The Global Marine Selenium Cycle: Insights from Measurements and Modeling

Robert P. Mason; Anne L. Soerensen; Brian P. DiMento; Prentiss H. Balcom
2018 | Global Biogeochem Cycles

Anthropogenic activities have increased the selenium (Se) concentration in the biosphere, but the overall impact on the ocean has not been examined. While Se is an essential nutrient for microorganisms, there is little information on the impact of biological processes on the concentration and speciation of Se in the ocean. Additionally, other factors controlling the distribution and concentration of Se species are poorly understood. Here we present data gathered in the subtropical Pacific Ocean during a cruise in 2011 and we used these field data and the literature, as well as laboratory photochemical experiments examining the stability and degradation of inorganic Se (both Se (IV) and Se (VI)) and dimethyl selenide, to further constrain the cycling of Se in the upper ocean. We also developed a multi‐box model for the biosphere to examine the impact of anthropogenic emissions on the concentration and distribution of Se in the ocean. The model concurs with the field data indicating that the Se concentration has increased in the upper ocean waters over the past 30 years. Our observational studies and model results suggest that Se (VI) is taken up by phytoplankton in the surface ocean, in contrast to the results of laboratory culture experiments. In conclusion, while anthropogenic inputs have markedly increased Se in the atmosphere (42%) and net deposition to the ocean (38%) and terrestrial landscape (41%), the impact on Se in the ocean is small (3% increase in the upper ocean). This minimal response reflects its long marine residence time.

Deciphering the Role of Water Column Redoxclines on Methyl mercury Cycling Using Speciation Modeling and Observations From the Baltic Sea

A.L. Soerensen; A.T. Schartup; A. Skrobonja; S. Bouchet; D. Amouroux; V. Liem-Nguyen; E. Björn
2018 | Global Biogeochem Cycles | 32

Oxygen-depleted areas are spreading in coastal and offshore waters worldwide, but the implication for production and bioaccumulation of neurotoxic methylmercury (MeHg) is uncertain. We combined observations from six cruises in the Baltic Sea with speciation modeling and incubation experiments to gain insights into mercury (Hg) dynamics in oxygen depleted systems. We then developed a conceptual model describing the main drivers of Hg speciation, fluxes, and transformations in water columns with steep redox gradients. MeHg concentrations were 2–6 and 30–55 times higher in hypoxic and anoxic than in normoxic water, respectively, while only 1–3 and 1–2 times higher for total Hg (THg). We systematically detected divalent inorganic Hg (HgII) methylation in anoxic water but rarely in other waters. In anoxic water, high concentrations of dissolved sulfide cause formation of dissolved species of HgII: HgS2H-(aq) and Hg (SH)20(aq) . This prolongs the lifetime and increases the reservoir of HgII readily available for
methylation, driving the high MeHg concentrations in anoxic zones. In the hypoxic zone and at the hypoxic-anoxic interface, Hg concentrations, partitioning, and speciation are all highly dynamic due to processes linked to the iron and sulfur cycles. This causes a large variability in bioavailability of Hg, and thereby MeHg concentrations, in these zones. We find that zooplankton in the summertime are exposed to 2–6 times higher MeHg concentrations in hypoxic than in normoxic water. The current spread of hypoxic zones in coastal systems worldwide could thus cause an increase in the MeHg exposure of food webs.

Organic matter drives high interannual variability in methylmercury concentrations in a subarctic coastal sea

A.L. Soerensen; A.T. Schartup; A. Skrobonja; E. Björn
2017 | Environ. Pollut. | 229 (531-538)

Levels of neurotoxic methylmercury (MeHg) in phytoplankton are strongly associated to water MeHg concentrations. Because uptake by phytoplankton is the first and largest step of bioaccumulation in aquatic food webs many studies have investigated factors controlling seasonal changes in water MeHg concentrations. However organic matter (OM), widely accepted as an important driver of MeHg production and uptake by phytoplankton, is known for strong interannual variability in concentrations and composition within systems. In this study, we explore the role of OM on spatial and interannual variability of MeHg in a subarctic coastal sea, the northern Baltic Sea. Using MeHg (2014: 80±25 fM; 2015:

Methylmercury mass budgets and distribution characteristics in the Western Pacific Ocean

Hyunji Kim; Anne L. Soerensen; Jin Hur; Lars-Eric Heimburger; Doshik Hahm; Tae Siek Rhee; Seam Noh; Seunghee Han
2017 | Environ. Sci. Technol. | 51 (3) (1186-1194)

Methylmercury (MeHg) accumulation in marine organisms poses serious ecosystem and human health risk, yet the sources of MeHg in the surface and subsurface ocean remain uncertain. Here, we report the first MeHg mass budget for the Western Pacific Ocean estimated based on cruise observations. 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 (1.8–5.7 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. We suggest 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 both surface and subsurface waters.

Eutrophication increases phytoplankton methylmercury concentrations in a coastal sea – a Baltic Sea case study

A.L. Soerensen; A.T. Schartup; E. Gustafsson; B. Gustafsson; E.M. Undeman; E. Bjorn
2016 | Environ. Sci. Technol. | 50 (11787-11796)

Eutrophication is expanding worldwide, but its implication for production and bioaccumulation of neurotoxic monomethylmercury (MeHg) is unknown. We developed a mercury (Hg) biogeochemical model for the Baltic Sea and used it to investigate the impact of eutrophication on phytoplankton MeHg concentrations. For model evaluation we measured total methylated Hg (MeHgT) in the Baltic Sea and found low concentrations (39±16 fM) above the halocline and high concentrations in anoxic waters (1249±369 fM). To close the Baltic Sea MeHgT budget we inferred an average normoxic water column MeHg production rate of 2×10-4 d-1. We used the model to compare Baltic Sea’s present-day (2005-2014) eutrophic state to an oligo/mesotrophic scenario. Eutrophication increases primary production and export of organic matter and associated Hg to the sediment effectively removing Hg from the active biogeochemical cycle; this results in a 27% lower present-day water column Hg reservoir. However, increase in organic matter production and remineralization stimulates microbial Hg methylation resulting in a seasonal increase in both water and phytoplankton MeHg reservoirs above the halocline. Previous studies of systems dominated by external MeHg sources or benthic production found eutrophication to decrease MeHg levels in plankton. This Baltic Sea study shows that in systems with MeHg production in the normoxic water column eutrophication can increase phytoplankton MeHg content.

A mass budget for mercury and methylmercury in the Arctic Ocean

Anne L. Soerensen; Daniel J. Jacob; Amina T. Schartup; Jenny A. Fisher; Igor Lehnherr; Vincent L. St. Louis; Lars-Eric Heimbürger; Jeroen E. Sonke; David P. Krabbenhoft; Elsie M. Sunderland
2016 | Global Biogeochem Cycles | 30 (560-575)

Elevated biological concentrations of methylmercury (MeHg), a bioaccumulative neurotoxin, are
observed throughout the Arctic Ocean, but major sources and degradation pathways in seawater are not well
understood. We develop a mass budget for mercury species in the Arctic Ocean based on available data since
2004 and discuss implications and uncertainties. Our calculations show that high total mercury (Hg) in Arctic
seawater relative to other basins reflect large freshwater inputs and sea ice cover that inhibits losses through evasion. We find that most net MeHg production (20 Mg a 1) occurs in the subsurface ocean (20–200 m). There it 1
is converted to dimethylmercury (Me2Hg: 17 Mg a ), which diffuses to the polar mixed layer and evades to the atmosphere (14 Mg a 1). Me2Hg has a short atmospheric lifetime and rapidly degrades back to MeHg.
We postulate that most evaded Me2Hg is redeposited as MeHg and that atmospheric deposition is the largest net MeHg source (8 Mg a 1) to the biologically productive surface ocean. MeHg concentrations in Arctic Ocean seawater are elevated compared to lower latitudes. Riverine MeHg inputs account for approximately 15% of inputs to the surface ocean (2.5 Mg a 1) but greater importance in the future is likely given increasing freshwater discharges and permafrost melt. This may offset potential declines driven by increasing evasion from ice-free surface waters. Geochemical model simulations illustrate that for the most biologically relevant regions of the ocean, regulatory actions that decrease Hg inputs have the capacity to rapidly affect aquatic Hg concentrations.

Top-down constraints on atmospheric mercury emissions and implications for global biogeochemical cycling

Song, S.; Selin, N.E.; Soerensen, A.L.; Angot, H.; Artz, R.; Brooks, S.; Brunke, E.-G., Conley, G.; Dommergie, A.; Ebinghaus, R.; Holsen, T.M.; Jaffe, D.A.; Kang, S.; Kelley, P.; Luke, W.T.; Magand, O.; Marumoto, K.; Pfaffhuber, K.A.; Ren, X.; Sheu, G.-R.; Slemr, F.; Warneke, T.; Weigelt, A.; Weiss-Penzias, P; Wip, D.C.; Zhang, Q.
2015 | Atmos. Chem. Phys. | 15 (7103-7125)

We perform global-scale inverse modeling to constrain present-day atmospheric mercury emissions and relevant physio-chemical parameters in the GEOS-Chem chemical transport model. We use Bayesian inversion methods combining simulations with
5 GEOS-Chem and ground-based Hg0 observations from regional monitoring networks and individual sites in recent years. Using optimized emissions/parameters, GEOS-Chem better reproduces these ground-based observations, and also matches regional over-water Hg0 and wet deposition measurements. The optimized global mercury emission to the atmosphere is 5.8 Gg/yr-1. The ocean accounts for 3.2 Gg/yr-1 (55% of 10 the total), and the terrestrial ecosystem is neither a net source nor a net sink of Hg0. The optimized Asian anthropogenic emission of Hg0 (gas elemental mercury) is 650– 1770 Mg/yr-1, higher than its bottom-up estimates (550–800 Mg/yr-1). The ocean parameter inversions suggest that dark oxidation of aqueous elemental mercury is faster, and less mercury is removed from the mixed layer through particle sinking, when compared with current simulations. Parameter changes affect the simulated global ocean mercury budget, particularly mass exchange between the mixed layer and subsurface waters. Based on our inversion results, we re-evaluate the long-term global biogeo- chemical cycle of mercury, and show that legacy mercury becomes more likely to reside in the terrestrial ecosystem than in the ocean. We estimate that primary anthropogenic 20 mercury contributes up to 23 % of present-day atmospheric deposition.

Differences in decadal trends of atmospheric mercury between the Arctic and northern mid-latitudes suggest a decline in Arctic Ocean mercury

Chen, L; Zhang, Y.; Jacob, D.J.; Soerensen, A.L.; Fisher, J.A.; Horowitz, H.M.; Corbitt, E.S.; Wang, X.
2015 | Geophys Res Lett | 42 (6076-6083)

Atmospheric mercury (Hg) in the Arctic shows much weaker or insignificant annual declines relative to northern mid-latitudes over the past decade (2000-2009), but with strong seasonality in trends. We use a global ocean-atmosphere model of Hg (GEOS-Chem) to simulate these observed trends and determine the driving environmental variables. The atmospheric decline at northern mid-latitudes can largely be explained by decreasing North Atlantic oceanic evasion. The mid-latitude atmospheric signal propagates to the Arctic but is there countered by rapid Arctic warming and declining sea ice, which suppresses deposition and promotes oceanic evasion over the Arctic Ocean. The resulting simulation implies a decline of Hg in the Arctic surface ocean that we estimate to be -0.67% yr-1 over the study period. Rapid Arctic warming and declining sea ice are projected for future decades and would drive a sustained decline in Arctic Ocean Hg, potentially alleviating the methylmercury exposure risk for northern populations.

Freshwater discharges drive high levels of methylmercury in Arctic marine biota

Amina T. Schartup; Prentiss H. Balcom; Anne L. Soerensen; Kathleen J. Gosnell; Ryan S. D. Calder; Robert P. Mason; Elsie M. Sunderland
2015 | Proc. Natl. Acad. Sci. U.S.A. | 112 (38) (11789-11794)

Elevated levels of neurotoxic methylmercury in Arctic food-webs pose health risks for indigenous populations that consume large quantities of marine mammals and fish. Estuaries provide critical hunting and fishing territory for these populations, and, until recently, benthic sediment was thought to be the main methylmercury source for coastal fish. New hydroelectric developments are being proposed in many northern ecosystems, and the ecological impacts of this industry relative to accelerating climate changes are poorly characterized. Here we evaluate the competing impacts of climate-driven changes in northern ecosystems and reservoir flooding on methylmercury production and bioaccumulation through a case study of a stratified sub-Arctic estuarine fjord in Labrador, Canada. Methylmercury bioaccumulation in zooplankton is higher than in midlatitude ecosystems. Direct measurements and modeling show that currently the largest methylmercury source is production in oxic surface seawater. Water-column methylation is highest in stratified surface waters near the river mouth because of the stimulating effects of terrestrial organic matter on methylating microbes. We attribute enhanced biomagnification in plankton to a thin layer ofmarine snowwidely observed in stratified systems that concentrates microbial methylation and multiple trophic levels of zooplankton in a vertically restricted zone. Large freshwater inputs and the extensive Arctic Ocean continental shelf mean these processes are likely widespread and will be enhanced by future increases in water-column stratification, exacerbating high biological methylmercury concentrations. Soil flooding experiments indicate that near-term changes expected from reservoir creation will increase methylmercury inputs to the estuary by 25–200%, overwhelming climate-driven changes over the next decade.

Elemental mercury concentrations and fluxes in the tropical atmosphere and ocean

Soerensen, A.L.; R.P. Mason: P.H. Balcom; D.J. Jacob; Y. Zhang; J. Kuss; E.M. Sunderland
2014 | Environ. Sci. Technol. | online

Air-sea exchange of elemental mercury (Hg0) is a critical component of the global biogeochemical Hg cycle. To better understand variability in atmospheric and oceanic Hg0, we collected high-resolution measurements across large gradients in seawater temperature, salinity, and productivity in the Pacific Ocean (20°N-15°S). We modeled surface ocean Hg inputs and losses using an ocean general circulation model (MITgcm) and an atmospheric chemical transport model (GEOS-Chem). Observed surface seawater Hg0 was much more variable than atmospheric concentrations. Peak seawater Hg0 concentrations (~130 fM) observed in the Pacific inter-tropical convergence zone (ITCZ) were ~3-fold greater than surrounding areas (~50 fM). This is similar to observations from the Atlantic Ocean. Peak evasion in the northern Pacific ITCZ was four times higher than surrounding regions and located at the intersection of high wind speeds and elevated seawater Hg0. Modeling results show high Hg inputs from enhanced precipitation in the ITCZ combined with the shallow ocean mixed layer in this region drive elevated seawater Hg0 concentrations. Modeled seawater Hg0 concentrations reproduce observed peaks in the ITCZ of both the Atlantic and Pacific Oceans but underestimate its magnitude, likely due to insufficient deep convective scavenging of oxidized Hg from the upper troposphere. Our results demonstrate the importance of scavenging of reactive mercury in the upper atmosphere driving variability in seawater Hg0 and net Hg inputs to biologically productive regions of the tropical ocean.

Elevated Methylmercury Concentrations in Baltic Sea Hypoxic and Anoxic Waters

2014 | American Geophysical Union Annual Fall Meeting

AGU 2014 | September 19, 2019 | San Francisco, USA

Drivers of seawater mercury concentrations in the surface ocean and air-sea exchange: six cruises in the West Atlantic Ocean

Soerensen, A.L.; R.P. Mason; P. Balcom; E.M. Sunderland
2013 | Environ. Sci. Technol. | 47 (7757-7765)
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