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).

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.

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 δ¹ ³C 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 K_OW below 4.3 were more likely to be released at low turbidity, whereas those with a log K_OW 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.

Permafrost thaw in peatlands risks increasing the production and mobilization of methylmercury (MeHg), a bioaccumulative neurotoxin that poses a health hazard to humans. We studied 12 peatlands on a trophic gradient in northwestern Canada, including permafrost peat plateaus and thawed bogs and fens, to determine the effects of thaw on MeHg production from measures of soil and porewater MeHg and in situ methylation assays. The production of MeHg was greater in thawed peatlands, especially rich fens, as indicated by higher potential rates of microbial methylation of inorganic mercury (Hg) to MeHg and higher soil %MeHg (MeHg:total Hg). Soil %MeHg was 0.1% in permafrost peat plateaus, 0.7% in bogs, and 2.0% in fens. Microbial analysis indicated three putative methylators (two methanogens and one novel bacteria) as most influential to the community composition, although their abundances were not consistently highest in fens. Fens had a greater range of porewater MeHg concentrations than bogs, potentially due to hydrological flushing, controls on MeHg solubility, or redox disequilibria in fens. MeHg in porewater was strongly associated with dissolved organic matter that had a low aromaticity and a low oxygen-to-carbon ratio. Regionally upscaling our results suggested that the expansion of bogs and fens due to thawing may increase the landscape-scale potential for MeHg production by 65% by 2100, representing a substantial risk to downstream aquatic ecosystems.

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.

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® 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.