of marine organisms and vertical carbon cycles through biological pumps, and 3) it considers production of methylated mercury in the water column followed by biotransfer from lower (i.e., particle organic matter) to higher (i.e., fishes) order consumers. The general chemical species of mercury and physical processes such as transformations in air, cloud water, seawater, and sediment; dry and wet depositions; and air-sea diffusive exchange of gaseous elemental mercury are also considered. We compiled monitoring data for mercury concentrations from peer reviewed literature to validate our results. We performed decadal simulation, and calculated the mercury budget in the atmosphere, ocean, sediment, and marine organisms. The result of the validation showed that our model could simulate levels and distributions of gaseous elemental mercury above the ocean, and dissolved elemental, reactive, and methylated mercury in the ocean.

Human activities like mining and fossil fuel combustion liberate mercury (Hg) sequestered in the lithosphere into biologically active surface reservoirs, where it can pose risks to human and ecological health. In our previous work, we showed that future trends in Hg concentrations and exposures are affected by the trajectory of past environmental releases due to the slow burial of Hg in coastal and deep ocean sediment. This work did not consider dynamics of ocean circulation and productivity specific to individual ocean basins that affect the lifetime and accumulation of Hg. Here we present a fully coupled 20-box biogeochemical model for the atmosphere, terrestrial ecosystems, and all major ocean basins globally. Global circulation between ocean basins is resolved using an ocean general circulation model (MITgcm) and exchange rates for Hg are constrained based on a synthesis of available ocean measurements. We find that the lifetimes of oceanic Hg are highly variable across basins, ranging from less than a decade for the upper Atlantic to hundreds of years in Pacific Deep Water. Results are highly sensitive to evasion rates, emphasizing the importance of resolving uncertainty in factors controlling ocean-atmosphere exchange of Hg0. We present an eigenanalysis of characteristic timescales that account for coupling between reservoirs and to track the fate of a pulse of atmospheric Hg through all environmental compartments. We use this analysis as the basis for examining potential responses in each ocean basin to future anthropogenic emissions trajectories. Previous atmospheric modeling without dynamic coupling to longer-lived terrestrial and oceanic reservoirs has shown only modest changes under future emission scenarios. Considering the legacy impacts in our fully coupled biogeochemical modeling framework by contrast suggests large increases in Hg deposition over all ocean basins relative to 2008 levels under business as usual emissions scenarios. In the North Pacific, seawater total Hg concentrations in 2050 relative to 2015 levels range from an increase of 40% under business as usual emissions to a decrease of more than 40% with zero anthropogenic emissions as a bounding scenario. Our results reinforce the benefits of global policies aimed at reducing emissions.

Human activities have greatly perturbed the natural biogeochemical cycle of mercury and released large quantitites of previously sequestered mercury into the oceans. Methylmercury (MeHg) is the only species that biomagnifies in aquatic food webs and is widely recognized as negatively impacting the behavioral and reproductive health of fish, marine mammals, and humans. The main objective of this work is to link previously modeled Hg deposition into the ocean and seawater reservoirs of Hg species to MeHg flows associated with harvests of commercially important fish stocks. We compare the most recent deposition estimates from the GEOS-Chem global CTM based on updated oxidant chemistry and modeled total and MeHg reservoirs (seawater and plankton) in the global oceans from the MITgcm, to reservoirs and flows of MeHg from commercial fisheries. Based on estimated fisheries catches between 2001-2010 from the Sea Around Us database, we construct a first estimate of MeHg levels in global ocean catches by fishing sectors (industrial, subsistence and artisanal) and taxonomic groups. For species without data on MeHg levels in their tissues, we use a regression model to relate Hg concentrations to trophic level and fish length. Results show domestic waters are the major source of MeHg for most world regions and industrial fisheries are the predominant sector transferring MeHg from fish in the global oceans to humans. We quantify in this work the proportion of the oceanic MeHg budget removed by fishery catches.