Impacts of climate change on methylmercury formation and bioaccumulation in the 21st century ocean

Abstract

•Seawater MeHg may increase in the polar oceans and decrease in the North Atlantic in 2100•Plankton MeHg may increase at high latitudes and decrease at mid to low latitudes•Ocean acidification leads to different spatial patterns compared with physical factors Climate change is altering primary production and plankton biomass in the global ocean, which in turn will influence the formation and bioaccumulation of the neurotoxin methylmercury (MeHg). Here we use a model to project how changes in the ocean impact MeHg. Results show an almost doubling of seawater MeHg in the polar oceans and a decrease in the North Atlantic Ocean due to changes in primary productivity. Phytoplankton MeHg may increase at high latitudes and decrease in the mid- and low-latitude oceans due to the shifts in phytoplankton communities. Ocean acidification might enhance the MeHg uptake by phytoplankton by promoting the growth of a small species that efficiently accumulates MeHg. Simulated changes in zooplankton MeHg differ from phytoplankton due to complex grazing relationships. These effects thus need to be considered when evaluating future trajectories of biological MeHg concentrations, including marine fish and shellfish that are consumed by humans. Climate change-driven alterations to marine biogeochemistry will impact the formation and trophic transfer of the bioaccumulative neurotoxin methylmercury (MeHg) in the global ocean. We use a 3D model to examine how MeHg might respond to changes in primary production and plankton community driven by ocean acidification and alterations in physical factors (e.g., ocean temperature, circulation). Productivity changes lead to significant increases in seawater MeHg in the polar oceans and a decrease in the North Atlantic Ocean. Phytoplankton MeHg may increase at high latitudes and decrease in lower latitudes due to shifts in community structure. Ocean acidification might enhance phytoplankton MeHg uptake by promoting the growth of a small species that efficiently accumulate MeHg. Non-linearities in the food web structure lead to differing magnitudes of zooplankton MeHg changes relative to those for phytoplankton. Climate-driven shifts in marine biogeochemistry thus need to be considered when evaluating future trajectories in biological MeHg concentrations. Climate change-driven alterations to marine biogeochemistry will impact the formation and trophic transfer of the bioaccumulative neurotoxin methylmercury (MeHg) in the global ocean. We use a 3D model to examine how MeHg might respond to changes in primary production and plankton community driven by ocean acidification and alterations in physical factors (e.g., ocean temperature, circulation). Productivity changes lead to significant increases in seawater MeHg in the polar oceans and a decrease in the North Atlantic Ocean. Phytoplankton MeHg may increase at high latitudes and decrease in lower latitudes due to shifts in community structure. Ocean acidification might enhance phytoplankton MeHg uptake by promoting the growth of a small species that efficiently accumulate MeHg. Non-linearities in the food web structure lead to differing magnitudes of zooplankton MeHg changes relative to those for phytoplankton. Climate-driven shifts in marine biogeochemistry thus need to be considered when evaluating future trajectories in biological MeHg concentrations. Mercury (Hg) is a global toxicant of concern. Its organic form, monomethylmercury (CH3Hg), has been associated with neurocognitive deficits in children and impaired cardiovascular health in adults.1Debes F. Weihe P. Grandjean P. Cognitive deficits at age 22 years associated with prenatal exposure to methylmercury.Cortex. 2016; 74: 358-369Crossref PubMed Scopus (82) Google Scholar,2Roman H.A. Walsh T.L. Coull B.A. Dewailly É. Guallar E. Hattis D. Mariën K. Schwartz J. Stern A.H. Virtanen J.K. et al.Evaluation of the cardiovascular effects of methylmercury exposures: current evidence supports development of a dose-response function for regulatory benefits analysis.Environ. Health Perspect. 2011; 119: 607-614Crossref PubMed Scopus (150) Google Scholar In most countries, human CH3Hg exposure occurs predominantly through seafood consumption, and thus cycling of CH3Hg in the ocean is of great interest.3Eagles-Smith C.A. Silbergeld E.K. Basu N. Bustamante P. Diaz-Barriga F. Hopkins W.A. Kidd K.A. Nyland J.F. Modulators of mercury risk to wildlife and humans in the context of rapid global change.Ambio. 2018; 47: 170-197Crossref PubMed Scopus (143) Google Scholar,4Sunderland E.M. Li M. Bullard K. Erratum: “decadal changes in the edible supply of seafood and methylmercury exposure in the United States.Environ. Health Perspect. 2018; 126: 029003Crossref PubMed Scopus (4) Google Scholar CH3Hg in the ocean is mainly formed in situ from atmospherically deposited inorganic Hg.5Obrist D. Kirk J.L. Zhang L. Sunderland E.M. Jiskra M. Selin N.E. A review of global environmental mercury processes in response to human and natural perturbations: changes of emissions, climate, and land use.Ambio. 2018; 47: 116-140Crossref PubMed Scopus (264) Google Scholar,6Zhang Y. Soerensen A.L. Schartup A.T. Sunderland E.M. A global model for methylmercury formation and uptake at the base of marine food webs.Glob. Biogeochem. Cycles. 2020; 34: 1-21Crossref Scopus (20) Google Scholar CH3Hg efficiently bioaccumulates in marine food webs with the largest magnification between seawater and plankton.7Lee C.S. Fisher N.S. Methylmercury uptake by diverse marine phytoplankton.Limnol. Oceanogr. 2016; 61: 1626-1639Crossref PubMed Scopus (43) Google Scholar,8Schartup A.T. Qureshi A. Dassuncao C. Thackray C.P. Harding G. Sunderland E.M. A model for methylmercury uptake and trophic transfer by marine plankton.Environ. Sci. Technol. 2018; 52: 654-662Crossref PubMed Scopus (43) Google Scholar The first global treaty aimed at reducing anthropogenic Hg releases (the Minamata Convention) entered into force in 2017 (http://mercuryconvention.org). Evaluating the effectiveness of this treaty requires diagnosing the roles of anthropogenic Hg emissions and climate-driven changes for future CH3Hg exposures. Excess radiative forcing associated with climate change is leading to increases in global sea surface temperature and altered ocean circulation, with secondary effects on nutrient and light availability.9Marinov I. Doney S.C. Lima I.D. Response of ocean phytoplankton community structure to climate change over the 21st century: partitioning the effects of nutrients, temperature and light.Biogeosciences. 2010; 7: 3941-3959Crossref Scopus (113) Google Scholar Higher atmospheric CO2 concentrations also lead to elevated carbonic acid and a lowering of seawater pH or ocean acidification (a projected drop of about 0.4 pH units by 2100).10Dutkiewicz S. Morris J.J. Follows M.J. Scott J. Levitan O. Dyhrman S.T. Berman-Frank I. Impact of ocean acidification on the structure of future phytoplankton communities.Nat. Clim. Chang. 2015; 5: 1002-1006Crossref Scopus (135) Google Scholar These changes are expected to substantially alter spatial patterns in primary productivity, carbon remineralization, and phytoplankton community structure,10Dutkiewicz S. Morris J.J. Follows M.J. Scott J. Levitan O. Dyhrman S.T. Berman-Frank I. Impact of ocean acidification on the structure of future phytoplankton communities.Nat. Clim. Chang. 2015; 5: 1002-1006Crossref Scopus (135) Google Scholar,11Dutkiewicz S. Scott J.R. Follows M.J. Winners and losers: ecological and biogeochemical changes in a warming ocean.Glob. Biogeochem. Cycles. 2013; 27: 463-477Crossref Scopus (83) Google Scholar thus indirectly affecting CH3Hg accumulation in food webs.12Krabbenhoft D.P. Sunderland E.M. Global change and mercury.Science. 2013; 341: 1457-1458Crossref PubMed Scopus (211) Google Scholar In particular, ocean acidification is expected to change spatial patterns in plankton community composition.10Dutkiewicz S. Morris J.J. Follows M.J. Scott J. Levitan O. Dyhrman S.T. Berman-Frank I. Impact of ocean acidification on the structure of future phytoplankton communities.Nat. Clim. Chang. 2015; 5: 1002-1006Crossref Scopus (135) Google Scholar Understanding the effects of climate change on CH3Hg formation in the marine environment is limited. Booth and Zeller13Booth S. Zeller D. Mercury, food webs, and marine mammals: implications of diet and climate change for human health.Environ. Health Perspect. 2005; 113: 521-526Crossref PubMed Scopus (145) Google Scholar predicted an increase in seawater total methylmercury [MeHg, the sum of CH3Hg and (CH3)2Hg] concentrations in a future climate because they assumed inorganic Hg methylation would be enhanced by warmer ocean temperatures.14Downs S.G. Macleod C.L. Lester J.N. Mercury in precipitation and its relation to bioaccumulation in fish: a literature review.Water Air Soil Pollut. 1998; 108: 149-187Crossref Scopus (170) Google Scholar Many studies have shown MeHg formation in the marine water column is associated with the activity of heterotrophic,15Sunderland E.M. Krabbenhoft D.P. Moreau J.W. Strode S.A. Landing W.M. Mercury sources, distribution, and bioavailability in the North Pacific Ocean: insights from data and models.Glob. Biogeochem. Cycles. 2009; 23: 1-14Crossref Scopus (317) Google Scholar, 16Cossa D. Averty B. Pirrone N. The origin of methylmercury in open mediterranean waters.Limnol. Oceanogr. 2009; 54: 837-844Crossref Scopus (162) Google Scholar, 17Cossa D. Heimbürger L.E. Lannuzel D. Rintoul S.R. Butler E.C.V. Bowie A.R. Averty B. Watson R.J. Remenyi T. Mercury in the Southern Ocean.Geochim. Cosmochim. Acta. 2011; 75: 4037-4052Crossref Scopus (161) Google Scholar, 18Gilmour C.C. Podar M. Bullock A.L. Graham A.M. Brown S.D. Somenahally A.C. Johs A. Hurt R.A. Bailey K.L. Elias D.A. Mercury methylation by novel microorganisms from new environments.Environ. Sci. Technol. 2013; 47: 11810-11820Crossref PubMed Scopus (405) Google Scholar and the expression of the key Hg methylating genes, hgcAB, has been found across all ocean basins.19Gionfriddo C.M. Tate M.T. Wick R.R. Schultz M.B. Zemla A. Thelen M.P. Schofield R. Krabbenhoft D.P. Holt K.E. Moreau J.W. Microbial mercury methylation in Antarctic sea ice.Nat. Microbiol. 2016; 1: 1-12Crossref Scopus (78) Google Scholar,20Villar E. Cabrol L. Heimbürger-Boavida L.E. Widespread microbial mercury methylation genes in the global ocean.Environ. Microbiol. Rep. 2020; 12: 277-287Crossref PubMed Scopus (29) Google Scholar It has been widely suggested that climate change will exacerbate MeHg production in the future, as higher temperatures facilitate organic carbon remineralization.5Obrist D. Kirk J.L. Zhang L. Sunderland E.M. Jiskra M. Selin N.E. A review of global environmental mercury processes in response to human and natural perturbations: changes of emissions, climate, and land use.Ambio. 2018; 47: 116-140Crossref PubMed Scopus (264) Google Scholar,12Krabbenhoft D.P. Sunderland E.M. Global change and mercury.Science. 2013; 341: 1457-1458Crossref PubMed Scopus (211) Google Scholar,21Stern G.A. Macdonald R.W. Outridge P.M. Wilson S. Chételat J. Cole A. Hintelmann H. Loseto L.L. Steffen A. Wang F. et al.How does climate change influence arctic mercury?.Sci. Total Environ. 2012; 414: 22-42Crossref PubMed Scopus (150) Google Scholar The effects of climate change on CH3Hg uptake at the base of marine food webs are similarly uncertain. Jonsson et al.22Jonsson S. Andersson A. Nilsson M.B. Skyllberg U. Lundberg E. Schaefer J.K. Åkerblom S. Björn E. Terrestrial discharges mediate trophic shifts and enhance methylmercury accumulation in estuarine biota.Sci. Adv. 2017; 3: 1-10Crossref Scopus (60) Google Scholar suggested that increases in terrestrial dissolved organic matter (DOM) discharges from rivers may elongate the trophic structure of estuarine food webs, leading to higher CH3Hg concentrations. However, in open ocean regions, DOM concentrations are much more uniform and will not be subject to the same magnitudes of changes as the shelf and slope.23Hansell D.A. Carlson C.A. Repeta D.J. Schlitzer R. Dissolved organic matter in the ocean a controversy stim ulates new insights.Oceanography. 2009; 22: 202-211Crossref Scopus (524) Google Scholar Schartup et al.8Schartup A.T. Qureshi A. Dassuncao C. Thackray C.P. Harding G. Sunderland E.M. A model for methylmercury uptake and trophic transfer by marine plankton.Environ. Sci. Technol. 2018; 52: 654-662Crossref PubMed Scopus (43) Google Scholar showed that shifts in phytoplankton community composition due to changing ecosystem productivity alter CH3Hg uptake at the base of marine food webs. As some ecosystems become less productive, phytoplankton communities favor smaller species with larger cell surface area-to-volume ratios that facilitate nutrient and CH3Hg uptake.7Lee C.S. Fisher N.S. Methylmercury uptake by diverse marine phytoplankton.Limnol. Oceanogr. 2016; 61: 1626-1639Crossref PubMed Scopus (43) Google Scholar,8Schartup A.T. Qureshi A. Dassuncao C. Thackray C.P. Harding G. Sunderland E.M. A model for methylmercury uptake and trophic transfer by marine plankton.Environ. Sci. Technol. 2018; 52: 654-662Crossref PubMed Scopus (43) Google Scholar Prior work has assumed that uptake rates for CH3Hg increase in warmer seawater.13Booth S. Zeller D. Mercury, food webs, and marine mammals: implications of diet and climate change for human health.Environ. Health Perspect. 2005; 113: 521-526Crossref PubMed Scopus (145) Google Scholar,24Zeller D. Reinert J. Modelling spatial closures and fishing effort restrictions in the Faroe Islands marine ecosystem.Ecol. Modell. 2004; 172: 403-420Crossref Scopus (37) Google Scholar,25Alava J.J. Cisneros-Montemayor A.M. Sumaila U.R. Cheung W.W.L. Projected amplification of food web bioaccumulation of MeHg and PCBs under climate change in the Northeastern Pacific.Sci. Rep. 2018; 8: 1-12Crossref PubMed Scopus (24) Google Scholar However, experimental data collected by Lee and Fisher7Lee C.S. Fisher N.S. Methylmercury uptake by diverse marine phytoplankton.Limnol. Oceanogr. 2016; 61: 1626-1639Crossref PubMed Scopus (43) Google Scholar did not show a significant change in CH3Hg uptake at higher seawater temperatures for most marine phytoplankton species. Instead, the cell surface area-to-volume ratio appears to be the most important factor driving CH3Hg uptake by phytoplankton,8Schartup A.T. Qureshi A. Dassuncao C. Thackray C.P. Harding G. Sunderland E.M. A model for methylmercury uptake and trophic transfer by marine plankton.Environ. Sci. Technol. 2018; 52: 654-662Crossref PubMed Scopus (43) Google Scholar illustrating the importance of potential shifts in phytoplankton community structure. Impacts of climate-driven changes on CH3Hg bioaccumulation are known to propagate to higher trophic-level fish.26Schartup A.T. Thackray C.P. Qureshi A. Dassuncao C. Gillespie K. Hanke A. Sunderland E.M. Climate change and overfishing increase neurotoxicant in marine predators.Nature. 2019; 572: 648-650Crossref PubMed Scopus (67) Google Scholar Prior modeling efforts have assumed a temperature-dependent increase in the grazing flux for herbivorous zooplankton under a warmer climate.13Booth S. Zeller D. Mercury, food webs, and marine mammals: implications of diet and climate change for human health.Environ. Health Perspect. 2005; 113: 521-526Crossref PubMed Scopus (145) Google Scholar,25Alava J.J. Cisneros-Montemayor A.M. Sumaila U.R. Cheung W.W.L. Projected amplification of food web bioaccumulation of MeHg and PCBs under climate change in the Northeastern Pacific.Sci. Rep. 2018; 8: 1-12Crossref PubMed Scopus (24) Google Scholar Alava et al.25Alava J.J. Cisneros-Montemayor A.M. Sumaila U.R. Cheung W.W.L. Projected amplification of food web bioaccumulation of MeHg and PCBs under climate change in the Northeastern Pacific.Sci. Rep. 2018; 8: 1-12Crossref PubMed Scopus (24) Google Scholar reported such bioenergetic shifts will result in an approximately 12% increase in CH3Hg concentrations in zooplankton in 2100 under the RCP 8.5 scenario (a high emission scenario often referred to as “business as usual”).27Moss R.H. Edmonds J.A. Hibbard K.A. Manning M.R. Rose S.K. Van Vuuren D.P. Carter T.R. Emori S. Kainuma M. Kram T. et al.The next generation of scenarios for climate change research and assessment.Nature. 2010; 463: 747-756Crossref PubMed Scopus (4071) Google Scholar However, these studies did not consider regional shifts in phytoplankton community structure in a warmer environment that would affect the grazing flux for herbivorous zooplankton, which depends on the biomass of both phytoplankton and zooplankton.11Dutkiewicz S. Scott J.R. Follows M.J. Winners and losers: ecological and biogeochemical changes in a warming ocean.Glob. Biogeochem. Cycles. 2013; 27: 463-477Crossref Scopus (83) Google Scholar These changes also potentially influence the food web dynamics of CH3Hg. Here we examine the effects of future changes in ocean biogeochemistry on CH3Hg concentration in seawater and its bioaccumulation at the base of the marine food webs. We simulate seawater MeHg concentrations and CH3Hg uptake by marine food webs using a global 3-dimensional model (MITgcm-Hg).6Zhang Y. Soerensen A.L. Schartup A.T. Sunderland E.M. A global model for methylmercury formation and uptake at the base of marine food webs.Glob. Biogeochem. Cycles. 2020; 34: 1-21Crossref Scopus (20) Google Scholar The model is driven by the output (e.g., carbon remineralization, plankton biomass, and grazing fluxes) of an ecosystem model (Darwin) that was used to simulate the ecological response to climate change in a “business as usual” emissions scenario over the 21st century.10Dutkiewicz S. Morris J.J. Follows M.J. Scott J. Levitan O. Dyhrman S.T. Berman-Frank I. Impact of ocean acidification on the structure of future phytoplankton communities.Nat. Clim. Chang. 2015; 5: 1002-1006Crossref Scopus (135) Google Scholar We perform three simulations: (1) a baseline simulation, in which MITgcm-Hg is driven by the output of the Darwin model for the year 2000; (2) a simulation for the year 2100, in which MITgcm-Hg is driven by the projection of the Darwin model as a consequence of changing ocean temperature, circulation, and sea-ice cover (i.e., physical changes) in 2100. We also explore the effects of changing ocean biogeochemistry on MeHg driven by ocean acidification. We do this by conducting (3) a simulation with both physical changes as in (2) and also with pH changes simulated in the work by Dutkiewicz et al.10Dutkiewicz S. Morris J.J. Follows M.J. Scott J. Levitan O. Dyhrman S.T. Berman-Frank I. Impact of ocean acidification on the structure of future phytoplankton communities.Nat. Clim. Chang. 2015; 5: 1002-1006Crossref Scopus (135) Google Scholar The difference between simulation 2 and 3 is attributed to ocean acidification and is referred to as “ocean acidification only” results. The impact of other climate-change factors (i.e., difference between 2 and 1) is referred to as “physical factors only” results, and the combined effects of all above-mentioned factors (i.e., difference between 3 and 1) are referred to as “all factors” results. We hold both the anthropogenic and natural emissions of Hg constant for the year 2010, as well as the current day circulation for MITgcm-Hg, to diagnose the sensitivity of the changing ocean biogeochemistry and ecology of the future ocean (see Experimental procedures for details). We focus on MeHg concentrations because most observational studies report the sum of CH3Hg and (CH3)2Hg in seawater. Modeled seawater MeHg concentrations for the upper 100 m of the water column (surface ocean) in the year 2000 (52 ± 55 fM) are consistent with available observations (67 ± 73 fM) during 1990–2015 (Figure 1A, data are from Zhang et al.,6Zhang Y. Soerensen A.L. Schartup A.T. Sunderland E.M. A global model for methylmercury formation and uptake at the base of marine food webs.Glob. Biogeochem. Cycles. 2020; 34: 1-21Crossref Scopus (20) Google Scholar Bowman et al.,28Bowman K.L. Lamborg C.H. Agather A.M. A global perspective on mercury cycling in the ocean.Sci. Total Environ. 2020; 710: 136166Crossref PubMed Scopus (24) Google Scholar and references therein). The modeled concentrations at peak MeHg level depth in the subsurface ocean (350 ± 310 fM, typically 300–500 m depth) also agree with observations (290 ± 280 fM, Figure 1B). For the year 2100, MITgcm-Hg suggests a 9% increase for the global mean seawater MeHg concentrations in the surface ocean (Figure 1G) and 6% decrease at the peak MeHg depth (Figure 1H) as a result of changing ocean biogeochemistry and ecology due to all factors. However, the directionality and magnitude of the projected changes differ substantially for different ocean regions and driving factors (Figures 1C–1F and Table 1).Table 1Percentage regional responses to different driving factors in 2100 relative to the present-day conditionsBasinsSeawateraFor MeHg.PlanktonbFor CH3Hg.BCFfDefined as the ratios of the CH3Hg concentration in all phytoplankton divided by that in seawater.ZMFgDefined as the ratio of the CH3Hg concentration in herbivorous zooplankton divided by that in phytoplankton.PhytoplanktonZooplanktonsurfacemaxdiatomsynprosmalllargeArctic−8cChanges due to physical factors only.−5−6648169−3621−756dChanges due to ocean acidification only.14553−133−1214−9−3eChanges due to all factorsy.−4−2210135−4835−84−33−30−61−29−40−39−7720−60NA42−1317−6022−87−12−29−28−75−11−100−17−8528−72−29−17−42−48−28−61−5729−25TA41−2727−7213−3−4513−25−17−69−21−100−48−61−16−12−10−119−15−5−6−20SA91−2054−8565−2012−1281−3064−10060−2611−12−5−6−15−13−10−14−1717−6NP80−1729−9041−10−1993−6−3216−10027−27−24−4−4−24−18−15−20−362−5TP51−31280−8515−18−54251−4−55262−100−5−53−5221−7−15−27−85−14−3215−11SP114−2364−10573−2−55394−11−5056−10058−33−39281−3−70−11−20−1012−9Indian112−2877−8925−8−22−1112−1−3577−1005−18−10−19681461163−43164−2623S7041118−57−3712−417514102281−100127−13−182−7−813−14−23−357−26Global71−1164−8625−12−3619−6−1977−1003−47−29−24The global ocean is divided into nine basins: Arctic, NA (North Atlantic), TA (Tropical Atlantic), SA (South Atlantic), NP (North Pacific), TP (Tropical Pacific), SP (South Pacific), Indian, and S (Southern) Oceans. Blank values indicate too little biomass to calculate robustly.a For MeHg.b For CH3Hg.c Changes due to physical factors only.d Changes due to ocean acidification only.e Changes due to all factorsy.f Defined as the ratios of the CH3Hg concentration in all phytoplankton divided by that in seawater.g Defined as the ratio of the CH3Hg concentration in herbivorous zooplankton divided by that in phytoplankton. Open table in a new tab The global ocean is divided into nine basins: Arctic, NA (North Atlantic), TA (Tropical Atlantic), SA (South Atlantic), NP (North Pacific), TP (Tropical Pacific), SP (South Pacific), Indian, and S (Southern) Oceans. Blank values indicate too little biomass to calculate robustly. First, we discuss the impacts of physical factors. In the surface ocean, the model suggests a decline in seawater MeHg concentrations in 2100 in the North and Tropical Atlantic Ocean (33% and 29%, respectively, Figure 1C). This reflects the reduced supply of nutrients projected by the Darwin model due to enhanced ocean stratification and changes in seawater circulation, which decreases primary production and carbon remineralization associated with Hg methylation.6Zhang Y. Soerensen A.L. Schartup A.T. Sunderland E.M. A global model for methylmercury formation and uptake at the base of marine food webs.Glob. Biogeochem. Cycles. 2020; 34: 1-21Crossref Scopus (20) Google Scholar In 2100, increases in stratification and reduced nutrient supply, as projected by the Darwin model, result in a shift in the phytoplankton community structure toward smaller size classes (such changes in community structure are typically simulated in climate change marine ecosystem models; e.g., Bopp et al.,29Bopp L. Aumont O. Cadule P. Alvain S. Gehlen M. Response of diatoms distribution to global warming and potential implications: a global model study.Geophys. Res. Lett. 2005; 32: 1-4Crossref Scopus (267) Google Scholar Steinacher et al.,30Steinacher M. Joos F. Frölicher T.L. Bopp L. Cadule P. Doney S.C. Gehlen M. Schneider B. Segschneider J. Projected 21st century decrease in marine productivity: a multi-model analysis.Biogeosci. Discuss. 2009; 6: 7933-7981Google Scholar Marinov et al.9Marinov I. Doney S.C. Lima I.D. Response of ocean phytoplankton community structure to climate change over the 21st century: partitioning the effects of nutrients, temperature and light.Biogeosciences. 2010; 7: 3941-3959Crossref Scopus (113) Google Scholar). This diminishes export production at 100 m depth by 20%.10Dutkiewicz S. Morris J.J. Follows M.J. Scott J. Levitan O. Dyhrman S.T. Berman-Frank I. Impact of ocean acidification on the structure of future phytoplankton communities.Nat. Clim. Chang. 2015; 5: 1002-1006Crossref Scopus (135) Google Scholar Seawater MeHg concentrations in the surface ocean are projected to increase in both the Arctic (87%, except for the region adjacent to the North Atlantic Ocean) and southern (130%) oceans in the year 2100 due to physical factors (Figure 1C). The Darwin model projects that increasing seawater temperature and altered light environment in the 21st century in the polar regions lead to higher primary productivity and carbon remineralization.10Dutkiewicz S. Morris J.J. Follows M.J. Scott J. Levitan O. Dyhrman S.T. Berman-Frank I. Impact of ocean acidification on the structure of future phytoplankton communities.Nat. Clim. Chang. 2015; 5: 1002-1006Crossref Scopus (135) Google Scholar The MITgcm-Hg model suggests that this could potentially increase surface ocean MeHg concentrations (Figures 1C and Table 1). Such changes are also likely to be affected by declines in inorganic Hg as the substrate for methylation due to enhanced oceanic evasion in ice-free waters31Fisher J.A. Jacob D.J. Soerensen A.L. Amos H.M. Corbitt E.S. Streets D.G. Wang Q. Yantosca R.M. Sunderland E.M. Factors driving mercury variability in the Arctic atmosphere and ocean over the past 30 years.Glob. Biogeochem. Cycles. 2013; 27: 1226-1235Crossref Scopus (30) Google Scholar, 32Fisher J.A. Jacob D.J. Soerensen A.L. Amos H.M. Steffen A. Sunderland E.M. Riverine source of Arctic Ocean mercury inferred from atmospheric observations.Nat. Geosci. 2012; 5: 499-504Crossref Scopus (133) Google Scholar, 33DiMento B.P. Mason R.P. Brooks S. Moore C. The impact of sea ice on the air-sea exchange of mercury in the Arctic Ocean.Deep. Res. Part Oceanogr. Res. Pap. 2019; 144: 28-38Crossref Scopus (21) Google Scholar and a potential increase in riverine discharge of inorganic Hg and MeHgz from the terrestrial environment due to thawing permafrost.34Sonke J.E. Teisserenc R. Heimbürger-Boavida L.E. Petrova M.V. Marusczak N. Le Dantec T. Chupakov A.V. Li C. Thackray C.P. Sunderland E.M. et al.Eurasian river spring flood observations support net Arctic Ocean mercury export to the atmosphere and Atlantic Ocean.Proc. Natl. Acad. Sci. U S A. 2018; 115: E11586-E11594Crossref PubMed Scopus (36) Google Scholar, 35Schaefer K. Elshorbany Y. Jafarov E. Schuster P.F. Striegl R.G. Wickland K.P. Sunderland E.M. Potential impacts of mercury released from thawing permafrost.Nat. Commun. 2020; 11: 1-6Crossref PubMed Scopus (19) Google Scholar, 36Schuster P.F. Schaefer K.M. Aiken G.R. Antweiler R.C. Dewild J.F. Gryziec J.D. Gusmeroli A. Hugelius G. Jafarov E. Krabbenhoft D.P. et al.Permafrost stores a globally significant amount of mercury.Geophys. Res. Lett. 2018; 45: 1463-1471Crossref Scopus (133) Google Scholar, 37Lim A.G. Jiskra M. Sonke J.E. Loiko S.V. Kosykh N. Pokrovsky O.S. A revised pan-Arctic permafrost soil Hg pool based on Western Siberian peat Hg and carbon observations.Biogeosciences. 2020; 17: 3083-3097Crossref Scopus (10) Google Scholar However, these changes cannot be quantitatively evaluated as the climate impacts on sea-ice fraction, permafrost dynamics, and riverine Hg discharge are not included in the MITgcm-Hg model. In contrast to the surface ocean, modeled global mean MeHg concentrations at peak depth are projected to slightly decrease by 6% in 2100 due to physical factors (Figures 1D and S1). However, the modeled changes are spatially heterogeneous with increases or almost no changes over iron-limited regions: e.g., the Southern Ocean (14%) and Tropical Pacific (−4%), as primary production and carbon remineralization over these regions increase as projected by the Darwin model.10Dutkiewicz S. Morris J.J. Follows M.J. Scott J. Levitan O. Dyhrman S.T. Berman-Frank I. Impact of ocean acidification on the structure of future phytoplankton communities.Nat. Clim. Chang. 2015; 5: 1002-1006Crossref Scopus (135) Google Scholar Declines in subsurface seawater MeHg concentrations are projected for all regions that are limited by dissolved inorganic fixed nitrogen: e.g., North Atlantic (30%), Tropical Atlantic (17%) (Figure 1D and Table 1). This reflects the modeled reduced remineralization of carbon at the subsurface ocean due to a decline in carbon export,10Dutkiewicz S. Morris J.J. Follows M.J. Scott J. Levitan O. Dyhrman S.T. Berman-Frank I. Impact of ocean acidification on the structure of future phytoplankton communities.Nat. Clim. Chang. 2015; 5: 1002-1006Crossref Scopus (135) Google Scholar which is used to parameterize microbial activity affecting MeHg formation. The modeled decline in carbon export is driven by less primary production and enhanced efficiency of carbon remineralization at shallower depths due to warmer temperatures (see discussion in Kwon et al.38Kwon E.Y. Primeau F. Sarmiento J.L. The impact of remineralization depth on the air-sea carbon balance.Nat. Geosci. 2009; 2: 630-635Crossref Scopus (242) Google Scholar). Although