Soils across permafrost regions are one of the largest terrestrial pools of mercury (Hg) in the world, storing an estimated 500-1500 Gg of Hg in the top three meters of soil. Ongoing climate-driven thaw threatens to release this legacy Hg into the environment. Efforts to quantify and model this pool have been hindered by a lack of harmonized, spatially resolved observations. To address this, we compiled a database of 117,802 Hg observations collected between 1988 and 2022 from 59 studies across Arctic, sub-Arctic, and alpine permafrost regions of the Northern Hemisphere, including North America, northern Europe, Eurasia and the Tibetan Plateau. The database includes Hg concentration measurements in solid materials - such as soil, leaves, roots, and wood - as well as in water samples from soil porewater, lakes, and rivers across the northern hemisphere permafrost domain. The database enables cross-site synthesis, model calibration and evaluation, and environmental assessments by standardizing and harmonizing data from diverse sources. Data standardization included unit conversion, categorization of observations by type, and quality-control procedures to ensure consistency across studies. Analytical uncertainty was preserved where reported in source studies, and quality control indicators - including range and outlier flags - were applied to support data screening and interpretation. Mercury concentrations vary widely across observations, with lake sediment showing the highest median values (70 ng g-1, IQR: 45-116), followed by soil (50 ng g-1, IQR: 32-90), and vegetation (15 ng g-1, IQR: 9-33). Water observations (total Hg) had a median of 2 ng L-1 (IQR: 2-6). Statistically significant differences in Hg concentrations among observation types were observed at both global and regional scales, generally following the pattern: lake sediment > soil > vegetation, although this ordering is sensitive to regional sampling distribution. These patterns, along with spatial and observation-type biases, highlight the need for improved coverage in underrepresented regions such as Eurasia. The database is freely accessible through Zenodo under the concept 10.5281/zenodo.18300989 (all versions; Olson et al., 2026a), to support ongoing research and model development in Arctic and sub-Arctic Hg cycle studies.

Mercury (Hg) has been stored in permafrost peatlands for millennia. As permafrost thaw is predicted to increase with ongoing climate warming, Hg is at risk to be remobilized from those peatlands and hotspots for Hg methylation could potentially form. Monomethylmercury (MeHg) is a known neurotoxin and a health concern to northern communities if Hg is remobilized, transformed to MeHg and subsequently bioaccumulated in the food chain. It is uncertain how Hg cycles in thawing permafrost systems and how much of it could potentially be remobilized by thaw processes. In this study, we have investigated Hg dynamics in a permafrost peatland of northern Sweden, where a snow fence field experiment was set up in 2005 to simulate accelerated permafrost thaw through winter warming. We compared total mercury (THg) and MeHg concentrations in soil plots representing intact and thawed permafrost conditions, investigated seasonal variations and examined the coupling between microbial community composition and MeHg concentration. Similar stocks of both total THg and MeHg were observed in intact and thawed permafrost conditions, suggesting that 17 years of winter warming manipulation and accelerated permafrost thaw had not led to substantial Hg loss from the peat nor extensive MeHg production. The apparent stability of the Hg stocks contrasted with our hypothesis and with many previous studies. While there was no difference in microbial communities between treatments with or without accelerated thaw, putative methylators were more abundant in thaw plots in the fall. This indicates that permafrost thaw has increased the potential for Hg methylation, although these shifts have not yet been strong enough to measurably affect MeHg stocks. Our study emphasizes the complexity of Hg dynamics in thaw-affected permafrost landscapes and the need to consider thaw-related perturbations of the Hg cycle on various timescales.

Differences in catchment properties may be major drivers in mercury (Hg) cycling. However, the complex interplay of these environmental drivers with Hg speciation, transport, and bioavailability is still not fully understood. To relate Hg speciation to different catchment types (tundra, birch, boreal) and their inherent differences in stream and lake chemistry, we studied Hg speciation and concentrations along a climate and vegetation gradient in sub‐arctic northern Sweden (including 18 streams and 8 lakes). We find differences in Hg concentrations aligning with differences in water chemistry between the studied catchment types. All observed differences between catchments align with the gradient in aquatic and terrestrial biological productivity (tundra < birch < boreal). Moreover, we find higher methylmercury (MeHg) concentrations in lakes compared to streams. Overall, our data suggests that dissolved organic matter (DOM) components play a crucial role in (a) the concentrations of total Hg and MeHg in the studied waters (especially allochthonous DOM) and (b) Hg methylation (especially autochthonous DOM).

The world's busiest ship-recycling hub, Gadani Beach in Baluchistan, Pakistan, may pollute the environment. This work provides the first complete and geographically resolved assessment of heavy metal pollution in Gadani shipbreaking region coastal sediments. Nine heavy metals were evaluated in 69 sediment samples from the shipbreaking site and a neighboring reference zone. Top 7 cm beach sand and sediments were collected from the yard zone and deconstruction zone at intertidal (0.3 m) and subtidal (2.4 m) depths. Additional sediment samples were taken from reference sites at 1.4-3 m depth. Two yard and dismantling zone replicates, and one reference site replicate were taken at each station. ICP-MS analyzed metals. Geoaccumulation Index and potential ecological risk index (PERI) were used to assess sediment contamination. Geoaccumulation Index and potential ecological risk index (PERI) were used to assess sediment contamination. Metal concentrations were 3-53 times greater in the shipbreaking area than at the reference location. The concentration hierarchy was Fe > Mn > Zn > Pb > Cr > Ni > As > Co > Cd, with Pb, Zn, and Ni above Effect Range Medium (ERM) criteria, implying ecological harm. Spearman correlation and cluster analysis showed substantial inter-metal correlations, indicating shared sources-primarily shipbreaking materials like paints, batteries, anodes, and lubricants. The yard zone was most polluted, followed by the seaward disassembly zone. Sediment Quality Guidelines (SQGs), Contamination Factor (Cf), Geoaccumulation Index (Igeo), and delta(1) C-3 isotopic fingerprints identified anthropogenic oil inputs and environmental hazards. This study provides essential baseline data for regional ship recycling regulatory frameworks and environmental management.

In surface waters, photodegradation is a major abiotic removal pathway of the neurotoxin monomethylmercury (MMHg), acting as a key control on the amounts of MMHg available for biological uptake. Different environmental factors can alter the rate of MMHg photodegradation. However, our understanding of how MMHg photodegradation pathways in complex matrixes along the land-to-ocean aquatic continuum respond to changes in salinity, dissolved organic carbon (DOC) concentration and dissolved organic matter (DOM) composition is incomplete. In a set of laboratory experiments combining several artificial and natural waters, we demonstrate that the interplay of DOC concentration, DOM composition, and salinity affects the photodegradation rate of MMHg. The presence of DOM was found to facilitate MMHg photodegradation, but degradation rates were not altered by varying DOC concentrations over two orders of magnitude. We found DOM composition to have a stronger effect on MMHg photodegradation rates than DOC concentration. However, at high DOC levels, where most UV radiation was lost within the first cm of the reaction vessels, lower MMHg photodegradation rates were observed. When moving from terrestrially influenced waters, characterized by a high degree of humification, towards marine conditions with a protein-rich DOM pool, MMHg photodegradation rates increased. In contrast, salinity had a stabilizing effect on MMHg. Hence, especially in systems with low salt and DOC concentrations, changes in either salinity or DOC concentration can impact the photodegradation rates of MMHg.

Sediments can act as reservoirs for hydrophobic contaminants, such as polycyclic aromatic hydrocarbons (PAHs), but can also release these pollutants into the water, posing risks to aquatic life. Conventional risk assessments typically focus on total sediment concentrations, even though the bioavailable fraction provides a more accurate measure of ecological risk. This study aimed to explore how sediment resuspension and contaminant hydrophobicity influence contaminant release into the water. We conducted an experiment where the release of four PAHs (acenaphthene, fluorene, phenanthrene, and fluoranthene) from an artificial sediment was studied at different resuspension treatments. Polyethylene passive samplers were used to sample PAHs released from sediment and water turbidity was used as a proxy for sediment resuspension. We identified a turbidity threshold at 2.7 NTU, below which PAH release was primarily driven by diffusion from sediment pore water, whereas at higher turbidity, resuspension and desorption processes played a more dominant role. Hydrophobicity was a critical factor for contaminant release: contaminants with a log KOW below 4.3 were more likely to be released at low turbidity, whereas those with a log KOW above 4.5 were released more at higher turbidity. Taken together, the results show that the release of contaminants from sediment into the water column is influenced by contaminant hydrophobicity and turbidity.

Environmental stressors, such as contaminants and physical factors, rarely act in isolation, and studying their joint effects provides a more accurate reflection of real-world scenarios. To capture these interactions and disentangle the direct and indirect influences on algal responses, we applied partial least squares structural equation modeling (PLS-SEM), allowing us to reveal the hierarchical relationships among stressors and their cumulative impact on algal physiology. We examined combined effects of microplastics (MP; presence/absence), polycyclic aromatic hydrocarbons (PAHs; a mixture of acenaphthene, fluorene, phenanthrene, and fluoranthene at a total chemical activity in the sediment of 0 or 0.14), and sediment resuspension (turbidity: 0.8-3.9 NTU) on Ceramium tenuicorne, a coastal macroalga that is likely to encounter all these stressors in its natural habitats. Mechanical mixing at two intensities (low and high) was applied as an experimental treatment to induce resuspension. The analysis separated the effects of mechanical mixing and turbidity, given their nonlinear relationship, as stronger mechanical mixing did not consistently result in proportional turbidity increases. The algal physiological responses were evaluated using changes in pigment composition (Chl a, Chl c, and carotenoids), photosystem II (PSII) performance, total antioxidant capacity, and algal stoichiometry measured as elemental (%C, %N, %H, and C/N) ratios. We found that PAH exposure was the main suppressor of pigment concentrations and PSII performance, underscoring the mechanisms of its adverse effects on the photosynthetic machinery and nutrient assimilation. Moreover, stronger turbulence further decreased pigment concentrations, while sediment resuspension increased antioxidant capacity in algae, possibly due to physical damage from abrasion and scouring. We also found that MP addition significantly increased turbidity, thus aggravating the effects of the sediment resuspension. In conclusion, we provide a mechanistic explanation of how the combined exposure to MPs, PAHs, and sediment resuspension can impact pigment composition, photosynthesis, and stoichiometry of the algae, leading to decreased productivity.

Hindukush (HK), Karakoram (KK), and Himalayan (HM) ranges (collectively called HKH), an extension of the Tibetan plateau, are sensitive areas for mercury (Hg) and other trace metals (TMs) contamination. These metals reach remote regions via long-range atmospheric transport from distant transboundary pollution sources, whereas local emissions, physiography, and climatic properties of alpine regions cause further enrichment of Hg and other TMs. Little is known about the chemical cycling of Hg and other TMs in the HKH region, which was investigated in the current study. Sediment and peat cores were taken from 10 remote lakes of the region, comprising three sediments and one peat core each from HM and KK, and two sediment cores from HK region. The mean concentration of total Hg in HM lakes was 13.08 µg/g, 8.46 µg/g in HK lakes, and 4.65 µg/g in KK lakes. Other metals, including iron (Fe), manganese (Mn), nickel (Ni), cobalt (Co), chromium (Cr), zinc (Zn), cadmium (Cd), arsenic (As), and selenium (Se), were also investigated in these cores. The mean concentrations of these metals in all three ranges were observed to be in decreasing order of Fe > Mn > Zn > Ni > Cr > As > Pb > Se > Cd. Overall, HM lakes were found to be highly enriched in Hg and other TMs, compared to KK and HK lakes. Both the mass burial rate (MBR) and mass burial flux (MBF) of Hg and other TMs were in decreasing order of HM > KK > HK. As a result of these findings, Hg might pose a potential risk within the remote lakes of HKH; therefore, further studies are highly recommended to understand the geochemistry, source apportionment, and bioaccumulation of Hg and other toxic metals in this pristine region.

Mono- and dimethylmercury (MMHg and DMHg, respectively) are the two primary organic forms of mercury (Hg) found in natural waters. While experimental approaches to characterize the environmental behavior of MMHg and inorganic forms of Hg are widely used today, few laboratories conduct experimental studies entailing the use of DMHg. In this paper, we have evaluated and developed different analytical and experimental approaches to quantify and use DMHg in laboratory studies. We demonstrate that DMHg can be analyzed from samples where MMHg is derivatized using sodium tetraethyl borate and where the matrix effects of dissolved sulfide are masked using copper sulfate. Tests, where the calibration curves of MMHg and DMHg were used, showed that MMHg may be used to calibrate for DMHg. For the pre-concentration of DMHg, both traps filled with Tenax (R) TA and Bond Elut ENV were found suitable. We observed good recoveries of DMHg added to different types of natural waters or purified water containing aquarium salt, sodium chloride and dissolved sulfide, iron sulfide, and cadmium sulfide at DMHg : sulfide molar ratios > 10(-6). In addition to evaluating these analytical aspects, we present suitable subsampling techniques for DMHg-containing solutions, the recovery of DMHg when filtering DMHg through different types of filters, and experimental data on the long-term stability of DMHg added to different types of waters and stored at different temperatures. Finally, we present and discuss a new synthetization protocol for preparing aqueous solutions containing DMHg free of organic solvents and where handling DMHg in a pure form is prevented.

Anoxic microniches in sinking particles in lakes have been identified as important water phase production zones of monomethylmercury (MeHg). However, the production and decay of MeHg during organic matter (OM) decomposition in the water column and its relation to the total Hg concentration in seston are poorly understood. We investigated total Hg and MeHg in relation to chemical changes in sinking seston and hydrochemical settings in a small and shallow (12 m deep) eutrophic lake during phytoplankton blooms from April to November 2019. The results show that MeHg proportions reach up to 22 % in seston in oxygen super saturation at the water surface and highest values (up to 26 %) at the oxic–suboxic redox boundary. MeHg concentrations were highest in May and November when algal biomass production was low and seston were dominated by zooplankton. Biodilution of MeHg concentrations could not be observed in the months of the highest algal biomass production; instead, MeHg and THg concentrations in seston were comparatively high. During suboxic OM decomposition and with decreasing redox potential (Mn and nitrate reduction), the concentration and proportion of MeHg in seston strongly decreased (<0.5 %), whereas total Hg concentrations show a 3.8- to 26-fold increase with water depth. Here, it remains unclear to which extent biodilution on the one hand and OM decomposition on the other alter the MeHg and THg concentration in seston. Changes in OM quality were most intense within or slightly below the redox transition zone (RTZ). The concentrations of MeHg and THg in seston from the RTZ were comparable to those found in the sediment trap material which integrated the changes in seston composition during the entire sampling period, suggesting that changes in the MeHg and THg content in the hypolimnion below the RTZ are comparatively small. Our study suggests that, in shallow eutrophic lakes, the water phase formation and decomposition of MeHg is intense and controlled by the decomposition of algal biomass and is, assumedly, largely disconnected from Hg methylation in sediments, similar to what has been observed in deep oligotrophic lakes.

Climate change driven increases in permafrost thaw and terrestrial runoff are expected to facilitate the mobilization and transport of mercury (Hg) from catchment soils to coastal areas in the Arctic, potentially increasing Hg exposure of marine food webs. The main aim of this study was to determine the impacts of seasonal riverine inputs on land-ocean Hg transport, zooplankton diet and Hg bioaccumulation in an Arctic estuary (Adventfjorden, Svalbard). The Adventelva River was a source of dissolved and particulate Hg to Adventfjorden, especially in June and July during the river's main discharge period. Stable isotope and fatty acid analyses suggest that zooplankton diet varied seasonally with diatoms dominating during the spring phytoplankton bloom in May and with increasing contributions of dinoflagellates in the summer months. In addition, there was evidence of increased terrestrial carbon utilization by zooplankton in June and July, when terrestrial particles contributed substantially to the particulate organic matter pool. Total (TotHg) and methyl Hg (MeHg) concentrations in zooplankton increased from April to August related to increased exposure to riverine inputs, and to shifts in zooplankton diet and community structure. Longer and warmer summer seasons will probably increase riverine runoff and thus Hg exposure to Arctic zooplankton.

Wetland area in agricultural landscapes has been heavily reduced to gain land for crop production, but in recent years there is increased societal recognition of the negative consequences from wetland loss on nutrient retention, biodiversity and a range of other benefits to humans. The current trend is therefore to re-establish wetlands, often with an aim to achieve the simultaneous delivery of multiple ecosystem services, i.e., multifunctionality. Here we review the literature on key objectives used to motivate wetland re-establishment in temperate agricultural landscapes (provision of flow regulation, nutrient retention, climate mitigation, biodiversity conservation and cultural ecosystem services), and their relationships to environmental properties, in order to identify potential for tradeoffs and synergies concerning the development of multifunctional wetlands. Through this process, we find that there is a need for a change in scale from a focus on single wetlands to wetlandscapes (multiple neighboring wetlands including their catchments and surrounding landscape features) if multiple societal and environmental goals are to be achieved. Finally, we discuss the key factors to be considered when planning for re-establishment of wetlands that can support achievement of a wide range of objectives at the landscape scale.

The central Arctic Ocean remains largely unexplored when it comes to the presence and cycling of mercury and its methylated forms including mono- and dimethylmercury (MMeHg and DMeHg, respectively). In this study, we quantified total Hg (HgT) and methylated Hg species in seawater, ice cores, snow, brine, and water from melt ponds collected during the SWEDARCTIC 2016 expedition to the Amerasian and Eurasian side of the Lomonosov Ridge. In the water column, concentrations of HgT, MMeHg and DMeHg ranged from 0.089 to 1.5 pM, <25 to 520 fM and from <1.6 to 160 fM, respectively. HgT was enriched in surface waters while MMeHg and DMeHg were low at the surface (i.e. in the polar mixed layer) and enriched at a water depth of around 200–400 m. A 1:2 ratio of DMeHg to MMeHg was observed in the water column suggesting a lower ratio in the central parts of the Arctic Ocean than what has previously been reported from other parts of the Arctic Ocean. At the ice stations, average HgT ranged from 0.97 ± 1.2 pM in the ice cores to 27 ± 17 pM in melt pond waters and average MeHgT (total MeHg) from 28 ± 15 fM in brine to 130 ± 18 fM in melt pond water. The HgT observed in melt ponds and brine was an order of magnitude greater than HgT observed in surface waters and HgT in the upper part of the ice-cores was ~4–8 times higher HgT in comparison to lower layers. Our study suggests that ice may act as a source of HgT to surface waters but not to be a likely source of the methylated Hg forms. Unlike elemental Hg, DMeHg did not enrich in surface waters covered by ice. Concentrations of DMeHg observed in the ice cores and other samples collected from the ice stations were low, suggesting ice to not act as a source of DMeHg to the atmosphere nor to surface waters.

Photochemical degradation of dimethylmercury (DMHg) could constitute an important source of monomethylmercury (MMHg) in surface waters, thus impacting Hg bioaccumulation and exposure risks. Despite this, few have studied this process, and no consensus has been reached on whether DMHg photodegradation occurs in nature. We used isotope labeling techniques to study DMHg and MMHg photodegradation in natural waters when exposed to artificial UV light. Our results confirm that DMHg degrades at rates comparable to those of MMHg for a variety of natural waters. We corroborated these findings in outdoor experiments, where samples containing DMHg and MMHg were exposed to natural sunlight. Comparison of the rates of photodecomposition for DMHg and MMHg in various water types imply differences in underlying reaction mechanisms for the species. To learn more about the factors controlling DMHg photodecomposition, we performed additional experiments where the effects of factors such as DOC, Cl- and O2 concentrations on DMHg and MMHg photodegradation rates were compared. Our findings indicate that the DMHg à MMHg flux through DMHg photodecomposition could represent a significant vector for MMHg production in surface oceans.

Sediments represent the main reservoir of mercury (Hg) in aquatic environments and may act as a source of Hg to aquatic food webs. Yet, accumulation routes of Hg from the sediment to benthic organisms are poorly constrained. We studied the bioaccumulation of inorganic and methylmercury (HgII and MeHg, respectively) from different geochemical pools of Hg into four groups of benthic invertebrates (amphipods, polychaetes, chironomids, and bivalves). The study was conducted using mesocosm experiments entailing the use of multiple isotopically enriched Hg tracers and simulation of estuarine systems with brackish water and sediment. We applied different loading regimes of nutrients and terrestrial organic matter and showed that the vertical localization and the chemical speciation of HgII and MeHg in the sediment, in combination with the diet composition of the invertebrates, consistently controlled the bioaccumulation of HgII and MeHg into the benthic organisms. Our results suggest a direct link between the concentration of MeHg in the pelagic planktonic food web and the concentration of MeHg in benthic amphipods and, to some extent, in bivalves. In contrast, the quantity of MeHg in benthic chironomids and polychaetes seems to be driven by MeHg accumulation via the benthic food web. Accounting for these geochemical and dietary drivers of Hg bioaccumulation in benthic invertebrates will be important to understand and predict Hg transfer between the benthic and the pelagic food web, under current and future environmental scenarios.

Increasing terrestrial run-off from melting glaciers and thawing permafrost to Arctic coastal areas is expected to facilitate re-mobilization of stored legacy persistent organic pollutants (POPs) and mercury (Hg), potentially increasing exposure to these contaminants for coastal benthic organisms. We quantified chlorinated POPs and Hg concentrations, lipid content and multiple dietary markers, in a littoral deposit-feeding amphipod Gammarus setosus and sediments during the melting period from April to August in Adventelva river estuary in Svalbard, a Norwegian Arctic Aarchipelago. There was an overall decrease in concentrations of ∑POPs from April to August (from 58 ± 23 to 13 ± 4 ng/g lipid weight; lw), Hg (from 5.6 ± 0.7 to 4.1 ± 0.5 ng/g dry weight; dw) and Methyl Hg (MeHg) (from 5 ± 1 to 0.8 ± 0.7 ng/g dw) in G. setosus. However, we observed a seasonal peak in penta- and hexachlorobenzene (PeCB and HCB) in May (2.44 ± 0.3 and 23.6 ± 1.7 ng/g lw). Sediment concentrations of POPs and Hg (dw) only partly correlated with the contaminant concentrations in G. setosus. Dietary markers, including fatty acids and carbon and nitrogen stable isotopes, indicated a diet of settled phytoplankton in May-July and a broader range of carbon sources after the spring bloom. Phytoplankton utilization and chlorobenzene concentrations in G. setosus exhibited similar seasonal patterns, suggesting a dietary uptake of chlorobenzenes that is delivered to the aquatic environment during spring snowmelt. The seasonal decrease in contaminant concentrations in G. setosus could be related to seasonal changes in dietary contaminant exposure and amphipod ecology. Furthermore, this decrease implies that terrestrial run-off is not a significant source of re-mobilized Hg and legacy POPs to littoral amphipods in the Adventelva river estuary during the melt season.