Widespread persistent contaminants are a global environmental problem. In the Baltic Sea, wildlife contamination was first noticed in the 1960s, prompting the Swedish Environmental Protection Agency to establish a comprehensive Swedish National Monitoring Programme for Contaminants in Marine Biota (MCoM) in 1978 run by the Swedish Museum of Natural History. Eight species have been analysed, four fish species (Atlantic herring, Atlantic cod, European perch, viviparous eelpout), one bivalve species (blue mussel), and egg from three bird species (common guillemot, common tern, Eurasian oystercatcher). Here, we present a dataset containing MCoM data from its start until 2021. It includes 36 sets of time-series, each analysed for more than 100 contaminants. The longest time-series is for common guillemot and starts in 1968. We describe the structure of MCoM including historic changes to the number of stations, sample treatment, analytical methods, instruments, and laboratories. The MCoM data is available at the Bolin Centre repository and on GitHub through our R package mcomDb. The latter will be updated yearly with new MCoM records.

Mercury (Hg) is a pollutant of global concern. Due to anthropogenic emissions, the atmospheric and surface ocean Hg burden has increased substantially since preindustrial times. Hg emitted into the atmosphere gets transported on a global scale and ultimately reaches the oceans. There it is transformed into highly toxic methylmercury (MeHg) that effectively accumulates in the food web. The international community has recognized this serious threat to human health and in 2017 regulated Hg use and emissions under the UN Minamata Convention on Mercury. Currently, the first effectiveness evaluation of the Minamata Convention is being prepared, and, in addition to observations, models play a major role in understanding environmental Hg pathways and in predicting the impact of policy decisions and external drivers (e.g., climate, emission, and land-use change) on Hg pollution. Yet, the available model capabilities are mainly limited to atmospheric models covering the Hg cycle from emission to deposition. With the presented model MERCY v2.0 we want to contribute to the currently ongoing effort to improve our understanding of Hg and MeHg transport, transformation, and bioaccumulation in the marine environment with the ultimate goal of linking anthropogenic Hg releases to MeHg in seafood.

Here, we present the equations and parameters implemented in the MERCY model and evaluate the model performance for two European shelf seas, the North and Baltic seas. With the model evaluation, we want to establish a set of general quality criteria that can be used for evaluation of marine Hg models. The evaluation is based on statistical criteria developed for the performance evaluation of atmospheric chemistry transport models. We show that the MERCY model can reproduce observed average concentrations of individual Hg species in water (normalized mean bias: HgT 17 %, Hg0 2 %, MeHg −28 %) in the two regions mentioned above. Moreover, it is able to reproduce the observed seasonality and spatial patterns. We find that the model error for HgT(aq) is mainly driven by the limitations of the physical model setup in the coastal zone and the availability of data on Hg loads in major rivers. In addition, the model error in calculating vertical mixing and stratification contributes to the total HgT model error. For the vertical transport we find that the widely used particle partitioning coefficient for organic matter of log(kd)=5.4 is too low for the coastal systems. For Hg0 the model performance is at a level where further model improvements will be difficult to achieve. For MeHg, our understanding of the processes controlling methylation and demethylation is still quite limited. While the model can reproduce average MeHg concentrations, this lack of understanding hampers our ability to reproduce the observed value range. Finally, we evaluate Hg and MeHg concentrations in biota and show that modeled values are within the range of observed levels of accumulation in phytoplankton, zooplankton, and fish. The model performance demonstrates the feasibility of developing marine Hg models with similar predictive capability to established atmospheric chemistry transport models. Our findings also highlight important knowledge gaps in the dynamics controlling methylation and bioaccumulation that, if closed, could lead to important improvements of the model performance.

Identifying Hg sources to aquatic ecosystems and processes controlling the levels of monomethylmercury (MMHg) is critical for developing efficient policies of Hg emissions reduction. Here we measured Hg concentrations and stable isotopes in sediment, seston, and fishes from the various basins of the Baltic Sea, a large brackish ecosystem presenting extensive gradients in salinity, redox conditions, dissolved organic matter (DOM) composition, and biological activities. We found that Hg mass dependent fractionation (Hg-MDF) values in sediments mostly reflect a mixing between light terrestrial Hg and heavier industrial sources, whereas odd Hg isotope mass independent fractionation (odd Hg-MIF) reveals atmospheric inputs. Seston presents intermediate Hg-MDF and odd Hg-MIF values falling between sediments and fish, but in northern basins, high even Hg-MIF values suggest the preferential accumulation of wet-deposited Hg. Odd Hg-MIF values in fish indicate an overall low extent of MMHg photodegradation due to limited sunlight exposure and penetration but also reveal large spatial differences. The photodegradation extent is lowest in the central basin with recurrent algal blooms due to their shading effect and is highest in the northern, least saline basin with high concentrations of terrestrial DOM. As increased loads of terrestrial DOM are expected in many coastal areas due to global changes, its impact on MMHg photodegradation needs to be better understood and accounted for when predicting future MMHg concentrations in aquatic ecosystems.

The Arctic terrestrial 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, d202Hg), 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.

Human-induced expansion of oxygen-deficient zones can have dramatic impacts on marine systems and itsresident biota. One example is the formation of the potent neurotoxic methylmercury (MeHg) that is mediated bymicrobial methylation of inorganic divalent Hg (HgII) under oxygen-deficient conditions. A negative consequenceof the expansion of oxygen-deficient zones could be an increase in MeHg production due to shifts in microbialcommunities in favor of microorganisms methylating Hg. There is, however, limited knowledge about Hg-methylatingmicrobes, i.e., those carrying hgc genes critical for mediating the process, from marine sediments. Here, weaim to study the presence of hgc genes and transcripts in metagenomes and metatranscriptomes from four surfacesediments with contrasting concentrations of oxygen and sulfide in the Baltic Sea. We show that potential Hgmethylators differed among sediments depending on redox conditions. Sediments with an oxygenated surface featuredhgc-like genes and transcripts predominantly associated with uncultured Desulfobacterota (OalgD group)and Desulfobacterales (including Desulfobacula sp.) while sediments with a hypoxic-anoxic surface included hgccarryingVerrucomicrobia, unclassified Desulfobacterales, Desulfatiglandales, and uncharacterized microbes. Ourdata suggest that the expansion of oxygen-deficient zones in marine systems may lead to a compositional changeof Hg-methylating microbial groups in the sediments, where Hg methylators whose metabolism and biology havenot yet been characterized will be promoted and expand.

Neurotoxic methylmercury (MeHg) is formed by microbial methylation of inorganic divalent Hg (HgII) and constitutes severe environmental and human health risks. The methylation is enabled by hgcA and hgcB genes, but it is not known if the associated molecular-level processes are rate-limiting or enable accurate prediction of MeHg formation in nature. In this study, we investigated the relationships between hgc genes and MeHg across redox-stratified water columns in the brackish Baltic Sea. We showed, for the first time, that hgc transcript abundance and the concentration of dissolved HgII-sulfide species were strong predictors of both the HgII methylation rate and MeHg concentration, implying their roles as principal joint drivers of MeHg formation in these systems. Additionally, we characterized the metabolic capacities of hgc+ microorganisms by reconstructing their genomes from metagenomes (i.e., hgc+ MAGs), which highlighted the versatility of putative HgII methylators in the water column of the Baltic Sea. In establishing relationships between hgc transcripts and the HgII methylation rate, we advance the fundamental understanding of mechanistic principles governing MeHg formation in nature and enable refined predictions of MeHg levels in coastal seas in response to the accelerating spread of oxygen-deficient zones.

Anthropogenic mercury (Hg) emissions have driven marked increases in Arctic Hg levels, which are now being impacted by regional warming, with uncertain ecological consequences. This Review presents a comprehensive assessment of the present-day total Hg mass balance in the Arctic. Over 98% of atmospheric Hg is emitted outside the region and is transported to the Arctic via long-range air and ocean transport. Around two thirds of this Hg is deposited in terrestrial ecosystems, where it predominantly accumulates in soils via vegetation uptake. Rivers and coastal erosion transfer about 80 Mg year−1 of terrestrial Hg to the Arctic Ocean, in approximate balance with modelled net terrestrial Hg deposition in the region. The revised Arctic Ocean Hg mass balance suggests net atmospheric Hg deposition to the ocean and that Hg burial in inner-shelf sediments is underestimated (up to >100%), needing seasonal observations of sediment-ocean Hg exchange. Terrestrial Hg mobilization pathways from soils and the cryosphere (permafrost, ice, snow and glaciers) remain uncertain. Improved soil, snowpack and glacial Hg inventories, transfer mechanisms of riverine Hg releases under accelerated glacier and soil thaw, coupled atmosphere–terrestrial modelling and monitoring of Hg in sensitive ecosystems such as fjords can help to anticipate impacts on downstream Arctic ecosystems.

Temporal and spatial trends of 15 per- and polyfluoroalkyl substances (PFAS) were determined in white-tailed sea eagle (WTSE) eggs (Haliaeetus albicilla) from two inland and two coastal regions of Sweden between 1969 and 2021. PFAS concentrations generally increased from ∼1969 to ∼1990s–2010 (depending on target and site) and thereafter plateaued or declined, with perfluorooctane sulfonamide (FOSA) and perfluorooctane sulfonate (PFOS) declining faster than most perfluoroalkyl carboxylic acids (PFCAs). The net result was a shift in the PFAS profile from PFOS-dominant in 1969–2010 to an increased prevalence of PFCAs over the last decade. Further, during the entire period higher PFAS concentrations were generally observed in coastal populations, possibly due to differences in diet and/or proximity to more densely populated areas. Fluorine mass balance determination in pooled samples from three of the regions (2019–2021) indicated that target PFAS accounted for the vast majority (i.e. 81–100%) of extractable organic fluorine (EOF). Nevertheless, high resolution mass-spectrometry-based suspect screening identified 55 suspects (31 at a confidence level [CL] of 1–3 and 24 at a CL of 4–5), of which 43 were substances not included in the targeted analysis. Semi-quantification of CL ≤ 2 suspects increased the identified EOF to >90% in coastal samples. In addition to showing the impact of PFAS regulation and phase-out initiatives, this study demonstrates that most extractable organofluorine in WTSE eggs is made up of known (legacy) PFAS, albeit with low levels of novel substances.

Air-sea exchange of gaseous elemental mercury (Hg0) is not well constrained, even though it is a major component of the global Hg cycle. Lack of Hg0 flux measurements to validate parameterizations of the Hg0 transfer velocity contributes to this uncertainty. We measured the Hg0 flux on the Baltic Sea coast using micrometeorological methods (gradient-based and relaxed eddy accumulation [REA]) and also simulated the flux with a gas exchange model. The coastal waters were typically supersaturated with Hg0 (mean ± 1σ = 13.5 ± 3.5 ng m−3; ca. 10% of total Hg) compared to the atmosphere (1.3 ± 0.2 ng m−3). The Hg0 flux calculated using the gas exchange model ranged from 0.1–1.3 ng m−2 h−1 (10th and 90th percentile) over the course of the campaign (May 10–June 20, 2017) and showed a distinct diel fluctuation. The mean coastal Hg0 fluxes determined with the two gradient-based approaches and REA were 0.3, 0.5, and 0.6 ng m−2 h−1, respectively. In contrast, the mean open sea Hg0 flux measured with REA was larger (6.3 ng m−2 h−1). The open sea Hg0 flux indicated a stronger wind speed dependence for the Hg0 transfer velocity compared to commonly used parameterizations. Although based on a limited data set, we suggest that the wind speed dependence of the Hg0 transfer velocity is more consistent with gases that have less water solubility than CO2 (e.g., O2). These pioneering flux measurements using micrometeorological techniques show that more such measurements would improve our understanding of air-sea Hg exchange.

Arctic mercury (Hg) concentrations respond to changes in anthropogenic Hg emissions and environmental change. This manuscript, prepared for the 2021 Arctic Monitoring and Assessment Programme Mercury Assessment, explores the response of Arctic Ocean Hg concentrations to changing primary Hg emissions and to changing sea-ice cover, river inputs, and net primary production. To do this, we conduct a model analysis using a 2015 Hg inventory and future anthropogenic Hg emission scenarios. We model future atmospheric Hg deposition to the surface ocean as a flux to the surface water or sea ice using three scenarios: No Action, New Policy (NP), and Maximum Feasible Reduction (MFR). We then force a five-compartment box model of Hg cycling in the Arctic Ocean with these scenarios and literature-derived climate variables to simulate environmental change. No Action results in a 51% higher Hg deposition rate by 2050 while increasing Hg concentrations in the surface water by 22% and <9% at depth. Both “action” scenarios (NP and MFR), implemented in 2020 or 2035, result in lower Hg deposition ranging from 7% (NP delayed to 2035) to 30% (MFR implemented in 2020) by 2050. Under this last scenario, ocean Hg concentrations decline by 14% in the surface and 4% at depth. We find that the sea-ice cover decline exerts the strongest Hg reducing forcing on the Arctic Ocean while increasing river discharge increases Hg concentrations. When modified together the climate scenarios result in a ≤5% Hg decline by 2050 in the Arctic Ocean. Thus, we show that the magnitude of emissions-induced future changes in the Arctic Ocean is likely to be substantial compared to climate-induced effects. Furthermore, this study underscores the need for prompt and ambitious action for changing Hg concentrations in the Arctic, since delaying less ambitious reduction measures–like NP–until 2035 may become offset by Hg accumulated from pre-2035 emissions.

This report present a graphic and statistical overview of temporal trends and spatial variations within the Swedish National Monitoring Programme for Contaminants in Marine Biota.

The aim of the report is three-fold. Firstly, to evaluate to what extent phase-out of certain per- and polyfluoroalkyl substances (PFAS) first initiated in the early 2000s are reflected in the biota concentration within the Baltic Sea. Secondly, to investigate the spatial differences of PFAS across the Baltic Sea, which has not previously been evaluated, and couple these results to PFAS observations in other matrices to better understand the flow of PFAS through the Baltic Sea ecosystem. Lastly, to investigate the implication of a proposed PFAS EQS dossier on the evaluation of Good Environmental Status in the Baltic Sea.

We use data on PFAS from the Swedish Monitoring Program for Contaminants in Marine Biota covering 40 years for the longest time series (four Time Trend Stations) and 26 stations at present day. The program covers stations from the Bothnian Bay in the north to Skagerrak at the Swedish west coast (referred to as the Greater Baltic Sea) and PFAS is analyzed in four fish species and three bird species. The target compounds are perfluoroalkyl sulfonic acid (PFSA; C4-C8), perfluoroalkyl carboxylic acid (PFCA; C6-C15) and FOSA. In the data evaluation, observations from other monitoring programmes and research campaigns are included (river, marine surface water, sediment, and top predators) to set the observations from the Swedish Monitoring Program for Contaminants in Marine Biota into context. 

After an initial exponential increase in PFAS concentrations, we found that PFAS displayed a rapid response to phase-out and regulations in the early 2000s. As a result, PFAS concentrations stabilized (rather than displaying an immediate decrease). This is linked to the few removal pathways of the long lived PFAS homologues in the Baltic Sea water column. However, within the last decade PFAS has started to show significant declines at many stations, the exception being a few PFAS homologues (PFOA (C8) and PFNA (C9)). 

Two distinct water masses are present in the Greater Baltic Sea, North Sea water and Baltic Sea water, the latter with origin in the Baltic Sea drainage basin. Differences in the PFAS loads of these water masses likely drives geographical differences seen in the PFAS concentrations and homologue distribution between fish in Kattegat and the Baltic Sea. Different water mass lifetimes in the Baltic Sea basins further affect the concentration and response time with regards to changes in external sources for individual homologues.

A proposed PFAS EQS dossier presents an EQS (with the PFAS sum expressed as PFOA-equivalents) a factor 100 lower than the current PFOS EQS. We find that PFOS, PFNA and PFUnDA contribute more than 80% to the sumPFAS-equivalent. PFNA is the only homologue that is currently increasing in some parts of the Baltic Sea and special attention should be on this homologue in the future. Despite the slow recovery with regards to PFAS concentrations over the past decade the biota PFAS concentrations are still 5-230 times higher than the threshold proposed in the PFAS EQS dossier. Screening studies has further identified a range of PFAS in the Greater Baltic Sea not currently part of the Swedish Monitoring Program for Contaminants in Marine Biota. As an example the cyclic PFECHS has been found at various trophic levels in the food web at concentrations indicating biomagnification potential. These findings indicate that PFAS is still affecting the Greater Baltic Sea environment negatively and the development in concentrations of individual homologues should be followed closely over the coming decade both for those PFAS included in the current program but also the emergence of novel PFAS through screening program.

Fish tissue levels have to comply with environmental quality standards (EQSs) within the European Water Framework Directive. However, within monitoring, contaminants are sometimes measured in a different tissue than the tissue for which the environmental (whole fish) or human (fillet (equivalent to muscle tissue)) quality standard is set. Tissue conversion factors (k), describing the relationship between concentrations in different tissues, can be used to obtain a quality standard for the appropriate tissue. Several different approaches have been suggested for the calculation of k. For monitoring purposes, we propose the use of a simple, easy reproducible approach that assumes proportionality between two tissue, or tissue and whole fish, concentrations. This allows for an easy comparison of studies and adoption of ks into independent monitoring programs. Here, we determined ks for three metals (mercury (Hg), lead (Pb), cadmium (Cd)) and nine per- and polyfluoroalkyl substances (PFAS) including perfluorooctanesulfonic acid (PFOS) across six marine and freshwater fish species from Northern European lakes and the Baltic Sea. We found significant species differences for Hg for k_muscle/whole fish, for Cd and Pb for k_{liver/whole fish} and for Cd for kliver/muscle. For perfluoroalkyl carboxylic acids (PFCA), we found a chain length dependence with lowest kliver/muscle at low and high chain lengths (C₈, C₁₃) and highest for median chain lengths (C₉-C₁₂). Further, there were differences between fish species with k_liver/muscle for PFOS almost doubling from eelpout (10.3) to herring (19.2) and increasing up to a factor 4 between eelpout and herring for other PFASs. FOSA had two distinctive groups, herring with a k_liver/muscle of 48.7 and a second group with ks of 2.3 to 5.9 for all other fish species. Our results suggest that differences in the tissue somatic index, and contaminant uptake, tissue transfer and metabolism result in the need for species-specific ks within monitoring

The aim of the report is to produce improved estimates of tissue conversion factors (k) to use within environmental monitoring. The contaminant distribution across a range of marine and freshwater fish species from Northern Europe freshwater and the Baltic Sea was investigated. New tissue conversion factors were established and converted threshold limits (C-EQS or C-QS) for tissues of relevance for monitoring (liver and muscle) presented. We further explored inter-species variability in contaminant distribution, which can result in the need to create species-specific conversion factors. 

                      In this study, we use conversion factors that assumes a proportional relation between two tissue concentrations. We recommend that this method is always used when the aim is to convert between tissue concentrations for monitoring purposes. This will allow for better transparency and allow for easy comparison between studies, something that is made difficult at the moment due to the use of different alternative methods for conversion.

                      Based on the analysis we recommend four new threshold value estimates that can be used within environmental monitoring of fish. For mercury (Hg) we only looked at the kmuscle/whole fish and did not consider liver measurements. We recommend the use of a C-EQSmuscle of 24 ng g^-1 ww across all fish species. For cadmium (Cd) and lead (Pb) we calculated C-QSliver estimates based on conversion from both the QSmuscle-human health and QSwhole fish-sec pois. For Cd, we found significant differences between the two species investigated (herring and perch) as well as ten times higher C-QSliver based on the QSmuscle-human health than the QSwhole fish-sec pois. From a precautionary principle we recommend to use the herring specific k and the QS for secondary poising set for whole fish concentration to set the C-QSliver, which lead to a C-QSliver for Cd of 2.6 μg g^-1 ww. For lead (Pb), the liver:muscle dataset was considered less robust than the liver:whole fish dataset. Also, for Pb significant differences were found between k’s in herring and perch. We recommend using a C-QSliver of 0.3 μg g^-1 ww for Pb based on perch data. For PFOS we only looked at the kliver/muscle and did not consider whole fish measurements. We recommend the use of a C-EQSliver of 153 ng g^-1 ww across all fish species despite some differences in k between the six species included in the estimate. For all contaminants, more data on additional fish species and data on the same species but better distributed between marine and freshwater environments (for species living in both like perch) is needed to elucidate the need for species specific and environment specific thresholds. In addition, more observations with concentrations close to the thresholds are needed for Cd and Pb to bring down the uncertainty on the QSliver estimates, which are currently, based on extrapolation of values more than a factor ten below the thresholds.

Selenium (Se) has a narrow range between nutritionally optimal and toxic concentrations for many organisms, including fish and humans. However, the degree to which humans are affecting Se concentrations in coastal food webs with diffuse Se sources is not well described. Here we examine large-scale drivers of spatio-temporal variability in Se concentration in herring from the Baltic Sea (coastal sea) to explore the anthropogenic impact on a species from the pelagic food web. We analyze data from three herring muscle time series covering three decades (1979–2010) and herring liver time series from 20 stations across the Baltic Sea covering a fourth decade (2009–2019). We find a 0.7–2.0% per annum (n = 26–30) Se decline in herring muscle samples from 0.34 ± 0.02 μg g−1 ww in 1979–1981 to 0.18 ± 0.03 μg g−1 ww in 2008–2010. This decrease continues in the liver samples during the fourth decade (6 of 20 stations show significant decrease). We also find increasing North-South and East-West gradients in herring Se concentrations. Using our observations, modelled Se deposition (spatio-temporal information) and estimated Se river discharge (spatial information), we show that the spatial variability in herring Se tracks the variability in external source loads. Further, between 1979 and 2010 we report a ∼5% per annum decline in direct Se deposition and a more gradual, 0.7–2.0% per annum, decline in herring Se concentrations. The slower rate of decrease for herring can be explained by stable or only slowly decreasing riverine inputs of Se to the Baltic Sea as well as recycling of Se within the coastal system. Both processes can reduce the effect of the trend predicted from direct Se deposition. We show that changing atmospheric emissions of Se may influence Se concentrations of a pelagic fish species in a coastal area through direct deposition and riverine inputs from the terrestrial landscape.

Biota sites have good geographical coverage, sediment sites are more limited. PBDEs in sediment and biota have been stable (54% of assessed areas) or declining (46%) for the past 20 years. Concentrations are below thresholds (FEQGs) for all assessment areas and congeners, except BDE209 in Irish Sea sediment, and should not cause adverse effects to marine wildlife.

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 theBeaufort Sea is a worrying issue. To help address this question, we analyzed >500 measurements ofHg and other waterproperties from 22 rivers collected in 2012–2018 by communities of the MRB. This new dataset provides a more comprehensiveview of riverine Hg variations across the basin than was previously available. We find that rivers issuedfrom mountains in the western MRB contribute the largest share of Hg in the Mackenzie River, 60–95 % of it beingcarried as fine suspended solids and probably sourced from riverbank erosion and thaw slumps. In contrast, lowlandrivers of the central and eastern MRB contribute larger shares of dissolved Hg (up to 78 %), likely from recent atmosphericdeposition through precipitation. Using load modelling constrained by the new water quality dataset, we estimatethat the three largest western tributaries (Liard, Peel and Arctic Red rivers) of the Mackenzie contribute 60 % ofthe annual MRB THg export and DHg export to the Beaufort Sea during freshet, as well as 51 % of DHg export, whilesupplying 60% of freshet discharge. Load modelling also reveals a sustained decline in DHg loads of ~13 kg yr⁻¹ between2001 and 2016 in the lower Mackenzie River, which likely reflect a decreasing trend in atmospheric Hg depositionover most of northwestern Canada during this period. This study highlights the value of community-based waterquality monitoring in helping to support assessments of riverine Hg in theMRB in support of the Minamata Conventionon Mercury.