Stable isotopes unveil ocean transport of legacy mercury into Arctic food webs

Domain: earth_science
Published: 2025-06-12
Editorial outcome: minor_revision

Abstract
Anthropogenic activities have caused large-scale mercury (Hg) pollution in the Arctic reaching toxic levels, but knowledge of sources and pathways is sparse. Here, we present Hg stable isotope data in peat and key aquatic predatory species collected across Greenland. We observed distinct regional differences with significantly lower total Hg and higher δ202Hg in central-western versus northern-eastern Greenland influenced by different ocean currents. While Δ200Hg shows that atmospheric Hg deposition occurs predominantly (60–97%) as Hg(0), Δ199Hg reveals marked photochemical demethylation in especially freshwater habitats. We find δ202Hg in muscle tissue to increase with trophic level linked to internal metabolic transformation. Finally, we observe significant increases in total Hg and δ202Hg for several species/sites during the past 40 years, suggesting an increase in anthropogenic Hg sources and/or change in environmental processes. These findings show that ocean currents carrying large inventories of legacy Hg may be the dominant pathway driving present Hg uptake in Arctic marine and coastal areas. This explains the discrepancy between decreasing atmospheric Hg deposition in the Arctic in recent decades due to reduced global anthropogenic emissions, and the lack of response or increases in Hg-loads in many Arctic species, with implications for effectiveness evaluation of the Minamata Convention.

Introduction
Mercury (Hg) is among the most toxic heavy metals and its occurrence in the Arctic environment has increased markedly since pre-industrial times1–3. Hg originates from both natural and anthropogenic sources and is transported to the Arctic via atmospheric pathways, rivers, and ocean currents4,5. Since the dawn of industrialization, an increasing fraction of Hg has been released from sources of anthropogenic air emission, such as small-scale gold mining and the burning of fossil fuels, resulting in a ~5-fold increase of Hg in the atmosphere6 and a ~3-fold increase in surface ocean waters7. Once Hg enters the Arctic, it biomagnifies in the long Arctic food webs, creating high Hg loads in long-lived top predators such as toothed whales, polar bears (Ursus maritimus) and humans, thereby constituting a threat to biodiversity and health due to adverse neuroendocrine disruption1,2. In recent decades, atmospheric Hg deposition has generally decreased across the Arctic8 due to global reduction of anthropogenic Hg emissions9, whereas predominantly increasing Hg trends occur for many biological matrices in the Arctic, including marine and freshwater fish and marine mammals3. This mismatch leaves important knowledge gaps regarding Hg sources and pathways to and within the Arctic10.

In recent years, Hg stable isotopes have proven to be an important tool in the study of Hg sources and pathways11,12. Hg exhibits both mass-dependent fractionation (MDF, δ202Hg) and mass-independent fractionation (MIF, Δ199Hg, Δ200Hg, Δ201Hg, and Δ204Hg)13. MDF occurs in all investigated biogeochemical transformation processes in the environment11,12, including foliar uptake of gaseous Hg(0)14 and methylation of inorganic Hg by bacteria15, and is seen internally in many species, including birds and mammals associated with metabolic detoxification16. Odd-isotope MIF (Δ199Hg and Δ201Hg) occurs in aquatic systems because of impact by natural sunlight13 and can be used to assess photochemical activity in the environment, including changes in sea-ice cover and turbidity prior to uptake of Hg into the aquatic (both marine and freshwater) food web12,17. Even-isotope MIF (Δ200Hg and Δ204Hg) is considered linked to photo-oxidation of gaseous Hg(0) in the tropopause18, and the Δ200Hg signature is commonly used as a conservative tracer of atmospheric Hg deposition pathways to both terrestrial surfaces and water bodies (i.e. deposition as Hg(0) or Hg(II))14,19,20. Despite scientific momentum in recent years, Hg stable isotopes in Arctic biota remain relatively unexplored14,17,21,22, and only scattered information exists for Greenland20,23,24.

Here, we report a Hg stable isotope study of key aquatic predatory species collected across several regions in Greenland over the past 40 years, to elucidate Hg sources and pathways, environmental processes, internal metabolism and trophic transfer across the Arctic food web, and to identify temporal changes. The species include land-locked Arctic char (Salvelinus alpinus) in the freshwater environment and shorthorn sculpin (Myoxocephalus scorpius), ringed seal (Pusa hispida), and polar bear in the marine environment collected at regular intervals as part of the Arctic Monitoring and Assessment Programme (AMAP) under the Arctic Council collaboration and previously included in total Hg assessments3. In addition, we include glaucous gull (Larus hyperboreus) despite not currently used for Hg monitoring in AMAP to represent seabirds. Furthermore, we include stable Hg isotopes in peat (palsa) profiles from two different coastal regions in Greenland (Central West and Northeast) to evaluate spatial differences and long-term changes in atmospheric Hg deposition. We observe significant regional differences in δ202Hg signatures of ~0.5-0.6‰ across three different sample types (sculpins, seals, and peat) for central-western versus northern-eastern Greenland influenced by different ocean currents. The Δ200Hg signatures in all samples show that atmospheric Hg deposition occurs predominantly (60-97%) as gaseous elemental Hg (i.e. Hg(0)), and the Δ199Hg reflect marked photochemical demethylation in the aquatic environment especially in freshwater habitats as identified in land-locked Arctic char. We find that δ202Hg in muscle tissue of the marine species (sculpin, ringed seal, glaucous gull, and polar bear) increases with trophic level linked to internal metabolism and isotopic fractionation. Finally, we identify significant increases in total Hg and δ202Hg for several species and sites during the past 40 years, suggesting increases in anthropogenic Hg sources and/or changes in environmental Hg transformation processes important for fractionation. Based on these results, we propose that ocean currents carrying large inventories of legacy Hg may be the main driver of Hg uptake in Arctic marine and coastal environments rather than air transport.

Results
Spatial trends in sources and pathways
In the peat profiles, Hg stable isotopes reveal significant spatial differences in δ202Hg signatures between Central West Greenland (Qeqertarsuaq) and Northeast Greenland (Zackenberg) (Fig. 1a, b; Supplementary Fig. 1b; Supplementary Data 1). The differences in δ202Hg are consistent throughout the upper 25 cm active layers (i.e. the periodically unfrozen layers), with a median of 0.62‰ higher δ202Hg in the litter and upper 5 cm active layer (representing the most recent deposition) in Central West Greenland (median, −1.16‰; IQR, −1.21 to −1.08‰) compared to Northeast Greenland (median, −1.78‰; IQR, −1.84 to −1.72‰). The δ202Hg signatures decrease with depth and reach near-similar values in the upper part of the permafrost at both sites (Supplementary Fig. 1b; Supplementary Data 1). The Δ199Hg signatures are near-similar for Central West and Northeast Greenland in the litter and upper 5 cm active layer (medians, −0.17 and −0.15‰) (Fig. 1b) and decrease with depth down to c. 50 cm (Supplementary Fig. 1c; Supplementary Data 1). The Δ200Hg signatures show no significant variation with depth and are significantly higher in the litter and upper 5 cm active layer in Central West Greenland (median, 0.02‰; IQR, −0.01 to 0.03‰) than in Northeast Greenland (median, −0.05‰; IQR, −0.06 to −0.03‰) (Supplementary Fig. 1d; Supplementary Data 1). Total Hg concentrations in the peat profiles are shown in Supplementary Fig. 1a.

In the marine species, Hg stable isotopes in muscle tissue of shorthorn sculpin and ringed seal reveal similar significant regional differences in δ202Hg signatures between Central West Greenland, Northwest Greenland (Qaanaaq), and Central East Greenland (Ittoqqortoormiit) (Fig. 1c, d). Shorthorn sculpin displays 0.46 to 0.58‰ higher median δ202Hg in Central West Greenland (median, 0.41‰; IQR, 0.33 to 0.47‰) compared to Northwest Greenland (median, −0.17‰; IQR, −0.20 to −0.10‰) and Central East Greenland (median, −0.05‰; IQR, −0.10 to 0.08‰). Ringed seal shows 0.59 to 0.63‰ higher median δ202Hg in Central West Greenland (median, 0.87‰; IQR, 0.63 to 1.24‰) compared to Northwest Greenland (median, 0.24‰; IQR, 0.09 to 0.45‰) and Central East Greenland (median, 0.28‰, IQR, 0.17 to 0.43‰). Δ199Hg and Δ200Hg signatures in shorthorn sculpin and ringed seal do not differ significantly between the three regions (Fig. 1c, d; Fig. 2). Total Hg concentrations are significantly lower in both sculpin liver and ringed seal muscle and liver in Central West Greenland compared to Northwest and Central East Greenland (Supplementary Fig. 2; Supplementary Data 1 and 2).

The consistent geographic differences in δ202Hg signatures in the peat profiles as well as in δ202Hg and total Hg in shorthorn sculpins and ringed seals are surprising and considered likely to reflect long-term regional differences in Hg source inputs to Greenland. Overall, Central West Greenland displays ~0.5 to 0.6‰ higher δ202Hg compared to similar matrices in Northwest, Northeast, and Central East Greenland, with consistent differences observed throughout the active layer in peat profiles (Fig. 1b) and through the last 40 years of sampling for ringed seal (Fig. 3a). Although the peat profiles are not dated, a similar peat profile (i.e. palsa consisting of alternating layers of segregated ice, peat, or mineral soil material) from Northeast Greenland is dated to 2623±123 BP at 50 cm depth25, implying that the Hg in the upper c. 25 cm thick active layer of the peat should represent hundreds of years of deposition though growth rates of palsas vary with time26. However, using δ202Hg as source tracers in peat profiles and biological tissue samples pose limitations. For peat, factors such as vegetation type and decomposition/leaching may influence δ202Hg27,28. Consequently, to minimize potential effects of vegetation type and decomposition on the δ202Hg between sites, we chose two representative palsas, one from each region, both considered ombrotrophic and fully covered with a similar vegetation type dominated by sedges including Carex and Eriophorum species as well as a diverse moss flora. We observe the same spatial difference in δ202Hg in the fresh litter layer and through the upper 25 cm of the active layer between the two regions (Fig. 1b; Supplementary Fig. 1b; Supplementary Data 1), indicating little/no effects from to varying decomposition between the two sites. Thus, the consistently higher δ202Hg signatures in Central West Greenland throughout the top 25 cm active layer compared to the Northeast Greenland profile indicate persistent long-term regional differences in δ202Hg inputs. For the biological tissue samples, δ202Hg signatures are influenced not only by the Hg source input to the environment but also by all biogeochemical transformation processes (e.g. microbial methylation/demethylation)11,12 as well as prey species/trophic level29,30, and internal metabolism in the species12,16. Here, we choose to compare δ202Hg signatures in similar species (shorthorn sculpin and ringed seal) and tissues (muscle) across the three regions to minimize effects related to differences in prey species/trophic level and internal metabolism. The consistent ~0.5 to 0.6‰ higher δ202Hg in Central West Greenland compared to the other Greenland regions for both peat (mainly influenced by atmospheric Hg(0) source uptake22,31) and marine species (considered mainly influenced by marine methyl-Hg source uptake32) suggests an overall difference in Hg source inputs between the regions. However, we cannot rule out that regional differences in environmental biochemical processes affecting the δ202Hg can have additional minor effects.

The significant spatial difference in δ202Hg for Greenland contrasts with recent model predictions for atmospheric Hg deposition in the Arctic showing regional differences in absolute deposition but relatively uniform source compositions, reflecting a relatively long lifetime of Hg in the atmosphere (~0.5–1 y)33 and effective mixing of source inputs4. Atmospheric Hg transport to the Arctic follows a general pattern dominated by northern Eurasian sources in winter and mid-latitude Asian and North American sources in spring4,34. The significantly higher δ202Hg in all sample matrices at the Central West Greenland site, located only ~1000 km from the Northwest Greenland site and both being exposed to the same overall atmospheric transport patterns, is therefore surprising. This suggests that direct atmospheric Hg deposition may not, as currently predicted5, be the dominant pathway for Hg pollution in marine and coastal areas in Greenland and perhaps also elsewhere in the Arctic. Inputs of Hg from rivers or glacial meltwater in Greenland are considered low35–37 and therefore not a likely cause of the observed δ202Hg variation.

Instead, we propose that ocean currents may be the main Hg pathway explaining the significantly lower total Hg concentrations and higher δ202Hg observed in marine and coastal areas in Central West Greenland than in Northwest, Northeast, and Central East Greenland. Central West Greenland is dominated by the relatively warm West Greenland Current, which is a mixed current comprising a branch of the Irminger Current from the North Atlantic Ocean and a branch of the East Greenland Current originating from the Arctic Ocean (Fig. 1a)38–40. The other Greenland regions are all dominated by currents from the Arctic Ocean39,40. The total Hg concentrations measured in North Atlantic waters are lower than in Arctic waters5,41,42, with North Atlantic-sourced water flowing into the Arctic having lower Hg (0.15±0.07 ng/l) than Arctic Ocean surface waters (0.24±0.12 ng/l)5. Further, the few studies available on Hg stable isotopes in seawater report higher δ202Hg in the North Atlantic waters (median, −0.45‰)19 than in Arctic Ocean waters (−2.85 to −1.10‰)43. Consequently, we hypothesize that ocean currents may be the main driver of Hg uptake in marine and coastal environments in Greenland, and other Arctic regions, either through direct uptake of Hg from ocean waters or through Hg evasion from the ocean surface followed by atmospheric redeposition and uptake, explaining the significantly lower total Hg in marine species and higher δ202Hg in both marine species and peat profiles in Central West Greenland influenced by Atlantic Ocean water.

It is interesting to note that the same regional difference in δ202Hg as recorded for marine species occurs in the peat profiles in Central West Greenland and Northeast Greenland, situated 800 m and 2 km inland, respectively. This may be due to a greater influence of atmospheric Hg deposition from sea spray and/or evasion/redeposition of Hg from regional ocean surface waters24 compared to long-distance atmospheric sources. The influence of sea spray on the vegetation in coastal zones is well known44, and laboratory studies have shown that sea salt aerosols have a high capacity for uptake of reactive gaseous Hg45. A recent study shows that oceanic evasion is likely the dominating source and pathway to atmospheric Hg(0) in the Arctic during summertime (~80%) driving the transport to Arctic terrestrial ecosystems46. Likewise, evasion from the ocean surface waters is known to constitute a major part (~43%) of the global Hg flux budget8.

The Hg transported by ocean currents includes Hg previously deposited from the atmosphere to the ocean and terrestrial surfaces, which is later transported to the ocean via rivers and glaciers. The Δ200Hg signatures in peat profiles and aquatic biota have been used as a conservative tracer of atmospheric deposition pathways (as elemental Hg (Hg(0)) or Hg(II) wet or dry deposition, respectively) in numerous previous studies19,28,47. The Δ200Hg signatures in the peat profiles and marine and freshwater species in Greenland indicate that the dominant atmospheric deposition pathway is as Hg(0) rather than Hg(II) (Fig. 2). Jiskra et al.19 applied a two-source mixing model for the northern hemisphere with a Δ200Hg value of −0.05 as end-member Hg(0) dry deposition and a Δ200Hg value of 0.14 as end-member Hg(II) of combined wet and dry deposition. Applying the same regression line for the peat litter layer and upper 5 cm active layer in Central West Greenland and Northeast Greenland indicates that ~64 and ~97% (medians) of the current/recent vegetation uptake and Hg deposition in these two regions occurred as Hg(0) deposition and the remaining ~36% and ~3% as Hg(II) wet or dry deposition. For the entire peat active layer profiles down to 25 cm depth, the median Hg depositions as Hg(0) are ~72 and ~92% for Central West Greenland and Northeast Greenland, respectively. These estimates include net deposition of Hg(II) from atmospheric mercury depletion events (AMDEs) occurring in the Arctic during springtime48 in the estimated Hg(0) deposition, as the Δ200Hg signatures are considered unaffected by AMDEs20,22. During AMDEs, sea salt-derived halogen radicals oxidize atmospheric Hg(0) to Hg(II), leading to substantial Hg deposition from the atmosphere48. The Δ200Hg signatures in the marine and freshwater species in this study indicate that between ~60 and ~87% of the Hg bioaccumulated in the species are atmospherically deposited as Hg(0), while the remaining ~13 to ~40% are deposited via wet or dry Hg(II) deposition (Fig. 2). These values are consistent with the range of recently observed Δ200Hg values for other areas in the Arctic14,19–21. No consistent spatial differences in Δ199Hg are observed between similar sample types across the studied Greenland regions in contrast to some Arctic studies21,23, likely because the regions cover a relatively limited latitude range (69-77°N) and therefore have comparatively similar photochemical exposure.

Environmental processes and trophic transfer
In the marine species, total Hg concentrations in muscle tissue are lowest in sculpin, ringed seal, and polar bear (medians, 0.09 to 0.36 μg/g wet weight (ww)), followed by glaucous gull (median, 0.61 μg/g ww), while land-locked Arctic char exhibits the highest concentrations (median, 0.66 μg/g ww) (Supplementary Data 2). Hg stable isotope signatures vary significantly between matrices (Fig. 4a; Supplementary Data 2). Peat has the lowest δ202Hg (medians in active layer, −1.4 and −1.7‰ for Central West and Northeast Greenland, respectively) and Δ199Hg (medians in active layer, −0.16 and −0.33‰). Marine species show a large variation in δ202Hg (medians, −0.17 to 2.2‰) and Δ199Hg (medians, 0.75 to 1.2‰) in muscle tissue. Land-locked Arctic char shows a δ202Hg in muscle tissue within the range of the marine species (median, 0.93‰), while Δ199Hg is significantly higher (median, 6.2‰). Δ200Hg signatures exhibit relatively little variation between sample matrices (medians, −0.038 to 0.038) (Fig. 2; Supplementary Data 2).

We observe trophic level differences in δ202Hg signatures with significantly increasing δ202Hg in muscle tissue between shorthorn sculpin, ringed seal, glaucous gull, and polar bear in Central East Greenland (Fig. 4d). We see a similar significant trophic level difference in δ202Hg for shorthorn sculpin and ringed seal in Central West Greenland and Northwest Greenland (Fig. 1c, d; Supplementary Data 2). Ringed seal has 0.33 to 0.46‰ higher median δ202Hg and glaucous gull has 0.94‰ higher median δ202Hg compared to shorthorn sculpin. Polar bear exhibits the highest δ202Hg, with 1.8 to 2.2‰ higher median δ202Hg compared to shorthorn sculpin and 1.4 to 1.9‰ higher median δ202Hg compared to ringed seal. Shorthorn sculpin and ringed seal in Central East Greenland have significantly lower Δ199Hg signatures (medians, 0.87 to 0.89‰) than glaucous gull and polar bear (medians, 0.98 to 1.0‰).

To assess the dominant photochemical processes in the aquatic environment prior to Hg uptake, we compared Δ199Hg and Δ201Hg ratios in the aquatic species to experimentally established ratios representing photochemical reduction (1.00±0.02) and photochemical demethylation (1.36±0.02)13 (Fig. 4b). Median Δ199Hg/Δ201Hg in muscle tissue of the five species at the four sites ranges from 1.18 to 1.36, corresponding to 50–100% uptake of residual methyl-Hg from photochemical demethylation, with the following species-specific ratios and percentages of photochemical demethylation: shorthorn sculpin (medians, 1.21 to 1.36, 58-100%), ringed seal (medians, 1.25 to 1.36, 69-100%), glaucous gull (median, 1.18, 50%), polar bear (medians, 1.22 to 1.30, 61-83%), and Arctic char (median, 1.31, 86%).

To evaluate species-specific internal metabolic transformation processes as the driving mechanism behind the trophic level differences in δ202Hg, we measured Hg stable isotopes in muscle and liver of Arctic char, shorthorn sculpin, ringed seal, glaucous gull, and polar bear (Fig. 4c; Supplementary Data 2). In polar bear, we also measured Hg stable isotopes in hair and kidney. Shorthorn sculpin shows no difference in δ202Hg between muscle and liver, but all other species exhibit higher δ202Hg in muscle than in liver, significantly so for ringed seal and polar bear. For these species, differences in δ202Hg between muscle and liver are lowest in glaucous gull (median, 0.25‰) and Arctic char (median, 0.33‰) and highest in ringed seal (median, 1.1‰) and polar bear (median, 2.6‰). Polar bear hair has a δ202Hg signature that is not significantly different from that of muscle, and the δ202Hg signature of kidney is not significantly different from that of liver.

The significantly lower δ202Hg signatures observed in peat compared to marine and freshwater species are likely predominantly caused by the large isotope fractionation by foliar uptake14,22. Obrist et al.14 found that foliar uptake produced a negative shift in δ202Hg in Arctic tundra vegetation of ~−2‰ compared to Hg deposited as Hg(0). Uptake of Hg(0) by vegetation is considered the dominant pathway of atmospheric Hg deposition to terrestrial surfaces, both in the Arctic22 and globally31.

The large difference in Δ199Hg signatures between atmospherically deposited Hg in peat and the Hg found in marine species and, especially, freshwater species (Fig. 4a), evidence that Hg is subject to photochemical processing in the aquatic environment after mobilization and before entrance in the food chain. Once Hg accumulates in the food chain, negligible MIF is expected since the bioaccumulated Hg is no longer exposed to direct sunlight12. The Δ199Hg/Δ201Hg ratios in the marine and freshwater species included in this study (Fig. 4b) demonstrate that photochemical demethylation followed by uptake of residual methyl-Hg (rather than photochemical reduction followed by uptake of residual Hg (II)) is the dominant photochemical process affecting the Hg uptake in both marine and freshwater habitats in Greenland. The significantly higher Δ199Hg values observed in land-locked Arctic char compared to the marine species are considered due to intense in-lake photochemical demethylation at the South Greenland sampling site. It is well documented that freshwater and open-ocean fish have high Δ199Hg values relative to coastal and benthic fish11,29.

We observe an increase in δ202Hg signatures in muscle tissue with increasing trophic level for shorthorn sculpin, ringed seal, glaucous gull, and polar bear in Central East Greenland. The four species represent different trophic levels, but the food chain dynamics are complex, and except for ringed seals and polar bears, the species in the food chain may not be directly linked. Furthermore, the species have different mobility and therefore take up Hg at varying distances from the sampling sites. Sculpins are known as a sedentary benthic fish species with a generalist feeding strategy encompassing amphipods, polychaetes, decapods, molluscs, and other fish in their diet49. Ringed seals in East and Northwest Greenland are known to feed mainly on polar cod (Boreogadus saida) and Arctic cod (Arctogadus glacialis), whereas ringed seals in Central West Greenland primarily prey on amphipods (Parathemisto spp.), capelin (Mallotus villosus), redfish (Sebastes sp.), and squid (Gonatus sp.)50. The ringed seal ecology is closely connected to the sea ice and most adult ringed seals are believed to be rather stationary, whereas juvenile ringed seals from different parts of the Arctic have been reported to travel over large distances (>1000 km)51. Glaucous gulls are top predators and scavengers, and feed on a variety of prey items, including marine invertebrates, fish, birds, and bird eggs as well as e.g. polar bear left-over seal carcases52. Glaucous gulls come to East Greenland in spring and leave in late October to winter at lower latitudes, possibly around Iceland53. Polar bears are top predators and feed predominantly on ringed seals, hooded seals (Cystophora cristata), and harp seals (Pagophilus groenlandicus) in East Greenland but occasionally also on other food items (e.g. bearded seals (Erignathus barbatus), narwhal (Monodon monoceros), and Atlantic walrus (Odobenus rosmarus))54. The Central East Greenland polar bear subpopulation is one of 20 recognized subpopulations of polar bears in the world and the movement patterns cover >2000 km of pack ice and fast ice along the East Greenland coast above 64°N55.

We consider the observed MDF and increase in δ202Hg between trophic levels to be mainly caused by internal metabolic transformation processes, involving demethylation in the liver/kidneys and excretion of inorganic Hg with the urine, leading to enhanced δ202Hg in the residual methyl-Hg16, which is then transferred via the diet from prey to predator. We detect significant internal fractionation for ringed seal and polar bear, with lower δ202Hg in the liver (and kidney for polar bear), which is known to contain significant proportions of inorganic Hg56, and higher δ202Hg in muscle tissue (and hair for polar bear), which is known to contain almost 100% methyl-Hg56. Previous studies have detected internal MDF in some fish species as well as in seabirds and marine mammals16. However, to our knowledge, this is the only study on internal Hg fractionation in these Arctic key species and no studies exist on Hg stable isotopes in the muscle tissue of shorthorn sculpins, ringed seal, glaucous gull, and polar bear from other areas in the Arctic that can be used for comparison. Previous studies have shown MDF shifts and increases in δ202Hg related to the trophic transfer in non-Arctic areas29,30.

The significantly lower Δ199Hg signatures in shorthorn sculpins and ringed seals than in glaucous gull and polar bear in Central East Greenland indicate that sculpins and ringed seals are more influenced by Hg from benthic prey items, which are less impacted by photochemical processes29,57. In contrast, glaucous gulls and polar bears may at least partly feed on more pelagic prey items.

Temporal trends and future perspectives
The peat profiles from Central West Greenland and Northeast Greenland, representing long-term time series of Hg deposition, show a general global trend for lake sediment60. The higher δ202Hg in the top layers is in line with higher δ202Hg reported for modern anthropogenic Hg emissions61, whereas increasing Δ199Hg in more recent deposition may reflect a general change in environmental conditions60. The significant differences in δ202Hg and Δ199Hg between the active layer and the upper permafrost in Central West and Northeast Greenland with low δ202Hg and Δ199Hg in the permafrost are noteworthy. These differences may enable detection of a potentially increasing uptake of legacy Hg in Arctic aquatic species from thawing permafrost and subsequent river transport, which is currently a concern in the Arctic. Thus, recent estimates show that permafrost regions contain 7% of Earth’s global soil Hg pool62, and studies from Arctic Canada and Alaska describe elevated Hg fluxes in rivers attributed to permafrost and glacier ice thawing63,64. Further studies are needed to determine if these differences in δ202Hg and Δ199Hg in active layers and upper permafrost apply to larger areas in the Arctic.

For the marine species, significantly increasing temporal trends of total Hg in liver tissue of shorthorn sculpin, ringed seal, and polar bear (and in hair for polar bear) during 1983–2024 indicate increased Hg-loading in the Greenland marine environment during the last four decades, which has now reached toxic levels for top-predators such as polar bears with increases in total Hg in hair above the reported effect concentration of 6 ng/g (ref. 2) (Supplementary Fig. 2d; Supplementary Data 1). Liver tissue is the most often used matrix for temporal assessment of Hg loading in Arctic biota3 and reflects accumulation during a longer timespan compared to muscle and hair65. The increasing Hg trends in Greenland biota contradict the decreasing atmospheric Hg deposition trends observed across the Arctic during the last decades8 and support a general global disagreement between biota time series and atmospheric deposition trends10.

The temporal trends with increasing δ202Hg signatures in both ringed seals in Central West and Northwest Greenland, and in polar bears in Central East Greenland, are notable and indicate a change in Hg source composition during the last 40 years but can also be influenced by changes in biogeochemical processes in the environment during this period (Fig. 3a and c). The observed trends in Greenland towards generally higher δ202Hg signatures in ringed seals and polar bears during the last 40 years are in line with long-term trends observed in Hg depositions globally caused by an increased influence of anthropogenic Hg emissions60,61 but may also reflect changes in environmental transformation processes affecting the δ202Hg in the methyl-Hg taken up in the food web11,12.

We propose that the disagreement between increases in total Hg in many biota time series in the marine environment in Greenland, and other regions in the Arctic, and the decreases in atmospheric Hg deposition over the Arctic during the last decades, is predominantly due to the large inventory of legacy Hg in the aquatic environment transported by ocean currents. This causes a substantial lag in the response time between changes in external input from the atmosphere and the biotic response. Changes in physical, geochemical, biological, and ecological conditions, affecting the complex biogeochemical Hg cycling10, may also be factors contributing to the disagreement. The influence of ocean currents and the aquatic Hg inventory is reflected by significant differences in total Hg and δ202Hg between Central West and Northwest Greenland, which are situated only 1000 km apart and exposed to the same overall atmospheric wind transport regime but are influenced by different ocean currents (i.e. the Irminger Current from the North Atlantic and the Arctic Ocean currents, Fig. 1a), with different total Hg and δ202Hg levels. The observed increase in δ202Hg in some Greenland biota series during the last 40 years indicates enhanced contribution from modern-time anthropogenic sources and/or changes in environmental Hg transformation processes, but there may be a considerable time lag. The lifetime of Hg in the ocean is much longer than in the atmosphere, with estimated turnover times of >300 years66. These findings have major implications for the current understanding of the Arctic Hg cycle and for the effectiveness evaluation of UN’s Minamata Convention as the high Hg concentrations observed in the Arctic marine biota are likely to persist for a much longer period than previously anticipated, regardless of atmospheric emissions, especially in larger deeper marine ecosystems. Biota in smaller water bodies with less capacity for legacy Hg, such as lakes or shallow coastal environments with limited water mass turnover, are likely to respond faster to emission control. However, sea spray and ocean ice evasion/redeposition of Hg will impact all Arctic marine and coastal ecosystems and may also dominate summertime transport to Arctic terrestrial ecosystems46. Consequently, biomonitoring to evaluate regulation efforts should be diverse and include multiple species occupying different ecological and geographic niches.

This study documents geographic differences and temporal trends in Hg sources and pathways, environmental processes, and trophic transfer across Greenland. Future studies using both total Hg and Hg stable isotopes are needed to document geographic trends in key species and samples covering the circumpolar Arctic, including Alaska, Arctic Canada, and Svalbard, to expand our knowledge of the Arctic Hg cycle. The influence of the ocean transport of legacy Hg should also be investigated in other global regions to help evaluate the effects of Hg regulations under the Minamata Convention and to fulfil UN’s Sustainable Development Goals.

Methods
Inclusion and ethics
We have engaged local subsistence hunters and other persons from the local communities in Greenland in the samplings and several persons from Aarhus University and University of Copenhagen including technical staff and students have been involved in the study but without qualifying for co-authorship (but are included in the Acknowledgements section). Authorship follows the Vancouver recommendations and individual contributions have been clarified in the Author contributions section. The analysed animal tissue samples were mainly harvested by local subsistence hunters in Greenland following local legislation, practices, ethics, and hunting quotas. All samples obtained from polar bears were under the international IUCN CITES regulations legally exported from Greenland (295/2000, 61/2001, 69/2001, 66/2003, 156/2004, 23/2005, 17/2006, 08GL0706599, 09GL0805839, 10GL0806166, 10GL0805167, 42 10GL0806333, 10GL0806334, 11GL1003311, 12GL1003376, 13GL1003418, 14GL1003515, 15GL1166847, 16GL1167015, 17GL1167116, 18GL1717224, 19GL1717365, 20GL1717427) to Denmark (IM0813-642/01, IM 0312-080/01, IM 1116-901/04, IM1116-954/05, IM0316-613/06, IM0811-266/06, IM0711-993/08, IM0630-137/09, IM0611288/10, IM0611-290/10, IM1223-856/10, IM1223-855/10, IM 1214-596/11, IM1199-728/12, IM0826-378/13, IM1202-211714, IM1222-060/15, IM1202-580/16, IM0818-549/17, IM0928-712/18, IM0816-496/19, DK2020-0017113-01).

Sampling
The sampling of Arctic char, shorthorn sculpin, ringed seal, glaucous gull, and polar bear was conducted in Greenland during 1983–2024; since 1994 in association with deliveries to the Arctic Monitoring and Assessment Programme (AMAP). Although glaucous gulls are not currently used for Hg monitoring in AMAP, they are included in this study to represent seabirds. More detailed information on sampling year(s) for each sample type is provided in Supplementary Data 2. The locations of the sampling sites are shown in Fig. 1a. The AMAP sampling sites with approximate coordinates are Ittoqqortoormiit (70º29′N, 21º58′W), Qaanaaq (77º28′N, 69º14′W), Qeqertarsuaq (69º15′N, 53º32′W), and Isortoq (60º54′N, 47º33′W). A thorough description of the programme is given in AMAP (2021)67.

Peat was sampled at palsas in Zackenberg and Qeqertarsuaq. A palsa is a typical feature of a permafrost-affected landscape and is common in poorly drained lowlands in Greenland68. Palsas are mounds that contain lenses of segregation ice and represent continuous peat formation and formation of ice in the core due to capillary water undergoing slow freezing68. Palsa formation and collapse is a dynamic feature68. Two complete (non-collapsing) palsas were sampled: one east of the Zackenberg River and north of Zackenberg Research Station (74°29'05“N; 20°33'58“W) in Northeast Greenland with a height of c. 1 m and one in Blæsedalen east of Qeqertarsuaq on Disko Island (69°16'05“N; 53°28'06“W) in West Greenland with a height of c. 0.7 m. The Zackenberg palsa was sampled in September 2012 and the Disko palsa in August 2022. Both palsas were fully covered by vegetation dominated by mosses and grasses. Roots occurred at both locations throughout the top 25 cm but dominated the top 5 cm. A nearby palsa in the Zackenberg area has been dated based on microfossils to 2623±123 BP (14C AMS) at 0.5 m depth25. The samples were collected from the litter layer and the active layer comprising the top 25 cm. From the permafrost table, a steel pipe was hammered down into the soil with 10 cm depth increments, after which the sample was carefully collected from the pipe before sampling the next 10 cm. The samples were stored in plastic bags and kept frozen (below −6°C). Prior to analyses, the samples were taken out of the freezer and the outer 0.5 to 1 cm cut off with a ceramic knife and discarded. The samples were subsequently cut into slices using a ceramic knife and put into plastic bags for further processing.

Chemical analyses
All samples were freeze dried prior to analysis, and the dry matter content was determined.

For muscle samples, subsamples of c. 0.4 g homogenized tissue were digested in a mixture of 4 ml nitric acid and 4 ml Milli-Q water in an Anton Paar 7000 Multiwave microwave oven (220°C, 30 min). For liver and kidney, subsamples of c. 0.3 g homogenized tissue, and for hair and peat subsamples of c. 0.075 g and c. 0.5 g, respectively, were applied following the same digestion procedure as above. One blank, one duplicate, and at least one sample of certified reference material (CRM) were included in every series of digestions (18 vials). Immediately after digestion, 1 ml HCl was added, followed by dilution to c. 50 g with Milli-Q water. The CRMs DORM-5 (fish protein), DOLT-5 (dogfish liver), and TORT-3 (lobster hepatopancreas) were included in muscle, liver, and kidney digestions, respectively, IAEA-086 (human hair) in hair digestions, and ERM-CD-281 (rye grass) and NIST-1515 (apple leaves) in peat digestions.

Digestion solutions were analyzed for total Hg using an Agilent 7900 ICP-MS. Results of the CRMs showed close to 100% recovery of Hg (mean recovery ± 2 SD; DORM-5 (n=46): 103±15%; DOLT-5 (n=9): 96±15%; TORT-3 (n=11): 105±16%; IAEA-086 (n=9): 100±13%; ERM-CD-281 (n=7): 111 ± 16%; and NIST-1515 (n=7): 105±4%).

Next, the digestion solutions were analyzed for Hg isotopes using a Thermo Neptune XT MC-ICP-MS equipped with a Cetac HGX-200 cold vapor introduction system using 3% SnCl in 10% HCl (v/v) as reductant and a flow rate of 0.5 ml/min69. Prior to analysis, the digestion solutions were diluted to 1 ppb Hg for muscle, liver, kidney, and hair samples and 0.5 ppb for peat using an acid mixture corresponding to the solutions. The digestion solutions of peat containing <0.5 ppb (0.2–0.5 ppb, corresponding to 20–50 ng/g in samples) were analyzed without further treatment. The masses 198, 199, 200, 201, 202, and 204 were measured using Faraday cups with 1013 Ω amplifiers on mass 198 and 199, and 1011 Ω amplifiers on the other masses. Sensitivity was typically 0.5 V on mass 202 for 1 ppb Hg solution. Measurements were done using std-sample-std bracketing with NIST 3133 CRM as standard with a similar concentration as the samples, and a measurement time of 10 min. Signal intensities measured in digestion solutions corresponded well the NIST 3133 CRM showing minimum matrix dependency. The delta values δ202Hg, Δ199Hg, Δ200Hg, Δ201Hg, and Δ204Hg were determined relative to NIST 3133 as described in ref. 13 One blank sample and one NIST 8610 CRM were analyzed for at least every six samples to assess contamination/carry-over and accuracy/precision. Blank samples had signal intensities of <1% of samples and CRMs. The following values (mean ± 2 SD in ‰) were measured on the NIST 8610 in 1 ppb solution (n=172) (δ202Hg: −0.54±0.14; Δ199Hg: −0.02±0.05; Δ200Hg: 0.01±0.05; Δ201Hg: −0.04±0.08, and Δ204Hg: 0.00±0.14), 0.5 ppb solution (n=10) (δ202Hg: −0.58±0.10; Δ199Hg: −0.04±0.05; Δ200Hg: 0.03±0.04; Δ201Hg: −0.04±0.07, and Δ204Hg: 0.05±0.13), and 0.2 ppb solution (n=5) (δ202Hg: −0.66±0.16; Δ199Hg: −0.09±0.10; Δ200Hg: 0.07±0.07; Δ201Hg: −0.09±0.09, and Δ204Hg: 0.19±0.25), which are all within the range of certified values. The CRM IAEA-086 was also measured for Hg isotopes (n=6) and the results (δ202Hg: 0.80±0.03; Δ199Hg: 0.31±0.03; Δ200Hg: 0.02±0.02; Δ201Hg: 0.21±0.02, and Δ204Hg: −0.10±0.06) were close to previously reported values70.

The total Hg results reported in Supplementary Fig. 2 were analyzed as part of the AMAP Core biota monitoring programme in Greenland with results most recently published in 2022 (ref. 3), and we refer to method descriptions in references herein.

Statistical analyses
Statistical analyses were done using R version 4.2.3. One-way analysis of variance (ANOVA) was performed to determine differences in Hg stable isotope signatures between sites and species. Prior to the ANOVA, a Shapiro-Wilk test was conducted to test for normality of data, and a Brown-Forsythe test was undertaken to test for equal variances between groups. A Welch ANOVA test was applied due to unequal variances identified for some groups, and a Games-Howell post hoc test was used to assess individual group differences (using R-package ‘rstatix 0.7.1’). Temporal trends in Hg stable isotope signatures were evaluated using the Mann-Kendall and Theil-Sen’s slope analysis on subadult ringed seals (≤4 years) and polar bears (males ≤5 years; females ≤4 years) (using R-packages ‘Kendall 2.2.1’ and ‘mblm 0.12.1’). Differences were regarded significant at p<0.05. Temporal trends in Hg stable isotope signatures in Central East Greenland polar bears were adjusted for changes in diet composition over time54,58 and Hg stable isotope signatures in prey items (see Supplementary Fig. 3; Supplementary Data 1).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of NCOMMS-24-75705\n\"Stable isotopes unveil ocean transport of legacy mercury into Arctic food webs\"\nDecember 18, 2024\nSummary: This study used mercury (Hg) stable isotopes to study numerous environmental processes in Arctic ecosystems.\nUsing both new and historical samples and data from long-term monitoring by the AMAP program, the study generates\nseveral notable findings. First, the isotopic modeling suggests that ocean currents are the main transport pathway of\nlegacy\nHg to Greenland. This is in notable contrast to our traditional understanding of Arctic Hg cycling, which suggests the majority\nof Hg is deposited from the atmosphere (through AMDE events, which are discussed in the manuscript). Secondly, this study\nsuggests that the dominant species deposited and accumulated is Hg(0), not Hg(II). Additionally, the authors support that\nphoto-demethylation continues to be an important process impacting Arctic Hg cycling. Their work further supports growing\nevidence that, while Hg emissions are reducing, concentrations (and, according to their study, the sources of this Hg) are not\nresponding similarly and are, in fact, increasing in some biota. Collectively, these findings have implications for long-term\nmercury trends and necessary monitoring, given that the lifetime of Hg in oceans is longer than atmospheric sources used in\nmost temporal models to-date.\nOverview: Overall, this is a strong paper that impacts our collective understanding of Hg cycling. The data are valuable and\nexpansive, both in a temporal and geographic sense. Their methods are appropriately detailed, particularly around the\nvarious lab analyses. However, the manuscript is particularly dense, assessing processes from atmospheric to ecological,\nwhich has resulted in deficient details (e.g., statistical information) and discussion content in some places (e.g.,\nphotochemical impacts on bioaccumulation, see detailed comments below). In general, the results and discussion around\nabiotic cycling are much stronger and more complete than the discussions around trophic transfer and metabolism. This may\nbe due, in part, to a firmer collective understanding of isotopic behavior during the former processes, but additional details\nare warranted in the latter, where possible. We additionally suggest the authors check their references to figure panels within\nthe text, as we found several errors (noted below).\nSpecific points:\n1. Line 55: Food webs cannot be slow-growing. We believe the authors may mean the biota within those food webs are slow\ngrowing, which We agree can increase mercury accumulation. We suggest some minor re-wording for clarification around\nthis point.\n2. Figure 1: What is the source of information for the ocean currents presented in part (a). Please include relevant citations.\n3. Line 98: Should the reference to Figure 1C be 1D in this line?\n4. Line 114: There is some confusing language in this line. You begin with “As for Peat” but discuss muscle results. You\nmight consider rewording for clarity.\n5. Jiskra et al. (2021) use their endpoints to study terrestrial environments. Can the authors comment on the relevance of\nthese endpoints when translated to aquatic ecosystems? Are there any differences in fractionation that should be accounted\nfor in the interpretation herein or would all the changes from atmospheric speciation be equally reflected in terrestrial and\naqueous environments?\n6. Lines 180-183: The results herein are important. Where can the data supporting this conclusion be seen? Figure 2, We\nassume. Please add a reference to relevant information to allow for independent reader interpretation.\n7. Line 246: “Residual Hg from photochemical demethylation” would inherently not be methylmercury, which is commonly\nassumed to be the dominate form bioaccumulated in the biota and tissues reference herein. Thus, these estimates are\nsurprising. The authors don’t explore this or explain potential mechanisms. We suggest some discussion is added around\nthese points within lines 242-250, or clarifications to your phrasing as we are mis-interpreting this sentence.\n8. Lines 305-307: More support for this statement is needed. Were benthic organisms tested in this study to make this\ninference, or is this based on previous work? If the latter, citations are needed.\n9. Lines 388-395: The direct link between water mass exchange, ecosystem size/location, and legacy Hg is unclear. Ocean\nspray and local atmospheric processes would surely impact coastal ecosystems, marine and freshwater alike. Could the\nauthors elaborate on these points?\n10. Line 521: Seabirds are used to make local inferences; how mobile is this gull species? Is there a confounding factor of\nmovement in the interpretation of their Hg concentrations and isotopes that should be accounted for herein? The same could\nbe asked of seals and polar bears.\n11. Line 569: We would argue that blanks test for contamination, not accuracy of precision. This is a minor point, but\nimportant nonetheless. Otherwise, We appreciate the authors clear reporting of their QA/QC procedures and results.\n12. Where possible, statistical details (e.g., p-values, and metrics of relationship strength) should be added to results. We\nfind the method statement about p-values in line 585 to be insufficient. In particular, line statistics (including slopes, r2, and\np-values) would be helpful in figures 4 and extended figure 2 and allow for more effective reader interpretations.\n13. Line 578: The program used for statistical analyses and graphing should be noted. If R was used, any specific packages\n(and their versions) should also be noted, as this can affect the background settings and statistical inferences.\n14. Line 332-334: Should the reference to Figure 4C be 4B? We suggest double-checking the figure references in this\nsection.\n15. Figure 4: Beyond diet, which only explains ~ 0.5 per mil of the differences in slopes, what other factors explains the\ndifference in temporal response between seal and bears, two species which should be tropically linked. If this trend is\notherwise geographically impacted, we would expect both species to show an impact.", "reviewer_2": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts.", "reviewer_3": "The authors provide an in-depth and comprehensive assessment on spatio-temporal variation in mercury source and\npathway dynamics in the Arctic using total Hg and Hg isotope measurements of peat and several key marine organisms.\nThe authors attribute increases in Hg for some species to spatial differences in ocean current transport of legacy Hg up the\nfood web. I found the methodological approach, experimental design and timeline investigated to be novel with results\nhaving high impact. I really enjoyed reading this well-written and well thought-out manuscript where the data and results\nsupport the main conclusions of the paper. This manuscript will definitely be of interest to readers of Nature Communications\nand the broader scientific community. I only have a few very minor comments.\n1) In the Summary section, suggest modifying “internal metabolic fractionation” to “internal metabolism and isotopic\nfractionation.”\n2) Bottom of page 9 where the authors are inferring species trophic level from d202Hg values – I’m curious whether the\nauthors investigated how well d202Hg correlates with d15N across species. D15N is an isotope that is very commonly used\nto estimate trophic level differences between species and this could be worthwhile to include in the supplementary materials\nsection.\n3) Top of page 10 when describing the foraging ecology of glaucous gulls – it’s also worthwhile to state that they are also\nscavengers.\n4) Top of page 10 when describing the foraging ecology of polar bears – it’s also worthwhile to briefly state some of the other\nfood items (i.e. bearded seals, narwhal, Atlantic walrus)", "reviewer_4": "The authors describe a study on total Hg concentrations and Hg isotopes in peat profiles and different key Arctic aquatic\npredators at different locations in Greenland. The major aims of this study are 1: to track Hg sources and pathways to\nGreenland 2: to show the development of total Hg and Hg isotopes in aquatic predator species in the past 40 years and how\nthe development of the Hg content compares to that of anthropogenic atmospheric Hg emissions. 3: to show if there is\ninternal metabolic fractionation of Hg isotopes in the different investigated aquatic species. 3:\nThe authors could show that there are distinct regional differences in total Hg and Hg isotopes (δ202Hg signatures) between\ncentral western Greenland and northern-eastern Greenland which they suggest to be caused by different marin currents with\ndifferent dissolved Hg concentration rather than by different atmospheric sources. Moreover they observed that Hg\nconcentrations in Arctic aquatic predatory species have increased in the past 40 years opposite to the decrease of\natmospheric Hg emissions and atmospheric Hg concentrations. From this they concluded that rather delayed mobilised\nlegacy Hg than direct atmospheric Hg deposition is the major pathway of Hg to arctic aquatic predators and Greenland\nsoils/peat (e.g. through seaspray aerosols).\nThis study is innovative and of importance and shows important findings on the distribution of Hg contamination in the\naquatic environment in Greenland. To my knowledge, this is the first and most comprehensive data-set of Hg concentrations\nand Hg isotopes from Greenland biota and from Arctic biota in general including monitoring data of aquatic species of the\npast 40 years, which I think is crucial to understand the biogeochemical Hg cycling in the arctic.\nFor me, the merit of this study lies in the large and unique data set of Hg concentrations and isotopes in Arctic biota, which\ngives new insights into the large variability in Hg enrichment and Hg isotope fractionation due to different food sources and\nthrough internal metabolic processes and indicates that processes and pathways of Hg and Hg isotope fractionation are less\nclear than previously assumed. The discussion here appears to be sound, although the authors clearly point out that more\nresearch is needed here.\nHowever, even if I think that it is a plausible assumption, the conclusion that the major pathway of Hg to the marine and the\nterrestrial Greenland foodchains is Hg evasion from different ocean currents is not really supported by the data. The\ndifferences in dissolved Hg data which the authors have drawn from the literature may explain at least partly the differences\nobserved in marine predatory species at different locations in Greenland, but cannot explain the differences in peat or\nlandlocked Arctic char.\nFrankly, I do not understand why the authors have included the Hg data from the peat/palsa profiles in this study. In the\nintroduction the authors do not to clearly explain why they believe that the approach of using Hg data of peat for Hg source\ntracing (marine versus atmosphere) is reasonable. First, the investigated peat deposits are quite different and as it seems,\nthat they are not ombrothrophic (all elements atmospherically derived), which makes a solely marine Hg source appear\ndoubtful. Moreover, instead of using Hg accumulation rates to estimate differences in Hg flux or Hg accumulation the authors\nbase their interpretation only on peat Hg concentrations. Many studies have shown that Hg concentrations in peat alone\ncannot be used to derive differences in atmospheric fluxes as they are strongly influenced by peat decomposition, plant\nspecies or hydrology etc.. Similar, the interpretation of δ202Hg in peat as a source proxy is doubtful as long as peat\ndecomposition and related Hg isotope fraction has not been considered. The authors actually mention this limitations.\nGenerally, δ202Hg as a source proxy appears to be doubtful regarding that the authors show intense internal metabolic\nδ202Hg fractionation in their biota.\nI suggest that the authors focus on the Hg data in biota as this data set is unique and important e.g. the finding that Hg\nconcentrations in hair and liver of polar bears show an increasing trend with time while Hg concentration in muscle\ndecrease, is remarkable. I also believe that the authors should attenuate their conclusions regarding the role of different Hg\ncurrents as different Hg sources because the data supporting this conclusion is rather weak. Discussing the invers trend of\nHg accumulation in biota and the atmosphere is the most important finding here. In this context, I further propose to remove\nthe peat data from the manuscript as it does not really support the discussion on Hg sources or transport pathways.\nPlease mention number of samples in figure captions, respectively."}, "author_response": "Author’s response: We are grateful for the positive and thorough feedback. We find\nthat the general comments have been elaborated in the specific comments below, so\nplease find our responses here.\nSpecific points:\n1. Line 55: Food webs cannot be slow-growing. We believe the authors may mean the\nbiota within those food webs are slow growing, which We agree can increase mercury\naccumulation. We suggest some minor re-wording for clarification around this point.\n#1: Author’s response: We agree, and the sentence has now been changed to: “Once\nHg enters the Arctic, it biomagnifies in the long Arctic food web, creating high Hg loads\nin long-lived top predators such as toothed whales, polar bears (Ursus maritimus) and\nhumans, thereby constituting a threat to biodiversity and health due to adverse\nneuroendocrine disruption.”\n2. Figure 1: What is the source of information for the ocean currents presented in part (a).\nPlease include relevant citations.\n#2: Author’s response: References are now inserted.” 25-27 (modified from ref. 28)”\n3. Line 98: Should the reference to Figure 1C be 1D in this line?\n#3: Author’s response: We find that the sentence: “The Δ199Hg signatures are near-\nsimilar for Central West and Northeast Greenland in the litter and upper 5 cm active\nlayer (medians, -0.17 and -0.15‰) (Fig 1b) and decrease with depth down to c. 50 cm\n(Supplementary Fig. 1c)” refers correctly to Supplementary Fig. 1c.\n4. Line 114: There is some confusing language in this line. You begin with “As for Peat”\nbut discuss muscle results. You might consider rewording for clarity.\n#4 Author’s response: ‘As for peat’ was meant as ‘Similar to peat’. The sentence has\nnow been reworded for clarity:\n“In the marine species, Hg stable isotopes in the muscle tissue of shorthorn sculpin\nand ringed seal reveal similar significant regional differences in δ202Hg signatures\nbetween Central West Greenland, Northwest Greenland (Qaanaaq), and Central East\nGreenland (Ittoqqortoormiit) (Fig. 1c and d).”\n5. Jiskra et al. (2021) use their endpoints to study terrestrial environments. Can the authors\ncomment on the relevance of these endpoints when translated to aquatic ecosystems? Are\nthere any differences in fractionation that should be accounted for in the interpretation\nherein or would all the changes from atmospheric speciation be equally reflected in\nterrestrial and aqueous environments?\n#5: Author’s response: Jiskra et al. (2021) used Δ200Hg endpoints to study atmospheric\nmercury sources to the ocean (i.e. not terrestrial environments) (Jiskra, M. et al. Nature\n597, 678-682 (2021)). Perhaps, it is a typing error, and the sentence should be reversed?\nHowever, regardless, Δ200Hg is considered a conservative trace for atmospheric\nmercury deposition as Hg(0) or Hg(II) to both marine and terrestrial environments.\nΔ200Hg is thought to be produced by photochemical Hg redox reactions at or above\nthe tropopause, and no relevant Hg transformations at the Earth’s surface have thus\nfar been shown to further result in changes in Δ200Hg. This is now elaborated in the\nintroduction:\n“Even-isotope MIF (Δ200Hg and Δ204Hg) is considered linked to photo-oxidation of\ngaseous Hg(0) high in the tropopause, and the Δ200Hg signature is therefore commonly\nused as a conservative tracer of atmospheric Hg deposition pathways to both terrestrial\nsurfaces and water bodies (i.e. deposition as Hg(0) or Hg(II)). ”\n6. Lines 180-183: The results herein are important. Where can the data supporting this\nconclusion be seen? Figure 2, We assume. Please add a reference to relevant information\nto allow for independent reader interpretation.\n#6: Author’s response: Yes, correct, the conclusion points to the data shown in Fig. 2\nand a reference to Fig. 2 has now been inserted:\n“The Δ200Hg signatures in the marine and freshwater species in this study indicate that\nbetween ~60 and ~87% of the Hg bioaccumulated in the species are atmospherically\ndeposited as Hg(0), while the remaining ~13 to ~40% are deposited via wet or dry Hg(II)\ndeposition (Fig. 2).”\n7. Line 246: “Residual Hg from photochemical demethylation” would inherently not be\nmethylmercury, which is commonly assumed to be the dominate form bioaccumulated in\nthe biota and tissues reference herein. Thus, these estimates are surprising. The authors\ndon’t explore this or explain potential mechanisms. We suggest some discussion is added\naround these points within lines 242-250, or clarifications to your phrasing as we are\nmisinterpreting this sentence.\n#7: Author’s response: We agree. The sentence should have been “Residual methyl-Hg\nfrom photochemical demethylation”. Now corrected.\n8. Lines 305-307: More support for this statement is needed. Were benthic organisms\ntested in this study to make this inference, or is this based on previous work? If the latter,\ncitations are needed.\n#8: Author’s response: We agree. Benthic organisms were not part of the study, but\nother studies have found that fish feeding on benthic organisms have lower Δ199Hg than\nfish feeding on pelagic organisms. This is due to less light penetration and\nphotochemical breakdown of methyl-Hg in the benthic environment. A reference to two\nof those studies (Perrot et al., 2012; Pinzone et al., 2021) is now provided in the text.\n9. Lines 388-395: The direct link between water mass exchange, ecosystem size/location,\nand legacy Hg is unclear. Ocean spray and local atmospheric processes would surely\nimpact coastal ecosystems, marine and freshwater alike. Could the authors elaborate on\nthese points?\n#9: Author’s response: We agree. It is a good point to mention that ocean spray and\nlocal atmospheric processes such as oceanic evasion/redeposition will impact all\ncoastal ecosystems, both marine and fresh water. Furthermore, since the manuscript\nwas submitted, a study was published in Nature Communications showing that oceanic\nevasion and redeposition of gaseous elemental Hg is likely to drive transport to Arctic\nterrestrial ecosystems (Huang S. et al. Oceanic evasion fuels Arctic summertime\nrebound of atmospheric mercury and drives transport to Arctic terrestrial ecosystems,\nNat. Commun. 16, 903 (2025)).\nWe have added the following sentence to the discussion: “However, sea spray and\noceanic evasion/redeposition of Hg will impact all Arctic marine and coastal\necosystems and may also dominate summertime transport to Arctic terrestrial\necosystems47.”\n10. Line 521: Seabirds are used to make local inferences; how mobile is this gull species?\nIs there a confounding factor of movement in the interpretation of their Hg concentrations\nand isotopes that should be accounted for herein? The same could be asked of seals and\npolar bears.\n#10: Author’s response: This is a good point, and we agree that the mobility of all\nspecies should be elaborated in the discussion. For the glaucous gulls, they come to\nEast Greenland in spring and leave in late October and go to wintering areas in warmer\nlatitudes. The wintering area for the East Greenland glaucous gull population is not\nknown but is likely to include wintering areas in Iceland. This is now elaborated in the\nmanuscript:\n“Sculpins are known as a sedentary benthic fish species with a generalist feeding\nstrategy encompassing amphipods, polychaetes, decapods, molluscs, and other fish\nin their diet50. Ringed seals in East and Northwest Greenland are known to feed mainly\non polar cod (Boreogadus saida) and Arctic cod (Arctogadus glacialis), whereas ringed\nseals in Central West Greenland primarily prey on amphipods (Parathemisto spp.),\ncapelin (Mallotus villosus), redfish (Sebastes sp.), and squid (Gonatus sp.)51. The ringed\nseal ecology is closely connected to the sea ice and most adult ringed seals are\nbelieved to be rather stationary, whereas juvenile ringed seals from different parts of\nthe Arctic have been reported to travel over large distances (>1,000 km)52. Glaucous\ngulls are top predators and scavengers, and feed on a variety of prey items, including\nmarine invertebrates, fish, bird eggs, and other birds53. Glaucous gulls come to East\nGreenland in spring and leave in late October to winter at lower latitudes, possibly\naround Iceland54. Polar bears are top predators and feed predominantly on ringed seals,\nhooded seals, and harp seals in East Greenland but occasionally also on other food\nitems (e.g. bearded seals (Erignathus barbatus), narwhal (Monodon monoceros), and\nAtlantic walrus (Odobenus rosmarus))55. The Central East Greenland polar bear\nsubpopulation is one of 20 recognized subpopulations of polar bears in the world and\nthe movement patterns cover >2,000 km of pack ice and fast ice along the East\nGreenland coast above 64°N56.”\n11. Line 569: We would argue that blanks test for contamination, not accuracy of\nprecision. This is a minor point, but important nonetheless. Otherwise, We appreciate the\nauthors clear reporting of their QA/QC procedures and results.\n#11: Author’s response: We agree and have now adjusted the sentence: “One blank\nsample and one NIST 8610 CRM were analyzed for at least every six samples to assess\ncontamination/carry-over and accuracy/precision”. We also updated the method\nsection with a description of the methodology for the liver, hair, and kidney samples\nthat were added to the manuscript late in the writing process along with QA/QC data\nfor those.\n12. Where possible, statistical details (e.g., p-values, and metrics of relationship strength)\nshould be added to results. We find the method statement about p-values in line 585 to be\ninsufficient. In particular, line statistics (including slopes, r2, and p-values) would be\nhelpful in figures 4 and extended figure 2 and allow for more effective reader\ninterpretations.\n#12: Author’s response: We agree and have now inserted trendline statistics in Fig. 4\nand Fig. S-2 to allow for a more effective reader interpretation. To accommodate the\nline statistics in Fig. 4, we enlarged the figure to two column width. We chose to also\ndisplay the individual points in the Fig. 4 (not only the medians) to visualize the\nvariation in the data. Finally, since the original manuscript was written, we received 9\nmore specimens of ringed seal from Qeqertarsuaq collected in 2023 and 2024 and have\nupdated the figures and text with data from those specimens (confirming the previous\ntrends but now with 66 data points instead of 57, providing more solid conclusions).\n13. Line 578: The program used for statistical analyses and graphing should be noted. If R\nwas used, any specific packages (and their versions) should also be noted, as this can affect\nthe background settings and statistical inferences.\n#13: Author’s response: Now inserted:\n“Statistical analyses were done using R version 4.2.3. One-way analysis of variance\n(ANOVA) was performed to determine differences in Hg stable isotope signatures\nbetween sites and species. Prior to the ANOVA, a Shapiro-Wilk test was conducted to\ntest for normality of data, and a Brown-Forsythe test was undertaken to test for equal\nvariances between groups. A Welch ANOVA test was applied due to unequal variances\nidentified for some groups, and a Games-Howell post hoc test was used to assess\nindividual group differences (using R-package ‘rstatix 0.7.1’). Temporal trends in Hg\nstable isotope signatures were evaluated using the Mann-Kendall and Theil-Sen’s slope\nanalysis on subadult ringed seals (≤4 years) and polar bears (males ≤5 years; females\n≤4 years) (using R-packages ‘Kendall 2.2.1’ and ‘mblm 0.12.1’).”\n14. Line 332-334: Should the reference to Figure 4C be 4B? We suggest double-checking\nthe figure references in this section.\n#14: Author’s response: The section has now been checked and the references\nupdated.\n15. Figure 4: Beyond diet, which only explains ~ 0.5 per mil of the differences in slopes,\nwhat other factors explains the difference in temporal response between seal and bears,\ntwo species which should be tropically linked. If this trend is otherwise geographically\nimpacted, we would expect both species to show an impact.\n#15: Author’s response: This is a good question. The δ202Hg is affected by both source\ninputs to the ecosystem (i.e. oceanic, atmospheric and/or riverine) as well as\nbiogeochemical transformation processes in the environment, and prey\nspecies/trophic levels. Consequently, the observed differences in δ202Hg trends\nbetween ringed seals and polar bears could reflect different changes in diet across\nspecies. We can adjust for changes in diet in the polar bears as we have reported\ndocumented diet changes for the polar bears along with δ202Hg data of the prey species.\nHowever, we do not have similar data available for the ringed seals. This is now\nmentioned:\n“Part of the increase in δ202Hg in the polar bears in Central East Greenland can be\nascribed to a documented shift in diet during the period with consumption of significant\nless ringed seals with lower δ202Hg (median, 0.28; n = 82) towards more hooded seals\nwith higher δ202Hg (median, 1.19; n = 27) (Supplementary Fig. 3)55,59. When accounting\nfor this change in diet composition, the increase in δ202Hg is reduced from ~1.7‰ to\n~1.2‰ (Fig. 4c). We do not have similar data for potential changes in diet for the ringed\nseals.”\nReviewer #2 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports.\nThis is part of the Nature Communications initiative to facilitate training in peer review\nand to provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\nReviewer #3 (Remarks to the Author):\nThe authors provide an in-depth and comprehensive assessment on spatio-temporal\nvariation in mercury source and pathway dynamics in the Arctic using total Hg and Hg\nisotope measurements of peat and several key marine organisms. The authors attribute\nincreases in Hg for some species to spatial differences in ocean current transport of legacy\nHg up the food web. I found the methodological approach, experimental design and\ntimeline investigated to be novel with results having high impact. I really enjoyed reading\nthis well-written and well thought-out manuscript where the data and results support the\nmain conclusions of the paper. This manuscript will definitely be of interest to readers of\nNature Communications and the broader scientific community. I only have a few very\nminor comments.\n1) In the Summary section, suggest modifying “internal metabolic fractionation” to\n“internal metabolism and isotopic fractionation.”\n#16: Author’s response: We agree. Now changed.\n2) Bottom of page 9 where the authors are inferring species trophic level from d202Hg\nvalues – I’m curious whether the authors investigated how well d202Hg correlates with\nd15N across species. D15N is an isotope that is very commonly used to estimate trophic\nlevel differences between species and this could be worthwhile to include in the\nsupplementary materials section.\n#17: Author’s response: This is a good suggestion, and we expect D15N to correlate well\nwith δ202Hg within sites as we see a very clear increase in δ202Hg with trophic level in\nIttoqqortoormiit, East Greenland (Fig. 3d). However, we do not have D15N data available\nfor the sculpins and glaucous gulls from Ittoqqortoormiit measured for δ202Hg. This is\ncertainly something we will consider for future work.\n3) Top of page 10 when describing the foraging ecology of glaucous gulls – it’s also\nworthwhile to state that they are also scavengers.\n#18: Author’s response: We agree and this is now added: “Glaucous gulls are top\npredators and scavengers, and feed on a variety of prey items, including marine\ninvertebrates, fish, birds and bird eggs as well as e.g. polar bear left-over seal\ncarcases53.”\n4) Top of page 10 when describing the foraging ecology of polar bears – it’s also\nworthwhile to briefly state some of the other food items (i.e. bearded seals, narwhal,\nAtlantic walrus).\n#19: Author’s response: We agree and this is now added: “Finally, polar bears are top\npredators and feed predominantly on ringed seals, hooded seals, and harp seals in East\nGreenland but occasionally also on other food items (e.g. bearded seals, narwhal, and\nAtlantic walrus)”.\nReviewer #4 (Remarks to the Author):\nThe authors describe a study on total Hg concentrations and Hg isotopes in peat profiles\nand different key Arctic aquatic predators at different locations in Greenland. The major\naims of this study are 1: to track Hg sources and pathways to Greenland 2: to show the\ndevelopment of total Hg and Hg isotopes in aquatic predator species in the past 40 years\nand how the development of the Hg content compares to that of anthropogenic\natmospheric Hg emissions. 3: to show if there is internal metabolic fractionation of Hg\nisotopes in the different investigated aquatic species. 3:\nThe authors could show that there are distinct regional differences in total Hg and Hg\nisotopes (δ202Hg signatures) between central western Greenland and northern-eastern\nGreenland which they suggest to be caused by different marin currents with different\ndissolved Hg concentration rather than by different atmospheric sources. Moreover they\nobserved that Hg concentrations in Arctic aquatic predatory species have increased in the\npast 40 years opposite to the decrease of atmospheric Hg emissions and atmospheric Hg\nconcentrations. From this they concluded that rather delayed mobilised legacy Hg than\ndirect atmospheric Hg deposition is the major pathway of Hg to arctic aquatic predators\nand Greenland soils/peat (e.g. through seaspray aerosols).\nThis study is innovative and of importance and shows important findings on the\ndistribution of Hg contamination in the aquatic environment in Greenland. To my\nknowledge, this is the first and most comprehensive data-set of Hg concentrations and Hg\nisotopes from Greenland biota and from Arctic biota in general including monitoring data\nof aquatic species of the past 40 years, which I think is crucial to understand the\nbiogeochemical Hg cycling in the arctic.\nFor me, the merit of this study lies in the large and unique data set of Hg concentrations\nand isotopes in Arctic biota, which gives new insights into the large variability in Hg\nenrichment and Hg isotope fractionation due to different food sources and through internal\nmetabolic processes and indicates that processes and pathways of Hg and Hg isotope\nfractionation are less clear than previously assumed. The discussion here appears to be\nsound, although the authors clearly point out that more research is needed here.\nHowever, even if I think that it is a plausible assumption, the conclusion that the major\npathway of Hg to the marine and the terrestrial Greenland foodchains is Hg evasion from\ndifferent ocean currents is not really supported by the data. The differences in dissolved\nHg data which the authors have drawn from the literature may explain at least partly the\ndifferences observed in marine predatory species at different locations in Greenland, but\ncannot explain the differences in peat or landlocked Arctic char.\n#20: Author’s response:\nWe thank for the positive feedback and appreciate the comment. Based on the comment\nwe find that we may not have explained properly how our data supports the conclusion\nthat oceanic source Hg, either through direct uptake in the marine environment or\nthrough evasion/atmospheric re-deposition in the terrestrial coastal environment, is\nlikely to be a major pathway to the marine and coastal/terrestrial food webs in\nGreenland (and why we consider the peat data important to include). We base this\nconclusion on the following:\n1) We observe significant regional difference in Hg isotopes in Greenland with\n~0.5‰ higher δ202Hg in Central West Greenland compared to\nNorthwest/Northeast/Central East Greenland (Fig. 1). This spatial trend\ncontrasts with modelling data for atmospheric deposition showing roughly\nsimilar source contributions (i.e. δ202Hg) for these regions but is in line with the\nregions being dominated by different ocean currents (i.e. Central West\nGreenland influenced by Atlantic Ocean inflow via the Irminger Current, and the\nother sites dominated by Arctic Ocean water). Previously reported δ202Hg values\nfor the Atlantic and Arctic Ocean show higher δ202Hg in the Atlantic Ocean in line\nwith the higher δ202Hg observed in Central West Greenland in this study\ninfluenced by Atlantic Ocean water. Importantly, we find the same regional\n~0.5‰ difference in both the aquatic species (considered dominated by aquatic\nmethyl-Hg uptake) and in peat (dominated by atmospheric Hg(0) uptake). This\nsupports the conclusion that the difference in δ202Hg is likely due to an overall\nsource difference rather than regional differences in environmental\ntransformation processes affecting the partitioning of Hg (although we cannot\nrule out the influence of that, which is now mentioned in the discussion). The\nspatial trend is consistent throughout the 1983-2024 period when ringed seals\nhave been collected (Fig 4a) and on a long-term basis revealed by the peat data\n(Fig. 1b; Supplementary Fig 1b).\n2) We find consistently lower total Hg in similar marine biota matrices in Central\nWest Greenland influenced by Atlantic Ocean inflow compared to the other\nGreenland regions consistent with lower total Hg concentration reported for\nAtlantic Ocean water compared with Arctic Ocean water.\n3) We observe increasing total Hg trends in most marine biota matrices in\nGreenland during the past 40 years in contrast to observed and modelled\natmospheric deposition trends for the Arctic showing decreasing trends due to\ndecreased global emissions. This discrepancy is an indication that ocean\ntransported Hg may now be a dominant source. The average retention time of\nHg in the ocean is much longer (>300 years) than in the atmosphere (approx. 0-\n5-1 year) and because of that, the marine species will show a much-delayed\nresponse to atmospheric variations. Due to the long retention time of Hg\nreported in the ocean, the ocean transported Hg will contain both recent and\nlegacy Hg.\nConsequently, we have updated and sharpened the discussion:\n“The consistent geographic difference in δ202Hg signatures in peat profiles as well as\nin δ202Hg and total Hg in shorthorn sculpin and ringed seal is surprising and considered\nto reflect long-term regional differences in Hg source inputs to Greenland. Overall,\nCentral West Greenland displays ~0.5 to 0.6‰ higher δ202Hg compared to similar\nmatrices in Northwest, Northeast, and Central East Greenland, with consistent\ndifferences observed throughout the active layer in peat profiles (Fig. 1b) and through\nthe last 40 years of sampling for ringed seal (Fig. 4a). Although the peat profiles are not\ndated, a similar peat profile (i.e. palsa consisting of alternating layers of segregated ice,\npeat, or mineral soil material) from Northeast Greenland is dated to 2,623 ± 123 BP at\n50 cm depth29, implying that the Hg in the upper c. 25 cm thick active layer of the peat\nshould represent hundreds of years of deposition even though growth rates of palsas\nvary with time30. However, using δ202Hg as source tracers in peat profiles and biological\ntissue samples pose limitations. For peat, factors such as vegetation type and\ndecomposition/leaching may influence δ202Hg31-32. Consequently, to minimize potential\neffects of vegetation type and decomposition on the δ202Hg between sites, we chose\ntwo representative palsas, one from each region, both considered ombrotrophic and\nfully covered with a similar vegetation type dominated by sedges including Carex and\nEriophorum species as well as a diverse moss flora. We observe the same spatial\ndifference in δ202Hg in the fresh litter layer and through the upper 25 cm of the active\nlayer between the two regions (Fig. 1b; Supplementary Fig. 1b), indicating little/no\neffects from to varying decomposition between the two sites. Thus, the consistently\nhigher δ202Hg signatures in Central West Greenland throughout the top 25 cm active\nlayer compared to the Northeast Greenland profile is considered a result of persistent\nlong-term regional differences in δ202Hg inputs. For the biological tissue samples,\nδ202Hg signatures are influenced not only by the Hg source input to the environment\nbut also by all biogeochemical transformation processes (e.g. microbial\nmethylation/demethylation)11,12 as well as prey species/trophic level33-34, and internal\nmetabolism in the species12,16. Here, we choose to compare δ202Hg signatures in similar\nspecies (shorthorn sculpin and ringed seal) and tissue (muscle) across the three\nregions to minimize effects related to differences in prey species/trophic level and\ninternal metabolism. The consistent ~0.5 to 0.6‰ higher δ202Hg in Central West\nGreenland compared to the other Greenland regions for both peat (mainly influenced\nby atmospheric Hg(0) source uptake22,35) and marine species (considered mainly\ninfluenced by marine methyl-Hg source uptake36) suggests an overall difference in Hg\nsource inputs between the regions. However, we cannot rule out that regional\ndifferences in environmental biochemical processes affecting the δ202Hg can have\nadditional minor effects.\nThe significant spatial difference in δ202Hg for Greenland contrasts with recent model\npredictions for atmospheric Hg deposition in the Arctic showing regional differences\nin absolute deposition but relatively uniform source compositions, reflecting a\nrelatively long lifetime of Hg in the atmosphere (~0.5-1 y)37 and effective mixing of\nsource inputs4. Atmospheric Hg transport to the Arctic follows a general pattern\ndominated by northern Eurasian sources in winter and mid-latitude Asian and North\nAmerican sources in spring4,38. The significantly higher δ202Hg in all sample matrices at\nthe Central West Greenland site, located only approximately 1,000 km from the\nNorthwest Greenland site and both being exposed to the same overall atmospheric\ntransport patterns, is therefore surprising. This suggests that direct atmospheric Hg\ndeposition may not, as currently predicted5, be the dominant pathway for Hg pollution\nin marine and coastal areas in Greenland and perhaps also elsewhere in the Arctic.\nInputs of Hg from rivers or glacial meltwater in Greenland are considered low39-41 and\ntherefore not a likely cause of the observed δ202Hg variation. Instead, we propose that\nocean currents may be the main Hg pathway explaining the significantly lower total Hg\nconcentrations and higher δ202Hg observed in marine and coastal areas in Central West\nGreenland than in Northwest, Northeast, and Central East Greenland. Central West\nGreenland is dominated by the relatively warm West Greenland Current, which is a\nmixed current comprising a branch of the Irminger Current from the North Atlantic\nOcean and a branch of the East Greenland Current originating from the Arctic Ocean\n(Fig. 1a)25. The other Greenland regions are all dominated by currents from the Arctic\nOcean26-27. The total Hg concentrations measured in North Atlantic waters are lower\nthan in Arctic waters42-43,54, with North Atlantic-sourced water flowing into the Arctic\nhaving lower Hg (0.15 ± 0.07 ng/l) than Arctic Ocean surface waters (0.24 ± 0.12 ng/l)5.\nFurther, the few studies available on Hg stable isotopes in seawater report higher\nδ202Hg in the North Atlantic waters (median, -0.45‰)19 than in Arctic Ocean waters (-2.85\nto -1.10‰)44. Consequently, we hypothesize that ocean currents are the main driver of\nHg uptake in marine and coastal environments in Greenland, and other Arctic regions,\neither through direct uptake of Hg from ocean waters or through Hg evasion from the\nocean surface followed by atmospheric redeposition and uptake, explaining the\nsignificantly lower total Hg in marine species and higher δ202Hg in both marine species\nand peat profiles in Central West Greenland influenced by Atlantic Ocean water. It is\ninteresting to note that the same regional difference in δ202Hg as recorded for marine\nspecies occurs in peat profiles in Central West Greenland and Northeast Greenland,\nsituated 800 m and 2 km inland, respectively. This may be due to a greater influence of\natmospheric Hg deposition from sea spray and/or evasion/redeposition of Hg from\nregional ocean surface waters compared to long-distance atmospheric sources. The\ninfluence of sea spray on the vegetation in coastal zones is well known45, and laboratory\nstudies have shown that sea salt aerosols have a high capacity for uptake of reactive\ngaseous Hg46. A recent study shows that oceanic evasion is likely the dominating\nsource and pathway to atmospheric Hg(0) in the Arctic during summertime (~80%)\ndriving the transport to Arctic terrestrial ecosystems47. Likewise, evasion from the\nocean surface waters is known to constitute a major part (~43%) of the global Hg flux\nbudget8.”\nFrankly, I do not understand why the authors have included the Hg data from the\npeat/palsa profiles in this study. In the introduction the authors do not to clearly explain\nwhy they believe that the approach of using Hg data of peat for Hg source tracing (marine\nversus atmosphere) is reasonable. First, the investigated peat deposits are quite different\nand as it seems, that they are not ombrothrophic (all elements atmospherically derived),\nwhich makes a solely marine Hg source appear doubtful. Moreover, instead of using Hg\naccumulation rates to estimate differences in Hg flux or Hg accumulation the authors base\ntheir interpretation only on peat Hg concentrations. Many studies have shown that Hg\nconcentrations in peat alone cannot be used to derive differences in atmospheric fluxes as\nthey are strongly influenced by peat decomposition, plant species or hydrology etc.. Similar,\nthe interpretation of δ202Hg in peat as a source proxy is doubtful as long as peat\ndecomposition and related Hg isotope fraction has not been considered. The authors\nactually mention this limitations. Generally, δ202Hg as a source proxy appears to be\ndoubtful regarding that the authors show intense internal metabolic δ202Hg fractionation\nin their biota.\nI suggest that the authors focus on the Hg data in biota as this data set is unique and\nimportant e.g. the finding that Hg concentrations in hair and liver of polar bears show an\nincreasing trend with time while Hg concentration in muscle decrease, is remarkable. I also\nbelieve that the authors should attenuate their conclusions regarding the role of different\nHg currents as different Hg sources because the data supporting this conclusion is rather\nweak. Discussing the invers trend of Hg accumulation in biota and the atmosphere is the\nmost important finding here. In this context, I further propose to remove the peat data from\nthe manuscript as it does not really support the discussion on Hg sources or transport\npathways.\n#21: Author’s response: We valuate the comment and agree that it is unusual to\ncombine aquatic biota data and peat data in the same paper. We also agree that we\nneed to better explain the reasoning behind this approach. However, after careful\nconsideration, and based on the reasons elaborated below, we suggest keeping the\nmost important peat data in the paper (i.e. the Hg isotope data from the litter/active layer\nrepresenting the most recent deposition). The peat Hg isotope data (δ202Hg) in\ncombination with the sculpin and ringed seals Hg isotope data show a similar spatial\ntrend across different Hg pools (representing predominately Hg uptake atmospheric Hg\n(0) and aquatic methyl-Hg, respectively), providing evidence that the difference in\nδ202Hg is likely to be due to overall differences in source input and most likely related\nto the ocean currents (see #20 pt. 1 above). Further, we use Δ200Hg in the peat to\ndescribe long-term pathways of atmospheric Hg deposition as Hg(0) or Hg(II), which we\nconsider a valuable addition to the paper. In contrast to δ202Hg, the Δ200Hg signature\nin peat is not considered influenced by decomposition etc. and Δ200Hg signature in peat\nis widely used as a conservative tracer for atmospheric deposition pathways (e.g.\nEnrico et al. Environ. Sci. Technol. 50, 2405−2412 (2016); Li, C. et al. Geochim.\nCosmochim. Acta 341, 62-74 (2023)).\nFor the peat total Hg concentrations, we agree that since we do not have a dating of the\nindividual peat layers, the total Hg cannot be used to conclude on absolute deposition\nrates and may therefore only be important as a reference in the Supplementary\nInformation section. Due to these changes, Fig. 1, 2 and 3 and Supplementary Table 1\nhave been revised accordingly along with changes to the text throughout the\nmanuscript (we refer to the track-changes version of the manuscript for these changes).\nFurther, we agree that indeed all the sample matrixes (in this case peat, sculpin muscle\nand ringed seal muscle) have limitations with regards to using δ202Hg as a Hg source\nproxy, and we have now elaborated more on that in the manuscript. All biogeochemical\nprocesses in the Hg cycle will results in changes in δ202Hg (e.g. Blum, J.D. et al. Annu.\nRev. Earth Planet. Sci. 42, 249-269 (2014)). Moreover, factors such as vegetation type\nand decomposition (for peat) and diet and internal metabolic fractionation (for biota)\nmay affect δ202Hg. However, by comparing the same vegetation type across regions and\nusing similar species and tissues, we can largely normalise for these influencing\nfactors, although we cannot rule out the regional differences in environmental Hg\ntransformations processes may also affect the δ202Hg. Both peat sites are considered\nombrotrophic and no indication of decomposition effects on the δ202Hg was observed\nas we see the same difference between regions throughout the upper profile (now\nadded to the discussion). Consequently, we have elaborated on the limitations for using\nthe δ202Hg as a source tracer in the all sample matrices in the discussion and have\nattenuated the conclusions regarding the regional differences in δ202Hg as a source\ntracer for the ocean currents, pointing out the regional differences in environmental Hg\ntransformation processes may also have an influence (although several trends in the\ndata suggest ocean currents as the driving source, see #20 pt. 1-3 above). Also, we\nmention that the increase in δ202Hg during the last 40 years in ringed seals and polar\nbears may also be influenced by changes in environmental Hg transformation\nprocesses in addition to anthropogenic sources. We refer to the track-changes version\nof the manuscript for these changes.\nPlease mention number of samples in figure captions, respectively.\n#22: Author’s response: Good point – thanks. The numbers of samples are now\ninserted in figure captions or figure legends."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Summary: We have re-reviewed NCOMMS-24-75705 \"Stable isotopes unveil ocean transport of legacy mercury into Arctic\nfood webs\" including the response to reviewers file. Overall, we find the authors responses to be thorough and detailed,\naddressing most of our original concerns and corrected areas where we believed were lacking with sufficient details.\nThe inclusion of more robust evidence and clarity on connections between the peat data and biological data is especially well\nwritten. We have one final point around the differences in temporal trends between polar bears and seals for consideration,\ndetailed below.\nRecommendation: Overall, we encourage publication of the manuscript.\nDetailed points:\n1. Our comments were addressed, largely through text modification and clarifications. This notably includes: (1)\nincorporating recently published literature (e.g., Huang et al. 2025); (2) a more detailed (i.e., well cited) paragraph around\nanimal migration impacts near line 521; (3) more robust statistical details (p-value reporting and method information); (4)\n2. For our (Review 1) point 15: In some ways, the authors’ answer contradicts itself: differences in trends are due to changes\nin polar bear diets, from rings seals to hooded seals, but what are the diet differences in the seals themselves. We\nunderstand that you do not have data on that, but there are past studies done that could build a much more robust discussion\naround this point, in lieu of direct data. E.g., Schiøtt, S., Holland, D.M., Holland, D. et al. The diet and diving behaviour of the\nringed seal (Pusa hispida) in Kangia (Ilulissat Icefjord), Greenland. Polar Biol 47, 1409–1422 (2024).\nhttps://doi.org/10.1007/s00300-024-03310-4.\na. More broadly, the authors also mention 3 sources of variability in their answer: sources, biogeochemistry, and diet. What\nabout species-specific metabolism? That has been important in fish recently (as well as other groups like birds), with some\ntaxonomic groups being identified as able to de-methylate MeHg or excrete high amounts of MeHg, relative to other species.\nE.g., see the recent paper: Tepper, K., King, J., Manuneedhi Cholan, P. et al. Methylmercury demethylation and volatilization\nby animals expressing microbial enzymes. Nat Commun 16, 1117 (2025). https://doi.org/10.1038/s41467-025-56145-w.\n3. The authors responses to review 4’s thorough comments are details and sufficient. While we do not disagree that the peat\ndata made the original paper somewhat complex, we agree with the authors’ holistic approach. Furthermore, we do not\nagree that the peat data is not inherently useful. While it may not serve as a direct measure of deposited Hg due to variations\nin decomposition rates and other factors, it remains a valuable data point when considered alongside biotic data. Given the\ndistance between sites (1000 km shortest) there may be differences in biogeochemical properties in the are but that also\noccurring with similar changes in biota (internal metabolism)seem to be slim. Therefore, I believe the inferences drawn are\nreasonable and well-supported.\n4. The added detail from Reviewer 4’s comments seem to be redundant at times. More specifically, they added a lot of\nsentences that end with “…and that’s why ocean currents seem to be the Hg source” The authors might consider compiling\nall of this evidence more concisely. However, we consider this a stylistic comment and defer to the editor on its\napplicability.", "reviewer_2": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts.", "reviewer_3": "The authors have done a nice job addressing and incorporating reviewer comments. I recommend acceptance.", "reviewer_4": "The authors have generally improved the clarity of the manuscript and have addressed most of my concerns. However, they\ndecided to keep the peat data in the manuscript although they now discuss the influence of peat decomposition etc.in more\ndepth.\nI still believe that the peat data is weak. The authors used only one record for each side of Greenland, respectively, and it\nremains doubtful that such a small data is representative. The variability of Hg uptake by peatland plants, and especially\nrelated Hg isotope fractionation, is not well understood and most existing data is from moss and not from vascular plants.\nMoreover, I don’t think that the peat data (n=1 only) does add much valuable and robust information to the overall message\nof the study and I still think it should be removed. However, it seems that the information received from the peat data is not\nessential for the conclusion drawn why I thing that it might be possible to published the ms in its present form."}, "author_response": "Author’s response: We are very pleased to hear that.\n2. For our (Review 1) point 15: In some ways, the authors’ answer contradicts itself: differences in trends are due to changes\nin polar bear diets, from rings seals to hooded seals, but what are the diet differences in the seals themselves. We\nunderstand that you do not have data on that, but there are past studies done that could build a much more robust discussion\naround this point, in lieu of direct data. E.g., Schiøtt, S., Holland, D.M., Holland, D. et al. The diet and diving behaviour of the\nringed seal (Pusa hispida) in Kangia (Ilulissat Icefjord), Greenland. Polar Biol 47, 1409–1422 (2024). https://doi.org/10.1007/s00300-024-03310-4.\nAuthor’s response: Diet differences between the seals species (as well as potential\nspecies-specific internal metabolism) are indeed reflected in the differences in Hg\nisotope signatures in the seal species tissues shown in Supplemental Fig. 3b and is\nthe basis for diet adjustments of the polar bear Hg isotope time trends shown in Fig.\n4c and d. We see that this was perhaps not adequately explained/elaborated in the\nprevious version of the manuscript. Consequently, we have adjusted the paragraph\nand added a new sentence and reference on documented stable isotope/diet\ndifferences between Greenland ringed seals, harp seals and hooded seals (ref. 60).\n‘Part of the increase in δ202Hg in the polar bears in Central East Greenland can be\nascribed to a documented shift in diet during the period with consumption of\nsignificant less ringed seals having lower δ202Hg (median in muscle tissue, 0.28; n =\n82) towards more hooded seals having higher δ202Hg (median in muscle tissue, 1.19; n\n= 27) (Supplementary Fig. 3)55,59. These differences in δ202Hg between seal species are\nconsidered to mainly reflect differences in prey species60.’\nWe also thank for the reference to the interesting paper from the Ilulissat Icefjord\narea. However, we are a bit reluctant to include the paper as reference in this study as\nthe Ilulissat Icefjord represents an area with very special environmental\ncharacteristics, and with a unique and adapted ringed seal and harp seal population.\nConsequently, the conclusions drawn may not apply to other Greenland sites and we\nhave therefore not included it. We hope the discussion on this topic is now\nsufficiently detailed, and we consider that a longer discussion on diet differences\nbetween seal species is beyond the scope of this study (which is also the reason why\nthe Hg isotope data on the different seal species is placed in the Supplementary\nInformation section). In our view, the main discussion should be focused on the\nchanges in diet over time for the East Greenland polar bears (and ringed seals) and\nthe differences in Hg isotopes between prey items to interpret/adjust the Hg isotope\ntime-trend series for the influence of diet changes. We can do these diet adjustments\nfor the East Greenland polar bears, but we do not currently have the data needed to\ndo it for the ringed seals (as also discussed).\na. More broadly, the authors also mention 3 sources of variability in their answer: sources, biogeochemistry, and diet. What about species-specific metabolism? That has been important in fish recently (as well as other groups like birds), with some taxonomic groups being identified as able to de-methylate MeHg or excrete high amounts of MeHg, relative to other species. E.g., see the recent paper: Tepper, K., King, J., Manuneedhi Cholan, P. et al. Methylmercury demethylation and volatilization by animals expressing microbial enzymes. Nat Commun 16, 1117 (2025). https://doi.org/10.1038/s41467-025-56145-w.\nAuthor’s response: Species-specific metabolism and internal Hg isotope fractionation\nis indeed an important aspect of the variability in Hg isotopes observed in the paper\nas presented and discussed in the section ‘Environmental processes and trophic\ntransfer’, especially for the mammals (ringed seals and polar bears). Species-specific\nmetabolism leading the internal Hg isotope fractionation in tissues is shown in Fig. 3c\nand the resulting difference in Hg isotope signatures with increasing δ202Hg shown in\nFig 3d. However, species-specific metabolism is not considered a reason for the\nspatial and temporal trends observed in our study as we compare similar species and\ntissues to normalise for this effect. Finally, thanks for the interesting reference.\nHowever, for the scope of our study with focus on Hg isotopes, and considering that\nwe already have 71 references, we consider the Li et al. (2022) review paper titled\n‘Internal dynamics and metabolism of mercury in biota: A review of insights from\nmercury stable isotopes’ (ref. 16) more appropriate and sufficient to cover the topic.\n3. The authors responses to review 4’s thorough comments are details and sufficient. While we do not disagree that the peat data made the original paper somewhat complex, we agree with the authors’ holistic approach. Furthermore, we do not agree that the peat data is not inherently useful. While it may not serve as a direct measure of deposited Hg due to variations in decomposition rates and other factors, it remains a valuable data point when considered alongside biotic data. Given the distance between sites (1000 km shortest) there may be differences in biogeochemical properties in the are but that also occurring with similar changes in biota (internal metabolism)seem to be slim. Therefore, I believe the inferences drawn are reasonable and well-supported.\nAuthor’s response: We are very pleased to hear that.\n4. The added detail from Reviewer 4’s comments seem to be redundant at times. More specifically, they added a lot of sentences that end with “…and that’s why ocean currents seem to be the Hg source” The authors might consider compiling all of this evidence more concisely. However, we consider this a stylistic comment and defer to the editor on its applicability.\nAuthor’s response: We appreciate the comment and have read through the\nmanuscript carefully again and made some minor changes. However, since this is\nconsidered a stylistic comment, we will leave it to the editor to decide if more\nadjustment is required.\nAuthor’s response: We agree that the peat profiles only represent two sites, one from\nCentral West Greenland and one from Northeast Greenland and therefore represent a\nlimited spatial coverage. However, the sites were carefully chosen to represent\ncomparable palsas fully covered with a similar vegetation type dominated by sedges\nincluding Carex and Eriophorum species as well as a diverse moss flora.\nConsequently, we expect that any difference of Hg isotope fractionation related to\nvegetation type is minimal. We find it important here that we observe a similar\ndifference in δ202Hg between the two peat profiles (~0.5-0.6‰) as in the aquatic\nspecies (muscle of sculpins and ringed seals) from the same regions (Central West\nGreenland vs. Northwest/Northeast/Central East Greenland, respectively). Hg uptake\nin vegetation is known to be dominated by Hg(0), whereas Hg uptake in aquatic\nspecies is dominated by methyl-Hg. We find the peat data important to include in the\npaper since the data adds evidence and support overall differences in Hg sources\nbetween these regions in Greenland (i.e. and not just differences in biogeochemical\nprocesses affecting Hg isotope fractionation in methyl-Hg as also pointed out by\nReviewer #1). Furthermore, we observe the same consistent difference in δ202Hg in the\npeat cores between the two regions throughout the upper 25 cm unfrozen active layer,\nfrom the litter layer to the permafrost table, indicating consistent long-term\ndifferences and making the conclusion more robust."}]

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