
==== Front
Nat Commun
Nat Commun
Nature Communications
2041-1723
Nature Publishing Group UK London

51852
10.1038/s41467-024-51852-2
Article
Near surface oxidation of elemental mercury leads to mercury exposure in the Arctic Ocean biota
http://orcid.org/0000-0003-0584-3755
Lim Seung Hyeon 1
Kim Younggwang 1
http://orcid.org/0000-0002-4098-904X
Motta Laura C. 2
http://orcid.org/0000-0002-8639-5968
Yang Eun Jin 3
http://orcid.org/0000-0001-8025-9431
Rhee Tae Siek 3
Hong Jong Kuk 3
http://orcid.org/0000-0002-3284-0106
Han Seunghee 4
http://orcid.org/0000-0001-8665-0327
Kwon Sae Yun saeyunk@postech.ac.kr

1
1 https://ror.org/04xysgw12 grid.49100.3c 0000 0001 0742 4007 Division of Environmental Science and Engineering, Pohang University of Science and Technology, 77 Cheongam-Ro, Nam-Gu, Pohang, 37673 Republic of Korea
2 https://ror.org/03zbnzt98 grid.56466.37 0000 0004 0504 7510 Marine Chemistry & Geochemistry, Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, MA 02543 USA
3 https://ror.org/00n14a494 grid.410913.e 0000 0004 0400 5538 Korea Polar Research Institute, Incheon, 21990 Republic of Korea
4 https://ror.org/024kbgz78 grid.61221.36 0000 0001 1033 9831 School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju, 61005 Republic of Korea
31 8 2024
31 8 2024
2024
15 759815 1 2024
20 8 2024
© The Author(s) 2024
2024
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Atmospheric mercury (Hg(0), Hg(II)) and riverine exported Hg (Hg(II)) are proposed as important Hg sources to the Arctic Ocean. As plankton cannot passively uptake Hg(0), gaseous Hg(0) has to be oxidized to be bioavailable. Here, we measured Hg isotope ratios in zooplankton, Arctic cod, total gaseous Hg, sediment, seawater, and snowpack from the Bering Strait, the Chukchi Sea, and the Beaufort Sea. The Δ200Hg, used to differentiate between Hg(0) and Hg(II), shows, on average, 70% of Hg(0) in all biota and differs with seawater Δ200Hg (Hg(II)). Since Δ200Hg anomalies occur via tropospheric Hg(0) oxidation, we propose that near-surface Hg(0) oxidation via terrestrial vegetation, coastally evaded halogens, and sea salt aerosols, which preserve Δ200Hg of Hg(0) upon oxidation, supply bioavailable Hg(II) pools in seawater. Our study highlights sources and pathways in which Hg(0) poses potential ecological risks to the Arctic Ocean biota.

This study finds that atmospheric mercury is rapidly oxidized near the surface via terrestrial vegetation and sea salt aerosols, generating bioavailable mercury pools for the Arctic Ocean biota.

Subject terms

Element cycles
Marine chemistry
https://doi.org/10.13039/501100003725 National Research Foundation of Korea (NRF) NRF-2021M1A5A1075513, KOPRI-NRF-2021M1A5A1075512 Kwon Sae Yun https://doi.org/10.13039/501100011705 Korea Institute of Marine Science and Technology promotion (Korea Institute of Marine Science & Technology promotion) 20210541 KIMST RS-2021-KS211500 20210541 Yang Eun Jin Han Seunghee Kwon Sae Yun Korea Institute of Marine Science and Technology promotion (Korea Institute of Marine Science & Technology promotion)Korea Institute of Marine Science and Technology promotion (Korea Institute of Marine Science & Technology promotion)issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

Mercury (Hg) is a globally distributed trace metal, which is mainly present in the atmosphere as gaseous elemental Hg (Hg(0))1. When Hg(0) is oxidized in the atmosphere, Hg(II) deposits to the biosphere via wet (precipitation) and dry deposition (particulate bound Hg; PBM)2. While the majority of anthropogenic Hg emissions occur in the mid-latitudes in industrialized regions of Asia and North America3, the long-range transport of Hg(0) and accelerated effects of global warming pose significant Hg threats to remote polar regions such as the Arctic. A recent estimate has suggested that atmospheric Hg deposition, derived from long-range transport, mounts to 65 ± 20 Mg yr−1 in the Arctic Ocean4. Ocean currents (55 ± 7 Mg yr−1)5 and continental export from rivers (41 ± 4 Mg yr−1)6–8 and coastal erosion (39 ± 30 Mg yr−1) also supply substantial amounts of Hg4. Owing to the volatile nature, Hg(0) re-emission into the atmosphere is anticipated to change upon surface warming9,10, decline in sea ice cover9,11, and via intensified wildfire events9,12.

There are substantial gaps in knowledge regarding the sources and environmental pathways governing Hg exposure to the Arctic biota, despite the elevated Hg concentrations frequently reported in the tissues of marine mammals13,14. In many remote oceans other than the Arctic, it has been suggested Hg(II) deposited from the atmosphere15,16 and Hg(II) introduced via continental export17 are subjected to microbial methylation in the deep water column18,19 and aquatic sediment20. Given the bioaccumulative nature, the resultant methylmercury (MeHg) biomagnifies through the aquatic food web. In the Arctic Ocean, it is possible that certain biogeochemical features and/or seasonal events enhance ecological Hg exposure. In addition to high surface seawater Hg level21, microbial and abiotic methylation are thought to take place both in the (sub)surface22–24 and deeper water columns in the Arctic Ocean25,26. Seasonally important events such as the Atmospheric Mercury Depletion Event (AMDE), mediated by coastally evaded halogen27, and spring freshet also cause substantial Hg deposition and releases along with increased primary productivity24. It is unclear, however, whether these features/events fully explain Hg levels found in marine mammals and health risks posed to the Arctic populations, who regularly harvest and consume marine products28. It is estimated that 63% of the Arctic Inuit population is subjected to food insecurity, which is much higher than the global estimates of food insecurity (9.2%)28. In the absence of local anthropogenic activities, the sources and pathways leading to spatially unequal Hg exposure and health risks posed to the Arctic biota and populations should be explored.

Hg stable isotopes have played an instrumental role in deciphering sources and processes governing Hg exposure in aquatic ecosystems. Given the absence of isotopic changes during bioaccumulation and trophic transfer29,30, mass-dependent (MDF; δ202Hg) and mass-independent Hg isotope fractionation signatures (MIF; Δ199Hg, Δ200Hg, Δ201Hg, Δ204Hg) of biota have provided multi-dimensional information regarding sources, chemical forms of Hg, and biogeochemical processes prior to exposure31. Among numerous applications in the Arctic8,32–42, Jiskra and co-workers43 recently employed Δ200Hg to model the relative Hg(0) and Hg(II) input to the global seawater. The Δ200Hg has been used as a tracer for atmospheric Hg(0) and Hg(II) deposition given that Δ200Hg anomalies occur exclusively via tropospheric oxidation, resulting in a positive Δ200Hg in Hg(II) and a negative to near-zero Δ200Hg in Hg(0)44. The model estimated that, in the Arctic, 70% of gross Hg input occurs via atmospheric gaseous Hg(0) dissolution into seawater relative to wet/dry Hg(II) deposition, leaving near-zero Δ200Hg in both the seawater and biota. Dissolution and evasion of Hg(0) from seawater, resulting in bi-directional gas exchange of Hg(0), has previously been measured in various oceanic basins45–47. Given the active Hg redox cycle in polar marine waters11,48 and the preferential assimilation of dissolved Hg(II) by plankton49–51, the oxidation of gaseous Hg(0) in seawater followed by passive Hg(II) diffusion into the plankton cell may explain the near-zero Δ200Hg in the Arctic Ocean seawater and biota. Alternatively, other sources or processes may be responsible for supplying bioavailable Hg(II) pools in the seawater.

We measured total Hg (THg) concentration and Hg isotope ratios in zooplankton, Arctic cods (Boreogadus saida), and other environmental matrices (total gaseous Hg; TGM, seawater, precipitation, sediment, snowpack) sampled from the Bering Strait (Section A), Chukchi Sea (Section B), and Beaufort Sea (Section C) (Fig. 1, Supplementary Fig. ﻿1). Given that our sampling sites vary in their distances away from land and major riverine basins, we compiled Hg isotope data reported previously from the Mackenzie River38. The sampling was performed during non-AMDE season to exclude the influence of unique seasonal events on ecological Hg exposure. This study aims to quantify potentially widespread Hg(0) influence in biota and environmental matrices of the Arctic Ocean and assess pathways in which Hg(0) is oxidized to become bioavailable. Our results would enable a greater understanding towards sources and exposure pathways of Hg to the Arctic Ocean food web.Fig. 1 Map of the sampling locations.

a Zooplankton collection sites at Section A (the Bering Strait to the Chukchi Sea) and zooplankton, Arctic cod, and snowpack collection sites at Section B (the Chukchi Sea). Red and orange boxes indicate sampling locations of beluga whale (Delphinapterus leucas), ringed seal (Pusa hispida) and polar bear (Ursus maritimus) from Masbou et al.34, respectively. b Zooplankton and environmental matrices (sediment, seawater, TGM, precipitation) sampled at Section C (the Mackenzie River estuaries to the Beaufort Sea). Source data are provided as a Source Data file. Figures were prepared using the dataset sourced from The GEBCO Grid109.

Results and discussion

Overall Hg pattern in the Arctic Ocean

In the Beaufort Sea, where diverse environmental media were sampled (Section C, Fig. ﻿1b), we observed an average TGM and precipitation THg of 1.38 ± 0.27 ng m−3 (n = 3; 1 SD) and 3.03 ng L−1 (n = 1), respectively. The seawater THg (0.38 ± 0.15 ng L−1; 1.89 ± 0.72 pM; n = 13) shows no significant relationship with distance (regression; p = 0.48), while the surface sediment THg (62.9 ± 14.4 ng g−1, n = 11) increase with increasing distances away from the Mackenzie River to the Beaufort Sea (Supplementary Fig. ﻿2; regression; p < 0.05). In the Chukchi Sea (Section B), the snowpack sampled on sea ice have THg concentrations (1.44 ± 0.42 ng L−1, n = 3) lower than that of AMDE-affected snowpack (104 ± 40.5 ng L−1)52. The Arctic cods display a wide THg range of 48.5 to 118 ng g−1 (n = 3) due to their varying body length (6.2–14.5 cm). The observed THg are within the range of THg of Arctic cod (85 ± 5 ng g g−1 to 190 ± 30 ng g g−1)53–55 sampled at similar locations.

The zooplankton sampled across the Arctic Ocean display notable THg and MeHg patterns with respective to their size fractions (0.2−1 mm, 1−5 mm, >5 mm), sampled locations, and distances away from land. The average THg are ranked in the order of medium (64.0 ± 48.4 ng g−1, 1 SD, n = 19) > small (62.2 ± 37.3 ng g−1, n = 14) > large size fractions (35.1 ± 26.3 ng g−1, n = 11), with no significant difference between small and medium size fractions (Mann–Whitney test; p = 0.87). The observed THg are within the ranges of zooplankton previously measured from the Chukchi Sea (42.0 ± 2.0 ng g−1) and the Beaufort Sea (68.0 ± 6.6 ng g−1)56. The % MeHg, measured in a subset of zooplankton, increase with size fractions at their individual sampled locations (Section A; 0.05 to 5%, Section C; <2 to >7%) (Source Data), similar to the trend of MeHg biomagnification in aquatic food webs29,30,57,58. With respect to the sampled location, the Section B zooplankton show the highest THg (90.4 ± 31.9 ng g−1, n = 6) relative to Section A (25.3 ± 20.4 ng g–1, n = 4) and C (57.0 ± 21.9 ng g–1, n = 4). In fact, there is a significant positive relationship between zooplankton THg and distances away from land across the studied regions (Supplementary Fig. 3; regression; p < 0.05), possibly due to the exposure of spatially different Hg sources.

In regard to Hg isotopes, all zooplankton, regardless of size and location, and the Arctic cods exhibit intermediate δ202Hg and Δ199Hg to that of various environmental matrices (TGM, snowpack, seawater, sediment) characterized in this study and with the global average precipitation43, atmospheric particulate bound Hg (PBM) from Alert, Canada37, and dissolved (River dHg) and particulate Hg phases (River pHg) of the Mackenzie River waters38 (Fig. 2a, Source Data). The relative importance of these environmental matrices acting as potential Hg sources to zooplankton is discussed below. The zooplankton exhibit increasing Δ199Hg with size fractions and % MeHg, but not with δ202Hg (Source Data, Supplementary Fig. 4). The Section B zooplankton, sampled in the open ocean of the Chukchi Sea, have positive δ202Hg and negative Δ199Hg, similar to that of TGM, when compared to the zooplankton at Section A and C (Fig. 2a, Supplementary Fig. 5﻿).Fig. 2 Hg isotope values of the Arctic biota and potential endmembers.

a δ202Hg and Δ199Hg, and b. Δ200Hg and Δ204Hg of zooplankton by section, total gaseous Hg (TGM), snowpack, sediment, and seawater from this study. Hg isotope values of riverine dissolved (River dHg) (n = 13, average, 1 SD) and particulate bound Hg (River pHg) (n = 13, average, 1 SD) are from the Mackenzie River38. Precipitation values are compiled by Jiskra et al.43, sampled at various regions of the world (n = 106, median, quartile), and particulate bound Hg (PBM) values are from Alert, Canada37 (n = 10, average, 1 SD). A green circle indicates Hg isotope ranges of surface zooplankton in the Central Pacific Ocean65 (n = 6, average, 1 SD). Solid line represents Δ200Hg/Δ204Hg slope of atmospheric species (Hg(0), Hg(II); n = 219)32,37,60,61,63–66, and dotted line represents Δ200Hg/Δ204Hg slope of zooplankton (Section A: n = 9, Section B: n = 22, Section C: n = 13), sediment (n = 11), seawater (n = 13), TGM (n = 3), Arctic cod (n = 3), snowpack (n = 3), PBM, river dHg, and pHg38 (average, 1 SD, respectively). Analytical errors by reference materials are listed. Source data are provided as a Source Data file.

The Δ200Hg and Δ204Hg values (referred to as even-MIF) have been used as indicators to quantify the relative deposition of atmospheric Hg(0) and Hg(II) to the biosphere. This is because measurable Δ200Hg and Δ204Hg changes occur only via Hg(0) photo-oxidation in the troposphere, resulting in a Δ200Hg/Δ204Hg slope of −0.51 ± 0.0444,59(p < 0.05, r2 = 0.76, n = 219; Fig.2b). This slope has previously been established by the collection and evaluation of Δ200Hg and Δ204Hg data measured from various atmospheric samples59. All analyzed samples, atmospheric PBM from Alert, Canada, and the river dHg and pHg from the Mackenzie River38 depict a slope (−0.40 ± 0.08, p < 0.05, r2 = 0.48, n = 113), similar to the theoretical Hg(0) photo-oxidation (Fig. 2b). The near-zero Δ200Hg and Δ204Hg of our zooplankton, Arctic cod, sediment, atmospheric PBM, and river pHg38 are higher than the measured and compiled TGM32,43,60–64 but lower than zooplankton from the Central Pacific Ocean65, global precipitation37,43,60,62,63,65,66, and our seawater, which mostly reflect Hg(II). The snowpack and river dHg38 have intermediate Δ200Hg values, reflecting mixtures of Hg(0) and Hg(II).

In summary, the near-zero Δ200Hg observed in all zooplankton and the Arctic cods are consistent with Hg(0) but contrast with the positive Δ200Hg of the seawater, reflecting Hg(II). The zooplankton in the open ocean of the Chukchi Sea (Section B) have Hg isotopic compositions particularly similar to that of TGM. Lastly, the % MeHg and Δ199Hg increase with zooplankton sizes to fish, which is a typical pattern of MeHg biomagnification in aquatic food web29,30. On the basis of contrasting Δ200Hg values between the biota and seawater, we hypothesize that there may be specific pools of Hg in seawater, which are preferentially bioavailable for zooplankton and fish. Since Hg(0) has to be oxidized to become bioavailable49–51, we also speculate that Hg(0) has undergone some oxidation processes, which do not impart significant even-MIF, prior to methylation and bioaccumulation in the Arctic Ocean.

Sources and pathways of Hg(0) uptake by zooplankton

While the prevalence of Hg(0) in the Arctic atmosphere is well established, mediated via global transport4,9,67, snowpack Hg(II) photo-reduction32, and Hg(0) evasion from the ocean surface4,25,32,68,69, our study documented the widespread Δ200Hg in the Arctic Ocean zooplankton and fish. Here, we apply the global end-member Δ200Hg of Hg(0) (median; −0.05‰, quartiles; −0.08‰, −0.03‰) and Hg(II) (median; 0.14‰, calculated based on precipitation, reactive Hg(II)), estimated by Jiskra et al. 202143, to the Eq. (1)-(2) to quantify % contributions of Hg species. The same end-members were used by Jiskra et al. 202143 to estimate Hg(0) and Hg(II) deposition to the global seawater. The proportions of Hg(0) and Hg(II) are represented as fHg(0) and fHg(II).1 Δ200Hgsample=Δ200HgII×fHg(II)+Δ200Hg(0)×fHg(0)

2 1=fHg(II)+fHg(0)

We estimate that the zooplankton and Arctic cod have, on average, 71 ± 20% (small; 70 ± 20%, medium; 74 ± 21%, large; 67 ± 20%) and 83 ± 15% of Hg originated in the form of Hg(0), respectively (Supplementary Data. 1). The site-specific Hg(0) contribution is difficult to estimate here, given the relatively small Δ200Hg variation in the zooplankton sampled across varying locations.

The high estimated Hg(0) contributions in the Arctic Ocean zooplankton and fish are puzzling since experimental studies have repeatedly demonstrated that Hg(0) uptake by zooplankton is impossible and it would require Hg(0) to be oxidized to modulate passive uptake via the cellular membrane49–51. The near-zero Δ200Hg of our biota also contrasts with the positive Δ200Hg of the seawater, reflecting Hg(II) that has undergone oxidation and deposition from the troposphere. Since even-MIF anomalies are generated exclusively in the troposphere under UVC-light and not in aqueous solution70,71, atmospheric Hg(0) dissolution followed by oxidation in seawater is unlikely to be the primary pathway generating bioavailable Hg(II) in the seawater. Instead, there are already well-established Hg(0) oxidation pathways near the surface in the Arctic63,72,73, which do not impart even-MIF anomalies and may supply bioavailable Hg(II). The first oxidation pathway is Hg(0) uptake by terrestrial media followed by oxidation within foliar tissues. Especially in the Arctic, it is estimated that Hg(0) uptake by tundra vegetation accounts for 70% of total atmospheric Hg deposition relative to direct Hg(II) input63,72. This pathway currently explains the globally observed near-zero Δ200Hg in foliage, litter72,74 and peat core, used to model historical Hg(0) levels in the Arctic atmosphere75. Given the large gross Hg(0) flux to terrestrial ecosystem (118 ± 20 Mg yr−1) followed by riverine discharge to the Arctic Ocean (41 ± 4 Mg yr−1)4,6–8, this may explain the near-zero Δ200Hg in our zooplankton, fish, and river pHg from the Mackenzie River (Fig. 2b). The second pathway is Hg(0) oxidation at the marine boundary layer or near the land surface given the abundances of sea salt aerosols52,76–78 and bromine radicals liberated from snowpack and sea ice in the Arctic52,77,79. Hg(0) oxidation via bromine radicals recorded at a few meters above the snow surface is what drives substantial Hg(II) deposition to the Arctic snowpack during AMDE76,80,81. In contrary to the tropospheric oxidation, which imparts positive Δ200Hg in Hg(II) 82,83, this process is thought to leave negative to near-zero Δ200Hg (−0.08 ± 0.04‰; n = 9)32,52,84 in the AMDE-affected snow. Isotopic characterizations of atmospheric samples in Alert, Canada have also found that Hg(0) oxidation followed by particulate Hg(II) scavenging preserves near-zero Δ200Hg in PBM32,37.

In sum, our proposed Hg sources are no different from the global consensus that Hg in the open ocean waters and biota originates from atmospheric Hg(II) deposition65,85 and that the Arctic Ocean receives a substantial amount of Hg(II) via riverine export7,8,17,69. The difference is the pathway/location at which Hg(0) is oxidized to supply bioavailable Hg(II) in the mid-latitude ocean and the Arctic Ocean biota. In the mid-latitude oceans, Hg(0) oxidation in the troposphere followed by rain and particulate scavenging explain positive Δ200Hg (0.07 ± 0.04‰) observed in both the seawater and biota19,58,65,86. While this process explains Hg(II) in the Arctic Ocean seawater, Hg(II) that is preferentially available for zooplankton uptake is supplied by Hg(0) oxidation via terrestrial vegetation and at the land/ocean surface, mediated by bromine radicals52,77,79 and sea salt aerosols52,76–78. Apart from the seawater measured in this study and in the Canadian Arctic Archipelago (0.23 ± 0.16‰)33, oxidation in the troposphere still plays some role in supplying Hg(II) to other abiotic matrices including snow and riverine water (Fig. 2b). Even then, these sources are not as actively utilized by the zooplankton and fish.

Evidence of Hg isotope difference between seawater and biota has previously been documented by Motta et al.87 in the Central Pacific Ocean. In that study, the authors recorded overlapping δ202Hg between marine particles and MeHg, estimated using the Pacific Ocean fish, to suggest that marine particles act as the main substrate for microbial methylation and bioaccumulation at the base of the food web. The consistent Δ200Hg and Δ204Hg between atmospheric PBM, marine particles, and zooplankton, which differed significantly from the values of precipitation Hg(II) and seawater, further implied that atmospheric PBM supplies marine particles available for methylation in the open ocean. Similarly, at Section C, the δ202Hg, Δ199Hg, and Δ200Hg of the seawater and sediment are within the ranges of dHg and pHg of the Mackenzie River waters, respectively38 (Fig. 3). Based on the overall positive Δ200Hg and the estimated % Hg(II) (63%) relative to Hg(0) in the river dHg, Campeau et al. 202238 suggested that river dHg is partly sourced from wet Hg(II) deposition, which has circulated through continental watersheds via throughfall. A portion of river pHg, reflecting Hg(0) sequestered by terrestrial vegetation, is deposited to the sediment during riverine export. The river dHg (originated from wet Hg(II) deposition) acting as a source of the nearshore Beaufort Sea water is further depicted by the relationships between surface seawater profiles and our seawater Δ199Hg and Δ200Hg (Supplementary Fig. 6﻿). The surface seawater Δ199Hg and Δ200Hg reveal significant negative relationships with increasing distances away from the Mackenzie River, salinity, and dissolved oxygen, and significant positive relationships with water temperature and chlorophyll A (all p < 0.05). The reduction in Δ199Hg and Δ200Hg toward the near-zero value of Hg(0) suggests that the Arctic Ocean seawater reflects mixtures of Hg(II) exported from the river in dissolved phases and Hg(0) oxidized near the surface. The uniform zooplankton Δ200Hg (Fig. 3b) indicates that they mostly integrate oxidized Hg(0) regardless of the distance away from the Mackenzie River.Fig. 3 Hg isotope ratios of the Beaufort Sea samples.

a δ202Hg and Δ199Hg and (b) Δ200Hg and Δ199Hg of total gaseous mercury (TGM), sediment, seawater, and zooplankton measured at Section C. Ranges of Hg isotope values of riverine dissolved (River dHg; n = 13) and riverine particulate-bound Hg (River pHg; n = 13) are from Campeau et al. 202238, and the median precipitation is from Jiskra et al. 202143 (n = 106, median, quartile). Source data are provided as a Source Data file.

As such, the Hg isotope difference between seawater and biota, documented both in the Central Pacific87 and the Beaufort Sea, would be well explained if bioavailable Hg(II) pools in seawater are in particulate phases. To further describe the overall processes, we propose that Hg(0) oxidized and introduced in the form of atmospheric PBM or riverine pHg act as a substrate for microbial aggregation and methylation in seawater, which further enhances Hg/MeHg bioaccumulation. The Arctic atmosphere is well-known for high proportions of PBM, due to the abundance of sea salt, relative to reactive gaseous Hg(II) (PBM/RGM = 1.42), making atmospheric PBM widely available for deposition to seawater88. A recent experimental study has also reported higher methylation rates in unfiltered seawater relative to filtered seawater, suggesting that methylation occurs more actively in the presence of particles89. In addition to atmospherically deposited PBM, river pHg has also been reported to serve as a substrate for microbial aggregation and methylation, resulting in up to 85% of MeHg on particles sampled near riverine basins of the Arctic Ocean90–93.

Thus, we designate atmospheric PBM and river pHg, both of which originated from Hg(0), and precipitation (Hg(II)) as potential Hg sources to the zooplankton and use a ternary mixing model to calculate % source contribution to our zooplankton (Eq. (3)–(5), Supplementary Table. 1). We estimate the isotopic composition of PBM using our TGM and the isotopic difference between PBM and TGM established in Alert, Canada (δ202Hg shift by −0.51‰, Δ199Hg shift by −0.05‰)37. Note that kinetic fractionation via oxidation and particle sorption both cause more negative δ202Hg and slightly more negative Δ199Hg in PBM relative to TGM37,70,94. While all zooplankton are used for the calculation of % Hg source contribution (Supplementary Data. 1), we report the zooplankton of small size fractions given that they best reflect environmental Hg sources at the base of the food web. We do not consider snowpack as an important Hg source based on the highly negative Δ199Hg (Fig. 2a) and prior modeling studies, which estimated a low Hg influx via snow and ice melting11,69.3 δ202Hgsample=δ202Hga×fa+δ202Hgb×fb+δ202Hgc×fc

4 Δ199Hgsample=Δ199Hga×fa+Δ199Hgb×fb+Δ199Hgc×fc

5 1=fa+fb+fc

In the open ocean of the Chukchi Sea (Section B), the zooplanktons reflect 72 ± 22% of PBM, 28 ± 22% of precipitation, and 0 ± 0% of river pHg (Kruskal-Wallis test; p < 0.05; Supplementary Table. 2). In the Beaufort Sea (Section C), the zooplankton reveal 55 ± 11% contribution of PBM, 30 ± 12% of precipitation, and 14 ± 12% of river pHg (Kruskal-Wallis test; p < 0.05). The zooplankton near the Bering Strait (Section A) show stronger river pHg contribution (42 ± 18%) and precipitation (46 ± 23%), followed by PBM (13 ± 16%), in line with the larger estimated annual Hg export from the Yukon River (3282 kg y−1) relative to the Mackenzie River (2610 kg y−1)8 (Kruskal-Wallis test; p = 0.08).

Implications to the Arctic Ocean food web

A compilation of Δ200Hg in the Arctic Ocean biota, particularly those located at high trophic positions (ringed seal, polar bear, beluga whale, murre egg), display uniform near-zero values (0.01 ± 0.03‰, n = 128; 1 SD)34,41,42 (Supplementary Fig. 7). Even freshwater fish surveyed across the Arctic lake systems have suppressed Δ200Hg (0.05 ± 0.05‰; n = 65)39 relative to fish from the Great Lakes, U.S (0.07 ± 0.03‰; n = 135)95, which mostly receive wet deposited Hg(II). While the even-MIF anomalies reveal the pathways of Hg(0) oxidation and incorporation into biota, other Hg isotope signatures (MDF, odd-MIF) also provide insights into the pathways of MeHg formation and degradation within the water column. From the perspective of Hg isotopic pattern, we further summarize sources and processes, leading to MeHg bioaccumulation across several studied marine food webs.

In the Central Pacific Ocean, Blum et al. 201319 suggested that atmospherically deposited Hg(II) acts as a single dominant source, which is primarily methylated at the oxygen minimum zone, circulated to the mixed layer and deeper depth via sinking, and photo-degraded at varying extents. This leaves zooplankton and fish with a Δ199Hg/δ202Hg slope (2.32 ± 0.23; p < 0.05, r2 = 0.82, n = 86)19,65 consistent with experimentally derived MeHg photo-degradation in aqueous solution (2.43 ± 0.10)96 (Fig. 4a). The positive Δ200Hg in the zooplankton and fish at the Pacific Ocean as well as the consistent Δ200Hg between the zooplankton and marine particles, originated from atmospheric PBM, now suggest that Hg(0) oxidized in the troposphere, scavenged by particles, and methylated in the water column is the main source to the mid-latitude ocean food web.Fig. 4 Trends of δ202Hg and Δ199Hg in marine biota across different latitudes.

a Zooplankton and fish from the Central Pacific Ocean19,65 (n = 86), (b) Zooplankton from Section A (Bering Strait), ringed seal, and polar bear from Norton Sound34 (n = 27), c Zooplankton and Arctic cod from Section B (Chukchi Sea), and the estimated prey item consumed by beluga whales are from the offshore of Utqiagvik, Alaska34 (n = 28). Particulate-bound Hg (PBM) is derived from the measured total gaseous Hg (TGM). Wet precipitation (n = 8) and riverine particulate-bound Hg (river pHg; n = 13) are from Motta et al. 201965 and Campeau et al. 202238, respectively. Source data are provided as a Source Data file.

A similar isotopic pattern is revealed in the Bering Strait food web (Section A), except for the substantially depressed Δ199Hg/δ202Hg slope (0.18 ± 0.05, p < 0.05, r2 = 0.62, n = 27). The Bering Strait food web shown in Fig. 4b encompasses the Section A zooplankton and the estimated isotopic compositions of prey (mostly cod) consumed by ringed seal and polar bear sampled at Norton Sound34 (red box; Fig. 1) (Supplementary Information). As estimated in this study, river pHg (Hg(0) oxidation via foliar tissue) is an important Hg source to the Section A zooplankton and supplies the site for microbial methylation and photo-degradation prior to ecological exposure. In addition to photo-degradation, the depressed Δ199Hg/δ202Hg slope indicates that there is active microbial demethylation, which typically enriches δ202Hg of the remaining MeHg97,98. Similarly depressed slopes have been recorded in the food webs of the Bohai Sea57, Gulf of Mexico58, and across the northeast estuaries in the U.S29., all influenced by riverine Hg sources. Terrestrial organic matter exported from the Arctic rivers is thought to promote the growth of microbial communities and primary productivity93, which may, in turn, enhance the extent of biotic demethylation and Hg biodilution90. This would also explain the low THg observed in the Section A zooplankton relative to other locations.

The unusual negative Δ199Hg/δ202Hg slope is observed in the open ocean of the Chukchi Sea food web (Fig. 4c), spanning our Section B zooplankton and Arctic cod, and the estimated isotopic compositions of fish consumed by beluga whales collected offshore of Utqiagvik, Alaska34 (orange box; Fig. 1) (Supplementary Information). We explain this isotopic pattern by Hg(0) oxidation and deposition in the form of PBM followed by rapid Hg(II) photo-reduction and uptake by surface-dwelling zooplankton. This is evident by the high THg concentration and a Δ199Hg/Δ201Hg slope (0.91 ± 0.12; p < 0.05, r2 = 0.73, n = 22), consistent with the experimental Hg(II) photo-reduction, of our Section B zooplankton. The Arctic Ocean has one of the highest THg content in the surface water column relative to deeper depths21, which may induce active plankton Hg(II) uptake near the surface. A portion of PBM is methylated and photo-degraded during sinking to impart negative δ202Hg and suppressed Δ199Hg in MeHg bioaccumulated into the Arctic cod and other fish utilizing deeper depths (>900 m)87,99. In polar marine waters, methylation takes place at both the chlorophyll maximum26,35 and in the oxycline25,26, contrasting the mid-latitude oceans where methylation occurs primarily at the oxygen minimum zone18,19. The unusually strong stratification in the Arctic Ocean could hinder the circulation of methylated Hg to the mixed layer for further photo-degradation22,100. The estimated MeHg δ202Hg prior to photo-degradation, using the average Arctic cod δ202Hg and Δ199Hg and the experimental MeHg photo-degradation slope (DOC; 1 mg L−1)96, reveal a value of −0.83‰. The δ202Hg difference between the estimated MeHg and atmospheric PBM, reflecting Hg(II) (0.86‰), in the Chukchi Sea is measurably higher compared to the Central Pacific Ocean (0.5‰ between marine particles and their estimated MeHg using fish), but similar to the experimental δ202Hg difference during near complete methylation (<0.9‰)101. The substantial microbial methylation and suppressed photo-degradation evaluated using the Hg isotope pattern are in strong agreement with the measured data from the Arctic Ocean21,26,35.

Apart from the Hg isotopic pattern, a vast number of studies from the Arctic Ocean have reported active formation and evasion of dimethylmercury (DMHg) from seawater11,26,102, which may serve as another important MeHg source to aquatic and terrestrial food webs. The photo-decomposition of DMHg into MeHg and its subsequent deposition back into the environment is estimated to be as high as 8 Mg/yr11,103, leading to secondary MeHg exposure to terrestrial vegetation (lichen) in coastal landscapes104. While the absence of isotopic data on DMHg is primarily attributed to its low concentration, future isotopic characterization would enable a comprehensive understanding of the Arctic Ocean Hg cycle. Isotopic characterization of MeHg in biota would also enhance understanding of processes driving MeHg production and bioaccumulation in the Arctic Ocean food web105.

It is surprising that much of the Hg(0) oxidized in the troposphere supplies the bioavailable Hg(II) to the mid-latitude oceans since the troposphere occurs at a higher atmospheric column compared to the Arctic. While this still explains the previously documented latitudinal Δ200Hg increase in precipitation sampled across various regions of the world44 and positive Δ200Hg anomalies in the Arctic seawater33, the prevalence of near-surface Hg(0) oxidation, caused by coastal halogen and sea salt aerosols, seems to drive rapid Hg(II) input and bioavailable pools to the surface-dwelling zooplankton in the Arctic Ocean. We speculate that this is exacerbated by microbial methylation on particles, which occurs at varying water depths, and suppressed photo-degradation, leading to Hg exposure to the Arctic Ocean food web. Hg(0) oxidation by terrestrial media followed by riverine export is also a relevant source since the Arctic Ocean is completely surrounded by land and receives larger riverine Hg export than other ocean basins. Given the importance of particulate phases acting as the site for microbial methylation and bioaccumulation, assessing the extent of Hg bioaccumulation during AMDEs and spring freshet would allow the understanding of ecological Hg exposures during seasonally unique periods in the Arctic. The contrasting Δ199Hg/δ202Hg pattern between the Arctic Ocean and mid-latitude ocean also merits the future need to study unique Hg oxidation and biogeochemical pathways leading to Hg(II) bioavailability.

Methods

Site description

All samples were collected in the Arctic Ocean on board of the R/V ARAON, operated by the Korea Polar Research Institute (KOPRI). Sampling durations are divided into two cruise campaigns of ARA13B (July 21st to August 20th, 2022) and ARA13C (August 26th to September 12th, 2022) (Fig. 1a, b). As illustrated in Fig. 1a, ARA13B is divided into two geographic sections of the Bering Strait and the Chukchi Sea (Section A, B). ARA13C is characterized as the Beaufort Sea and the Mackenzie River estuaries, located 650-850 km away from Utqiagvik (Section C; Fig. 1b). All sampling procedures were approved by Alaska Fisheries Science Center, National Oceanic and Atmospheric Administration, USA (U2022-005) for Section A and B. All sampling procedures conducted in Section C were approved by the Inuvialuit Environmental Impact Screening Committee (EISC Registry File: 01-22-08), the Government of Northwest Territories (License No. 16995), the Government of Yukon (License No. 22-11S&E) and Trade and Development Canada (Permit-IGR-1283).

Sample collection

During ARA13C (Section C; Fig. 1b), atmospheric samples of total gaseous mercury (TGM) and precipitation were sampled during the entire cruise campaign and at the compass deck to avoid vessel emissions. Samples of surface sediment, seawater, and zooplankton were collected at varying depths and at locations away from the Mackenzie River (Source Data, Supplementary Data 2). TGM was collected onto a gold trap (Brooksrand instruments) by pumping air through a PTFE syringe filter (pore size: 0.45 um, ϕ: 25 mm) and soda trap (Sigma Aldrich) at a flow rate of 1.94 L min−1. Rain event occurred once on the 25th of August, which was sampled for two hours in a 1 L FEP bottle pre-treated with 10 mL of 50% HCl. Each bottle was connected to an acid-washed 136 mm diameter Pyrex funnel and a p-trap. Precipitation was then preserved at 4°C prior to analyses.

Sediment was collected at the surface (0 to 3 cm depth) and at 11 sites (C.01-C.11; Fig. 1b) using a multi-corer (MUC 8 multi-corer, Oktopus GmbH, Germany), equipped with eight polycarbonate coring tubes (length: 80 cm, ϕ: 10.5 cm). Samples were placed in an acid-washed 50 mL polypropylene conical tube and stored at −70°C. Seawater was collected at the surface and the subsurface Chlorophyll Maxima (SCM) at five sites (C.01-C.04, C.07) using a CTD/rosette, equipped with acid-cleaned 10 L Niskin bottles with SBE9plus CTD profiler. Each site and depth were profiled with temperature, dissolved oxygen, salinity, and fluorescence (Supplementary Table 3). Seawater was transferred into a 20 L polycarbonate bottle or a 10 L Pyrex glass bottle, precleaned with 40% HNO3 and 10% HCl and equipped with a Teflon cap. All samples were kept in the dark after being treated with 5% BrCl. Zooplankton were collected at the surface and the SCM from 2 sites (C.02, C.04; Fig. 1b) using a Bongo plankton net (mesh size: 150 um). At each sampling location and depth, seawater properties were measured using a CTD. The bongo net was operated at a speed of 40 m min−1 upward, 60 m min−1 downward and maintained for 20 seconds at targeted depths. Each sampling was conducted with a flowmeter mounted in the mouth of a bongo net to calculate the abundance of zooplankton. Sampled zooplankton were sieved into three size fractions of 0.2–1 mm, 1–5 mm, and >5 mm with an acid-washed filtering device (47 mm diameter, 0.2 mm synthetic nylon mesh filters) and preserved at −70 °C in petri dish.

During ARA13B (Section A, B; Fig. 1a, Supplementary Fig. 1), snowpack, zooplankton, and fish samples were collected at varying locations. Zooplankton were collected using the same method as Section C at varying depths including the surface (Source Data, Supplementary Data.2). Three individuals of Arctic cod were sampled regardless of gender at Section B (Beaufort Sea) to evaluate the extent of Hg bioaccumulation. All fish were euthanized using 95% ethanol and preserved at −25 °C following the protocol106. Surface snowpack (0 to 1 cm) was collected on an Arctic Ocean ice sheet (Supplementary Fig. 1), using an acid-washed Teflon scoop. Samples were placed in a 2.2 L FEP bottle, cleaned with acetone, Citranox soap, 10% HNO3, and BrCl (Douglas and Blum 2019107), and stored at −25 °C.

Hg and MeHg concentration analyses

All samples were transported to the Environmental Health and Assessment Laboratory, Pohang University of Science and Technology (POSTECH) on the cruise. For solid samples of zooplankton, fish, and sediment, the samples were lyophilized, homogenized, and measured for total Hg (THg) concentration using a Nippon Instruments MA-3000 Hg analyzer. NRC-TORT-3 (lobster) and NIST-2711A (Montana soil) were used to verify the accuracy of the analysis, which resulted in THg recoveries of 97.4 ± 6.4% (n = 5, 1 SD) and 94.1 ± 0.3% (n = 2). MeHg concentration was measured for zooplankton by digesting the lyophilized sample in 30% HNO3 for 12 h at 75 °C and measuring them using a CVAFS connected with a gas chromatography (GC). NRC-TORT-3 was digested in the same way to verify the procedure, which has MeHg recovery of 93.0% ± 0.1% (n = 2, 1 SD).

TGM sampled onto the gold traps were heated, purged, and trapped into 1% KMnO4 in 10% H2SO4 (wt/wt) via Hg-free air. Snow was thawed at room temperature in the dark and digested with 10% HCl and 1% BrCl for 1 month. Precipitation, which had a total mass of 91.5 g, and seawater were treated with HCl and BrCl to achieve 2% and 4% by volume, respectively, and measured for THg using a cold vapor atomic fluorescence spectrophotometry (CVAFS; Brooksrand instruments, 99.4 ± 4.1%; n = 11, 1 SD).

Hg isotope analyses

Solid samples of zooplankton, fish, and surface sediment were loaded onto a dual-stage thermal combustion furnace to release all Hg in the form of Hg(0). The released Hg was transferred into a 1% KMnO4 in a 10% H2SO4 (wt/wt) solution. Seawater sampled at each site and depth was allocated into four 10 L Pyrex bottles and preconcentrated into a 6 mL 40% reverse aqua regia (HNO3/HCl=2:1 (v/v)), following the method by Jiskra et al. 202143. To describe the procedure, each seawater bottle was purged with high purity argon gas using a bubbler glass post (P3 porosity frit) for 16 h at 300 mL min−1. During purging, 10% NH2OH ∙ HCl was added to neutralize excess BrCl and 100 mL of SnCl2 was added to reduce all Hg(II) to Hg(0). Trapped solutions were preconcentrated into a 6 mL (volume) of 40% reverse aqua regia by purging with high purity argon gas for 3 h. Snow samples (1 L) were treated the same way and trapped into a 40% reverse aqua regia.

All samples in either 1% KMnO4 solutions or 40% reverse aqua regia were neutralized with 30% NH2OH ∙ HCl. 40% reverse aqua regia solutions were then diluted to 20% reverse aqua regia. Each sample was measured for THg concentration using a CVAFS to calculate the THg recoveries from combustion and preconcentration steps. The average THg recovery of zooplankton, fish, and sediment was 103.8 ± 19.9% (n = 17, 1 SD) for the available samples and the snow had an average THg recovery of 111.8 ± 7.4% (n = 3). NRC-TORT-3 (lobster) and NIST-2711A (Montana soil) for solid samples had THg transfer recoveries of 99.6 ± 8.0% (n = 7, 1 SD) and 89.9 ± 2.0% (n = 4, 1 SD). The THg recoveries for the seawater preconcentration steps were estimated by spiking 12 ng of Hg from NIST SRM 8610 into 20 L distilled water. The preconcentration steps yielded an average THg recovery of 88.5 ± 5.5% (n = 4, 1 SD).

Hg stable isotopes were measured using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS; Nu instruments) with a reducing agent of 2% SnCl2 connected to a gas-liquid separator. Instrumental mass bias was corrected using an internal Thallium standard (NIST SRM 997) introduced with a nebulizer and NIST SRM 3133 was used to bracket each sample with the same matrix and THg concentrations. MDF is reported as δ202Hg (‰) referenced to NIST SRM 3133 and MIF is reported as Δ199Hg, Δ200Hg, Δ201Hg, Δ204Hg (‰)108 (Supplementary Information, Supplementary Table. 4).

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.

Supplementary information

Supplementary Information

Peer Review File

Description of Additional Supplementary Files

Supplementary Data 1

Supplementary Data 2

Reporting Summary

Source data

Source Data

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-024-51852-2.

Acknowledgements

We want to thank the captain and crew of RV Araon for excellent collaboration and support during the Arctic expedition ARA13B and C in 2022. This work was supported by Korea Institute of Marine Science & Technology Promotion (KIMST) funded by the Ministry of Oceans and Fisheries (Korea) [Grant number 20210541 (Air-sea exchange, sediment methylation, and ecosystem sources of mercury in the Arctic Ocean; S.K., S.H.), KIMST RS-2021-KS211500(Korea-Arctic Ocean Warming and Response of Ecosystem, KOPRI; E.Y.)] Also, this research was supported by a National Research Foundation of Korea Grant from the Korean Government (MIST; the Ministry of Science and (ICT) NRF-2021M1A5A1075513; S.K.) (KOPRI-NRF-2021M1A5A1075512.).

Author contributions

S.L. analyzed and interpreted the data and wrote the paper. S.K. and L.C.M. contributed to project ideas, sample planning, and data interpretation. S.K., E.Y., and S.H acquired the funding. S.L., Y.K., E.Y., T.R, J.H., S.H. performed fieldwork and/or managed the cruise. All authors contributed to the final data validation and feedback on writing.

Peer review

Peer review information

Nature Communications thanks the anonymous reviewers for their contribution to the peer review of this work. A peer review file is available.

Data availability

Bathymetric data was extracted from the GEBCO_2022 Grid, GEBCO Compilation Group (2022) GEBCO 2022 Grid [10.5285/e0f0bb80-ab44-2739-e053-6c86abc0289c] for Fig. 1 and Supplementary Fig. 1. All data generated in this study are provided in the Source Data file and are available under accession code [10.6084/m9.figshare.24995960]. Detailed information regarding the calculation of % Hg source contribution and zooplankton sampling information (time, location) are shown in Supplementary Data 1 and 2, respectively. Additional information is shown in the Supplementary Information file, which are all available for download from Nature Communications. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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