No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
The interplay between total mercury, methylmercury and dissolved organic matter in fluvial systems: A latitudinal study across Europe
Abstract
Large-scale studies are needed to identify the drivers of total mercury (THg) and monomethyl-mercury (MeHg) concentrations in aquatic ecosystems. Studies attempting to link dissolved organic matter (DOM) to levels of THg or MeHg are few and geographically constrained. Additionally, stream and river systems have been understudied as compared to lakes. Hence, the aim of this study was to examine the influence of DOM concentration and composition, morphological descriptors, land uses and water chemistry on THg and MeHg concentrations and the percentage of THg as MeHg (%MeHg) in 29 streams across Europe spanning from 41°N to 64 °N. THg concentrations (0.06-2.78 ng L-1) were highest in streams characterized by DOM with a high terrestrial soil signature and low nutrient content. MeHg concentrations (7.8-159 pg L-1) varied non-systematically across systems. Relationships between DOM bulk characteristics and THg and MeHg suggest that while soil derived DOM inputs control THg concentrations, autochthonous DOM (aquatically produced) and the availability of electron acceptors for Hg methylating microorganisms (e.g. sulfate) drive %MeHg and potentially MeHg concentration. Overall, these results highlight the large spatial variability in THg and MeHg concentrations at the European scale, and underscore the importance of DOM composition on mercury cycling in fluvial systems.
Supplementary resource (1)
... The Hg level in water is affected mainly by the concentration of dissolved organic matter, which is a link between land and the aquatic environment. It is also influenced by physicochemical parameters, land cover or watershed disturbances (Bravo et al. 2018;Bishop et al. 2020). In comparison to soils, the level of Hg in the water of rivers and lakes in Poland is understudied, which is mainly due to the methodological difficulties of Hg analysis in liquid samples versus solid matrices. ...
... The results of a large-scale study conducted in streams across Europe by Bravo et al. (2018), showed that the measured THg concentrations were lower than in Polish rivers, and ranged from 0.1 to 2.8 ng L -1 (Table S1a). These differences result mainly from the type of catchment area, i.e. streams enriched in terrestrial dissolved organic matter revealed the elevated total Hg concentrations, as in the case of Sweden or UK (Bravo et al. 2018). ...
... The results of a large-scale study conducted in streams across Europe by Bravo et al. (2018), showed that the measured THg concentrations were lower than in Polish rivers, and ranged from 0.1 to 2.8 ng L -1 (Table S1a). These differences result mainly from the type of catchment area, i.e. streams enriched in terrestrial dissolved organic matter revealed the elevated total Hg concentrations, as in the case of Sweden or UK (Bravo et al. 2018). This may therefore indicate that one of the factors influencing higher THg concentrations in Polish rivers is the agriculturedominated catchment. ...
The goal of this paper is to assess the current status and trends of total mercury (THg) contamination of the atmosphere and terrestrial ecosystems in Poland. The study shows that the reduced domestic and worldwide atmospheric emission of Hg resulted in decreased THg level in the terrestrial biotope and biosphere. Considering that Poland is one of the main Hg emitters in Europe, the THg concentrations in its abiotic environment are still elevated. However, the THg level in terrestrial organisms is relatively low, which is because a large proportion of Hg deposited on land is accumulated in organic-rich soils. Regarding the THg concentration, consumption of wildlife and livestock from Poland is safe for humans. Nevertheless, the authors indicate the need for effective environmental monitoring, based on selected bioindicators, which is crucial considering the slowing reduction of Hg emission combined with the consequences of the changing climate.
... Multi-watershed Hg source analyses are useful as they allow for the isolation of potentially universal ecosystem parameters predictive of MeHg concentrations rather than those which may be site specific. Multi-watershed analyses are used less frequently than single system studies but have been employed in different areas around the world Bravo et al., 2018 ;Chen et al., 2014 ;Driscoll et al., 2012 ;Grigal, 2002 ). For example, a latitudinal study in Europe ( Bravo et al., 2018 ) led the authors to observe a pattern of elevated %MeHg in streams high in sulfate and autochthonous DOM, suggesting that MeHg production is in situ but tied to watershed land use. ...
... Multi-watershed analyses are used less frequently than single system studies but have been employed in different areas around the world Bravo et al., 2018 ;Chen et al., 2014 ;Driscoll et al., 2012 ;Grigal, 2002 ). For example, a latitudinal study in Europe ( Bravo et al., 2018 ) led the authors to observe a pattern of elevated %MeHg in streams high in sulfate and autochthonous DOM, suggesting that MeHg production is in situ but tied to watershed land use. Balcom et al. (2015) focused on estuarine sediment MeHg sources. ...
... Total OC has been suggested as a proxy for net methylation assuming that demethylation is essentially constant ( Lambertsson and Nilsson, 2006 ) which is not true at contaminated sites ( Schaefer et al., 2004 ), supporting the stronger relationships in the Reduced Model. More detailed water column studies suggest that OC composition directly impacts the %MeHg measured in riverine waters ( Bravo et al., 2018 ), which is likely the case in the systems used for this analysis. Indeed, Schartup et al., (2015b) showed that the composition of DOC in estuarine systems influenced Hg reactivity and bioaccumulation into plankton, processes that could also impact %MeHg. ...
Mercury (Hg) is a global and persistent pollutant which can be methylated to more toxic forms (methylmercury; MeHg) in natural systems. Both forms pose a health risk to humans and wildlife, and exposure often begins in aquatic environments. Therefore, quantifying aquatic concentrations and identifying source pathways is important for understanding biotic exposure. In this study, data from estuaries in the Northeast United States were combined to evaluate how point source contamination impacts the concentration and source dynamics of water column total and MeHg with an emphasis on sediment versus non-sediment sources. Partial least squares regression models were implemented to identify a set of variables most related to water column MeHg and total Hg (HgT) across the estuaries. The main findings suggest that contaminated sites have strong internal recycling of HgT that dominates over external inputs, and this leads to elevated concentrations of HgT and MeHg in the local water columns. However, HgT sources in uncontaminated estuarine systems have a strong connection to the local watershed with dissolved HgT linked to dissolved organic carbon, and particulate HgT linked to watershed land use and estuarine mixing. There was little correlative evidence that water column MeHg concentrations were linked to sediment in such systems, but unlike HgT, the concentrations were also not clearly linked to the watershed. Instead, in situ methylation of dissolved water column HgT appeared to dominate the MeHg source pathway. The results suggest that Hg point-source contaminated sites should be considered independently from non-contaminated sites in terms of management, and that land use plays an important indirect role in coastal MeHg dynamics.
... Recent studies have further demonstrated a link between dissolved OM (DOM) composition and the formation of MeHg, where the availability of microbial-and algal-derived DOM was associated with higher methylation rates within the system, and hence to higher MeHg concentrations (Bravo et al., 2017;Herrero Ortega et al., 2018). Total mercury (THg) concentrations, on the other hand, have been shown to be mostly associated with terrestrially derived DOM inputs, likely due to the tendency of Hg to bind to sulfur sites of DOC therefore travel together (Bravo, Kothawala, et al., 2018;Grigal, 2002). As a whole, current evidence suggests that flooding may concurrently alter biogeochemical transformations of Hg and C. ...
... Recent studies have found strong relationships between total DOM concentrations, and in particular, terrestrial DOM pools with THg concentrations (Bravo, Kothawala, et al., 2018;Lavoie et al., 2019), because Hg is bound to DOM and they are comobilized from land to water. Interestingly, when all sites were considered (including tributaries and groundwater that had higher values), we observed a slope of 0.28 ng/mg in the linear regression between THg and DOC, comparable to the average global slope identified in a recent meta-analysis for freshwater (Lavoie et al., 2019). ...
... First, this is surprising since warmer temperature typically enhance the conditions that favor methylation (i.e., anoxia; Ullrich et al., 2001;Paranjape & Hall, 2017) and export of MeHg by hydroelectric reservoirs in other regions typically peak during summer (Canavan et al., 2000;Zhao et al., 2017). Moreover, we expected DOM pools associated to recent production (e.g., protein-like C3 and C5; see to be related to concentrations of MeHg, as has been shown in recent studies (Bravo et al., 2017;Bravo, Kothawala, et al., 2018;Herrero Ortega et al., 2018), because high abundance or proportion of these DOM pools are often found in warm, productive sites and would represent high biological activity in the water column or in the sediments that may co-occur with Hg methylation. However, no DOM quantity or quality (i.e., PARAFAC components) variable was related to MeHg concentrations or proportions. ...
Reservoirs are known to accelerate the mobilization and cycling of mercury and carbon as a result of flooding of terrestrial organic matter, which can lead to environmental concerns at local and broader spatial scales. We explored the covariation of mercury (Hg) and carbon (C) functional pools in natural and recently dammed portions of the aquatic network of the Romaine River watershed in Northern Quebec, Canada, to understand how the fate of these elements varies across systems with contrasting hydrology and environmental conditions. We found that total Hg (THg) concentrations in surface waters were relatively constant along the network, whereas both the concentrations and proportions of MeHg tended to increase in reservoirs compared to surrounding non‐flooded systems, and along the cascade of reservoirs. Whereas THg was related to total and terrestrial pools of dissolved organic carbon (DOC), MeHg was weakly related to DOC but strongly linked to surface concentrations of CO2, as well as to concentrations of iron and manganese. The latter are proxies of cumulative organic matter processing within the network, presumably in anoxic portions of shallow bays, deep reservoir waters and river sediments, as well as in prior seasons (e.g. under ice). Our results suggest that these deep boreal reservoirs acted more as transformation sites for Hg that was already present than as mobilizers of new Hg, and that under ice metabolism plays a role in MeHg production in these systems as we found strong dichotomies in MeHg patterns between spring and summer.
... Fish populations may face reduced food availability and impaired foraging due to lower visibility (Craig et al. 2017). Greater [DOC] may also raise pollutant concentrations in surface water, including mercury, as DOC acts as a transport vector (Isidorova et al. 2016, Bravo et al. 2018. Additionally, ecosystem services may be affected, raising the costs of drinking water treatment and enhancing the risk of harmful disinfection byproducts (Kritzberg et al. 2020). ...
... Conversely, a substantial decrease in [DOC] might reverse some conditions but could introduce new issues, such as increased ultraviolet light penetration and surface warming, which could impact cold-water fish (Schindler et al. 1996). Greater light availability may boost primary productivity and elevate the proportion of autochthonous bioavailable DOC, potentially enhancing the bacteria-mediated formation of neurotoxic methylmercury (Bravo et al. 2018). However, predicting the precise outcomes of these shifts is challenging due to their often non-linear and context-specific nature, which underscores the need for further empirical studies at specific locations (Solomon et al. 2015). ...
The increase in dissolved organic carbon (DOC) concentrations in freshwater systems has received considerable attention due to its implications for drinking water treatment and numerous limnological processes. While past studies have documented the influence of recovery from acidification and climate change on long-term DOC trends, the emerging importance of these explanatory factors remains less understood. In addition, few studies have followed up on recent trends in sites that have undergone increases in DOC. Using a dataset from 1980 to 2020, we investigated interannual variations in DOC and dissolved organic nitrogen (DON) in 49 lakes across four eastern Canadian regions with a history of increases in DOC. We identified recent shifts in DOC patterns using LOESS smoothing and piecewise regression. We observed a stabilizing pattern or even a decrease (p < 0.001) in high acidification regions (Dorset and Nova Scotia), where increases in DOC were previously documented. At the low acid deposition region, IISD-Experimental Lakes Area, an increasing pattern in DOC stabilized in the early 2000s; however, DOC appears to be increasing again in recent years (p = 0.03). Our analysis identified precipitation and SO4 deposition as the primary explanatory variables for DOC patterns (explaining 56–71% of variance). However, because acid deposition has declined substantially, climate and local watershed factors are becoming increasingly influential, leading to the emergence of new DOC patterns. Long-term changes in DOC and DON were not always synchronous, as these were often correlated with different factors (e.g., DON with ammonium deposition). This resulted in observable shifts in DOC:DON ratios, indicative of changes in dissolved organic matter (DOM) composition. We underscore the importance of ongoing monitoring in diverse regions because of the changing nature of environmental variables and new emerging trends.
... The absorption properties of chromophoric dissolved organic matter (CDOM) were considered as predictors since dissolved organic matter from autochthonous and allochthonous sources are thought to play different roles in controlling Hg concentrations and methylation in freshwater environments (Bravo et al. 2018). We selected a number of commonly used absorbance optical properties (see Hansen et al. 2016 for further details). ...
... We calculated the total Hg enrichment ratio (contemporary : preindustrial concentrations) in addition to the contemporary total and MeHg concentrations (ng g À1 dry sediment). Given that Hg binds to organic matter as it moves through the environment (Haitzer et al. 2002;Bravo et al. 2018), we also calculated the Hg concentrations as a fraction of the organic matter in the sediment (ng g À1 dry organic sediment). Finally, the contemporary and preindustrial organic matter corrected concentrations were used to calculate the total Hg enrichment ratio of the organic fraction of the sediment. ...
Mercury (Hg) contamination in the environment is a persistent issue as emissions from industry are ongoing and legacy cycling is prolonged. To quantify total mercury (total Hg) and methylmercury (MeHg) concentrations in lake sediments across a wide suite of temperate to subarctic lakes, measurements and modeling were conducted for 320 sites across Canada. Total Hg varied from 7 to 567 ng g ⁻¹ dry sediment with a mean (median) value of 148 (128) ng g ⁻¹ dry sediment, with lowest concentrations observed in eutrophic lakes of the Prairies and Boreal Plains. Average total Hg concentrations have increased twofold in contemporary sediments relative to their preindustrial pair. MeHg concentrations were generally low, with a mean of 0.8 ng g ⁻¹ dry sediment and this form representing on average 0.6% of the total contemporary mercury concentrations. Water column variables related to eutrophication were among the strongest predictors of contemporary total Hg, MeHg, and total Hg enrichment in the sediments. In lakes where sediment radiometric data were collected, contemporary total Hg accumulation rates were strongly related to longitude(+) and Hg atmospheric deposition(+). Latitude(−) and Hg atmospheric deposition(+) were the best predictors of MeHg accumulation rates. Chromophoric dissolved organic matter optical properties were key predictors of both total Hg concentrations and accumulation rates. Our work highlights the paramount importance of lake trophic state and the nature of organic matter as key predictors of lake sediment mercury metrics.
... Most of the MeHg and THg were in the dissolved form, which has the potential to be more bioavailable than particle-bound forms (Le Faucheur et al. 2014), with 86% AE 13% of MeHg and 81% AE 18% of THg in dissolved form in streams and 70% AE 18% of MeHg and 75% AE 21% of THg in lakes. The %MeHg (18% AE 18% in streams, 10% AE 11% in lakes, unfiltered) was also relatively high throughout the transect (Fig. 2C, F) compared to European streams (Bravo et al. 2018a) and eastern Arctic lakes (Richardson et al. 2021) and of similar magnitude to productive Arctic thaw ponds (MacMillan et al. 2015). The high MeHg concentrations and %MeHg observed in this study are consistent with the numerous fish consumption advisories in the Interior Plains (Evans et al. 2005). ...
... Stream DOC concentrations were higher in smaller catchments with greater peatland extent, whereas lake DOC concentrations were higher in warmer climates with less permafrost (Fig. 4). The link between DOC concentrations and % MeHg is not universal, for example, it was not observed in European streams (Bravo et al. 2018a), but has previously been observed in boreal regions, for example, among lakes within the Mackenzie River Basin (Evans et al. 2005). Corresponding patterns of increasing stream DOC concentrations with greater catchment peatland cover and smaller catchment size have been documented in other boreal regions (Mattsson et al. 2005;Kothawala et al. 2014). ...
Permafrost thaw may increase the production of neurotoxic methylmercury (MeHg) in northern peatlands, but the downstream delivery of MeHg is uncertain. We quantified total mercury (THg) and MeHg concentrations in lakes and streams along a 1700 km permafrost transect in boreal western Canada to determine the influence of regional permafrost extent compared to local lake and catchment characteristics. In lakes, we assessed sediment microbial communities and modeled potential rates of water column photodemethylation (PD). Regardless of permafrost conditions, peatlands were the primary sources of MeHg across the transect as MeHg concentrations in streams increased with aromatic dissolved organic carbon (DOC), iron, and lower pH. Higher DOC and greater catchment peatland extent were further associated with higher stream %MeHg (MeHg/THg). Peatland lakes were potential MeHg sinks, with lower MeHg concentrations than streams (mean±1SD: 0.19±0.23 and 0.47±0.77 ng MeHg L cm-1, respectively), and larger stream catchments had lower %MeHg where PD may occur in abundant small lakes. Microbial communities in lake sediments showed that abundance of Hg reducing genes (merA) predominated over Hg methylating (hgcA) and MeHg demethylating (merB) genes. The effects of permafrost extent on MeHg processes in lakes were secondary to the influence of local catchment characteristics, but lakes in regions with less permafrost had higher DOC concentrations, higher %MeHg, and lower potential rates of PD. Our study highlights a need to understand the impacts of climate change on MeHg source and sink processes, particularly as mediated through changes to peatland DOC, to improve projections of future MeHg concentrations in northern catchments.
... Among the aquatic compartments, lowland lakes have been widely studied regarding Hg pollution, speciation and species transformations (Chételat, 2015;Braaten et al., 2014;Klapstein and O'Driscoll, 2018;Rolfhus et al., 2003). Several studies highlighted a link between organic matter and Hg in lake ecosystems (Braaten et al., 2014;Bravo et al., 2018), with evidence of the catchment influence on the Hg levels. Significant photo-reduction and -demethylation were observed in lakes (Seller et al., 1996;Bouchet et al., 2022), exceeding the external inputs of MMHg (rain, snow, runoff) and therefore suggesting MMHg sources from bottom sediments. ...
... The particles in the water have a key role in the transport and fate of Hg species (Bravo et al., 2018). Thus, to evaluate their influence on the distribution of Hg species, two kinds of samples were collected and further analysed: filtered (F, dissolved fraction) and unfiltered (UF, total fraction). ...
While mercury (Hg) is a major concern in all aquatic environments because of its methylation and biomagnification pathways, very few studies consider Hg cycling in remote alpine lakes which are sensitive ecosystems. Nineteen high-altitude pristine lakes from Western/Central Pyrenees were investigated on both northern (France) and southern (Spain) slopes (1620–2600 m asl.). Subsurface water samples were collected in June 2017/2018/2019 and October 2017/2018 for Hg speciation analysis of inorganic mercury (iHg(II)), monomethylmercury (MMHg), and dissolved gaseous mercury (DGM) to investigate spatial and seasonal variations. In June 2018/2019 and October 2018, more comprehensive studies were performed in four lakes by taking water column depth profiles. Besides, in-situ incubation experiments using isotopically enriched Hg species (¹⁹⁹iHg(II), ²⁰¹MMHg) were conducted to investigate Hg transformation mechanisms in the water column. While iHg(II) (0.08–1.10 ng L⁻¹ in filtered samples; 0.11–1.19 ng L⁻¹ in unfiltered samples) did not show significant seasonal variations in the subsurface water samples, MMHg (<0.03–0.035 ng L⁻¹ in filtered samples; <0.03–0.062 ng L⁻¹ in unfiltered samples) was significantly higher in October 2018, mainly because of in-situ methylation. DGM (0.02–0.68 ng L⁻¹) varies strongly and can exhibit higher levels in comparison with other pristine areas. Depth profiles and incubation experiments highlighted the importance of in-situ biotic methylation triggered by anoxic conditions in bottom waters. In-situ incubations confirm that significant methylation, demethylation and photoreduction extents are taking place in the water columns. Overall, drastic environmental changes occurring daily and seasonally in alpine lakes are providing conditions that can both promote Hg methylation (stratified anoxic waters) and MMHg photodemethylation (intense UV light). In addition, light induced photoreduction is a major pathway controlling significant gaseous Hg evasion. Global warming and potential eutrophication may thus have direct implications on Hg turnover and MMHg burden in those remote ecosystems.
... Among all potential factors affecting MeHg production, researchers have primarily focused on dissolved organic matter (DOM), since it can be easily regulated by shifts in human activities (e.g., soil erosions and eutrophication) (Borrelli et al., 2017;Bravo et al., 2018). DOM has been recognized for emerging importance in biogeochemical processes affecting the environmental fates of the trace contaminants, such as copper, lead, and polycyclic aromatic hydrocarbons (Inaba and Takenaka, 2005;Yamada and Katoh, 2020;Sun et al., 2021). ...
... Positive correlations between ALOM and Hg concentrations in boreal rivers and streams have been previously reported, and the remobilization of Hg-enriched soil organic matter is a major process impacting the Hg concentrations in streams and lakes (Bravo et al., 2017). Bravo et al. (2018) revealed that terrestrial DOM was a vital source of THg in European running waters. Moreover, correlations between ALOM and Hg and MeHg concentrations could highlight the catchment inputs of the terrigenous Hg and MeHg. ...
... Among all potential factors affecting MeHg production, researchers have primarily focused on dissolved organic matter (DOM), since it can be easily regulated by shifts in human activities (e.g., soil erosions and eutrophication) (Borrelli et al., 2017;Bravo et al., 2018). DOM has been recognized for emerging importance in biogeochemical processes affecting the environmental fates of the trace contaminants, such as copper, lead, and polycyclic aromatic hydrocarbons (Inaba and Takenaka, 2005;Yamada and Katoh, 2020;Sun et al., 2021). ...
... Positive correlations between ALOM and Hg concentrations in boreal rivers and streams have been previously reported, and the remobilization of Hg-enriched soil organic matter is a major process impacting the Hg concentrations in streams and lakes (Bravo et al., 2017). Bravo et al. (2018) revealed that terrestrial DOM was a vital source of THg in European running waters. Moreover, correlations between ALOM and Hg and MeHg concentrations could highlight the catchment inputs of the terrigenous Hg and MeHg. ...
Mercury (Hg) input into ecosystems is estimated to have increased by twofold to fivefold since the industrial revolution. In aquatic ecosystems, methylmercury (MeHg) receives the most attentions of all the Hg species due to its neurotoxicity and strong bioaccumulation capacity in food chain. Dissolved organic matter (DOM) is crucial in impacting aquatic Hg transformation. However, only few spatially constrained studies have attempted to quantify the relative importance of DOM and other factors (e.g., Hg availability, temperature, pH, and land-use type) on MeHg concentration. In this study, we collected data of 585 water samples at 373 sites globally, including lakes, rivers, estuaries, and wetlands, and characterized the global pattern of MeHg distribution and environmental drivers of aquatic MeHg concentration. Our results showed that MeHg concentrations ranged from detection limits to 11 (geometric mean 0.11 and average 0.29) ng/L, and the highest MeHg concentration and Hg methylation potential were observed in wetlands. A positive relationship was observed between MeHg fraction in the total mercury (THg) and DOM for all the aquatic ecosystems. Using the structural equation modeling, we found that Hg availability was a dominant factor in impacting water MeHg concentration followed by DOM. According to 129 samples of specific DOM source information, we found that the percentage of THg as MeHg (%MeHg) in water dominated by the autochthonous DOM was higher than that dominated by the allochthonous DOM. Our results could advance understanding of aquatic Hg cycling and their environmental drivers, which are fundamental for predicting and mitigating MeHg productions and its potential health risks for humans.
... Inland waters are a suitable environment for MeHg production and have gained considerable attentions worldwide (Bravo et al., 2017;Kim et al., 2013). Hg methylation is influenced by various environmental factors (Bravo et al., 2018;Seelen et al., 2021). Cossa et al. (2014) established a strong positive correlation between MeHg and Hg in aquatic sediment. ...
... Meanwhile, with the Hg methylation, the interannual variations of DGT-MeHg and% MeHg in a reservoir were stronger than that of DGT-Hg ( Fig. 5c; Fig. 6c). DGT-MeHg and%MeHg dynamics were affected in many ways with the extension of HRT due to river damming (Bravo et al., 2018;Cha et al., 2017;Evans et al., 2017). Firstly, the weakening of hydrodynamic condition enhances the retention of Hg and OC; secondly, more time for Hg methylation is provided; thirdly, the produced MeHg is less released downstream (Evans et al., 2017;Nogueira et al., 2002;Shi et al., 2017). ...
Methylmercury (MeHg) can be bioaccumulated through food chains and adversely affect human health. Reservoirs are reported to stimulate MeHg production, however, the characteristics of MeHg dynamics in cascade reservoirs and the associated relations to sedimentation as well as reservoir properties remained unclear. Here we investigated sediment MeHg dynamics in eight cascade reservoirs in the upper Mekong River. We found significant differences in sediment MeHg concentration between the reservoirs, showing an increase with fluctuations along the reservoirs cascade. However, a novel relationship was found between sediment %MeHg (MeHg/Hg) and the ratio of reservoir hydraulic residence time (HRT) to reservoir age. This relationship is formed by the joint effects of the original deposit of Hg and organic carbon (OC) before impoundment and the subsequent sedimentation of Hg and OC after impoundment. The original deposit is continuously transformed as the reservoir ages, whereas the latter is driven by the annual hydrological cycle and HRT, of which the HRT is dominant. This finding cannot be easily revealed in a single reservoir or by comparing multiple reservoirs in different rivers. The discovery is of great significance to understand Hg geochemical cycling in reservoirs, which is quickly increasing in rivers worldwide.
... Formation of complexes with terrestrial organic matter greatly limits the solubility, mobility and toxicity of Hg in the water column and in sediments (Obrist et al., 2011;O'Connor et al., 2019;Ravichandran, 2004;Wallschlager et al., 1998). Marine organic matter stimulates mobility and availability of Hg, meaning that higher Hg SPM-M concentrations are often found in marine water not impacted by rivers (Bravo et al., 2018;Schartup et al., 2015). ...
... The proportion of individual Hg fractions in riverine particulate matter reflected the mercury fractions present in the soils of the southern Baltic catchment area (Gębka et al., 2019). Independently of the quantity of Hg transported by the rivers, a similar share of the individual Hg fractions in the Vistula and the Reda (in both periods) rivers suggests that the terrestrial particulate matter washed out from the land was the main source for the particulate Hg in the rivers (Bravo et al., 2018). In the sea the share of the Hg fractions varied within a wide range. ...
Rivers play an important role in the coastal biogeochemical cycling of mercury (Hg), however, the fate of Hg in these environments remains poorly understood. In this study, we determine how processes such as mixing, flocculation, and settling of particles in the river mouths of the Vistula and the Reda rivers in the southern Baltic Sea, affect the transformation and migration of mercury. The most important process was the mixing of river water with seawater. This process led to a decrease in the concentration of total mercury and particulate mercury with increasing distance from the river mouth. The second most important process affecting the distribution of Hg was flocculation and particle settling. This process most effectively occurred in the middle part of the mouth, where the highest salinity gradient was observed. It was responsible for approximately 32% loss of Hg from the water column with a simultaneous increase in Hg concentration in sediments. In consequence, it significantly limited the transport of Hg to the open Baltic Sea. The assimilation of Hg by marine plankton was also observed. As riverine organic matter stimulated the growth of marine plankton, they actively accumulated Hg and then transferred this metal to the higher trophic level. In both of the river mouths the processes affected the fate of Hg in a similar way. We therefore concluded that the size of the river did not affect the Hg transformation in the river mouth. However, the area of the river catchment, depth of the mouth and hydrological conditions had a significant impact on Hg cycling in the coastal zone in the vicinity of the river mouth.
... Riverine DOC and Hg concentrations tend to be positively correlated with discharge, indicating common hydrological controls (Brigham et al., 2009;Schuster et al., 2011;Sonke et al., 2018). In recent years, optical properties of dissolved organic matter (DOM), such as UV-Vis absorbance or fluorescence, were found to improve the explanatory power of riverine [DOC] for Hg concentrations (Bravo et al., 2018;Dittman et al., 2009;Eklof et al., 2012;Grigal, 2003;Lescord et al., 2018). ...
... This adds to an existing body of research having reported positive relationships between [Hg] and SUVA abs254 or total absorbance at 254 mm in surface waters. (Bravo et al., 2018;Dittman et al., 2009;Eklof et al., 2012;Grigal, 2003;Lescord et al., 2018) However, most of these studies also report a co-existing positive correlation between [DOC] and [Hg] in the same waters. In these studies, SUVA abs254 improved the predictability of riverine [DHg] over an existing relationship with the bulk DOC (Lavoie et al., 2019). ...
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, 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.
... However, these differences were not clearly reflected in water chemistry, including MeHg concentrations (Figure 3). Similarly, on a European scale, Bravo et al. (2018) highlighted the large, non-systematic spatial variability in MeHg concentrations in stream systems (0.0078-0.159 ng L −1 ) [87]. The rate of Hg methylation is linked to factors controlling the abundance and activity of sulphate-reducing bacteria and other microorganisms capable of methylating Hg, such as a suboxic microenvironments, the availability of sulphate (electron acceptor), high-quality organic matter (electron donor), inorganic Hg and temperature [88]. ...
... However, these differences were not clearly reflected in water chemistry, including MeHg concentrations (Figure 3). Similarly, on a European scale, Bravo et al. (2018) highlighted the large, non-systematic spatial variability in MeHg concentrations in stream systems (0.0078-0.159 ng L −1 ) [87]. The rate of Hg methylation is linked to factors controlling the abundance and activity of sulphate-reducing bacteria and other microorganisms capable of methylating Hg, such as a suboxic microenvironments, the availability of sulphate (electron acceptor), high-quality organic matter (electron donor), inorganic Hg and temperature [88]. ...
Forest management activities in boreal and hemiboreal environments have been found to increase the concentration of carbon, nutrients, and methylmercury (MeHg) in runoff water, thus contributing to environmental quality issues. We evaluated carbon, nutrient, and MeHg concentrations in water at eight small, forested catchments on organic soils in Latvia, subject to ditch cleaning and beaver dam removal. These management-induced disturbances were classified into a major, minor, or no disturbance classes. The concentrations of dissolved organic carbon and total nitrogen were elevated in disturbed catchments (both major and minor) compared to the catchments with no disturbance. The concentrations of MeHg in the water displayed a clear seasonal variation with higher concentrations in spring and summer, but there were no significant differences in MeHg concentrations between catchments with major, minor, and no disturbances. However, the higher concentrations of SO4²⁻ in the disturbed catchments compared to those undisturbed may promote MeHg formation if the conditions become more reduced further downstream. While most former studies of forest management effects on water quality have focused on forest harvest, our research contributes to the currently rather scarce pool of data on the impact of less-studied management operations, such as ditch cleaning and beaver dam removal, on carbon, nutrient, and MeHg concentrations in runoff water.
... The positive relationship between T-Hg or MeHg and OM (particularly DOC) has been demonstrated in a number of river systems, although it has not been observed in all lotic systems, and the underlying principles were not studied (Lavoie et al., 2019, for a global analysis; Zolkos et al., 2020, for major arctic rivers; De Bonville et al., 2020, for the boreal Romaine river watershed). Bravo et al. (2018) studied 29 streams across Europe at latitudes ranging from 41 to 64°N and demonstrated that autochthonous DOM and the availability of electron acceptors for Hgmethylating microorganisms (e.g., sulfate) drive the percentage of MeHg, and potentially MeHg concentrations, in water. As has been observed in lakes and reservoirs, autochthonous carbon seems to stimulate bacterial metabolism and community plasticity in riverine systems (Comte and Del Giorgio, 2011). ...
... Through a crossover effect, this local carbon source would increase mercury methylation by enhancing the assimilation of allochthonous DOM (the main carbon source for prokaryotic communities in inland waters), which gradually becomes less reactive along the river continuum (Massicotte et al., 2017;LaBrie et al., 2020). On the other hand, Bravo et al. (2018) also found that allochthonous DOM inputs, i.e., dissolved organic matter probably originating from organic-rich catchment soils, control T-Hg concentrations in streams; however the authors do not mention the probable mechanisms underlying this interaction, described below. ...
The chemistry of mercury in freshwater systems, particularly man-made reservoirs, has received a great deal of attention owing to the high toxicity of the most common organic form, methylmercury. Although methylmercury bioaccumulation in reservoirs and natural lakes has been extensively studied at all latitudes, the fate of the different forms of mercury (total vs. dissolved; organic vs. inorganic) along the entire river-estuary continuum is less well documented. In fact, the difficulty of integrating the numerous parameters involved in mercury speciation in such large study areas, combined with the technical difficulties in sampling and analyzing mercury, have undoubtedly hindered advances in the field. At the same time, carbocentric science has grown exponentially in the last 25 years, and the common fate of carbon and mercury in freshwater has become increasingly clear with time. This literature review, by presenting the knowledge acquired in these two fields, aims to better understand the extent of mercury export from boreal inland waters to estuaries and to investigate the possible downstream ecotoxicological impact of reservoir creation on mercury bioavailability to estuarine food webs and local communities.
... We sampled sediments from 16 streams across 10 European countries (Table S1, Appendix S2), using the EuroRun network Bravo et al., 2018). Within a representative 50 m section of the investigated stream three patches were sampled based on their sediment grain size: (a) coarse (gravel, >0.2 cm to 2 cm), (b) medium (sand/mud, >6 μm to 2 mm), and (c) fine (silt/loam/clay, <6 μm; Figure 1). ...
... The upstream catchment area, elevation (metres above sea level), Strahler stream order (Strahler, 1952), and land use within the catchment were taken from Bravo et al. (2018), which covered the same sites (Table S1, Appendix S2). Land use within the catchment was further simplified after inspection via principal component analysis (PCA; see section 2.7) by unifying the classes 'urban' and 'agriculture' as well as 'forest' and 'others' into 'anthropogenically altered' and 'natural' respectively. ...
Aim
Although running waters are getting recognized as important methane sources, large‐scale geographical patterns of microorganisms controlling the net methane balance of streams are still unknown. Here we aim at describing community compositions of methanogenic and methanotrophic microorganisms at large spatial scales and at linking their abundances to potential sediment methane production (PMP) and oxidation rates (PMO).
Location
The study spans across 16 European streams from northern Spain to northern Sweden and from western Ireland to western Bulgaria.
Taxon
Methanogenic archaea and methane‐oxidizing microorganisms.
Methods
To provide a geographical overview of both groups in a single approach, microbial communities and abundances were investigated via 16S rRNA gene sequencing, extracting relevant OTUs based on literature; both groups were quantified via quantitative PCR targeting mcrA and pmoA genes and studied in relation to environmental parameters, sediment PMP and PMO, and land use.
Results
Diversity of methanogenic archaea was higher in warmer streams and of methanotrophic communities in southern sampling sites and in larger streams. Anthropogenically altered, warm and oxygen‐poor streams were dominated by the highly efficient methanogenic families Methanospirillaceae, Methanosarcinaceae and Methanobacteriaceae, but did not harbour any specific methanotrophic organisms. Contrastingly, sediment communities in colder, oxygen‐rich waters with little anthropogenic impact were characterized by methanogenic Methanosaetaceae, Methanocellaceae and Methanoflorentaceae and methanotrophic Methylococcaceae and Cd. Methanoperedens. Representatives of the methanotrophic Crenotrichaceae and Methylococcaceae as well as the methanogenic Methanoregulaceae were characteristic for environments with larger catchment area and higher discharge. PMP increased with increasing abundance of methanogenic archaea, while PMO rates did not show correlations with abundances of methane‐oxidizing bacteria.
Main conclusions
Methanogenic and methanotrophic communities grouping into three habitat types suggest that future climate‐ and land use changes may influence the prevailing microbes involved in the large‐scale stream‐related methane cycle, favouring the growth of highly efficient hydrogenotrophic methane producers. Based on these results, we expect global change effect on PMP rates to especially impact rivers adjacent to anthropogenically disturbed land uses.
... Besides the unequivocal importance of the molecular composition of DOM on Hg cycling (Bravo et al., 2017(Bravo et al., , 2018aHerrero Ortega et al., 2018;Jiang et al., 2018;Lescord et al., 2018), the amount of OM itself is a very important parameter. DOM enhances the dissolution and inhibits the precipitation of highly insoluble HgS (Graham et al., 2012a(Graham et al., , 2012bGraham et al., 2013;Ravichandran, 2004), contributing to higher concentrations of Hg aq in high DOM lakes than those with clear waters. ...
... The complex geochemistry of lakes regulates interactions in an intricate network of microorganisms that collectively create the environment conducive to HgII methylation. Net MeHg production is not a process governed by the activity of specific groups, but by the interactions among them and the chemical characteristics of their (micro)environment (Bravo et al., 2018a). These relationships play out most profoundly in periphyton and biofilms, where MeHg production is regulated by aquatic geochemistry, extracellular substances, and photosynthetic exudates in what may be one of the most under-recognized sources of MeHg to freshwater ecosystems. ...
The widely accepted conceptual model of mercury (Hg) cycling in freshwater lakes (atmospheric deposition and runoff of inorganic Hg, methylation in bottom sediments and subsequent bioaccumulation and biomagnification in biota) is practically accepted as common knowledge. There is mounting evidence that the dominant processes that regulate inputs, transformations, and bioavailability of Hg in many lakes may be missing from this picture, and the fixation on the temperate stratified lake archetype is impeding our exploration of understudied, but potentially important sources of methylmercury to freshwater lakes. In this review, the importance of understudied biogeochemical processes and sites of methylmercury production are highlighted, including the complexity of redox transformations of Hg within the lake system itself, the complex assemblage of microbes found in biofilms and periphyton (two vastly understudied important sources of methylmercury in many freshwater ecosystems), and the critical role of autochthonous and allochthonous dissolved organic matter which mediates the net supply of methylmercury from the cellular to catchment scale. A conceptual model of lake Hg in contrasting lakes and catchments is presented, highlighting the importance of the autochthonous and allochthonous supply of dissolved organic matter, bioavailable inorganic mercury and methylmercury and providing a framework for future convergent research at the lab and field scales to establish more mechanistic process-based relationships within and among critical compartments that regulate methylmercury concentrations in freshwater ecosystems.
... We sampled 16 streams across 10 European countries (Tab. S1), using the network of the EuroRun project [66][67][68]. Within a representative 50 m section of the investigated stream [69] three areas were selected for sampling based on their differing sediment grain size; (1) coarse (gravel, >0.2 cm to 2 cm), (2) medium (sand and mud, >6 μm to 2 mm), and (3) fine (silt/loam/clay, <6 μm). If one sediment type was not present or could not be sampled, sediment patches with the widest possible range of sediment grain size were selected (coarse, medium, and fine sediment). ...
... The upstream catchment area, altitude (meters above sea level), Strahler Stream order [70], and land use within the catchment were taken from [66], which covered the same sites (Tab. S1). ...
Background: Globally, streams emit significant amounts of methane, a highly potent greenhouse gas. However, little is known about the stream sediment microbial communities that control the net methane balance in these systems, and in particular about their distribution and composition at large spatial scales. This study investigated the diversity and abundance of methanogenic archaea and methane-oxidizing microorganisms across 16 European streams (from northern Spain to northern Sweden and from western Ireland to western Bulgaria) via 16S rRNA gene sequencing and qPCR. Furthermore, it examined environmental factors influencing both abundance and community composition and explored the link to measured potential methane production and oxidation rates of the respective sediments.
Results: Our results demonstrated that the methanogenic and methanotrophic microbiomes of the studied European streams were linked to both the temperature and degree of anthropogenic alteration. The microbiomes could be separated into two to three groups according to environmental factors at both stream and catchment scales. Main methanogenic taxa found within more anthropogenically-altered, warm, and oxygen-poor environments were either Methanospirillum spp. or members of the families Methanosarcinaceae and Methanobacteriaceae . Within such environments, methane oxidizing communities were strongly characterized by members of the family Methylobacteriaceae ( Meganema spp. and Microvirga spp.). Contrastingly, communities in colder environments rich in oxygen and with relatively little anthropogenic impact at the catchment scale were characterized by the methanogenic Methanosaetaceae , Methanocellaceae and Methanoregulaceae and the methanotrophic Methyloglobulus spp ., members of the CABC2E06 group (all Methylococcaceae ) and by various Candidatus Methanoperedens. Overall, diversity of methanogenic archaea increased with increasing water temperature. Methane oxidizing communities showed higher diversities in southern sampling sites and in streams with larger stream areas and widths. Potential methane production rates significantly increased with increasing abundance of methanogenic archaea, while potential methane oxidation rates did not show significant correlations with abundances of methane oxidizing bacteria, presumably due to the more diverse physiological capabilities of this group.
Conclusions: We present the first large scale overview of the large-scale microbial biogeography of two microbial groups driving the methane cycle dynamics within stream sediments and deduce the impact that future anthropogenic alterations may cause.
... Taking all this into consideration, it is plausible that the increase in MeHg concentrations in overlying waters observed in this study resulted from an enhanced Hg methylation (Fig. 3), fostered by the release of available Hg, associated to the increase in DOC [11,12,16], and labile dissolved organic matter (peak T) (Fig. 4) [15]. The low oxygen concentrations found in the studied sediments together with an enrichment in labile dissolved organic matter have been associated to a higher presence of Hg methylating microorganisms [69]. ...
... Furthermore, mercury concentration strongly correlates to nutrient input, such as all forms of organic carbon (i.e., TOC, DOC), resulting from decomposition (Erickson and Lin. 2015;Bravo et al. 2018). ...
Mercury is a highly toxic element present in water, soil, air, and biota. Anthropogenic activities, such as burning fossil fuels, mining, and deforestation, contribute to the presence and mobilization of mercury between environmental compartments. Although current research on mercury pathways has advanced our understanding of the risks associated with human exposure, limited information exists for remote areas with high diversity of fauna, flora, and indigenous communities. This study aims to deepen our understanding of the presence of total mercury in water, sediments, and fish, within aquatic ecosystems of two indigenous territories: Gomataon (Waorani Nationality) and Sinangoé (Ai´Cofán Nationality) in the Ecuadorian Amazon. Our findings indicate that, for most fish (91.5%), sediment (100%) and water (95.3%) samples, mercury levels fall under international limits. For fish, no significant differences in mercury levels were detected between the two communities. However, eight species exceeded recommended global limits, and one surpassed the threshold according to Ecuadorian legislation. Piscivore and omnivore fish exhibited the highest concentrations of total mercury among trophic guilds. Only one water sample from each community’s territory exceeded these limits. Total mercury in sediments exhibited greater concentrations in Gomataon than Sinangoé. Greater levels of mercury in sediments were associated with the occurrence of total organic carbon. Considering that members of the communities consume the analyzed fish, an interdisciplinary approach, including isotopic analysis, methylmercury sampling in humans, and mercury monitoring over time, is imperative for a detailed risk assessment of mercury exposure in Amazonian communities.
... Dissolved organic matter (DOM) and DOC can influence Hg speciation, methylation and photoreduction within marine and fresh waters and sediments (Ullrich et al., 2001;Mason et al., 2006;Bravo et al., 2018;Luo et al., 2020). The DOC concentrations were higher in covering waters from core experiments compared to those measured in the water column. ...
Mercury (Hg) fulminate was used as a primary fuse in World War (WW) munitions, and may consequently be a Hg source for impacted environments. Mercury is a conspicuous and persistent pollutant, with methylmercury (MeHg) acting as a notorious neurotoxin. Considerable amounts of munitions were intentionally dumped in the North Sea and Baltic Sea following the First and Second WWs. After more than 70 years on the seafloor many munitions have corroded and likely release explosive compounds, including Hg fulminate. The Germany coastal city of Kiel was a manufacturing centre for submarines, and accordingly a prominent target for bombing and post-war disarmament. We collected water and sediment samples around Kiel Bay to assess regional levels and quantify any Hg contamination. The munition dump site Kolberger Heide (KH) and a former anti-aircraft training center Dänisch-Nienhof are situated in Kiel Bay, and were targeted for sampling. Sediment Hg concentrations around KH were notably elevated. Average Hg concentrations in KH sediments were 125 ± 76 ng/g, compared to 14 ± 18 ng/g at background (control) sites. In contrast, dissolved Hg in the water column exhibited no site variations, all ranging between 0.8 – 2.1 pM. Methylmercury in sediments and waters did not have enhanced concentrations amongst sites (<30 pg/g and <50 fM, respectively). Sediment-water exchange experiments showed elevated Hg and MeHg fluxes (i.e. >400 pmol m-2 d-1 MeHg) at one KH location, however remaining cores had low to no Hg and MeHg output (<0 to 27 pmol m-2 d-1 MeHg). Thus, sediments in Kiel Bay proximate to WW munitions could harbor and form a source of Hg, however water column mixing and removal processes attenuate any discharge from the seafloor to overlying waters.
... The MMHg/THg ratios, phase distribution and partitioning of THg and MMHg in waters of our SOs and CSs were highly variable. Similar to other surface aquatic systems (Hurley et al., 1995;Noh et al., 2013;Mao et al., 2016;Bravo et al., 2018;Liu et al., 2021a), the IHg (the difference between THg and MMHg) was the most abundant Hg speciation in both dissolved and particulate phases in waters of CSs and SOs (Table 2). In our CSs and some major rivers (Noh et al., 2013;Liu et al., 2021a), the THg and MMHg were mainly bound to SPMs. ...
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 ~10 2 to 10 3 times of coastal surface waters and 10 to 10 2 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.
... Bravo et al., 2017;Lavoie et al., 2019). Bravo et al. (2018) highlighted the strong relationship between autochthonous DOM of algal origin and Hg methylation in lake water, while Millera Ferriz et al. (2021) suggest the crucial role played by terrestrial carbon for Hg methylation in a dammed river. Moreover, Lavoie et al. (2019) emphasized the stronger relation for both Hg and MeHg and the aromatic fraction of DOM compared to the bulk DOC. ...
... Algal and microbially-derived autochthonous DOC is more difficult to remove during treatment (Liao et al., 2017) and a greater proportion of autochthonous DOC can contribute significantly to the formation of disinfectant byproducts (Kraus et al., 2011). Also, a combination of greater DOC load and increase in autochthonous DOC may result in increased mercury methylation in rivers (Bravo et al., 2018). We recommend continued monitoring of the effect of reservoirs on riverine DOM properties. ...
Diverse environmental and anthropogenic factors, such as the ongoing reservoir constructions may influence riverine dissolved organic matter (DOM) properties. This has important implications for river water quality, particularly when reservoirs are a source of drinking water. Simultaneous studies of multidecadal trends in dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) are scarce. We studied the patterns in DOC and DON concentration in two major rivers of the South Saskatchewan River (SSR) basin over a 42-year period (1978–2019). We also examined the impact of a large reservoir on riverine DOC properties. Contrary to many studies, we did not find a long-term increase in DOC and DON concentration, and DOC and DON patterns were not always synchronous. In an agriculture dominated watershed like the SSR basin, agricultural land use (e.g., nitrogen-fertilizer application) could influence DOC and DON concentration differently, potentially resulting in asynchronous patterns over time. River discharge was an important driver of DOM patterns. Regional precipitation in the lower SSR basin may also influence DOM patterns in locations where runoff contribution is greater. These regional factors explained greater variability in DOM compared to global scale indices (e.g., Pacific decadal oscillation) due to their direct control on DOM. A travel time corrected approach to account for the lengthy reservoir turnover time showed that a large reservoir caused a reduction in allochthonous DOC characteristics through photodegradation and perhaps, an increase in autochthonous characteristics. Our results illustrate: 1) the increase in DOM concentrations seen in the northern hemisphere is not present in semi-arid prairie rivers, 2) Controls on different DOM components could be different, and 3) large reservoirs may modify riverine DOC composition due to longer water residence time.
... Our estimate of the releases from soils to water is sizable but relatively small. Bravo et al., 2018, based on an observed correlation between dissolved organic matter of terrestrial origin in rivers and Hg concentration at 21 sites in Europe, argue that mobilization of Hg bound to soil organic matter may be an important source of Hg in rivers. Based on their dataset, they come to an order of magnitude of releases of 10 3 kg year −1 of total Hg for the whole of Europe, which is reasonably in line with our calculations. ...
Mercury pollution is a cause of high concern for European freshwaters. In this study, we use modelled atmospheric deposition and novel high-resolution water cover and impervious urban areas data to quantify the input of Mercury to European rivers and lakes. This information, combined with estimates of releases from industrial installations and urban wastewater and from soils, yields an overall European budget of water Mercury. Compared to previous estimates, the calculation highlights that direct deposition to permanent and temporary inland water surfaces is a dominant source of pollution. We also show that an important source is the washout of impervious urban surfaces, while releases from soil and industrial and urban wastewater play a lesser, albeit sizable role. The contribution of wastewater to Mercury releases is expected to decrease over time because of more stringent regulations on Mercury use. The analysis confirms that reducing atmospheric deposition (hence air releases) of Mercury remains the single key action to control pollution. However, we show that control of urban runoff discharges to water bodies may be the most effective water management measure in order to reduce Mercury input to coastal and in-land water bodies.
... However, no study has demonstrated a quantitative relationship between the expression of hgc genes (i.e., hgc transcripts) and its associated reaction, (i.e., Hg II methylation rates), neither in environmental samples nor in laboratory culture experiments. [20][21][22]75 It has thus remained uncertain if rates of Hg II methylation are constrained by the molecular-level methylation processes mediated by the hgc genes. In general, a positive relationship between the abundance of genes or transcripts and the corresponding process rates is often assumed but rarely demonstrated in natural systems, mainly because of a lack of fundamental understanding of the factors that control the activity of the microorganisms and thus the gene transcription. ...
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.
... In the highly colored streams, diel CO 2 patterns can additionally be influenced by DOC shading diminishing benthic primary production 36 . In October, we measured DOC concentrations in a subset of the investigated streams for another study 37 where an agricultural stream in Sweden and peatland-dominated streams in Great Britain had high DOC concentrations (>10 mg L −1 ) whereas the median DOC was much lower with 2.6 mg L −1 37 . Due to the limited data, we could not test the effect of DOC on pCO 2 changes and we can neither confirm nor exclude that photomineralization might play a role for diel pCO 2 and consequently CO 2 flux variability in the studied streams. ...
Globally, inland waters emit over 2 Pg of carbon per year as carbon dioxide, of which the majority originates from streams and rivers. Despite the global significance of fluvial carbon dioxide emissions, little is known about their diel dynamics. Here we present a large-scale assessment of day- and night-time carbon dioxide fluxes at the water-air interface across 34 European streams. We directly measured fluxes four times between October 2016 and July 2017 using drifting chambers. Median fluxes are 1.4 and 2.1 mmol m−2 h−1 at midday and midnight, respectively, with night fluxes exceeding those during the day by 39%. We attribute diel carbon dioxide flux variability mainly to changes in the water partial pressure of carbon dioxide. However, no consistent drivers could be identified across sites. Our findings highlight widespread day-night changes in fluvial carbon dioxide fluxes and suggest that the time of day greatly influences measured carbon dioxide fluxes across European streams. Diel patterns can greatly impact total stream carbon dioxide emissions, with 39% greater carbon dioxide flux during the night-time relative to the day-time, according to a study of 34 streams across Europe.
... 42 However, future researchers may want to investigate the effects of agricultural land use on DOM as a driver of Hg patterns in stream and riparian food webs further since DOM may better characterize terrestrial Hg contributions from intensive human land use. 66 Evidence of increased Hg bioaccumulation in riparian food webs with increased agricultural land use is equivocal. Nutrient inputs leading to enhanced primary productivity can lead to bloom dilution resulting in lower Hg bioaccumulation, as shown in experimental streams. ...
Aquatic-to-terrestrial subsidies have the potential to provide riparian consumers with benefits in terms of physiologically important organic compounds like omega-3 long-chain polyunsaturated fatty acids (n-3 LCPUFAs). However, they also have a "dark side" in the form of exposure to toxicants such as mercury. Human land use intensity may also determine whether subsidies provide benefits or come at a cost for riparian predators. We sampled insects as well as Eastern Phoebe (Sayornis phoebe) chicks in 2015-2016 within the southern Finger Lakes region to understand how food quality, in terms of n-3 LCPUFAs and methylmercury (MeHg), of emergent freshwater insects compared with that of terrestrial insects and how land use affected the quality of prey, predator diet composition, and MeHg exposure. Across the landscape, freshwater insects had a significantly higher percentage of the n-3 LCPUFA eicosapentaenoic acid (EPA) compared to terrestrial insects and contained significantly more MeHg than terrestrial insects did. In spite of differences in MeHg concentrations between aquatic and terrestrial insects, chick MeHg concentrations were not related to diet composition. Instead, chick MeHg concentrations increased with several metrics of human land use intensity, including percent agriculture. Our findings suggest that freshwater subsidies provide predators with both risks and benefits, but that predator MeHg exposure can vary with human land use intensity.
... In the highly colored streams, diel CO 2 patterns can additionally be influenced by DOC shading diminishing benthic primary production 36 . In October, we measured DOC concentrations in a subset of the investigated streams for another study 37 where an agricultural stream in Sweden and peatland-dominated streams in Great Britain had high DOC concentrations (>10 mg L −1 ) whereas the median DOC was much lower with 2.6 mg L −1 37 . Due to the limited data, we could not test the effect of DOC on pCO 2 changes and we can neither confirm nor exclude that photomineralization might play a role for diel pCO 2 and consequently CO 2 flux variability in the studied streams. ...
... Lakes in NE are located at high latitudes, under low-temperature conditions, and some are barrier or dammed lakes, which caused the higher MeHg concentrations and MeHg:THg ratios. Flooding associated with reservoirs causes a long-term increase in MeHg production in many rivers as a result of the decomposing of terrestrial organic matter released from the reservoir, which leads to increased methylating bacteria activity and net MeHg production in flooded soil (Bravo et al., 2018). MX is characterized also by a cold climate and is much drier than NE. ...
Lake sediments are key materials for mercury deposition and methylation. To understand the mercury concentrations in China's lakes, 100 lake surface sediment samples were collected from 35 lakes in 2014. Total mercury (THg), methylmercury (MeHg) concentrations and the annual Hg burial rates in lake sediments were measured. THg and MeHg concentrations in the sediment ranged from 13.6 to 1488 ng‧g⁻¹ and 0.05 to 1.70 ng‧g⁻¹, respectively, and urban lakes reported most high values, indicating direct anthropogenic inputs. The Inner Mongolia-Xinjiang Region (MX) and Qinghai-Tibet Plateau Region (QT) reported relatively lower mercury burial rates, while the Eastern Plain Region (EP), Northeast Mountain and Plain Region (NE), and Yunnan-Guizhou Plateau Region (YG) reported higher mercury burial rates. Regional variances of THg burial fluxes were dominated by atmospheric deposition, terrestrial input, and sediment accumulation rates in different lakes. In 2014, the estimated average THg burial rate in China's lakes was 139 μg‧m⁻²‧yr⁻¹, comparable to the average in mid-latitude North America in recent years; however, due to China's much smaller lake area relative to NA, the annual THg burial flux in China was much lower than that in North America. EP and NE, where most freshwater aquatic products in China are harvested, accounted for 58.2% and 22.9%, respectively, of the THg burial flux. High sedimentary MeHg concentrations and MeHg:THg ratios were reported in most of the NE but low MeHg concentrations and MeHg:THg ratios were reported in EP. MeHg concentrations and MeHg:THg ratios were positively correlated with water COD levels and negatively correlated with average temperature. The results of this study indicate that in addition to the adjacent seas, lake sediments are an important mercury sink in China's aquatic environment, which could cause health risks due to MeHg intake, especially in NE.
... Previous meta-analyses in upland freshwater ecosystems, particularly, lakes, streams, and rivers, have indicated a wide array of factors influencing Hg dynamics and fate (Chen et al., 2005;Eagles-Smith et al., 2016a;Lavoie et al., 2019;Lavoie et al., 2013;Wu et al., 2019). Multiple studies in freshwater ecosystems have indicated relationships (both positive and negative) between organic matter, mercury loading, methylation, and biomagnification (Bravo et al., 2018;Lavoie et al., 2013). Land use has also been indicated as a factor influencing Hg bioaccumulation, with loons (Gavia immer) showing increased blood Hg in lakes surrounded by shrubland or wetlands and decreased in agricultural systems (Kramar et al., 2005) and with walleye (Sander vitreus) best predicted by habitat and watershed features rather than water chemistry (Hayer et al., 2011). ...
Biogeochemical conditions and landscape can have strong influences on mercury bioaccumulation in fish, but these effects across regional scales and between sites with and without point sources of contamination are not well understood. Normal means clustering, a type of unsupervised machine learning, was used to analyze relationships between forage fish (Fundulus heteroclitus and Menidia menidia) mercury (Hg) concentrations and sediment and water column Hg and methylmercury (MeHg) concentrations, ancillary variables, and land classifications within the sub-watershed. The analysis utilized data from 38 sites in 8 estuarine systems in the Northeast US, collected over five years. A large range of mercury concentrations and land use proportions were observed across sites. The cluster correlations indicated that for Fundulus, benthic and pelagic Hg and MeHg concentrations were most related to tissue concentrations, while Menidia Hg was most related to water column MeHg, reflecting differing feeding modes between the species. For both species, dissolved MeHg was most related to tissue concentrations, with sediment Hg concentrations influential at contaminated sites. The models considering only uncontaminated sites showed reduced influence of bulk sediment MeHg for both species, but Fundulus retained sediment drivers at some sites, with dissolved MeHg still highly correlated for both. Dissolved organic carbon (DOC), chlorophyll, land use, and other ancillary variables were of lesser importance in driving bioaccumulation, though DOC was strongly related within some clusters, likely in relation to dissolved Hg. Land use, though not of primary importance, showed relationships opposite to those observed in freshwater, with development positively correlated and forests and agriculture negatively correlated with tissue concentrations across clusters and species. Clusters were composed of sites from geographically distinct systems, indicating the greater importance of small scale drivers of MeHg formation and uptake into the food web over system or region-wide influences.
... The extent of how DOM is modified is still not well understood. The characterization of DOM has received increasing attention in the last few years in many fields of environmental sciences as DOM participates in a plethora of relevant processes such as the transportation of contaminants (Chiou et al., 1986;Chowdhury et al., 2020;Leuther et al., 2020), the microbial metabolism and its composition (McCarren et al., 2010;Forsström et al., 2013), the complexation and speciation of metal cations (Bravo et al., 2018;Kikuchi et al., 2017), the binding and aggregation of nanoparticles (Chen and Elimelech, 2007;Jiménez-Lamana and Slaveykova, 2016) and microplastics (Chen et al., 2018), the scavenging of radical species (Romera-Castillo and Jaffe, 2015), multiple photochemical reactions (Benner and Kaiser, 2011) and biogeochemical cycles (Ksionzek et al., 2016;Lusk and Toor, 2016;Porcal and Kopáček, 2018), all of which play relevant roles on aquatic ecosystems and their equilibrium. Additionally, water reclamation processes introduce anthropogenic chemicals into the water reuse cycle, some of which pose potential risks for the environment and the human health (Etchepare and van der Hoek, 2015;López-Serna et al., 2012;Siegrist and Joss, 2012). ...
High resolution mass spectrometry (HRMS) was used to investigate the dissolved organic matter (DOM) profile of a reclamation water trial performed in the Llobregat River (Spain) during summer 2019. 23 water samples (including tertiary effluents, surface river and drinking water), taken during five sampling campaigns, were analysed and their van Krevelen diagrams were compared. The reclaimed water fingerprint was substantially different from the natural profile of the river, showing a higher number of heteroatomic signals (i.e. CHON, CHOS and CHONS) and the presence of high-intensity S-containing features. As a result, reclaimed water discharge introduced substantial changes in the signature of the lignin-like and soot-like compositional-spaces of the river DOM fingerprint. However, the effect on the drinking water fingerprint was, ultimately, very limited. Only a limited number of features (up to 34) were detected as exclusively emitted with the reclaimed water.
During the second phase of the trial, the tertiary effluent was chlorinated for disinfection purposes. This process triggered the unexpected formation of a myriad of new features along the Llobregat River. Notably, 109 brominated/chlorinated features were detected, probably generated as a consequence of the photochemical decay of the emitted chloramines and their free-radical reaction with DOM, and three of them persisted in the final drinking water. The formation of halogenated species in situ in the Llobregat River entails uncertainty at ecological and water treatment levels and should be studied carefully to fully disclose the risks associated to wastewater effluent disinfection.
... The estimated maximum MeHg uptake potential may be over or underestimated compared to environmental conditions. The experiment does not take into account natural DOM, free thiol groups, periphyton matrix, invertebrates and bacteria that may compete for MeHg absorption (Leclerc et al., 2015;Pickhardt and Fisher, 2007;Wang et al., 2018) or changes in MeHg availability for algae and the periphyton itself (Bravo et al., 2018;Skrobonja et al., 2019). Consequently, it must be emphasized that such an estimate is just a potential, and caution is needed for environmental extrapolations. ...
Anthropogenic pressure in the high altitude lakes such as Titicaca and Uru (Bolivia) may favor the production of methylmercury (MeHg) known to accumulate in trophic chains. Periphyton associated with emerged aquatic plants (totoras) from the lake shores accumulates and demethylates MeHg providing a potential cost-effective water treatment technique. In this laboratory study, we measured the MeHg uptake kinetics of a consortium of green algae isolated from Lake Titicaca totora's periphyton. The most abundant algal consortium, composed of Oedogonium spp., Chlorella spp., Scenedesmus spp., was exposed to rising MeHg concentrations (from 5 to 200 ng⋅L − 1) to assess their maximum potential capacity for MeHg accumulation. Various algal biomass concentrations were tested to choose the optimal one. Results provided a net MeHg uptake rate by this algal consortium of 2.38 amol ng − 1 ⋅h − 1 ⋅nM − 1 (the total uptake was 2863 ng MeHg⋅g − 1) for an initial concentration of 200 ng MeHg⋅L − 1 with an algal biomass concentration of 0.02 g⋅L − 1. This initial MeHg concentration is 1000 times higher than the one measured in the eutrophic Cohana Bay of Lake Titicaca, which shows the high accumulation potential of these green algae. Our data suggest that periphyton has a high potential for the treatment of Hg contaminated waters in constructing wetlands in the Andean Altiplano.
... Positive relationships between FTHg and FMeHg with DOC have been observed in numerous fluvial Hg studies (Balogh et al. 2008;Brigham et al. 2009;Schuster et al. 2011;Tsui and Finlay 2011;Bravo et al. 2018;Lavoie et al. 2019) because DOM is presumed to be the dominant carrier of inorganic Hg and MeHg in surface water through complexation with thiol groups (Ravichandran 2004). ...
Torrential rain and extreme flooding caused by Atlantic hurricanes mobilize a large pool of organic matter (OM) from coastal forested watersheds in the southeastern United States. However, the mobilization of toxic metals such as mercury (Hg) that are associated with this vast pool of OM are rarely measured. This study aims to assess the variations of total Hg (THg) and methylmercury (MeHg) levels and the isotopic compositions of Hg in a blackwater river (Waccamaw River, SC, U.S.A.) during two recent extreme flooding events induced by Hurricane Joaquin (October 2015) and Hurricane Matthew (October 2016). We show that extreme flooding considerably increased filtered THg and MeHg concentrations associated with aromatic dissolved organic matter. During a 2‐month sampling window each year (October–November), we estimate that about 27% (2015) and 78% (2016) of the average amount of Hg deposited atmospherically during these 2 months was exported via the river. The isotopic composition of Hg in the river waters was changed only slightly by the substantial inputs of runoff from surrounding landscapes, in which mass‐dependent fractionation (as δ202Hg) decreased from −1.47 to −1.67‰ and mass‐independent fractionation (as ∆199Hg) decreased from −0.15 to −0.37‰. The slight variations in Hg isotopic composition during such extreme flooding events imply that sources of Hg in the river are nearly unchanged even under the very high wet deposition of Hg derived from the intensive rainfall. The majority of Hg exported by the river (74–85%) is estimated to have been derived from dry deposition to the watersheds. An increase in frequency and intensity of Atlantic hurricanes is expected in the next few decades due to further warming of ocean surface waters. We predict that increased hurricanes will mobilize more dry‐deposited Hg and in situ produced MeHg from these coastal watersheds where MeHg can be extensively bioaccumulated and biomagnified in the downstream aquatic food webs.
... However, as DOC increased, absorbance at 420 nm remained unchanged, suggesting that Hg responded to the amount of humic matter rather than bulk DOM. In a cross-European study, Bravo et al. (2018) showed that stream THg was terrestriallyderived based on the high fluorescence index of stream DOM. However, Bravo et al. (2017) found that terrestrial DOM was less likely than other DOM fractions to promote MeHg formation in lakes. ...
This review documents recent advances in terrestrial mercury cycling. Terrestrial mercury (Hg) research has matured in some areas, and is developing rapidly in others. We summarize the state of the science circa 2010 as a starting point, and then present the advances during the last decade in three areas: land use, sulfate deposition, and climate change. The advances are presented in the framework of three Hg “gateways” to the terrestrial environment: inputs from the atmosphere, uptake in food, and runoff with surface water. Among the most notable advances:
• The Arctic has emerged as a hotbed of Hg cycling, with high stream fluxes and large stores of Hg poised for release from permafrost with rapid high-latitude warming.
• The bi-directional exchange of Hg between the atmosphere and terrestrial surfaces is better understood, thanks largely to interpretation from Hg isotopes; the latest estimates place land surface Hg re-emission lower than previously thought.
• Artisanal gold mining is now thought responsible for over half the global stream flux of Hg.
• There is evidence that decreasing inputs of Hg to ecosystems may bring recovery sooner than expected, despite large ecosystem stores of legacy Hg.
• Freshly deposited Hg is more likely than stored Hg to methylate and be incorporated in rice.
• Topography and hydrological connectivity have emerged as master variables for explaining the disparate response of THg and MeHg to forest harvest and other land disturbance.
These and other advances reported here are of value in evaluating the effectiveness of the Minamata Convention on reducing environmental Hg exposure to humans and wildlife.
Water bodies around the world are currently warming with unprecedented rates since observations started, but warming occurs highly variable among ecoregions. So far, mountain rivers were expected to experience attenuated warming due to cold water input from snow or ice. However, air temperatures in mountain areas are increasing faster than the global average, and therefore warming effects are expected for cold riverine ecosystems.
In decomposing multi-decadal water temperature data of two Central European mountain rivers with different discharge and water source regime this work identified so far unreported a) long-term warming trends (with river-size dependent rates between +0.24 and +0.44 °C decade-1), but also b) seasonal shifts with both rivers warming not only during summer, but also in winter months (i.e., up to +0.52 °C decade in November), c) significantly increasing minimum and maximum temperatures (e.g., temperatures in a larger river no longer reach freezing point since 1996 and maximum temperatures increased at rates between 0.4 and 0.7°C decade-1) and d) an expanding of warm-water periods during recent decades in these ecosystems.
Our results show a substantial warming effect of mountain rivers with significant month-specific warming rates not only during summer but also in winter, suggesting that mountain river phenology continues to change with ongoing atmospheric warming. Further, this work demonstrates that apart from a general warming, also seasonal shifts, changes in extreme temperatures and expanding warm periods will play a role for ecological components of mountain rivers and should be considered in climate change assessments and mitigation management.
Increased concentration of mercury, particularly methylmercury, in the environment is a worldwide concern because of its toxicity in severely exposed humans. Although the formation of methylmercury in oxic water columns has been previously suggested, there is no evidence of the presence of microorganisms able to perform this process, using the hgcAB gene pair (hgc⁺ microorganisms), in such environments. Here we show the prevalence of hgc⁺ microorganisms in sinking particles of the oxic water column of Lake Geneva (Switzerland and France) and its anoxic bottom sediments. Compared to anoxic sediments, sinking particles found in oxic waters exhibited relatively high proportion of hgc⁺ genes taxonomically assigned to Firmicutes. In contrast hgc⁺ members from Nitrospirae, Chloroflexota and PVC superphylum were prevalent in anoxic sediment while hgc⁺ Desulfobacterota were found in both environments. Altogether, the description of the diversity of putative mercury methylators in the oxic water column expand our understanding on MeHg formation in aquatic environments and at a global scale.
The molecular composition of dissolved organic matter (DOM) is increasingly recognized as fundamentally important to mercury transport and transformations, with numerous approaches undertaken to examine DOM characteristics beyond dissolved organic carbon concentrations. In this study, we use a high-resolution mass spectrometry approach, Fourier-transform ion cyclotron resonance mass spectrometry, to characterize DOM compound classes, DOM aromaticity (AImod), and the nominal oxygenation state of carbon (NOSC) across thirteen small boreal forest streams in central Canada. We then relate the relative abundance of hundreds of different DOM molecules with inorganic mercury and methylmercury (MeHg) concentrations across late spring and fall seasons. The number of significant correlations and the classes of DOM compounds significantly correlating with inorganic mercury and MeHg concentrations differs substantially across seasons and between mercury forms. For inorganic mercury, the abundance of nitrogen and sulfur containing DOM are most often positively correlated (mean ρ = 0.80) in the late spring, whereas during the fall, the abundance of low-oxidized lignins is more important, though with weaker correlations (mean ρ = 0.51). For MeHg, low-oxidized lignins and hydrolysable tannins, likely sourced from conifer throughfall and litter, account for up to 83% of all DOM-MeHg correlations regardless of season. Further network analyses reveal that the strongest and most significant inorganic mercury-DOM correlations are found across a wide range of NOSC values, indicating that DOM involved with the transport of inorganic mercury encompasses a wide range of polarities and thermodynamic stabilities. In contrast, DOM molecules exclusively correlated with MeHg concentrations have more positive NOSC and AImod values, implying the preferential transport of MeHg with more thermodynamically stable and aromatic DOM molecules. DOM molecules significantly correlated with both inorganic mercury and MeHg concentrations are found exclusively in the late spring. Overall, this non-targeted approach may help to inform further targeted investigations, especially as it relates to the underrepresented importance of plant biomolecules in facilitating mercury transport.
Graphical abstract
Aqueous transport of mercury (Hg) across the landscape is closely linked to dissolved organic matter (DOM). Both quantity and quality of DOM affect Hg mobility, as well as the formation and transport of toxic methylmercury (MeHg), but only a limited number of field studies have investigated Hg and MeHg with respect to specific DOM components. We investigated these interactions at the 41‐ha forested W‐9 catchment at Sleepers River, Vermont, which has a long history of mercury and other biogeochemical research. We examined spatial and temporal patterns of filtered Hg fractions and dissolved organic carbon (DOC) concentration, DOM quality, and major solutes at 12 stream sites within W‐9 and the downstream W‐3 gage (837 ha) over five sampling campaigns including a large (79 mm) fall storm, spring snowmelt, and three seasonally contrasting base flow periods. Filtered total Hg (THg), MeHg, and DOC concentrations increased in order base flow < snowmelt < fall storm, except that MeHg remained at baseflow levels during snowmelt. Ranges of median concentrations across sites for the five campaigns were: THg, <0.2 to 4.1 ng L‐1; MeHg, <0.03 to 0.45 ng L‐1; and DOC, 0.8 to 14.0 mg L‐1. Humic‐like DOM fluorescence components, as determined by Parallel Factor Analysis (PARAFAC), dominated the fluorescence across sites and sampling campaigns. THg correlated strongly (r > 0.94) with these humic components, but even more strongly with bulk DOC and absorbance at 254 nm (UV254) (r > 0.96), and less strongly with protein‐like DOM (0.7 < r <0.9). MeHg correlated in the same order but less strongly with humic‐ (0.8 <r < 0.9) and protein‐like (0.6 < r < 0.8) DOM. MeHg increased in summer, potentially in response to enhanced microbial production in warmer periods. MeHg formation may have been linked to protein‐like DOM, but its transport was linked to humic‐like DOM. This article is protected by copyright. All rights reserved.
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 hgcA genes and MeHg across redox stratified water columns in the brackish Baltic Sea. We found that the abundance of hgcA genes and transcripts combined with the concentration of dissolved HgII-sulfide species were strong predictors of both HgII methylation rate and MeHg concentration, implying their roles as principal joint drivers of MeHg formation in these systems. In establishing relationships between hgcA genes and MeHg, 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.
Dissolved organic matter (DOM) is recognized for its importance in freshwater ecosystems, but historical reliance on DOM quantity rather than indicators of DOM composition has led to an incomplete understanding of DOM and an underestimation of its role and importance in biogeochemical processes. A single sample of DOM can be composed of tens of thousands of distinct molecules. Each of these unique DOM molecules has their own chemical properties and reactivity or role in the environment. Human activities can modify DOM composition and recent research has uncovered distinct DOM pools laced with human markers and footprints. Here we review how land use change, climate change, nutrient pollution, browning, wildfires, and dams can change DOM composition which in turn will affect internal processing of freshwater DOM. We then describe how human-modified DOM can affect biogeochemical processes. Drought, wildfires, cultivated land use, eutrophication, climate change driven permafrost thaw, and other human stressors can shift the composition of DOM in freshwater ecosystems increasing the relative contribution of microbial-like and aliphatic components. In contrast, increases in precipitation may shift DOM towards more relatively humic-rich, allochthonous forms of DOM. These shifts in DOM pools will likely have highly contrasting effects on carbon outgassing and burial, nutrient cycles, ecosystem metabolism, metal toxicity, and the treatments needed to produce clean drinking water. A deeper understanding of the links between the chemical properties of DOM and biogeochemical dynamics can help to address important future environmental issues, such as the transfer of organic contaminants through food webs, alterations to nitrogen cycling, impacts on drinking water quality, and biogeochemical effects of global climate change.
The monomethylmercury (MMHg) concentrations, water quality parameters (e.g., pH, suspended particles, total phosphorus, sulfate, and chlorophyll-a), and compositions of dissolved organic matter (DOM) were analyzed to understand how the quality of DOM is related to the MMHg distributions in the surface waters of 14 reservoirs. The excitation-emission matrix (EEM) fluorescence spectroscopy identified six fluorescence peaks, and a parallel factor analysis (PARAFAC) of EEM spectra identified three components of DOM: microbial humic-like (C1), terrestrial humic-like (C2), and protein-like DOM (C3). Using the observation data, the principal component analysis (PCA) were carried out to understand the relative importance of the fluorescence properties of DOM, representing DOM quality, on the MMHg distribution. The loading plot of PCA showed a strong positive correlation between the MMHg and protein regions of the EEM spectra and no correlation between MMHg and the terrestrial humic regions of the EEM, suggesting that autochthonous DOM production is a key factor in increasing MMHg concentration in reservoir water. The preliminary mass flux estimation, which was carried out to identify the major sources of MMHg in Okjeong reservoir, revealed that the major sources are sediment diffusion and water column methylation. Because the studied reservoirs are located remotely from a large-sized river and industrial region, most MMHg in reservoir water is likely diffused from the surface sediment or produced in the water column, and these sources tend to increase in reservoirs enriched with autochthonous DOM. It is suggested that EEM fluorescence can improve our ability to trace the major sources of MMHg in diverse reservoirs.
Levels of neurotoxic methylmercury (MeHg) in phytoplankton are strongly associated with water MeHg concentrations. Because uptake by phytoplankton is the first and largest step of bioaccumulation in aquatic food webs many studies have investigated factors driving seasonal changes in water MeHg concentrations. Organic matter (OM) is widely accepted as an important driver of MeHg production and uptake by phytoplankton but is also known for strong interannual variability in concentration and composition within systems. In this study, we explore the role of OM on spatial and interannual variability of MeHg in a subarctic coastal sea, the northern Baltic Sea. Using MeHg (2014: 80 ± 25 fM; 2015: <LOD; 2016: 21 ± 9 fM) and OM measurements during late summer/early fall, we find that dissolved organic carbon (DOC) and humic matter content explain 60% of MeHg variability. We find that while labile DOC increases MeHg levels in the water, humic content reduces it. We propose that the positive association between MeHg and labile DOC shows that labile DOC is a proxy for OM remineralization rate in nearshore and offshore waters. This is consistent with other studies finding that in situ MeHg production in the water column occurs during OM remineralization. The negative association between water humic content and MeHg concentration is most likely due to humic matter decreasing inorganic mercury (HgII) bioavailability to methylating microbes. With these relationships, we develop a statistical model and use it to calculate MeHg concentrations in late summer nearshore and offshore waters between 2006 and 2016 using measured values for water DOC and humic matter content. We find that MeHg concentrations can vary by up to an order of magnitude between years, highlighting the importance of considering interannual variability in water column MeHg concentrations when interpreting both short and long term MeHg trends in biota.
The boreal ecoregion supports about one-third of the world’s forest. Over 90% of boreal forest streams are found in headwaters, where terrestrial–aquatic interfaces are dominated by organic matter (OM)-rich riparian zones (RZs). Because these transition zones are key features controlling catchment biogeochemistry, appropriate RZ conceptualizations are needed to sustainably manage surface water quality in the face of a changing climate and increased demands for forest biomass. Here we present a simple, yet comprehensive, conceptualization of RZ function based on hydrological connectivity, biogeochemical processes, and spatial heterogeneity. We consider four dimensions of hydrological connectivity: (1) laterally along hillslopes, (2) longitudinally along the stream, (3) vertically down the riparian profile, and (4) temporally through event-based and seasonal changes in hydrology. Of particular importance is the vertical dimension, characterized by a ‘Dominant Source Layer’ that has the highest contribution to solute and water fluxes to streams. In addition to serving as the primary source of OM to boreal streams, RZs shape water chemistry through two sets of OM-dependent biogeochemical processes: (1) transport and retention of OM-associated material and (2) redox-mediated transformations controlled by RZ water residence time and availability of labile OM. These processes can lead to both retention and release of pollutants. Variations in width, hydrological connectivity, and OM storage drive spatial heterogeneity in RZ biogeochemical function. This conceptualization provides a useful theoretical framework for environmental scientists and ecologically sustainable and economically effective forest management in the boreal region and elsewhere, where forest headwaters are dominated by low-gradient, OM-rich RZs.
A detailed understanding of the formation of the potent neurotoxic methylmercury is needed to explain the large observed variability in methylmercury levels in aquatic systems. While it is known that organic matter interacts strongly with mercury, the role of organic matter composition in the formation of methylmercury in aquatic systems remains poorly understood. Here we show that phytoplankton-derived organic compounds enhance mercury methylation rates in boreal lake sediments through an overall increase of bacterial activity. Accordingly, in situ mercury methylation defines methylmercury levels in lake sediments strongly influenced by planktonic blooms. In contrast, sediments dominated by terrigenous organic matter inputs have far lower methylation rates but higher concentrations of methylmercury, suggesting that methylmercury was formed in the catchment and imported into lakes. Our findings demonstrate that the origin and molecular composition of organic matter are critical parameters to understand and predict methylmercury formation and accumulation in boreal lake sediments.
Aquatic ecosystems are an essential component of the biogeochemical cycle of mercury (Hg), as inorganic Hg can be converted to toxic methylmercury (MeHg) in these environments and reemissions of elemental Hg rival anthropogenic Hg releases on a global scale. Quantification of effluent Hg releases to aquatic systems globally has focused on discharges to the global oceans, rather than contributions to freshwater systems that affect local exposures and risks associated with MeHg. Here we produce a first-estimate of sector-specific, spatially resolved global aquatic Hg discharges to freshwater systems. We compare our release estimates to atmospheric sources that have been quantified elsewhere. By analyzing available quantitative and qualitative information, we estimate that present-day global Hg releases to freshwater environments (rivers and lakes) associated with anthropogenic activities have a lower bound of ~1000 Mg·a⁻¹. Artisanal and small-scale gold mining (ASGM) represents the single largest source, followed by disposal of mercury-containing products and domestic waste water, metal production, and releases from industrial installations such as chlor-alkali plants and oil refineries. In addition to these direct anthropogenic inputs, diffuse inputs from land management activities and remobilization of Hg previously accumulated in terrestrial ecosystems are likely comparable in magnitude. Aquatic discharges of Hg are greatly understudied and further constraining associated data gaps is crucial for reducing the uncertainties in the global biogeochemical Hg budget.
Significance
Estuaries are the predominant hunting and fishing territory for northern indigenous populations whose way of life is threatened by both climate change and industrial development. Direct measurements and modeling conducted as part of this study show enhanced production of methylmercury, a potent neurotoxin, and uptake by plankton in stratified oxic seawater. Enhanced climate-driven stratification of ocean margin areas with sea-ice melt will likely elevate biological methylmercury concentrations in the Arctic. Elevated biological methylmercury levels will be exacerbated by hydroelectric development planned throughout many northern regions. Our experimental measurements indicate that, over the next decade, regional increases in methylmercury concentrations resulting from flooding associated with hydroelectric development will be greater than those expected from climate change.
Effects of distribution of natural organic matter (NOM) on formation and distribution of trihalomethanes (THMs) in municipal water were investigated. Water samples were fractionated using serial ultrafiltration with membranes of molecular weight cut-off (MWCO) values of 500, 1 000 and 3 000 Da. The resulting 4 fractions of water with NOM of (i) < 500 Da; (ii) 500 Da - 1 kDa; (iii) 1 kDa - 3 kDa; and (iv) > 3 kDa were separated. Variable amounts of bromide ion (0, 40, 80, 120 and 200 mu g/l were added to these samples. The samples were chlorinated at pH of 6 and 8.5 and held at 20 degrees C for various reaction periods (3, 8, 28, 48 and 96 h). The results demonstrate that the higher molecular weight NOM is strongly correlated with UV254 and specific ultraviolet absorbance (SUVA), while the lower molecular weight NOM is weakly correlated with UV254 and SUVA. Increase in bromide ion concentration increases total THM formation. Fractions of brominated THMs decrease with increasing NOM molecular size. Lower molecular weight NOM forms more brominated THMs than the corresponding higher molecular weight NOM. Increase of bromide to chlorine ratio decreases chloroform and increases brominated THMs. Increase in pH increases chloroform and decreases brominated THMs. This study demonstrates that the distribution of NOM and bromide ion can have important role on the distribution of THMs in drinking water.
Methylmercury bioaccumulates in aquatic food chains and is able to cross
the blood-brain barrier, making this organometallic compound a much more
worrisome pollutant than inorganic mercury. We know that methylation of
inorganic mercury is carried out by microbes in the anoxic layers of
sediments and water columns, but the factors that control the extent of
this methylation are poorly known. Mercury methylation is generally
thought to be catalysed accidentally by some methylating enzyme, and it
has been suggested that cellular mercury uptake results from passive
diffusion of neutral mercury complexes. Here, we show that mercury
methylation by the bacterium Geobacter sulfurreducens is greatly
enhanced in the presence of low concentrations of the amino acid
cysteine. The formation of a mercury-cysteine complex promotes both the
uptake of inorganic mercury by the bacteria and the enzymatic formation
of methylmercury, which is subsequently released to the external medium.
Our results suggest that mercury uptake and methylation by microbes are
controlled more tightly by biological mechanisms than previously
thought, and that the formation of specific mercury complexes in anoxic
waters modulates the efficiency of the microbial methylation of mercury.
Microbial mercury methylation transforms a toxic trace metal into the highly bioaccumulated neurotoxin methylmercury (MeHg). The lack of a genetic marker for microbial MeHg production has prevented a clear understanding of the distribution of Hg-methylating organisms in nature. Recently, a specific gene cluster (hgcAB) was recently linked to mercury methylation in two bacteria1. Here we test whether the presence of hgcAB orthologs is a reliable predictor of Hg methylation capability in microorganisms, a necessary confirmation for the development of molecular probes for Hg-methylation in nature. Although orthologs of hgcAB are rare among the several thousand available microbial genomes, the encoding organisms are much more phylogenetically and environmentally diverse than previously thought. By directly measuring MeHg production in several bacterial and archaeal strains encoding hgcAB we confirmed that the presence of hgcAB predicts Hg methylation capability. For the first time, we demonstrated Hg methylation in species other than sulfate- (SRB) and iron- (FeRB) reducing bacteria, including methanogens, and syntrophic, acetogenic and fermentative Firmicutes. Several of these species occupy novel environmental niches for Hg methylation, including methanogenic habitats such as rice paddies, the animal gut, and extremes of pH and salinity. Identification of these organisms as Hg methylators now links methylation to discrete gene markers in microbial communities.
This review focuses on mercury (Hg) inputs and outputs in temperate and boreal terrestrial systems. It covers deposition via throughfall and litterfall, whose sum (ca. 38 μg m-2 a-1) is greater than that via precipitation (ca. 10 μg m-2 a-1). Outputs considered include volatilization, soil sequestration, and streamflow. The former is highly uncertain, but the mean rate (11 ng m-2 h-1) over a growing season is equivalent to about 32 μg m-2 a-1. Modern rates of soil sequestration (ca. 5 μg m-2 a-1) and streamflow fluxes (ca. 2 μg m-2 a-1) balance the annual budget. The majority of the uncertainty in the budget is related to volatilization. Nonetheless, a large fraction of atmospheric Hg is likely a product of continuing deposition and volatilization. Watershed characteristics related to streamflow fluxes of both Hg and methylmercury (MeHg) are discussed. Both runoff concentration and flux of Hg are weakly and inversely related to watershed size. Dissolved organic carbon (DOC) and particulates are important carriers of Hg; watershed activities that affect either affect Hg flux. Runoff flux of MeHg is skewed with about 80% of observations less than 0.15 μg m-2 a-1. Although there is no pattern of MeHg flux with watershed size, there is a strong positive relationship between flux and wetland area. Wetlands are a site of MeHg production and their presence increases water residence time; both increase MeHg flux. Concentrations of MeHg in streamflow from watersheds with wetlands are near the current water quality criterion, and effective control measures in those watersheds appear problematic.
Dissolved organic matter (DOM) originating from the extensive Arctic tundra is an important source of organic material to the Arctic Ocean. Chemical characteristics of whole water dissolved organic matter (DOM) and the fulvic acid fraction of DOM were studied from nine surface waters in the Arctic region of Alaska to gain insight into the extent of microbial and photochemical transformation of this DOM. All the fulvic acids had a strong terrestrial/higher plant signature, with uniformly depleted δ13C values of −28‰, and low fluorescence indices around 1.3. Several of the measured chemical characteristics of the Arctic fulvic acids were related to water residence time, a measure of environmental exposure to sunlight and microbial activity. For example, fulvic acids from Arctic streams had higher aromatic contents, higher specific absorbance values, lower nitrogen content, lower amino acid–like fluorescence and were more depleted in δ15N relative to fulvic acids isolated from lake and coastal surface waters. The differences in the nitrogen signature between the lake and coastal fulvic acids compared to the stream fulvic acids indicated that microbial contributions to the fulvic acid pool increased with increasing water residence time. The photo-lability of the fulvic acids was positively correlated with water residence time, suggesting that the fulvic acids isolated from source waters with larger water residence times (i.e., lakes and coastal waters) have experienced greater photochemical degradation than the stream fulvic acids. In addition, many of the initial differences in fulvic acid chemical characteristics across the gradient of water residence times were consistent with changes observed in fulvic acid photolysis experiments. Taken together, results from this study suggest that photochemical processes predominantly control the chemical character of fulvic acids in Arctic surface waters. Our findings show that hydrologic transport in addition to biogeochemical alteration of the organic matter must be considered in order to predict the ultimate fate of Arctic DOM.
Eastern North America receives elevated atmospheric mercury deposition from a combination of local, regional, and global sources.
Anthropogenic emissions originate largely from electric utilities, incinerators, and industrial processes. The mercury species
in these emissions have variable atmospheric residence times, which influence their atmospheric transport and deposition patterns.
Forested regions with a prevalence of wetlands and of unproductive surface waters promote high concentrations of mercury in
freshwater biota and thus are particularly sensitive to mercury deposition. Through fish consumption, humans and wildlife
are exposed to methylmercury, which markedly bioaccumulates up the freshwater food chain. Average mercury concentrations in
yellow perch fillets exceed the Environmental Protection Agency's human health criterion across the region, and mercury concentrations
are high enough in piscivorous wildlife to cause adverse behavioral, physiological, and reproductive effects. Initiatives
are under way to decrease mercury emissions from electric utilities in the United States by roughly 70%.
The biogeochemical processing of dissolved organic matter (DOM) in inland waters is inherently related to its molecular structure and ecological function. Controlled bioassays are a valuable tool to analyze these relationships, but are seldom conducted and compared at temporal scales that typically prevail in natural inland waters. Here we incubated water from six boreal lakes in the dark and examined changes to the initial fluorescence and absorbance after 3.5 years. We identified five fluorescence components with parallel factor (PARAFAC) analysis (C(C,) C(M), C(A), C(X) and C(T)) and found a consistent change in the relative intensity of two dominant PARAFAC components (increase in C(A):C(C), corresponding to Peak A:Peak C), commonly found in lake water, that represent terrestrially-derived DOM. Surprisingly, we only found minor changes to specific absorbance (SUVA), and did not find any changes to other spectral indexes including the fluorescence index, humification index and freshness index. By incorporating lakes spanning a wide range of initial total organic carbon concentrations (3.7 to 32.5 mg L(-1)), water residence times, and spectral characteristics (e.g. SUVA 1.13 to 3.77 L·mg C(-1)·m(-1)), we found that the relative intensities of two humic-like peaks were the most revealing of changes to DOM structure during dark incubations. We also verified that inner filter effects were adequately corrected within the concentration range of incubated samples. Thus, the processing of DOM under dark conditions, including microbial decomposition and flocculation, may have a greater influence on the humic-like peaks, particularly C(C) (Peak C), with negligible changes to more commonly used spectral indexes.
Because it is very toxic and accumulates in organisms, particularly in fish, mercury is an important pollutant and one of the most studied. Nonetheless we still have an incomplete understanding of the factors that control the bioconcentration of mercury. Elemental mercury is efficiently transported as a gas around the globe, and even remote areas show evidence of mercury pollution originating from industrial sources such as power plants. Besides elemental mercury, the major forms of mercury in water are ionic mercury (which is bound to chloride, sulfide, or organic acids) and organic mercury, particularly methylmercury. Methylmer- cury rather than inorganic mercury is bioconcentrated because it is better retained by organisms at various levels in the food chain. The key factor determining the concentration of mercury in the biota is the methylmercury concentration in water, which is controlled by the relative efficiency of the methylation and demethylation processes. Anoxic waters and sediments are an important source of methylmercury, apparently as the result of the methylating activity of sulfate- reducing bacteria. In surface waters, methylmercury may originate from anoxic
1] We develop an empirically constrained multicompartment box model for mercury cycling in open ocean regions to investigate changes in concentrations resulting from anthropogenic perturbations of the global mercury cycle. Using Monte Carlo simulations, we explicitly consider the effects of variability in measured parameters on modeled seawater concentrations. Our simulations show that anthropogenic enrichment in all surface (25%) and deep ocean waters (11%) is lower than global atmospheric enrichment (300–500%) and varies considerably among geographic regions, ranging from >60% in parts of the Atlantic and Mediterranean to <1% in the deep Pacific. Model results indicate that open ocean mercury concentrations do not rapidly equilibrate with atmospheric deposition and on average will increase if anthropogenic emissions remain at their present level. We estimate the temporal lag between changes in atmospheric deposition and ocean mercury concentrations will vary from decades in most of the Atlantic up to centuries in parts of the Pacific.
Dissolved organic matter (DOM) includes an array of carbon-based compounds that vary in size and structure and have complex interactions with mercury (Hg) cycling in aquatic systems. While many studies have examined the relationship between dissolved organic carbon concentrations ([DOC]) and methyl Hg bioaccumulation, few studies have considered the effects of DOM composition (e.g., protein-content, aromaticity). The goal of this study was to explore the relationships between total and methyl [Hg] in water, invertebrates, and fish, and optically-derived measures of DOM composition from 47 lake and river sites across a boreal watershed. Results showed higher aqueous total [Hg] in systems with more aromatic DOM and higher [DOC], potentially due to enhanced transport from upstream or riparian areas. Methyl [Hg] in biota were all positively related to the amount of microbial-based DOM and, in some cases, to the proportions of labile and protein-like DOM. These results suggest that increased Hg bioaccumulation is related to the availability of labile DOM, potentially due to enhanced methylation. DOM composition explained 68% and 54% more variability in [Hg] in surface waters and large-bodied fish, respectively, than [DOC] alone. These results show that optical measures of DOM characteristics are a valuable tool for understanding DOM-Hg biogeochemistry.
Suspended matter (SM) plays an important role in the transport of mercury (Hg) in aquatic systems. Information about Hg-species in this material is crucial to understand risk potential, especially for Hg methylation and bioavailability. In the Idrija Hg mine (Slovenia) cinnabar (α-HgS) was mined and processed for centuries. These activities caused contamination of the Idrijca river system by dumping of ore roasting residues, as well as atmospheric Hg deposition in soils attributed to excessive Hg emissions from the roasting plant. Cinnabar is the dominant Hg-species in the coarse-grained sediments of the Idrijca river where Hg methylation is generally low, whereas natural-organic-matter-bound Hg (NOM-bound) has caused intense Hg methylation in the Gulf of Trieste (GT), the final sink for Hg released from the Idrijca catchment. Hydrology of the Idrijca river is characterized by high discharge events during heavy rains and snowmelt, which transports large amounts of SM towards the GT. However, the dominant Hg-species transported in SM and their specific source under varying hydrological conditions is largely unknown, yet crucial to predict future transport of bioavailable Hg forms from the mining area to the GT.
As the methylation of inorganic divalent mercury (Hg(II)) to neurotoxic methylmercury (MeHg) has been attributed to the activity of anaerobic bacteria, the formation of MeHg in the oxic water column of marine ecosystems has puzzled scientists over the past years. Here we show for the first time that MeHg can be produced in particles sinking through oxygenated water column of lakes. Total mercury (THg) and MeHg concentrations were measured in settling particles and in surface sediments of the largest freshwater lake in Western Europe (Lake Geneva). Whilst THg concentration differences between sediments and settling particles were not significant, MeHg concentrations were up to ten-fold greater in settling particles. MeHg demethylation rate constants (kd) were of similar magnitude in both compartments. In contrast, Hg methylation rate constants (km) were one order of magnitude greater in settling particles. The net potential for MeHg formation, assessed by the ratio between the two rate constants (km kd(-1)), was therefore up to ten times higher in settling particles, denoting that in situ transformations likely contributed to the high MeHg concentrations found in settling particles. Hg methylation was inhibited (~80 %) in settling particles amended with molybdate, demonstrating the prominent role of biological sulfate-reduction in the process.
The forms of inorganic mercury (HgII) taken up and methylated by bacteria in sediments still remain largely unknown. From pure cultures studies, it has been suggested that dissolved organic matter (DOM) may facilitate the uptake either by acting as a shuttle molecule, transporting the HgII atom to divalent metal transporters, or by binding HgII and then being transported into the cell as a carbon source. Enhanced availability of Hg complexed to DOM has however not yet been demonstrated in natural systems. Here, we show that HgII complexed with DOM of marine origin was up to 2.7 times more available for methylation in sediments than HgII added as a dissolved inorganic complex (HgII(aq)). We argue that the DOM used to complex HgII directly facilitated the bacterial uptake of HgII whereas the inorganic dissolved HgII-complex adsorbed to the sediment matrix before forming bioavailable dissolved HgII complexes. We further demonstrate that differences in net methylation in sediments with high and low organic carbon content may be explained by differences in the availability of carbon to stimulate the activity of Hg methylating bacteria rather than, as previously proposed, be due to differences in HgII binding capacities between sediments.
There is an ongoing debate on the fate of mercury (Hg) in areas affected by artisanal and small-scale gold mining (ASGM). Over the last 30 years, ASGM has released 69 tons of Hg into the southeastern Peruvian Amazon. To investigate the role of suspended matter and hydrological factors on the fate of ASGM-Hg, we analysed riverbank sediments and suspended matter along the partially ASGM-affected Malinowski-Tambopata river system and examined Hg accumulation in fish. In addition, local impacts of atmospheric Hg emissions on aquatic systems were assessed by analysing a sediment core from an oxbow lake. Hg concentrations in riverbank sediments are lower (20–53 ng g⁻¹) than in suspended matter (∼400–4000 ng g⁻¹) due to differences in particle size. Elevated Hg concentrations in suspended matter from ASGM-affected river sections (∼1400 vs. ∼30–120 ng L⁻¹ in unaffected sections) are mainly driven by the increased amount of suspended matter rather than increased Hg concentrations in the suspended matter. The oxbow lake sediment record shows low Hg concentrations (64–86 ng g⁻¹) without evidence of any ASGM-related increase in atmospheric Hg input. Hg flux variations are mostly an effect of variations in sediment accumulation rates. Moreover, only 5% of the analysed fish (only piscivores) exceed WHO recommendations for human consumption (500 ng g⁻¹). Our findings show that ASGM-affected river sections in the Malinowski-Tambopata system do not exhibit increased Hg accumulation, indicating that the released Hg is either retained at the spill site or transported to areas farther away from the ASGM areas. We suspect that the fate of ASGM-Hg in such tropical rivers is mainly linked to transport associated with the suspended matter, especially during high water situations. We assume that our findings are typical for ASGM-affected areas in tropical regions and could explain why aquatic systems in such ASGM regions often show comparatively modest enrichment in Hg levels.
Annual stream loads of mercury (Hg) and inputs of wet and dry atmospheric Hg deposition to the landscape were investigated in watersheds of the Western United States and the Canadian-Alaskan Arctic. Mercury concentration and discharge data from flow gauging stations were used to compute annual mass loads with regression models. Measured wet and modeled dry deposition were compared to annual stream loads to compute ratios of Hg stream load to total Hg atmospheric deposition. Watershed land uses or cover included mining, undeveloped, urbanized, and mixed. Of 27 watersheds that were investigated, 15 had some degree of mining, either of Hg or precious metals (gold or silver), where Hg was used in the amalgamation process. Stream loads in excess of annual Hg atmospheric deposition (ratio>1) were observed in watersheds containing Hg mines and in relatively small and medium-sized watersheds with gold or silver mines, however, larger watersheds containing gold or silver mines, some of which also contain large dams that trap sediment, were sometimes associated with lower load ratios (<0.2). In the non-Arctic regions, watersheds with natural vegetation tended to have low ratios of stream load to Hg deposition (<0.1), whereas urbanized areas had higher ratios (0.34-1.0) because of impervious surfaces. This indicated that, in ecosystems with natural vegetation, Hg is retained in the soil and may be transported subsequently to streams as a result of erosion or in association with dissolved organic carbon. Arctic watersheds (Mackenzie and Yukon Rivers) had a relatively elevated ratio of stream load to atmospheric deposition (0.27 and 0.74), possibly because of melting glaciers or permafrost releasing previously stored Hg to the streams. Overall, our research highlights the important role of watershed characteristics in determining whether a landscape is a net source of Hg or a net sink of atmospheric Hg.
The aim of this work is to estimate background concentrations of organometallic compounds, such as tributyltin (TBT), dibutyltin (DBT), monobutyltin (MBT), triphenyltin (TPhT), diphenyltin (DPhT), monophenyltin (MPhT), methylmercury (MeHg), inorganic mercury (iHg) and diethyllead (Et2Pb) in the aquatic environment at the French national scale.
Sewage treatment plants (STPs) are important point sources of mercury (Hg) to the environment. STPs are also significant sources of iron when hydrated ferric oxide (HFO) is used as a dephosphatation agent during water purification. In this study, we combined geochemical and microbiological characterization with Hg speciation and sediment amendments to evaluate the impact of STP's effluents on monomethylmercury (MMHg) production. The highest in-situ Hg methylation was found close to the discharge pipe in subsurface sediments enriched with Hg, organic matter, and iron. There, ferruginous conditions were prevailing with high concentrations of dissolved Fe(2+) and virtually no free sulfide in the porewater. Sediment incubations demonstrated that the high MMHg production close to the discharge was controlled by low demethylation yields. Inhibition of dissimilatory sulfate reduction with molybdate led to increased iron reduction rates and Hg-methylation, suggesting that sulfate-reducing bacteria (SRB) may not have been the main Hg methylators under these conditions. However, Hg methylation in sediments amended with amorphous Fe(III)-oxides was only slightly higher than control conditions. Thus, in addition to iron-reducing bacteria, other non-SRB most likely contributed to Hg methylation. Overall, this study highlights that sediments impacted by STP discharges can become local hot-spots for Hg methylation due to the combined inputs of i) Hg, ii) organic matter, which fuels bacterial activities and iii) iron, which keeps porewater sulfide concentration low and hence Hg bioavailable.
Copyright © 2015 Elsevier Ltd. All rights reserved.
The complexity of mercury (Hg) biogeochemistry has made it difficult to model surface water concentrations of both total Hg (THg) and especially methylmercury (MeHg), the species of Hg having the highest potential for bioaccumulation. To simulate THg and MeHg variation in low-order streams we have adapted a conceptual modelling framework where a continuum of lateral flows through riparian soils determines streamflow concentrations. The model was applied to seven forest catchments located in two boreal regions in Sweden spanning a range of climatic, soil, and forest management conditions. Discharge, and simulated riparian soil water concentrations profiles, represented by two calibrated parameters, were able to explain much of the variability of THg and MeHg concentrations in the streams issuing from the catchments (Nash Sutcliffe (NS) up to 0.54 for THg and 0.58 for MeHg). Model performance for all catchments was improved (NS up to 0.76 for THg and 0.85 for MeHg) by adding two to four parameters to represent seasonality in riparian soil water THg and MeHg concentrations profiles. These results are consistent with the hypothesis that riparian flow-pathways and seasonality in riparian soil concentrations are the major controls on temporal variation in low-order stream Hg concentrations.
Methylmercury (MeHg) is the only species of mercury (Hg) to biomagnify in aquatic food-webs to levels that are a widespread concern for human and ecological health. Here we investigate the association between dissolved organic matter (DOM) in seawater and Hg speciation and uptake using experimental data and field measurements from Long Island Sound (LIS) and the Northwestern Atlantic continental margin (NWA). We measured differences in DOM composition across sampling stations using excitation emission matrix (EEM) fluorescence spectroscopy and further separated DOM into terrestrial and marine components using Parallel Factor Analysis (PARAFAC). Highest MeHg concentrations were found in the estuarine stations (LIS) with highest DOM concentrations due to enhanced external inputs from the watershed and rivers. For stations on the shelf and slope, MeHg in plankton increased linearly with a decreasing fraction of fluorescence attributable to DOM components with a terrestrial rather than marine origin. These results are corroborated by experimental data showing higher MeHg uptake by cells in the presence of predominantly marine DOM compared to terrestrial DOM. Highest fractions of dissolved gaseous mercury were also found at stations with the highest marine DOM content, suggesting a greater reducible fraction of divalent inorganic Hg. These data suggest DOM composition is a critical driver of Hg reactivity and bioavailability in offshore marine waters.
The biochemical composition of dissolved organic matter (DOM) strongly influences its biogeochemical role in freshwater ecosystems, yet DOM composition measurements are not routinely incorporated into ecological studies. To date, the majority of studies of freshwater ecosystems have relied on bulk analyses of dissolved organic carbon and nitrogen to obtain information about DOM cycling. The problem with this approach is that the biogeochemical significance of DOM can only partially be elucidated using bulk analyses alone because bulk measures cannot detect most carbon and nitrogen transformations. Advances in fluorescence spectroscopy provide an alternative to traditional approaches for characterizing aquatic DOM, and allow for the rapid and precise characterization of DOM necessary to more comprehensively trace DOM dynamics. It is within this context that we discuss the use of fluorescence spectroscopy to provide a novel approach to tackling a long-standing problem: understanding the dynamics and biogeochemical role of DOM. We highlight the utility of fluorescence characterization of DOM and provide examples of the potential range of applications for incorporating DOM fluorescence into ecological studies in the hope that this rapidly evolving technique will further our understanding of the biogeochemical role of DOM in freshwater ecosystems.
Neurotoxic methylmercury (MeHg) formed from inorganic divalent mercury (Hg(II)) accumulates in aquatic biota and remains at high levels worldwide. It is poorly understood to what extent different geochemical Hg pools contribute to these levels. Here we report quantitative data on MeHg formation and bioaccumulation, in mesocosm water-sediment model ecosystems, using five Hg(II) and MeHg isotope tracers simulating recent Hg inputs to the water phase and Hg stored in sediment as bound to natural organic matter or as metacinnabar. Calculations for an estuarine ecosystem suggest that the chemical speciation of Hg(II) solid/adsorbed phases control the sediment Hg pool's contribution to MeHg, but that input of MeHg from terrestrial and atmospheric sources bioaccumulates to a substantially greater extent than MeHg formed in situ in sediment. Our findings emphasize the importance of MeHg loadings from catchment runoff to MeHg content in estuarine biota and we suggest that this contribution has been underestimated.
Rivers are an important source of mercury (Hg) to marine ecosystems. Based on an analysis of compiled observations, we estimate global present-day Hg discharges from rivers to ocean margins are 27 ± 13 Mmol a(-1) (5500 ± 2700 Mg a(-1)), of which 28% reaches the open ocean and the rest is deposited to ocean margin sediments. Globally, the source of Hg to the open ocean from rivers amounts to 30% of atmospheric inputs. This is larger than previously estimated due to accounting for elevated concentrations in Asian rivers and variability in offshore transport across different types of estuaries. Riverine inputs of Hg to the North Atlantic have decreased several-fold since the 1970s while inputs to the North Pacific have increased. These trends have large effects on Hg concentrations at ocean margins but are too small in the open ocean to explain observed declines of seawater concentrations in the North Atlantic or increases in the North Pacific. Burial of Hg in ocean margin sediments represents a major sink in the global Hg biogeochemical cycle that has not been previously considered. We find that including this sink in a fully coupled global biogeochemical box model helps to balance the large anthropogenic release of Hg from commercial products recently added to global inventories. It also implies that legacy anthropogenic Hg can be removed from active environmental cycling on a faster time scale (centuries instead of millennia). Natural environmental Hg levels are lower than previously estimated, implying a relatively larger impact from human activity.
Elevated levels of mercury in aquatic environments remote from industrial sources have been broadly attributed to long-range atmospheric transport and deposition of anthropogenic Hg. Evidence in support of this prevailing scientific viewglobal biogeochemical Hg models, sedimentary archives of historic Hg fluxes, and geographic trends in soil Hghave been challenged as being insuf ficiently rigorous to rule out the alternative explanation that natural geologic sources are the principal contributors of Hg in remote locations. In this review, we examine the weaknesses in interpretation and the choice of information that has been used to argue against atmospheric Hg contamination. Analytical advances in measuring trace levels of environmental Hg have greatly narrowed estimates of natural Hg fluxes, providing a clear measure of the relative magnitude of anthropogenic Hg emissions and deposition. Recent experimental results indicate that diagenetic processes cannot explain the mounting number of lake sediment and peat profiles showing substantial increases in Hg flux during the past century. Geologic sources of Hg may be important in specific localities but cannot explain corresponding geographic trends in soil Hg and industrial emission sources. Despite uncertainties in current understanding, there is a broad and geochemically consistent data base indicating that, over large regions of the globe, human-related Hg emissions have increased relative to natural sources since the onset of the industrial period.
This volume provides a modern assessment of the state of the world's
water resources. It collects in one place a huge amount of data. A
uniform methodological approach for each continent is then built upon to
analyse the dynamics of water resources, water use and water
availability for the Earth as a whole, and for all natural-economic
regions and selected countries, as well as river water inflow to the
World Ocean. A dependence of water resources and water use upon climate
factors and socio-economic conditions is demonstrated. Regions of water
stress and deficit are highlighted, and a great increase of such regions
in the future is forecast if present trends towards water use remain.
The effect of anthropogenic climate change on water resources and water
needs is also considered. This volume will prove to be an invaluable
resource for all researchers and policy-makers involved in water
resource issues.
[1] Elevated mercury (Hg) in marine and terrestrial ecosystems is a global health concern because of the formation of toxic methylmercury. Humans have emitted Hg to the atmosphere for millennia, and this Hg has deposited and accumulated into ecosystems globally. Here we present a global biogeochemical model with fully coupled atmospheric, terrestrial, and oceanic Hg reservoirs to better understand human influence on Hg cycling and timescales for responses. We drive the model with a historical inventory of anthropogenic emissions from 2000 BC to present. Results show that anthropogenic perturbations introduced to surface reservoirs (atmosphere, ocean, or terrestrial) accumulate and persist in the subsurface ocean for decades to centuries. The simulated present-day atmosphere is enriched by a factor of 2.6 relative to 1840 levels, consistent with sediment archives, and by a factor of 7.5 relative to natural levels (2000 BC). Legacy anthropogenic Hg re-emitted from surface reservoirs accounts for 60% of present-day atmospheric deposition, compared to 27% from primary anthropogenic emissions, and 13% from natural sources. We find that only 17% of the present-day Hg in the surface ocean is natural and that half of its anthropogenic enrichment originates from pre-1950 emissions. Although Asia is presently the dominant contributor to primary anthropogenic emissions, only 17% of the surface ocean reservoir is of Asian anthropogenic origin, as compared to 30% of North American and European origin. The accumulated burden of legacy anthropogenic Hg means that future deposition will increase even if primary anthropogenic emissions are held constant. Aggressive global Hg emission reductions will be necessary just to maintain oceanic Hg concentrations at present levels.
Predicting the bioavailability of inorganic mercury (Hg) to bacteria that produce the potent bioaccumulative neurotoxin monomethylmercury remains one of the greatest challenges in predicting the environmental fate and transport of Hg. Dissolved organic matter (DOM) affects mercury methylation due to its influence on cell physiology (as a potential nutrient) and its influence on HgII speciation in solution (as a complexing agent), therefore controlling Hg bioavailability. We assessed the role of DOM on HgII bioavailability to a gram-negative bacterium bioreporter under oxic pseudo- and non-equilibrium conditions, using defined media and field samples spanning a wide range of DOM levels. Our results showed that HgII was considerably more bioavailable under non-equilibrium conditions than when DOM was absent or when HgII and DOM had reached pseudo-equilibrium (24h) prior to cell exposure. Under these enhanced uptake conditions, HgII bioavailability followed a bell shaped curve as DOM concentrations increased, both for defined media and natural water samples, consistent with bioaccumulation results in a companion paper (this issue) observed for amphipods. Experiments also suggest that DOM may not only provide shuttle molecules facilitating Hg uptake but also alter cell wall properties to facilitate the first steps towards HgII internalization. We propose the existence of a short-lived yet critical time window (<24h) during which DOM facilitates the entry of newly deposited HgII into aquatic food webs, suggesting that the bulk of mercury incorporation in aquatic food webs would occur within hours following its deposition from the atmosphere.
The corrinoid protein, HgcA has been shown to be essential for Hg methylation in anaerobic bacteria. We investigated the diversity of hgcA from temperate and tropical wetland soils where Hg methylation is demonstrated. Sequences obtained from both environments clustered with those from the δ-Proteobacteria, Chloroflexi, and Methanomicrobia with significant overlap in hgcA phylogeny between libraries. Clear differences in hgcA distribution were observed between two highly contrasting sites within a tropical wetland in Everglades National Park, USA. hgcA sequences obtained from the northern site clustered primarily with those of methanogens, while seequences from the estuarine site clustered primarily with sulfate-reducing bacteria and syntrophs in the δ-Proteobacteria. Libraries obtained from soils collected from a temperate swamp in Sweden were dominated by hgcA sequences within the δ-Proteobacteria with hgcA sequences clustering primarily with iron-reducers in the upstream portion of the swamp and with sulfate-reducers in the downstream portion of the swamp. Interestingly, enrichments prepared from the lower portion of this temperate wetland contained a high abundance of hgcA sequences clustering with methanogens. This first report on hgcA diversity in environmental samples suggests a role in Hg methylation for various phenotypic groups in different portions of wetlands.
We examined mercury (Hg) biogeochemistry and biomagnification in the Babeni Reservoir, a system strongly affected by the release of Hg from a chlor-alkali plant. Total mercury (THg) concentrations in river water reached 88 ng L(-1) but decreased rapidly in the reservoir (to 9 ng L(-1)). In contrast, monomethylmercury (MMHg) concentrations increased from the upstream part of the reservoir to the central part (0.7 ng L(-1)), suggesting high methylation within the reservoir. Moreover, vertical water column profiles of THg and MMHg indicated that Hg methylation mainly occurred deep in the water column and at the sediment-water interface. The discharge of Hg from a chlor-alkali plant in Valcea region caused the highest MMHg concentrations ever found in non-piscivorous fish worldwide. MMHg concentrations and bioconcentration factors (BCF) of plankton and macrophytes revealed that the highest biomagnification of MMHg takes place in primary producers.
We know that methylation of inorganic mercury is effected by microbes in the anoxic layers of sediments and water columns. But the parameters that control the extent of this methylation are still poorly known. We show that mercury methylation by the versatile bacterium Geobacter sulfurreducens is greatly enhanced in the presence of low concentrations of the amino-acid cysteine. This enhancement results from complexation of inorganic mercury by cysteine, and bacterial uptake of the complex, followed by its efficient methylation and release to the external medium. Those results indicate that mercury uptake and methylation by microbes are under much tighter biological control than heretofore considered and that the formation of specific Hg complexes in anoxic waters likely modulate the efficiency of the microbial methylation of mercury.
Despite methylmercury (MeHg) production in boreal wetlands being a research focus for decades, little is known about factors
in control of methylation and demethylation rates and the effect of wetland type. This is the first study reporting potential
Hg methylation (k
m
) and MeHg demethylation rate constants (k
d
) in boreal wetland soils. Seven wetlands situated in northern and southern Sweden were characterized by climatic parameters,
nutrient status (e.g. type of vegetation, pH, C/N ratio, specific UV-absorption), iron and sulfur biogeochemistry. Based on
nutrient status, the wetlands were divided into three groups; (I) three northern, nutrient poor fens, (II) a nutrient gradient
ranging from an ombrotrophic bog to a fen with intermediate nutrient status, and (III) southern, more nutrient rich sites
including two mesotrophic wetlands and one alder (Alnus) forest swamp. The k
m
/k
d
ratio in general followed %MeHg in soil and both measures were highest at the fen site with intermediate nutrient status.
Northern nutrient poor fens and the ombrotrophic bog showed intermediate values of %MeHg and k
m
/k
d
. The two mesotrophic wetlands showed the lowest %MeHg and k
m
/k
d
, whereas the alder swamp had high k
m
and k
d
, resulting in an intermediate k
m
/k
d
and %MeHg. Molybdate addition experiments suggest that net MeHg production was mainly caused by the activity of sulfate reducing
bacteria. A comparison with other studies, show that k
m
and %MeHg in boreal freshwater wetlands in general are higher than in other environments. Our results support previous suggestions
that the highest MeHg net production in boreal landscapes is to be found in fens with an intermediate nutrient status.
Concentrations of methyl mercury, CH3Hg (II), total mercury, Hgtot = CH3Hg (II) + Hg (II), and organic sulphur species were determined in soils, soil solutions and streams of a small (50 ha) boreal forest catchment in northern Sweden. The CH3Hg (II)/Hgtot ratio decreased from 1.2–17.2% in the peaty stream bank soils to 0.4–0.8% in mineral and peat soils 20 m away from the streams, indicating that conditions for net methylation of Hg (II) are most favourable in the riparian zone close to streams. Concentrations of CH3Hg (II) bound in soil and in soil solution were significantly, positively correlated to the concentration of Hgtot in soil solution. This, and the fact that the CH3Hg (II)/Hgtot ratio was higher in soil solution than in soil may indicate that Hg (II) in soil solution is more available for methylation processes than soil bound Hg (II). Reduced organic S functional groups (Org-SRED) in soil, soil extract and in samples of organic substances from streams were quantified using S K-edge X-ray absorption near-edge structure (XANES) spectroscopy. Org-SRED, likely representing RSH, RSSH, RSR and RSSR functionalities, made up 50 to 78% of total S in all samples examined. Inorganic sulphide [e.g. FeS2 (s)] was only detected in one soil sample out of 10, and in none of the stream samples. Model calculations showed that under oxic conditions nearly 100% of Hg (II) and CH3Hg (II) were complexed by thiol groups (RSH) in the soil, soil solution and in the stream water. Concentrations of free CH3Hg+ and Hg2+ ions in soil solution and stream were on the order of 10-18 and 10-32M, respectively, at pH 5. For CH3Hg (II), inorganic bi-sulphide complexes may contribute to an overall solubility at concentrations of inorganic sulphides higher than 10-9M, whereas considerably higher concentrations of inorganic sulphides (lower redox-potential) are required to increase the solubility of Hg (II).
The estimation of soil carbon content is of pressing concern for soil protection and in mitigation strategies for global warming. This paper describes the methodology developed and the results obtained in a study aimed at estimating organic carbon contents (%) in topsoils across Europe. The information presented in map form provides policy‐makers with estimates of current topsoil organic carbon contents for developing strategies for soil protection at regional level. Such baseline data are also of importance in global change modelling and may be used to estimate regional differences in soil organic carbon (SOC) stocks and projected changes therein, as required for example under the Kyoto Protocol to the United Nations Framework Convention on Climate Change, after having taken into account regional differences in bulk density.
The study uses a novel approach combining a rule‐based system with detailed thematic spatial data layers to arrive at a much‐improved result over either method, using advanced methods for spatial data processing. The rule‐based system is provided by the pedo‐transfer rules, which were developed for use with the European Soil Database. The strong effects of vegetation and land use on SOC have been taken into account in the calculations, and the influence of temperature on organic carbon contents has been considered in the form of a heuristic function. Processing of all thematic data was performed on harmonized spatial data layers in raster format with a 1 km × 1 km grid spacing. This resolution is regarded as appropriate for planning effective soil protection measures at the European level. The approach is thought to be transferable to other regions of the world that are facing similar questions, provided adequate data are available for these regions. However, there will always be an element of uncertainty in estimating or determining the spatial distribution of organic carbon contents of soils.
Dissolved organic matter (DOM) is a key component of fate and transport models for most metals, including mercury (Hg). Utilizing a suite of diverse DOM isolates, we demonstrated that DOM character, in addition to concentration, influences inorganic Hg (Hg(II)i) bioavailability to Hg-methylating bacteria. Using a model Hg-methylating bacterium, Desulfovibrio desulfuricans ND132, we evaluated Hg-DOM-sulfide bioavailability in washed-cell assays at environmentally relevant Hg/DOM ratios (~1-8 ng Hg/mg C) and sulfide concentrations (1-1000 µM). All tested DOM isolates significantly enhanced Hg methylation above DOM-free controls (from ~2 to >20-fold for 20 mg C/L DOM solutions), but high molecular weight/highly aromatic DOM isolates and/or those with high sulfur content were particularly effective at enhancing Hg methylation. Because these experiments were conducted under conditions of predicted super-saturation with respect to metacinnabar (β-HgS(s)), we attribute the DOM-dependent enhancement of Hg(II)i bioavailability to steric and specific chemical (e.g., DOM thiols) inhibition of β-HgS(s) growth and aggregation by DOM. Experiments examining the role of DOM across a wide sulfide gradient revealed that DOM only enhances Hg methylation under fairly low sulfide conditions (< ~30 µM), conditions that favor HgS nanoparticle/cluster formation relative to dissolved HgS species. <sup></sub><sub><sub><sub>
The isolation, characterization and study of the properties of aquatic dissolved organic matter (DOM) still represent a challenge because of the heterogeneity, complexity and low concentration of organic material in natural waters. Based on its ability to interact with contaminants and thus to modify their transport and bioavailability, DOM is of interest for environmental purposes. The objective of this work was to better characterize DOM in the Gironde Estuary (southwestern France). The estuary represents an exchange zone between the continent and the Atlantic Ocean and conditions the transfer of organic and inorganic substances from the continental to the oceanic environment. Several samples were collected along the estuary during three cruises in 2002 and 2006. They were analysed using excitation–emission matrix (EEM) spectroscopy, a sensitive technique that allows direct analysis of water samples. Fluorescent DOM and dissolved organic carbon (DOC) did not behave conservatively in this estuarine system, i.e. the organic material did not undergo simple dilution from the upstream to the downstream part of the estuary. A seasonal variability in DOC content was pointed out, whereas few seasonal variations in DOM fluorescence were observed. DOM sources and processing in the estuary were further evaluated by determining two fluorescence indices – the humification index (HIX) and the index of recent autochthonous contribution (BIX). By applying these indices, the relative degree of humification (HIX) and autotrophic productivity (BIX) could be assessed. Based on the fluorescence and DOC results, the estuary was divided into three zones depending on salinity (S) and characterized by specific DOM: (i) A turbid zone of low salinity (S
Methylmercury (MeHg) affects wildlife and human health mainly through marine fish consumption. In marine systems, MeHg is formed from inorganic mercury (HgII) species primarily in sediments then accumulates and biomagnifies in the food web. Most of the fish consumed in the US are from estuarine and marine systems highlighting the importance of understanding MeHg formation in these productive regions. Sediment organic matter has been shown to limit mercury methylation in estuarine ecosystems, as a result it is often described as the primary control over MeHg production. In this paper, we explore the role of organic matter by looking at the effects of its changing sediment concentrations on the methylation rates across multiple estuaries. We measured sedimentary MeHg production at eleven estuarine sites that were selected for their contrasting biogeochemical characteristics, mercury (Hg) content, and location in the Northeastern US (ME, NH, CT, NY, and NJ). Sedimentary total Hg concentrations ranged across five orders of magnitude, increasing in concentration from the pristine, sandy sediments of Wells (ME), to industrially contaminated areas like Portsmouth (NH) and Hackensack (NJ). We find that methylation rates are the highest at locations with high Hg content (relative to carbon), and that organic matter does not hinder mercury methylation in estuaries.
Analysis of Hg(II) complexed by a soil humic acid (HA) using synchrotron-based X-ray absorption spectroscopy (XAS) revealed the importance of reduced sulfur functional groups (thiol (R−SH) and disulfide (R−SS−R)/disulfane (R−SSH)) in humic substances in the complexation of Hg(II). A two-coordinate binding environment with one oxygen atom and one sulfur atom at distances of 2.02 and 2.38 Å, respectively, was found in the first coordination shell of Hg(II) complexed by humic acid. Model calculations show that a second coordination sphere could contain one carbon atom and a second sulfur atom at 2.78 and 2.93 Å, respectively. This suggests that in addition to thiol S, disulfide/disulfane S may be involved with the complexation of Hg(II) in soil organic matter. The appearance of carbon atom in the second coordination shell suggests that one O-containing ligand such as carboxyl and phenol ligands rather than H2O molecule is bound to the Hg(II). The involvement of oxygen ligand in addition to the reduced S ligands in the complexation of Hg(II) is due to the low density of reduced S ligands in humic substances. The XAS results from this experiment provided direct molecular level evidence for the preference of reduced S functional groups over oxygen ligands by Hg(II) in the complexation with humic substances.
Emissions of Hg, Pb, and Cd to air aretransported over wide areas in Europe and deposited far awayfrom their sources. About 80% of the atmospheric depositionof these metals in south Sweden originate from emissions inother countries. As a result of the increased anthropogenicdeposition the concentrations of Hg, Pb, and Cd in the morlayer of forest soils have increased considerably, mainlyduring the 20th century. Although the atmosphericdeposition of these elements has declined during the mostrecent decades, the reduction of the input of Hg and Pb isnot sufficient to prevent a further accumulation. Theconcentrations of Hg and Pb are still increasing by ca. 0.5and ca. 0.2% annually in the surface layer of forest soils.In contrast, the Cd concentration is currentlydecreasing in a large part of Sweden as a result of bothdeposition decreases and enhanced leaching induced by soilacidification. The accumulation factors of Hg and Pb,especially in the forest topsoils of south Sweden, arealready above those at which adverse effects on soilbiological processes and organisms have been demonstrated instudies of gradients from local emission sources andlaboratory assessment. There are also indications of sucheffects at the current regional concentrations of Hg and Pbin mor layers from south Sweden, judging from observationsin field and laboratory studies. There is an apparent riskof Pb induced reduction in microbial activity over parts ofsouth Sweden. This might cause increased accumulation oforganic matter and a reduced availability of soil nutrients.At current concentrations of Hg in Swedish forest soils,effects similar to those of Pb are likely. Increasedconcentrations of these elements in organs of mammals andbirds have also been measured, though decreases have beendemonstrated in recent years, related to changes inatmospheric deposition rates. As a result of current andpast deposition in south Sweden, concentrations of Hg infish have increased about fivefold during the 20thcentury. This implies risks for human health, when fish frominland waters are used for food. Although the concentrationof Hg in fish has decreased ca. 20% during the last decade,probably as a result of the reduced deposition, the levelstill exceeds the general limit (0.5 mg kg-1) in about half(ca. 40 000) of the Swedish lakes. In order to reduceconcentrations in fish to the level recommended, and avoidfurther accumulation of Hg in soils, the atmosphericdeposition has to be reduced to ca. 20% of the current deposition rate. This can only be achieved by international co-operation.
A number of recent studies have documented elevated concentrations of mercury (Hg) in fish caught in remote lakes and a pattern of increased concentrations of Hg in fish tissue with decreasing water column pH. Because of the potential linkage between fish Hg and surface water acidification, factors regulating water column concentrations and bioavailability of Hg were investigated in Adirondack lakes through a field study and application of the Mercury Cycling Model (MCM). Concentrations of total Hg and total MeHg were highly variable, with concentrations of total MeHg about 10% of total Hg in lakes which did not show anoxic conditions. In lakes exhibiting anoxic conditions in the hypolimnion during summer stratification, concentrations of total MeHg were elevated. Concentrations of total Hg and total MeHg increased with decreasing pH in remote Adirondack lakes. However, more importantly, concentrations of total Hg and total MeHg increased with increasing concentrations of dissolved organic carbon (DOC) and percent near-shore wetlands in the drainage basin. Mercury concentrations in muscle tissue of yellow perch from Adirondack lakes were elevated above the U.S. FDA action level (1 g/g Hg) in 7% of the fish sampled or in one or more individual fish from 9 of the 16 lakes sampled. Fish Hg concentrations generally increased with increasing fish length, weight and age. Patterns of increasing Hg concentration with age likely reflect shifts in prey of yellow perch and the bioconcentration of Hg along the food chain. For age 3 to 5 perch, concentrations of Hg increased with increasing concentrations of DOC and percent near-shore wetlands in the drainage basin. However, for a lake with very high DOC concentrations, fish concentrations of Hg declined. Calculations with the MCM also show that concentrations of Hg species increase with increasing DOC due to complexation reactions. Increases in DOC result in increasing concentrations of Hg in biota but decreases in the bioconcentration factor of Hg in fish tissue. This research suggests that DOC is important in the transport of Hg to lake systems. High concentrations of DOC may complex MeHg, diminishing its bioavailability. At high concentrations of monomeric Al, the complexation of MeHg with DOC apparently decreases, enhancing the bioavailability of MeHg.
Anthropogenic activities have enriched mercury in the biosphere by at least a factor of three, leading to increases in total mercury (Hg) in the surface ocean. However, the impacts on ocean fish and associated trends in human exposure as a result of such changes are less clear. Here we review our understanding of global mass budgets for both inorganic and methylated Hg species in ocean seawater. We consider external inputs from atmospheric deposition and rivers as well as internal production of monomethylmercury (CH(3)Hg) and dimethylmercury ((CH(3))(2)Hg). Impacts of large-scale ocean circulation and vertical transport processes on Hg distribution throughout the water column and how this influences bioaccumulation into ocean food chains are also discussed. Our analysis suggests that while atmospheric deposition is the main source of inorganic Hg to open ocean systems, most of the CH(3)Hg accumulating in ocean fish is derived from in situ production within the upper waters (<1000m). An analysis of the available data suggests that concentrations in the various ocean basins are changing at different rates due to differences in atmospheric loading and that the deeper waters of the oceans are responding slowly to changes in atmospheric Hg inputs. Most biological exposures occur in the upper ocean and therefore should respond over years to decades to changes in atmospheric mercury inputs achieved by regulatory control strategies. Migratory pelagic fish such as tuna and swordfish are an important component of CH(3)Hg exposure for many human populations and therefore any reduction in anthropogenic releases of Hg and associated deposition to the ocean will result in a decline in human exposure and risk.
There are inadequate measurements of surface ambient concentrations of mercury species and their deposition rates for the UK deposition budget to be characterized. In order to estimate the overall mercury flux budget for the UK, a simple long-term 1D Lagrangian trajectory model was constructed that treats emissions (1998), atmospheric transformation and deposition across Europe. The model was used to simulate surface concentrations of mercury and deposition across Europe at a resolution of 50 km×50 km and across the UK at 20 km×20 km. The model appeared to perform adequately when compared with the few available measurements, reproducing mean concentrations of elemental gaseous mercury at particular locations and the magnitude of regional gradients. The model showed that 68% of the UK's mercury emissions are exported and 32% deposited within the UK. Of deposition to the UK, 25% originates from the Northern Hemisphere/global background, 41% from UK sources and 33% from other European countries. The total mercury deposition to the UK is in good agreement with other modelling, 9.9 tonne yr−1 cf. 9.0 tonne yr−1, for 1998. However, the attribution differs greatly from the results of other coarser-scale modelling, which allocates 55% of the deposition to the UK from UK sources, 4% from other European countries and 60% from the global background atmosphere. The model was found to be sensitive to the speciation of emissions and the dry deposition velocity of elemental gaseous mercury. The uncertainties and deficiencies are discussed in terms of model parameterization and input data, and measurement data with which models can be validated. There is an urgent requirement for measurements of removal terms, concentrations, and deposition with which models can be parameterized and validated.
Here we report on an investigation of the three-dimensional excitation-emission-matrix (EEM) fluorescence spectra of unconcentrated water samples collected in 1996, 1998 and 1999 at a site particularly propitious for macro-algae development. The degradation of these macro-algae was studied to determine the influence of their exudates on natural water EEM fluorescence spectra. This work demonstrates that biological activity is one of the major factors involved in the formation of the blue-shifted fluorescence band observed in marine waters (β component Ex/Em=310–320 nm/380–410 nm); our study also shows that fluorescence can be used to evaluate the biological activity both quantitatively and to determine its different phases.