Mercury Export from Arctic Great Rivers

Properties of inflowing Pacific and Atlantic water govern total and methylated mercury profiles in the Arctic Ocean

High methylmercury (MeHg) concentrations in Arctic marine biota have been linked to high MeHg uptake driven by shallow MeHg peaks at water depths of 100-300 m in the Arctic Ocean. To understand how the biogeochemical characteristics of each basin affects the distribution of total Hg (THg) and MeHg across the Arctic Ocean, the spatial patterns of THg and MeHg were investigated using data from new transects in the Beaufort Sea (BS) and previously published Arctic Ocean expeditions covering the Canada Basin and Makarov Basin in the Pacific sector, and Amundsen Basin and Nansen Basin in the Atlantic sector. In the BS, the THg concentration in the polar mixed water increased with salinity (r = 0.87, p < 0.01), which was linked to THg transport from the Chukchi Shelf. Transport of Hg from the Chukchi Shelf also drove elevated THg concentrations in the polar mixed water and halocline water in the Canada Basin and Makarov Basin compared to other Arctic basins. The MeHg concentration in the BS was positively correlated with the biological index in the Pacific summer water (r = 0.86, p < 0.01), demonstrating that intrusion of warm and nutrient-rich Pacific water promotes MeHg production in the BS. In line with this result, chlorophyll-a showed a comparable cross-basin trend to that of MeHg, with the highest values in the Nansen Basin. In the halocline water, MeHg concentrations were highest in the Canada Basin likely due to the largest availability of Hg(II). On the contrary, MeHg concentration was highest in the Nansen Basin in the Atlantic water layer, which could be related to the higher seawater temperature and enhanced biological production. The results of this study underscore the critical role of Pacific and Atlantic inflows in modulating the profiles of THg and MeHg in the Arctic Ocean.

Toxic trace elements transport in stream sediments from the world-class Monte Amiata Hg mining district: Potential impact to the Mediterranean Sea

The spatial and temporal variability of Hg, As, and Sb contents were assessed in stream sediments across the Fiora River catchment (Italy), which drains the dismissed Monte Amiata Mining District, the 3rd largest Hg producer worldwide. Mercury, As, and Sb concentrations of samples collected in 2022 along Fiora River tributaries were compared to data collected in 1985 after mine decommissioning. In 2022, the Fiora River showed downstream magnification of Hg pollution, close to the outflow into the Mediterranean Sea. At several sampling sites, concentrations are above the Italian safety limit of 1 mg/kg (up to 3300 mg/kg). Arsenic and Sb concentrations are high in the mid-catchment tributaries (up to 84 and 79 mg/kg, respectively). The river contributes to the pollutant budget of the Mediterranean Sea, and showed low resilience to Hg pollution in the period 1985-2022, whereas for As and Sb some attenuation occurred. The mass loads of Hg, As, and Sb discharged into the Mediterranean Sea from the Fiora River are at least 0.9, 2.2, and 1.8 t/y respectively. The estimated potential sediment-bound Hg flux from the whole Monte Amiata district to the Mediterranean Sea (1.2-6.6 t/y) is in the same range of that calculated for the Idrija district, highlighting the pivotal role of the Mt. Amiata district on the Mediterranean Sea pollution.

Mercury bioaccumulation and assimilation in marine plankton in meltwater influenced fjords and shelf waters along the east coast of Greenland

The rapid melting of the Arctic cryosphere due to climate change will result in significant freshwater input into Arctic marine ecosystems. This might also cause the release of legacy mercury (Hg) stored in the cryosphere, increasing Hg concentration and its subsequent effects on the marine biota. However, there is scarce knowledge on the concentration of Hg in the lower trophic level organisms at the base of the Arctic pelagic food web. This is particularly important since these organisms modulate the transfer of Hg to higher trophic levels, including fish and marine mammals. We quantified the total Hg (THg) concentration in two plankton size classes (>200 and 50-200 μm) in coastal waters along the east Greenland coast and investigated the potential assimilation efficiency of both inorganic Hg (IHg) and methyl Hg (MeHg) in mesozooplankton and their faecal pellets in experimental incubations. The concentration of THg in plankton ranged from 12 to 109 ng (g dw)-1 without clear trends between geographic locations or between fjords and coastal areas. Also, the concentrations did not vary between the different plankton size fractions. MeHg concentrations were lower in the mesozooplankton faecal pellets than IHg, which may be due to the higher assimilation of MeHg than IHg in mesozooplankton tissue. Our results confirm that Arctic zooplankton assimilates MeHg more efficiently than IHg and may contribute significantly to the partitioning and cycling of different Hg types in Arctic marine ecosystems.

Oceanic evasion fuels Arctic summertime rebound of atmospheric mercury and drives transport to Arctic terrestrial ecosystems

Mercury (Hg) contamination poses a persistent threat to the remote Arctic ecosystem, yet the mechanisms driving the pronounced summer rebound of atmospheric gaseous elemental Hg (Hg0) and its subsequent fate remain unclear due to limitations in large-scale seasonal studies. Here, we use an integrated atmosphere-land-sea-ice-ocean model to simulate Hg cycling in the Arctic comprehensively. Our results indicate that oceanic evasion is the dominant source (~80%) of the summer Hg0 rebound, particularly driven by seawater Hg0 release facilitated by seasonal ice melt (~42%), with further contributions from anthropogenic deposition and terrestrial re-emissions. Enhanced Hg0 dry deposition across the Arctic coastal regions, especially in the Arctic tundra, during the summer rebound highlights the potential transport of Hg from the pristine Arctic Ocean to Arctic terrestrial ecosystems. Arctic warming, with a transition from multi-year to first-year ice and tundra greening, is expected to amplify oceanic Hg evasion and intensify Hg0 uptake by the Arctic tundra due to increased vegetation growth, underlining the urgent need for continued research to evaluate Hg mitigation strategies effectively in the context of a changing Arctic.

Recent advances in the study of mercury biogeochemistry in Arctic permafrost ecosystems

Permafrost predominates in polar and high mountain regions, encompassing nearly 15 % of the exposed land in the Northern Hemisphere. It denotes soil or rock that remains at or below 0 °C for the duration of at least two consecutive years. These frozen soils serve as a barrier to contaminants that are stored and accumulated in permafrost over extended periods of time. One of these chemical compounds is mercury (Hg), a heavy metal well recognized for its severe toxic effects. Mercury presents a major risk worldwide to ecosystems, biota and human health and is strengthened by the Minamata Convention on Mercury. The International Panel on Climate Change (IPCC) scientific group monitors and assesses the science related to climate change and highlights the significant impacts of global warming. The phenomenon known as Arctic amplification has accentuated warming of the Arctic in recent years and has led to the degradation and rapid thawing of permafrost. This process has significant implications in hydrology of the ecosystems and for the mobility of previously sequestered carbon and trace metals, such as Hg, with possible adverse environmental and human health impacts. In this article, we provide a comprehensive review of the current understanding of the Hg cycle in permafrost regions, exploring the effects of global warming on these intricate processes. Additionally, we highlight existing research gaps and propose directions for future investigations.

Organic carbon and mercury exports from pan-Arctic rivers in a thawing permafrost context - A review

Climate change affects more than elsewhere the northern circumpolar permafrost region. This zone comprises large rivers flowing mainly to the Arctic Ocean, delivering about 10 % of the global riverine water flux. These pan-Arctic Rivers drive the dynamics of northern organic carbon (OC) and mercury (Hg) cycling. Permafrost degradation may release substantial amounts of OC and Hg, with potential regional and global impacts. In this review, we summarise the main findings in the last three decades about the role of the pan-Arctic Rivers in OC and Hg cycling and the effect of climate change on these dynamics. Total DOC and POC fluxes delivered by the pan-Arctic rivers presently reach 34.4 ± 1.2 TgC·yr-1 and 7.9 ± 0.5 TgC·yr-1, while the export of Hg reaches 38.9 ± 1.7 Mg·yr-1. This review highlights future challenges for the scientific community in evaluating spatial and temporal dynamics of the processes involved in OC and Hg cycling in permafrost-affected areas. Permafrost thawing could lead to greater fluxes of OC and Hg with ill-known resulting risks for food chains. Within this context, efforts should be made to study OC effects on Hg methylation. Moreover, assessing the spatial variability of OC and Hg mobilisation and transport within the pan-Arctic watersheds may help understand the future OC and Hg cycling dynamics in the northern circumpolar permafrost region.

Substantial Mercury Releases and Local Deposition from Permafrost Peatland Wildfires in Southwestern Alaska

Increasing wildfire activity at high northern latitudes has the potential to mobilize large amounts of terrestrial mercury (Hg). However, understanding implications for Hg cycling and ecosystems is hindered by sparse research on peatland wildfire Hg emissions. In this study, we used measurements of soil organic carbon (SOC) and Hg, burn depth, and environmental indices derived from satellite remote sensing to develop machine learning models for predicting Hg emissions from major wildfires in the permafrost peatland of the Yukon-Kuskokwim Delta (YKD) in southwestern Alaska. Wildfire Hg emissions during summer 2015─estimated as the product of Hg:SOC (0.38 ± 0.17 ng Hg g C1-), predicted SOC stores (mean [5th-95th] = 9.1 [5.3-11.2] kg C m-2), and burn depth (11.3 [8.2-13.9] cm)─were 556 [164-1138] kg Hg or approximately 6% of Hg emissions from wildfire activity >60°N. Modeling estimates suggest that wildfire nearly doubled summertime Hg deposition within 10 km, despite advection of more than 75% of total emissions beyond Alaska. YKD areal emissions combined with remote sensing estimates of burned area suggest that wildfire Hg emissions from northern peatlands (25.4 [14.9-33.6] Mg y-1) are an important component of the northern Hg budget. Additional research is needed to refine these estimates and understand the implications for Arctic and global Hg cycling.

Exploring Drivers of Historic Mercury Trends in Beluga Whales Using an Ecosystem Modeling Approach

While mercury occurs naturally in the environment, human activity has significantly disturbed its biogeochemical cycle. Inorganic mercury entering aquatic systems can be transformed into methylmercury, a strong neurotoxicant that builds up in organisms and affects ecosystem and public health. In the Arctic, top predators such as beluga whales, an ecologically and culturally significant species for many Inuit communities, can contain high concentrations of methylmercury. Historical mercury concentrations in beluga in the western Canadian Arctic's Beaufort Sea cannot be explained by mercury emission trends alone; in addition, they could potentially be driven by climate change impacts, such as rising temperatures and sea ice melt. These changes can affect mercury bioaccumulation through different pathways, including ecological and mercury transport processes. In this study, we explore key drivers of mercury bioaccumulation in the Beaufort Sea beluga population using Ecopath with Ecosim, an ecosystem modeling approach, and scenarios of environmental change informed by Western Science and Inuvialuit Knowledge. Comparing the effect of historical sea ice cover, sea surface temperature, and freshwater discharge time series, modeling suggests that the timing of historical increases and decreases in beluga methylmercury concentrations can be better explained by the resulting changes to ecosystem productivity rather than by those to mercury inputs and that all three environmental drivers could partially explain the decrease in mercury concentrations in beluga after the mid-1990s. This work highlights the value of multiple knowledge systems and exploratory modeling methods in understanding environmental change and contaminant cycling. Future work building on this research could inform climate change adaptation efforts and inform management decisions in the region.

Near surface oxidation of elemental mercury leads to mercury exposure in the Arctic Ocean biota

Atmospheric mercury (Hg(0), Hg(II)) and riverine exported Hg (Hg(II)) are proposed as important Hg sources to the Arctic Ocean. As plankton cannot passively uptake Hg(0), gaseous Hg(0) has to be oxidized to be bioavailable. Here, we measured Hg isotope ratios in zooplankton, Arctic cod, total gaseous Hg, sediment, seawater, and snowpack from the Bering Strait, the Chukchi Sea, and the Beaufort Sea. The Δ200Hg, used to differentiate between Hg(0) and Hg(II), shows, on average, 70% of Hg(0) in all biota and differs with seawater Δ200Hg (Hg(II)). Since Δ200Hg anomalies occur via tropospheric Hg(0) oxidation, we propose that near-surface Hg(0) oxidation via terrestrial vegetation, coastally evaded halogens, and sea salt aerosols, which preserve Δ200Hg of Hg(0) upon oxidation, supply bioavailable Hg(II) pools in seawater. Our study highlights sources and pathways in which Hg(0) poses potential ecological risks to the Arctic Ocean biota.

The processes and transport fluxes of land-based macroplastics and microplastics entering the ocean via rivers

Approximately 80% of marine plastic waste originates from land-based sources and enters oceans through rivers. Hence, to create effective regulations, it is crucial to thoroughly examine the processes by which land-based plastic waste flows into marine environments. To this end, this review covers the complete journey of macro- and microplastics from their initial input into rivers to their ultimate release into oceans. Here, we also discuss the primary influencing factors and current popular research topics. Additionally, the principles, applicability, accuracy, uncertainty, and potential improvement of the standard methods used for flux estimation at each stage are outlined. Emission estimates of land-based macro- and microplastics are typically assessed using the emission factor approach, coefficient accounting approach, or material flow analysis. Accurately estimating mismanaged plastic waste is crucial for reducing uncertainty in the macroplastic emission inventory. In our review of the processes by which land-originating plastics enter rivers, we categorized them into two major types: point-source and diffuse-source pollution. Land surface hydrological models simulate transport from diffuse sources to rivers, necessitating further research. Riverine (micro)plastic flux to the ocean is often estimated using monitoring statistics and watershed hydrological models at the watershed scale; however, standardized monitoring methods have not yet been established. At the global scale, algorithms based on river datasets are often used, which require further improvements in river data selection and microplastic number-mass conversion factors. Furthermore, the article summarizes the accuracy and sources of uncertainty of various methods. Future research efforts should focus on quantifying and mitigating uncertainties in resultant projections. Overall, this review deepens our understanding of the processes by which land-based plastic waste enters the ocean and helps scholars efficiently select or improve relevant methods when studying land-ocean transport fluxes.

Seasonal riverine inputs may affect diet and mercury bioaccumulation in Arctic coastal zooplankton

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.

Climate change dynamics and mercury temporal trends in Northeast Arctic cod (Gadus morhua) from the Barents Sea ecosystem

The Northeast Arctic cod (Gadus morhua) is the world's northernmost stock of Atlantic cod and is of considerable ecological and economic importance. Northeast Arctic cod are widely distributed in the Barents Sea, an environment that supports a high degree of ecosystem resiliency and food web complexity. Here using 121 years of ocean temperature data (1900-2020), 41 years of sea ice extent information (1979-2020) and 27 years of total mercury (Hg) fillet concentration data (1994-2021, n = 1999, ≥71% Methyl Hg, n = 20) from the Barents Sea ecosystem, we evaluate the effects of climate change dynamics on Hg temporal trends in Northeast Arctic cod. We observed low and consistently stable, Hg concentrations (yearly, least-square means range = 0.022-0.037 mg/kg wet wt.) in length-normalized fish, with a slight decline in the most recent sampling periods despite a significant increase in Barents Sea temperature, and a sharp decline in regional sea ice extent. Overall, our data suggest that recent Arctic amplification of ocean temperature, "Atlantification," and other perturbations of the Barents Sea ecosystem, along with rapidly declining sea ice extent over the last ∼30 years did not translate into major increases or decreases in Hg bioaccumulation in Northeast Arctic cod. Our findings are consistent with similar long-term, temporal assessments of Atlantic cod inhabiting Oslofjord, Norway, and with recent investigations and empirical data for other marine apex predators. This demonstrates that Hg bioaccumulation is highly context specific, and some species may not be as sensitive to current climate change-contaminant interactions as currently thought. Fish Hg bioaccumulation-climate change relationships are highly complex and not uniform, and our data suggest that Hg temporal trends in marine apex predators can vary considerably within and among species, and geographically. Hg bioaccumulation regimes in biota are highly nuanced and likely driven by a suite of other factors such as local diets, sources of Hg, bioenergetics, toxicokinetic processing, and growth and metabolic rates of individuals and taxa, and inputs from anthropogenic activities at varying spatiotemporal scales. Collectively, these findings have important policy implications for global food security, the Minamata Convention on Mercury, and several relevant UN Sustainable Development Goals.

Glacial Erosion Drives High Summer Mercury Exports from the Yukon River, Canada

Mercury concentrations and yields in the Yukon River are the highest of the world's six largest panarctic drainages. Permafrost thaw has been implicated as the main driver of these high values. Alternative sources include mercury released from glacial melt and erosion, atmospheric mercury pollution, or surface mining. To determine the summer source and speciation of mercury across the Yukon River basin within Canada, we sampled water from 12 tributaries and the mainstem during July 2021. The total (unfiltered) mercury concentration in the glacier-fed White River was 57 ng/L, >10 times higher than all other sampled tributaries. The White River's high total mercury concentrations were driven by suspended sediment and persisted ∼300 km downstream of glacierized headwaters. Total mercury concentrations were lowest (typically <2 ng/L) in tributaries downstream of still-water landscape features (e.g., lakes and settling ponds), suggesting these features are effective sinks for sediment-bound mercury. Low total mercury concentrations (∼2 ng/L) were also observed in five tributaries across diverse thawing permafrost landscapes. These results suggest that glacial erosion and meltwater transport, not permafrost, drive enhanced exports of mercury with suspended sediment. Mercury exports may decline as glacial watersheds pass peak water. Other factors, including mercury released from permafrost thaw, are minor components at present.

Coastal streams and sewage outfalls: Hot spots of mercury discharge, pollution and cycling in nearshore environments

The coastal streams (CSs) and sewage outfalls (SOs) are widely distributed and direct anthropogenic stress on global coastal ecosystems. However, the CS/SO-associated mercury (Hg) discharge, pollution and cycle in nearshore environment are less quantified. Here, we report that total Hg (THg) and methylmercury (MMHg) concentrations in waters of CSs (n = 8) and SOs (n = 15) of the northern China were ∼102 to 103 times of coastal surface waters and 10 to 102 times of major rivers in China and other regions. The CS/SO discharges resulted in the increase of total organic carbon (TOC) contents, THg and MMHg concentrations and TOC-normalized THg and MMHg concentrations in sediments of CS/SO-impacted coasts. The laboratory experiments further illustrated that the CS/SO-impacted sediments characterized with high potentials of dissolved THg and MMHg productions and releases. Our findings indicate that the layout optimization of SOs is able to reduce the Hg risk in coastal environment.

Riverine Discharge Fuels the Production of Methylmercury in a Large Temperate Estuary

Estuaries are an important food source for the world's growing population, yet human health is at risk from elevated exposure to methylmercury (MeHg) via the consumption of estuarine fish. Moreover, the sources and cycling of MeHg in temperate estuarine ecosystems are poorly understood. Here, we investigated the seasonal and tidal patterns of mercury (Hg) forms in Long Island Sound (LIS), in a location where North Atlantic Ocean waters mix with the Connecticut River. We found that seasonal variations in Hg and MeHg in LIS followed the extent of riverine Hg delivery, while tides further exacerbated the remobilization of earlier deposited riverine Hg. The net production of MeHg near the river plume was significant compared to that in other locations and enhanced during high tide, possibly resulting from the enhanced microbial activity and organic carbon remineralization in the river plume. Statistical models, driven by our novel data, further support the hypothesis that the river-delivered organic matter and inorganic Hg drive net MeHg production in the estuarine water column. Our study sheds light on the significance of water column biogeochemical processes in temperate tidal estuaries in regulating MeHg levels and inspires new questions in our quest to understand MeHg sources and dynamics in coastal oceans.

Mercury budgets in the suspended particulate matters of the Yangtze River

Riverine processes are crucial for the biogeochemical cycle of mercury (Hg). The Yangtze River, the largest river in East Asia, discharges a substantial amount of Hg into the East China Sea. However, the influencing factors of the Hg budget and its recent trends remain unclear. This study quantitatively analyzed the total Hg concentration (THg) in suspended particulate matter (SPM) in the Yangtze River and calculated the Hg budget in 2018 and 2021. The results showed that the total Hg concentrations varied substantially along the river, with concentrations ranging from 23 to 883 μg/kg in 2018 and 47 to 146 μg/kg in 2021. The average Hg flux to China Sea in 2018 and 2021 were approximately 10 Mg/yr, lower than in 2016 (48 Mg/yr). Over 70% of the SPM was trapped in the Three Gorges Dam (TGD), and 22 Mg/yr of Hg settled in the TGD in 2018 and 10 Mg/yr in 2021. Hg fluxes in the Yangtze River watershed were driven by various factors, including decreased industrial emissions, increased agriculture emissions, and decreased soil erosion flux. We found that in the upper reach of the Yangtze River changed from sink to source of Hg possibly due to the resuspension of sediments, which implies that the settled sediments could be a potential source of Hg for downstream. Overall, emission control policies may have had a positive impact on reducing Hg flux to the East China Sea from 2016 to 2021, but more efforts are needed to further reduce Hg emissions.

Ocean current redistributed the currently using Organoamine Pesticides in Arctic summer water

In a comprehensive study on the presence and distribution of Currently Using Organoamine Pesticides (CUOAPs) in the Arctic Ocean, this study collected and analyzed 36 surface seawater samples during the summer of 2021. The study detected 36 CUOAPs, 17 of these compounds at levels exceeding the Method Detection Limits (MDLs). Concentrations of CUOAPs ranged from 0.11 to 2.94 ng/L, exhibiting an average of 1.83 ± 0.83 ng/L. Spatial distribution analysis revealed lower CUOAP concentrations in the central Arctic Ocean, with Cycloate constituting the most abundant component (23.66 %). The investigation identified terrestrial inputs and long-range atmospheric transport as potential sources of CUOAPs in the Arctic Ocean region. The origins of individual CUOAPs appeared to be associated with application procedures and their propensity for co-occurrence at low latitudes. The study also examined the role of ocean currents in the transport and redistribution of CUOAPs in surface seawater across different regions. While ocean currents played a significant role, the influence of sea ice cover on CUOAP distribution was minimal. An ecological risk assessment analysis underscored the need for regional attention to the presence of CUOAPs in the Arctic Ocean.

Geochemistry aspects of modern mercury accumulation in bottom sediments from the south-western Chukchi Sea

Mercury (Hg) having a high migration capacity reach the Arctic region via the atmosphere. The absorbers for Hg are sea bottom sediments. Sedimentation in the Chukchi Sea occurs under the influence of highly productive Pacific waters entering through the Bering Strait and the inflow of a terrigenous component from the western direction with the Siberian Coastal Current. The Hg concentrations ranged from 12 μg kg^-1 to 39 μg kg^-1 in bottom sediments of study polygon. Based on dating sediment core the background concentration was 29 μg kg^-1. Concentration of Hg in fine sediment fractions was 82 μg kg^-1, in sandy fractions (>63 μm) varied from 8 to12 μg kg^-1. In recent decades the Hg accumulation in bottom sediments has been controlled by the biogenic component. The Hg in the studied sediments presents as sulfide form.

Mercury geochemistry of marine sediments from the eastern Laptev Sea: The spatial distribution, levels, and contamination assessment

Twenty-seven sediment samples from the eastern Laptev Sea were analyzed for mercury and total organic carbon as well as grain-size distribution. The average total mercury (THg) concentrations in sediments are 29 ± 14 μg kg^-1. A significant correlation of THg content with total organic carbon and clay and silt fractions was shown. The ²¹⁰Pb-dated sediment core was used to evaluate the contamination degree and flux of THg in sediments from the eastern Laptev Sea. The average sedimentation rate for the all dated intervals was 0.17 cm/year. The THg flux increased from 20 to 28 μg/m²/year in the period of 1892-1950 to 53-59 μg/m²/year in the modern period of 2011-2015. According to various indices, the ecological risk from THg in studied sediment was low.

Spatial and temporal variations in riverine mercury in the Mackenzie River Basin, Canada, from community-based water quality monitoring data

Arctic rivers deliver ~40 t yr^-1 of mercury (Hg) to the Arctic Ocean, ~6 % of which is from the Mackenzie River Basin (MRB), a region warming at ~3 times the mean hemispheric rate. How this will affect Hg transfer to ecosystems of the Beaufort Sea is a worrying issue. To help address this question, we analyzed >500 measurements of Hg and other water properties from 22 rivers collected in 2012-2018 by communities of the MRB. This new dataset provides a more comprehensive view of riverine Hg variations across the basin than was previously available. We find that rivers issued from mountains in the western MRB contribute the largest share of Hg in the Mackenzie River, 60-95 % of it being carried as fine suspended solids and probably sourced from riverbank erosion and thaw slumps. In contrast, lowland rivers of the central and eastern MRB contribute larger shares of dissolved Hg (up to 78 %), likely from recent atmospheric deposition through precipitation. Using load modelling constrained by the new water quality dataset, we estimate that the three largest western tributaries (Liard, Peel and Arctic Red rivers) of the Mackenzie contribute 60 % of the annual MRB THg export and DHg export to the Beaufort Sea during freshet, as well as 51 % of DHg export, while supplying 60 % of freshet discharge. Load modelling also reveals a sustained decline in DHg loads of ~13 kg yr^-1 between 2001 and 2016 in the lower Mackenzie River, which likely reflect a decreasing trend in atmospheric Hg deposition over most of northwestern Canada during this period. This study highlights the value of community-based water quality monitoring in helping to support assessments of riverine Hg in the MRB in support of the Minamata Convention on Mercury.

Contributions and perspectives of Indigenous Peoples to the study of mercury in the Arctic

Arctic Indigenous Peoples are among the most exposed humans when it comes to foodborne mercury (Hg). In response, Hg monitoring and research have been on-going in the circumpolar Arctic since about 1991; this work has been mainly possible through the involvement of Arctic Indigenous Peoples. The present overview was initially conducted in the context of a broader assessment of Hg research organized by the Arctic Monitoring and Assessment Programme. This article provides examples of Indigenous Peoples' contributions to Hg monitoring and research in the Arctic, and discusses approaches that could be used, and improved upon, when carrying out future activities. Over 40 mercury projects conducted with/by Indigenous Peoples are identified for different circumpolar regions including the U.S., Canada, Greenland, Sweden, Finland, and Russia as well as instances where Indigenous Knowledge contributed to the understanding of Hg contamination in the Arctic. Perspectives and visions of future Hg research as well as recommendations are presented. The establishment of collaborative processes and partnership/co-production approaches with scientists and Indigenous Peoples, using good communication practices and transparency in research activities, are key to the success of research and monitoring activities in the Arctic. Sustainable funding for community-driven monitoring and research programs in Arctic countries would be beneficial and assist in developing more research/monitoring capacity and would promote a more holistic approach to understanding Hg in the Arctic. These activities should be well connected to circumpolar/international initiatives to ensure broader availability of the information and uptake in policy development.

Occurrence, allocation and geochemical controls for mercury in a typical estuarine ecosystem: Implications for the predictability of mercury species

In this study, surface seawater, bottom seawater and surface sediments were collected from the Yellow River Estuary Area (YREA) and the Laizhou Bay (LB) to investigate the occurrence, spatial distribution and geochemical control factors for total mercury (THg) and methylmercury (MeHg) in different phases. The geochemical characteristics of seawater and sediments suggested significant variances in the YREA and the LB. The high contamination of Hg in the YREA showed the discharge of the Yellow River (YR) contributed significantly to the Hg contamination in the LB. The partial least squares regression (PLSR) model was utilized to explore the complicated interactions between geochemical controls and methylation potentials in different phases. Although the ecological risk (ER) of Hg was not significant in this study area, the higher values of ER in the YREA suggested that the YR was the primary Hg contributor to LB. Therefore, the potential Hg risk should not be ignored.

Arctic atmospheric mercury: Sources and changes

Global anthropogenic and legacy mercury (Hg) emissions are the main sources of Arctic Hg contamination, primarily transported there via the atmosphere. This review summarizes the state of knowledge of the global anthropogenic sources of Hg emissions, and examines recent changes and source attribution of Hg transport and deposition to the Arctic using models. Estimated global anthropogenic Hg emissions to the atmosphere for 2015 were ~2220 Mg, ~20% higher than 2010. Global anthropogenic, legacy and geogenic Hg emissions were, respectively, responsible for 32%, 64% (wildfires: 6-10%) and 4% of the annual Arctic Hg deposition. Relative contributions to Arctic deposition of anthropogenic origin was dominated by sources in East Asia (32%), Commonwealth of Independent States (12%), and Africa (12%). Model results exhibit significant spatiotemporal variations in Arctic anthropogenic Hg deposition fluxes, driven by regional differences in Hg air transport routes, surface and precipitation uptake rates, and inter-seasonal differences in atmospheric circulation and deposition pathways. Model simulations reveal that changes in meteorology are having a profound impact on contemporary atmospheric Hg in the Arctic. Reversal of North Atlantic Oscillation phase from strongly negative in 2010 to positive in 2015, associated with lower temperature and more sea ice in the Canadian Arctic, Greenland and surrounding ocean, resulted in enhanced production of bromine species and Hg(0) oxidation and lower evasion of Hg(0) from ocean waters in 2015. This led to increased Hg(II) (and its deposition) and reduced Hg(0) air concentrations in these regions in line with High Arctic observations. However, combined changes in meteorology and anthropogenic emissions led to overall elevated modeled Arctic air Hg(0) levels in 2015 compared to 2010 contrary to observed declines at most monitoring sites, likely due to uncertainties in anthropogenic emission speciation, wildfire emissions and model representations of air-surface Hg fluxes.

Mercury isotope evidence for Arctic summertime re-emission of mercury from the cryosphere

During Arctic springtime, halogen radicals oxidize atmospheric elemental mercury (Hg⁰), which deposits to the cryosphere. This is followed by a summertime atmospheric Hg⁰ peak that is thought to result mostly from terrestrial Hg inputs to the Arctic Ocean, followed by photoreduction and emission to air. The large terrestrial Hg contribution to the Arctic Ocean and global atmosphere has raised concern over the potential release of permafrost Hg, via rivers and coastal erosion, with Arctic warming. Here we investigate Hg isotope variability of Arctic atmospheric, marine, and terrestrial Hg. We observe highly characteristic Hg isotope signatures during the summertime peak that reflect re-emission of Hg deposited to the cryosphere during spring. Air mass back trajectories support a cryospheric Hg emission source but no major terrestrial source. This implies that terrestrial Hg inputs to the Arctic Ocean remain in the marine ecosystem, without substantial loss to the global atmosphere, but with possible effects on food webs.

Arctic warming thaws permafrost, leading to enhanced soil mercury transport to the Arctic Ocean. Mercury isotope signatures in arctic rivers, ocean and atmosphere suggest that permafrost mercury is buried in marine sediment and not emitted to the global atmosphere

Total Mercury Mass Load from the Paglia-Tiber River System: The Contribution to Mediterranean Sea Hg Budget

The Mediterranean Sea is characterized by a marked mercury (Hg) geochemical anomaly, arising in part from large Hg deposits. Mercury mass loads discharged from the Monte Amiata mining district (Central Italy) to the Mediterranean Sea through the Paglia–Tiber River system were estimated. Data from two seasons showed that up to 40 kg year⁻¹ of Hg are drained to Tiber River and finally to the Mediterranean Sea. The mercury mass loads varied in different seasons, from 3 mg day⁻¹ in the upper section of Paglia River in November to 42 g day⁻¹ before the confluence with Tiber River in June. Along Tiber River, up to 15 ng L⁻¹ of the total Hg found at a site after Rome showed that Hg can be discharged to the sea. The Alviano reservoir along Tiber River acts as a temporary trap for Hg-rich particulate, while dam operations may promote Hg release (up to 223 g day⁻¹). The combination of hydrologic factors controlling Hg transport, the torrential regime in the upper catchment of Paglia River, the waterway steepness, together with Hg-contaminated legacy sediments in the Paglia River floodplain, make the Paglia–Tiber River system a long-lasting intermittent source of Hg to Tiber River and the Mediterranean Sea.

Ecotoxicology of mercury in burbot (Lota lota) from interior Alaska and insights towards human health

Fish consumption has many health benefits, but exposure to contaminants, such as mercury (Hg), in fish tissue can be detrimental to human health. The Tanana River drainage, Alaska, USA supports the largest recreational harvest of burbot (Lota lota) in the state, yet information to evaluate the potential risks of consumption by humans is lacking. To narrow this knowledge gap, we sought to (i) quantify the concentrations of total Hg ([THg]) in burbot muscle and liver tissue and the ratio between the two tissues, (ii) assess the effect of age, length, and sex on [THg] in muscle and liver tissue, (iii) evaluate if [THg] in muscle tissue varied based on trophic information, and (iv) compare observed [THg] to consumption guidelines and statewide baseline data. The mean [THg] was 268.2 ng/g ww for muscle tissue and 62.3 ng/g ww for liver tissue. Both muscle [THg] and liver [THg] values were positively associated with fish length. Trophic information (δ¹⁵N and δ¹³C) was not significantly related to measured [THg] in burbot muscle, which is inconsistent with typical patterns of biomagnification observed in other fishes. All burbot sampled were within the established categories for consumption recommendations determined by the State of Alaska for women of childbearing age and children. Our results provide the necessary first step towards informed risk assessment of burbot consumption in the Tanana drainage and offer parallels to fisheries and consumers throughout the subarctic and Arctic region.

Climate change and mercury in the Arctic: Abiotic interactions

Dramatic environmental shifts are occuring throughout the Arctic from climate change, with consequences for the cycling of mercury (Hg). This review summarizes the latest science on how climate change is influencing Hg transport and biogeochemical cycling in Arctic terrestrial, freshwater and marine ecosystems. As environmental changes in the Arctic continue to accelerate, a clearer picture is emerging of the profound shifts in the climate and cryosphere, and their connections to Hg cycling. Modeling results suggest climate influences seasonal and interannual variability of atmospheric Hg deposition. The clearest evidence of current climate change effects is for Hg transport from terrestrial catchments, where widespread permafrost thaw, glacier melt and coastal erosion are increasing the export of Hg to downstream environments. Recent estimates suggest Arctic permafrost is a large global reservoir of Hg, which is vulnerable to degradation with climate warming, although the fate of permafrost soil Hg is unclear. The increasing development of thermokarst features, the formation and expansion of thaw lakes, and increased soil erosion in terrestrial landscapes are increasing river transport of particulate-bound Hg and altering conditions for aquatic Hg transformations. Greater organic matter transport may also be influencing the downstream transport and fate of Hg. More severe and frequent wildfires within the Arctic and across boreal regions may be contributing to the atmospheric pool of Hg. Climate change influences on Hg biogeochemical cycling remain poorly understood. Seasonal evasion and retention of inorganic Hg may be altered by reduced sea-ice cover and higher chloride content in snow. Experimental evidence indicates warmer temperatures enhance methylmercury production in ocean and lake sediments as well as in tundra soils. Improved geographic coverage of measurements and modeling approaches are needed to better evaluate net effects of climate change and long-term implications for Hg contamination in the Arctic.

Spatial variation in mercury concentrations in polar bear (Ursus maritimus) hair from the Norwegian and Russian Arctic

We examined spatial variation in total mercury (THg) concentrations in 100 hair samples collected between 2008 and 2016 from 87 polar bears (Ursus maritimus) from the Norwegian (Svalbard Archipelago, western Barents Sea) and Russian Arctic (Kara Sea, Laptev Sea, and Chukchi Sea). We used latitude and longitude of home range centroid for the Norwegian bears and capture position for the Russian bears to account for the locality. We additionally examined hair stable isotope values of carbon (δ¹³C) and nitrogen (δ¹⁵N) to investigate feeding habits and their possible effect on THg concentrations. Median THg levels in polar bears from the Norwegian Arctic (1.99 μg g^-1 dry weight) and the three Russian Arctic regions (1.33-1.75 μg g^-1 dry weight) constituted about 25-50% of levels typically reported for the Greenlandic or North American populations. Total Hg concentrations in the Norwegian bears increased with intake of marine and higher trophic prey, while δ¹³C and δ¹⁵N did not explain variation in THg concentrations in the Russian bears. Total Hg levels were higher in northwest compared to southeast Svalbard. δ¹³C and δ¹⁵N values did not show any spatial pattern in the Norwegian Arctic. Total Hg concentrations adjusted for feeding ecology showed similar spatial trends as the measured concentrations. In contrast, within the Russian Arctic, THg levels were rather uniformly distributed, whereas δ¹³C values increased towards the east and south. The results indicate that Hg exposure in Norwegian and Russian polar bears is at the lower end of the pan-Arctic spectrum, and its spatial variation in the Norwegian and Russian Arctic is not driven by the feeding ecology of polar bears.

Multidecadal declines in particulate mercury and sediment export from Russian rivers in the pan-Arctic basin

Russian rivers are the predominant source of riverine mercury to the Arctic Ocean, where methylmercury biomagnifies to high levels in food webs. Pollution controls are thought to have decreased late–20th-century mercury loading to Arctic watersheds, but there are no published long-term observations on mercury in Russian rivers. Here, we present a unique hydrochemistry dataset to determine trends in Russian river particulate mercury concentrations and fluxes in recent decades. Using hydrologic and mercury deposition modeling together with multivariate time series analysis, we determine that 70 to 90% declines in particulate mercury fluxes were driven by pollution reductions and sedimentation in reservoirs. Results suggest that Russian rivers likely dominated over all other sources of mercury to the Arctic Ocean until recently.

High levels of methylmercury accumulation in marine biota are a concern throughout the Arctic, where coastal ocean ecosystems received large riverine inputs of mercury (Hg) (40 Mg⋅y⁻¹) and sediment (20 Tg⋅y⁻¹) during the last decade, primarily from major Russian rivers. Hg concentrations in fish harvested from these rivers have declined since the late 20th century, but no temporal data on riverine Hg, which is often strongly associated with suspended sediments, were previously available. Here, we investigate temporal trends in Russian river particulate Hg (PHg) and total suspended solids (TSS) to better understand recent changes in the Arctic Hg cycle and its potential future trajectories. We used 1,300 measurements of Hg in TSS together with discharge observations made by Russian hydrochemistry and hydrology monitoring programs to examine changes in PHg and TSS concentrations and fluxes in eight major Russian rivers between ca. 1975 and 2010. Due to decreases in both PHg concentrations (micrograms per gram) and TSS loads, annual PHg export declined from 47 to 7 Mg⋅y⁻¹ overall and up to 92% for individual rivers. Modeling of atmospheric Hg deposition together with published inventories on reservoir establishment and industrial Hg release point to decreased pollution and sedimentation within reservoirs as predominant drivers of declining PHg export. We estimate that Russian rivers were the primary source of Hg to the Arctic Ocean in the mid to late 20th century.

Downstream Modification of Mercury in Diverse River Systems Underscores the Role of Local Conditions in Fish Bioaccumulation

Fish consumption advisories for mercury (Hg) are common in rivers, highlighting connections between landscape sources of Hg and downstream fluvial ecosystems. Though watershed conditions can influence concentrations of Hg in smaller streams, how Hg changes downstream through larger rivers and how these changes associate with Hg concentrations in fish is not well understood. Here we present a continuum of concentrations and yields of total mercury (THg) and methylmercury (MeHg) from small tributary systems draining diverse western Canadian headwater landscapes through to major transboundary rivers. We associate these downstream patterns with THg concentrations in tissues of resident fish in major rivers. Mean concentrations and yields of unfiltered THg from over 80 monitored tributaries and major rivers were highly variable in space ranging from 0.28 to 120 ng L−1 and 0.39 to 170 µg ha−1 d−1, respectively. Using spatial data and a hierarchical cluster analysis, we identified three broad categories of tributary catchment conditions. Linear mixed modeling analysis with water quality variables revealed significantly lower THg concentrations in tributaries draining cordillera-foothills (geometric mean: 0.76 ng L−1) regions relative to those draining forested (1.5 ng L−1) and agriculturalized landscapes (2.4 ng L−1), suggesting that sources and mobility of THg in soils and surface waters were different between landscapes. However, these concentration differences were not sustained downstream in major rivers as local sources and sinks of THg in river channels smoothed differences between landscape types. Extensive fish tissue monitoring in major rivers and ANCOVA analysis found that site-specific, river water THg and MeHg concentrations and local catchment conditions were stronger associates of THg concentrations in fish than broader trends in rivers within and across landscape classes. Consequently, site-specific, targeted monitoring of THg and MeHg concentrations in water and fish is a preferred study design when assessing regional-level patterns in fish tissue concentrations.

Looping Mercury Cycle in Global Environmental-Economic System Modeling

The Minamata Convention on Mercury calls for Hg control actions to protect the environment and human beings from the adverse impacts of Hg pollution. It aims at the entire life cycle of Hg. Existing studies on the Hg cycle in the global environmental-economic system have characterized the emission-to-impact pathway of Hg pollution. That is, Hg emissions/releases from the economic system can have adverse impacts on human health and ecosystems. However, current modeling of the Hg cycle is not fully looped. It ignores the feedback of Hg-related environmental impacts (including human health impacts and ecosystem impacts) to the economic system. This would impede the development of more comprehensive Hg control actions. By synthesizing recent information on Hg cycle modeling, this critical review found that Hg-related environmental impacts would have feedbacks to the economic system via the labor force and biodiversity loss. However, the interactions between Hg-related activities in the environmental and economic systems are not completely clear. The cascading effects of Hg-related environmental impacts to the economic system throughout global supply chains have not been revealed. Here, we emphasize the knowledge gaps and propose possible approaches for looping the Hg cycle in global environmental-economic system modeling. This progress is crucial for formulating more dynamic and flexible Hg control measures. It provides new perspectives for the implementation of the Minamata Convention on Mercury.

Sources of riverine mercury across the Mackenzie River Basin; inferences from a combined HgC isotopes and optical properties approach

The Arctic environment harbors a complex mosaic of mercury (Hg) and carbon (C) reservoirs, some of which are rapidly destabilizing in response to climate warming. The sources of riverine Hg across the Mackenzie River basin (MRB) are uncertain, which leads to a poor understanding of potential future release. Measurements of dissolved and particulate mercury (DHg, PHg) and carbon (DOC, POC) concentration were performed, along with analyses of Hg stable isotope ratios (incl. ∆¹⁹⁹Hg, δ²⁰²Hg), radiocarbon content (∆¹⁴C) and optical properties of DOC of river water. Isotopic ratios of Hg revealed a closer association to terrestrial Hg reservoirs for the particulate fraction, while the dissolved fraction was more closely associated with atmospheric deposition sources of shorter turnover time. There was a positive correlation between the ∆¹⁴C-OC and riverine Hg concentration for both particulate and dissolved fractions, indicating that waters transporting older-OC (¹⁴C-depleted) also contained higher levels of Hg. In the dissolved fraction, older DOC was also associated with higher molecular weight, aromaticity and humic content, which are likely associated with higher Hg-binding potential. Riverine PHg concentration increased with turbidity and SO₄ concentration. There were large contrasts in Hg concentration and OC age and quality among the mountain and lowland sectors of the MRB, which likely reflect the spatial distribution of various terrestrial Hg and OC reservoirs, including weathering of sulfate minerals, erosion and extraction of coal deposits, thawing permafrost, forest fires, peatlands, and forests. Results revealed major differences in the sources of particulate and dissolved riverine Hg, but nonetheless a common positive association with older riverine OC. These findings reveal that a complex mixture of Hg sources, supplied across the MRB, will contribute to future trends in Hg export to the Arctic Ocean under rapid environmental changes.

Degrading permafrost river catchments and their impact on Arctic Ocean nearshore processes

Arctic warming is causing ancient perennially frozen ground (permafrost) to thaw, resulting in ground collapse, and reshaping of landscapes. This threatens Arctic peoples' infrastructure, cultural sites, and land-based natural resources. Terrestrial permafrost thaw and ongoing intensification of hydrological cycles also enhance the amount and alter the type of organic carbon (OC) delivered from land to Arctic nearshore environments. These changes may affect coastal processes, food web dynamics and marine resources on which many traditional ways of life rely. Here, we examine how future projected increases in runoff and permafrost thaw from two permafrost-dominated Siberian watersheds-the Kolyma and Lena, may alter carbon turnover rates and OC distributions through river networks. We demonstrate that the unique composition of terrestrial permafrost-derived OC can cause significant increases to aquatic carbon degradation rates (20 to 60% faster rates with 1% permafrost OC). We compile results on aquatic OC degradation and examine how strengthening Arctic hydrological cycles may increase the connectivity between terrestrial landscapes and receiving nearshore ecosystems, with potential ramifications for coastal carbon budgets and ecosystem structure. To address the future challenges Arctic coastal communities will face, we argue that it will become essential to consider how nearshore ecosystems will respond to changing coastal inputs and identify how these may affect the resiliency and availability of essential food resources.

Observation-Based Mercury Export from Rivers to Coastal Oceans in East Asia

Globally, the consumption of coastal fish is the predominant source of human exposure to methylmercury, a potent neurotoxicant that poses health risks to humans. However, the relative importance of riverine inputs and atmospheric deposition of mercury into coastal oceans remains uncertain owing to a lack of riverine mercury observations. Here, we present comprehensive seasonal observations of riverine mercury and methylmercury loads, including dissolved and particulate phases, to East Asia's coastal oceans, which supply nearly half of the world's seafood products. We found that East Asia's rivers annually exported 95 ± 29 megagrams of mercury to adjacent seas, 3-fold greater than the corresponding atmospheric deposition. Three rivers alone accounted for 71% of East Asia's riverine mercury exports, namely: Yangtze, Yellow, and Pearl rivers. We further conducted a metadata analysis to discuss the mercury burden on seawater and found that riverine export, combined with atmospheric deposition and terrestrial nutrients, quantitatively elevated the levels of total, methylated, and dissolved gaseous mercury in seawater by an order of magnitude. Our observations support that massive amounts of riverine mercury are exported to coastal oceans on a continental scale, intensifying their spread from coastal seawater to the atmosphere, marine sediments, and open oceans. We suggest that the impact of mercury transport along the land-ocean aquatic continuum should be considered in human exposure risk assessments.

Mercury sources and physicochemical characteristics in ice, snow, and meltwater of the Laohugou Glacier Basin, China

In this work, samples of surface snow, surface ice, snow pit and meltwater from the Laohugou Glacier No. 12 on the northern edge of Tibetan Plateau (TP) were collected during the summer of 2015. The average concentration of Hg in surface snow/ice was 22.41 ng L^-1, while the percentage of dissolved mercury (Hg_D) was observed to be around 26%. An altitudinal magnification of Hg was not observed for surface snow; however, in contrast, a significant positive magnification of Hg with altitude was observed in the surface ice. A higher concentration of Hg corresponded with the dust layer of the snow pit. It was observed that about 42% of Hg was lost from the surface snow when the glacier was exposed to sunlight within the first 24 h indicating some Hg was emitted back to the atmosphere while some were percolated downwards. The result from the principal component analysis (PCA) showed that the sources of Hg in Laohugou Glacier No. 12 were from crustal and biomass burning. Finally, it was estimated that total export of Hg from the outlet river of Laohugou glacier No. 12 in the year 2015 was about 1439.46 g yr^-1 with yield of 22.77 μg m² yr^-1. This study provides valuable insights for understanding the behavior of Hg in the glacier of the northern Tibetan Plateau.

Assessment of mercury levels in modern sediments of the East Siberian Sea

Mercury (Hg) is an important environmental indicator of anthropogenic pollution. In this study, the Hg content in the bottom sediments of the East Siberian Sea was observed to range from 13 to 92 ppb, with an average of 36 ppb. Facies dependence was also observed and expressed as an increase in the Hg concentration in fine-sized sediments on the shelf edge and continental slope, compared to that in the sandy silts and sands of the inner shelf. The Hg accumulation in bottom sediments of the eastern part has increased over the past 150 years due to an increase in global emissions of anthropogenic Hg, which is caused by the transboundary transport of Hg to the Arctic. Moreover, changes in the Hg value, which occur due to the plankton arriving at the bottom sediments because of changes in hydrology and primary production, are thought to be associated with global warming.

Surface-air mercury fluxes and a watershed mass balance in forested and harvested catchments

Forest soils are among the world's largest repositories for long-term accumulation of atmospherically deposited mercury (Hg), and understanding the potential for remobilization through gaseous emissions, aqueous dissolution and runoff, or erosive particulate transport to down-gradient aquatic ecosystems is critically important for projecting ecosystem recovery. Forestry operations, especially clear-cut logging where most of the vegetaiton is removed, can influence Hg mobility/fluxes, foodweb dynamics, and bioaccumulation processes. This paper measured surface-air Hg fluxes from catchments in the Pacific Northwest, USA, to determine if there is a difference between forested and logged catchments. These measurements were conducted as part of a larger project on the impact of forestry operations on Hg cycling which include measurements of water fluxes as well as impacts on biota. Surface-air Hg fluxes were measured using a commonly applied dynamic flux chamber (DFC) method that incorporated diel and seasonal variability in elemental Hg (Hg⁰) fluxes at multiple forested and harvested catchments. The results showed that the forested ecosystem had depositional Hg⁰ fluxes throughout most of the year (annual mean: -0.26 ng/m²/h). In contrast, the harvested catchments showed mostly emission of Hg⁰ (annual mean: 0.63 ng/m²/h). Differences in solar radiation reaching the soil was the primary driver resulting in a shift from net deposition to emission in harvested catchments. The surface-air Hg fluxes were larger than the fluxes to water as runoff and accounted for 97% of the differences in Hg sequestered in forested versus harvested catchments.