Hg Pools Research Articles

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

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

Important accumulated mercury pool in a remote alpine forest and dynamic accumulation revealed by tree rings in China's Qilian Mountains

Quantification mercury (Hg) pools in forests is crucial for understanding the Hg assimilation, flux and even biogeochemical cycle in forest ecosystems. While several investigations focused on Hg pools among broad-leaved, coniferous and mixed forests, there was still absent information on alpine forest. We sampled soil, moss and various tissues of the dominant Qinghai spruce (Picea crassifolia Kom.) to investigate Hg concentrations and pools, and assess Hg accumulation dynamics in the Qilian Mountains, northwestern China. The mean Hg concentration increased in the following order: trunk wood (1.8 ± 0.7 ng g−1) < branch (4.6 ± 0.8 ng g−1) < root (12.2 ± 2.9 ng g−1) < needle (19.3 ± 5.6 ng g−1) < bark (28.7 ± 9.0 ng g−1) < soil (34.1 ± 7.7 ng g−1) < litterfall (42.9 ± 2.9 ng g−1) < moss (62.5 ± 5.0 ng g−1). The soil contained Hg pools two orders of magnitude higher than vegetation and accounted for 92.2 % of the total Hg pool in the alpine forest ecosystem. Moss, despite representing only 2.7 % of total vegetation biomass, contained a disproportionate 16.7 % of the Hg pool. Although species-specific, aboveground spruce tissues exhibited higher Hg pools in alpine forests compared to other forests in China and America. The dynamic accumulation indicated that increasing atmospheric Hg concentration and enhancing tree productivity contributed to rising Hg assimilation in remote alpine forests, particularly after the 1960s. Our results highlight the relatively high levels of Hg pools in aboveground tree tissues of alpine forest and reveal a significant increase in Hg accumulation. We recommend that when assessing Hg dynamics in forest ecosystems, it is crucial to consider both the variability in atmospheric Hg exposure levels and the forest productivity.

Global Distribution of Mercury in Foliage Predicted by Machine Learning.

Foliar assimilation of elemental mercury (Hg0) from the atmosphere plays a critical role in the global Hg biogeochemical cycle, leading to atmospheric Hg removal and soil Hg insertion. Recent studies have estimated global foliar Hg assimilation; however, large uncertainties remained due to coarse accounting of observed foliar Hg concentrations, posing a substantial challenge in constraining the global Hg budget. Here, we integrated a comprehensive observation database of foliar Hg concentrations and machine learning algorithms to predict the first spatial distribution of foliar Hg concentrations on a global scale, contributing to the first estimate of global Hg pools in foliage. The global average of foliar Hg concentrations was estimated to be 24.0 ng g-1 (7.5-56.5 ng g-1), and the global total in foliar Hg pools reached 4561.3 Mg (1455.2-9062.8 Mg). The spatial distribution showed the hotspots in tropical regions, including the Amazon, Central Africa, and Southeast Asia. A range of 2268.5-2727.0 Mg yr-1 was estimated for annual foliar Hg assimilation accounting for the perennial continuous assimilation by evergreen vegetation foliage. The first spatial maps of foliar Hg concentrations and Hg pools may aid in understanding the global biogeochemical cycling of Hg, especially in the context of climate change and global vegetation greening.

Key factors influencing Hg levels and trends in unperturbed oligotrophic temperate and boreal lakes

Mercury (Hg) is a toxic metal that presents a major risk to ecosystems, biota, human health, and remains a priority concern. In temperate and boreal lakes Hg and methylmercury (MMHg) are expected to vary as a function of atmospheric Hg deposition, lake water chemistry, catchment characteristics and climate variables. The aim of this study was to quantify Hg and MMHg in unperturbed oligotrophic lakes and to identify the factors controlling their distribution. We first hypothesized that lake Hg (and MMHg to lesser extent) spatial variations are linked to atmospheric deposition, catchment characteristics, and terrestrial exportation of dissolved organic carbon (DOC). We secondly examined if lake Hg concentrations have followed the decrease in atmospheric Hg emission observed between the mid-1990s to the end-2010s. We found that overall, atmospheric Hg has little impact on lake Hg and MMHg concentrations, which are both primarily influenced by DOC input originating from the forest catchment. The relationship between DOC and Hg differed between the spring and the fall, with a Hg-to-DOC ratio twice as high in spring. This seems related to snowmelt input of Hg (with a relatively reduced input of DOC) or the internal lake build-up of Hg during the ice-covered period. Of the 10 lakes intensively visited over a 20-year period, only 3 showed significant lake Hg decreases despite significant negative trends in atmospheric Hg concentrations, suggesting a lag between atmospheric and surface water temporal trends. Overall, terrestrial catchments retain around 80% of atmospheric Hg implying that large Hg pools have been built up in soils in the last decades. As such, the reduction of atmospheric Hg alone will not necessarily result in Hg decreases in lakes, since the Hg concentrations may be modulated by DOC export trends and catchment characteristics. This stresses the need to improve our understanding of the processes governing Hg transfers from catchments into lakes.

Above- and belowground plant mercury dynamics in a salt marsh estuary in Massachusetts, USA

Abstract. Estuaries are a conduit of mercury (Hg) from watersheds to the coastal ocean, and salt marshes play an important role in coastal Hg cycling. Hg cycling in upland terrestrial ecosystems has been well studied, but processes in densely vegetated salt marsh ecosystems are poorly characterized. We investigated Hg dynamics in vegetation and soils in the Plum Island Sound estuary in Massachusetts, USA, and specifically assessed the role of marsh vegetation for Hg deposition and turnover. Monthly quantitative harvesting of aboveground biomass showed strong linear seasonal increases in Hg associated with plants, with a 4-fold increase in Hg concentration and an 8-fold increase in standing Hg mass from June (3.9 ± 0.2 µg kg−1 and 0.7 ± 0.4 µg m−2, respectively) to November (16.2 ± 2.0 µg kg−1 and 5.7 ± 2.1 µg m−2, respectively). Hg did not increase further in aboveground biomass after plant senescence, indicating physiological controls of vegetation Hg uptake in salt marsh plants. Hg concentrations in live roots and live rhizomes were 11 and 2 times higher than concentrations in live aboveground biomass, respectively. Furthermore, live belowground biomass Hg pools (Hg in roots and rhizomes, 108.1 ± 83.4 µg m−2) were more than 10 times larger than peak standing aboveground Hg pools (9.0 ± 3.3 µg m−2). A ternary mixing model of measured stable Hg isotopes suggests that Hg sources in marsh aboveground tissues originate from about equal contributions of root uptake (∼ 35 %), precipitation uptake (∼ 33 %), and atmospheric gaseous elemental mercury (GEM) uptake (∼ 32 %). These results suggest a more important role of Hg transport from belowground (i.e., roots) to aboveground tissues in salt marsh vegetation than upland vegetation, where GEM uptake is generally the dominant Hg source. Roots and soils showed similar isotopic signatures, suggesting that belowground tissue Hg mostly derived from soil uptake. Annual root turnover results in large internal Hg recycling between soils and plants, estimated at 58.6 µg m−2 yr−1. An initial mass balance of Hg indicates that the salt marsh presently serves as a small net Hg sink for environmental Hg of 5.2 µg m−2 yr−1.

Reconsidering mercury sources and exposure pathways to bivalves: Insights from mercury stable isotopes

Identifying mercury (Hg) sources and exposure pathways to bivalves, particularly in relation to sediment, is important for expanding the utility of bivalves as a monitoring organism for sediment quality. Here we use Hg isotope ratios to decipher Hg sources accumulated into bivalves by conducting field studies and in situ experiments. In the first part of this study, we characterized Hg isotope ratios in individual geochemical fractions of riverine sediment, contaminated by liquid Hg in South Korea (Hyeongsan River; HS). Asian clams (Corbicula fluminea) were then deployed at the contaminated sites to evaluate the isotopic turnover. Over the two-month period, the isotope ratios of the clams shifted toward the labile/exchangeable Hg pools (F1, F2 fractions) of the sediment. Conversely, in the control site where sediment Hg is low, we observed similar Hg isotope ratios between Asian clams and the samples of precipitation and dissolved phase of water column. In East Fork Poplar Creek, (Oak Ridge) U.S., Asian clams also displayed similar Hg isotope ratios with the dissolved phase of water column, which have undergone substantial in-stream processing or input from Hg-contaminated groundwater from the hyporheic zones and riparian tributary during high hydrologic flow seasons. Our study demonstrates that the dissolved Hg phases within the water column, whether originating via sediment diffusion or derived externally, act as the primary source and exposure pathways to bivalves. The results of our study also shed new light to the prior Hg isotope measurement in bivalves collected from estuarine, lake, and coastal systems, which showed significant isotopic deviation from bulk sediment. The fact that bivalves are sensitive to in situ and external dissolved Hg phases provides additional insight into the existing biomonitoring program, which uses bivalves as a bioindicator for sediment quality.

Variation of Hg concentration and accumulation in the soil of maritime pine plantations along a coast-inland transect in SW Europe

Climatic conditions have been shown as a major driver of the fate of Hg in forest ecosystems at a global scale, but less is known about climatic effects at shorter scales. This study assesses whether the concentration and pools of Hg in soils collected from seventeen Pinus pinaster stands describing a coastal-inland transect in SW Europe vary along a regional climatic gradient. In each stand, samples of the organic subhorizons (OL, OF + OH) and the mineral soil (up to 40 cm) were collected and some general physico-chemical properties and total Hg (THg) were analyzed. Total Hg was significantly higher in the OF + OH than in the OL subhorizons (98 and 38 μg kg−1, respectively), favored by a greater organic matter humification in the former. In the mineral soil, mean THg values decreased with depth, ranging from 96 μg kg−1 in the 0–5 cm layers to 54 μg kg−1 in the deepest layers (30–40 cm), respectively. The average Hg pool (PHg) was 0.30 mg m−2 in the organic horizons (92% accumulated in the OF + OH subhorizons), and 27.4 mg m−2 in the mineral soil. Changes in climatic factors, mainly precipitation, along the coast-inland transect resulted in a remarkable variation of THg in the OL subhorizons, consistent with their role as the first receiver of atmospheric Hg inputs. The high precipitation rate and the occurrence of fogs in coastal areas characterized by the oceanic influence would explain the higher THg found in the uppermost soil layers of pine stands located close to the coastline. The regional climate is key to the fate of mercury in forest ecosystems by influencing the plant growth and subsequent atmospheric Hg uptake, the atmospheric Hg transference to the soil surface (wet and dry deposition and litterfall) and the dynamics that determine net Hg accumulation in the forest floor.

Mercury Content and Pools in Complex Polycyclic Soils From a Mountainous Area in Galicia (NW Iberian Peninsula)

Atmospheric mercury (Hg) usually tends to accumulate in the upper horizons of soils. However, the physico-chemical characteristics of some soils, as well as pedogenetic processes, past climate changes, or soil degradation processes, can lead to a redistribution of mercury through the soil profile. In this work, the presence and accumulation of mercury was studied in three deep polycyclic soils from a mountainous area in NW Iberia Peninsula. The highest total Hg values (HgT) were found in the organic matter-rich O and A horizons of FL and MF profiles (169 and 139 μg kg−1, respectively) and in the illuvial horizon of RV (129.2 μg kg−1), with the latter two samples showing the maximum Hg reservoirs (29.3 and 29.0 mg m−2, respectively). Despite finding the highest Hg content in the surface horizons, considerable Hg reservoirs were also observed in depths higher than 40–50 cm, indicating the importance of taking into account these soil layers when Hg pools are evaluated at a global scale. Based on the mass transfer coefficients, we can rule out the contribution of parent material to the Hg accumulation in most of the horizons, thus indicating that pedogenetic processes are responsible for the Hg redistribution observed along the soil profiles. Finally, by means of principal component analysis (PCA) and stepwise linear regression we could assess the main soil components involved in the Hg accumulation in each soil horizon. Therefore, PC1 (organic matter and low stability Al-hummus complexes) showed a higher influence on the surface horizons, whereas PC2 (reactive Al-Fe complexes and medium-high Al-hummus complexes) and PC4 (crystalline Fe compounds and pHw) were more relevant in the Hg distribution observed in the deepest soil layers.

Needle age and precipitation as drivers of Hg accumulation and deposition in coniferous forests from a southwestern European Atlantic region

Vegetation and climate are critical in the biogeochemical cycle of Hg in forest ecosystems. The study assesses the influence of needle age and precipitation on the accumulation of Hg in needle biomass and its deposition by litterfall in thirty-one pine plantations spread throughout two biogeographical regions in SW Europe. Well-developed branches of Pinus pinaster were sampled and pine needles were classified according to 4 age classes (y0, y1, y2, y3). The concentration of total Hg (THg) was analyzed in the samples and Hg content in needle biomass and its deposition by litterfall were estimated. The concentration of total Hg (THg) increased with needle age ranging from 9.1 to 32.7 μg Hg kg−1 in the youngest and oldest needles, respectively. The rate of Hg uptake (HgR) three years after needle sprouting was 10.2 ± 2.3 μg Hg kg−1 yr−1, but it decreased with needle age probably due to a diminution in photosynthetic activity as needles get older. The average total Hg stored in needle biomass (HgWt) ranged from 5.6 to 87.8 mg Hg ha−1, with intermediate needle age classes (y1 and y2) accounting for 70% of the total Hg stored in the whole needle biomass. The average deposition flux of Hg through needle litterfall (HgLt) was 1.5 μg Hg m−2 yr−1, with the y2 and y3 needles contributing most to the total Hg flux. The spatial variation of THg, HgWt and HgLt decreased from coastal pine stands, characterized by an oceanic climate, to inland pine stands, a feature closely related to the dominant precipitation regime in the study area. Climatic conditions and needle age are the main factors affecting Hg accumulation in tree foliage, and should be considered for an accurate assessment of forest Hg pools at a regional scale and their potential consequences in the functioning of terrestrial ecosystems.

Geochemical and Dietary Drivers of Mercury Bioaccumulation in Estuarine Benthic Invertebrates

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

Uncovering geochemical fractionation of the newly deposited Hg in paddy soil using a stable isotope tracer

The newly deposited mercury (Hg) is more readily methylated to methylmercury (MeHg) than native Hg in paddy soil. However, the biogeochemical processes of the newly deposited Hg in soil are still unknown. Here, a field experimental plot together with a stable Hg isotope tracing technique was used to demonstrate the geochemical fractionation (partitioning and redistribution) of the newly deposited Hg in paddy soils during the rice-growing period. We showed that the majority of Hg tracer (200Hg, 115.09 ± 0.36 μg kg−1) was partitioned as organic matter bound 200Hg (84.6–89.4%), followed by residual 200Hg (7.6–8.1%), Fe/Mn oxides bound 200Hg (2.8–7.2%), soluble and exchangeable 200Hg (0.05–0.2%), and carbonates bound 200Hg (0.04–0.07%) in paddy soils. Correlation analysis and partial least squares path modeling revealed that the coupling of autochthonous dissolved organic matter and poorly crystalline Fe (oxyhydr)oxides played a predominant role in controlling the redistribution of the newly deposited Hg among geochemical fractions (i.e., fraction changes). The expected aging processes of the newly deposited Hg were absent, potentially explaining the high bioavailability of these Hg in paddy soil. This study implies that other Hg pools (e.g., organic matter bound Hg) should be considered instead of merely soluble Hg pools when evaluating the environmental risks of Hg from atmospheric depositions.

Distribution and pools of mercury in forest soils near recent and historical mercury emission sources in the central Czech Republic

The fate of atmospherically deposited mercury (Hg) was studied in forest soils situated near various Hg anthropogenic emission sources, including chlor-alkali plants, cement production, and pig iron and steel factories in the Czech Republic. Some of these emission sources were more active in the past, while others continue operation with lowered dust and Hg emissions up to the present day. The impact of Hg emission sources on forest soil was assessed with respect to other soil parameters, including organic carbon, soil nitrogen, soil sulfur, and soil oxalate-extractable aluminum and iron concentrations. The site-specific mean Hg concentrations in organic horizons (174–479 μg kg−1) were greater than mean Hg concentrations in mineral soil (15–88 μg kg−1). Site specific mean Hg/C ratios in organic horizons at four study sites ranged from 0.8 to 2.4 μg g−1, while mean mineral soil Hg/C varied from 2.0 to 3.4 μg g−1. Near cement plants, an 8- to 30-cm thick layer composed of dust particles was identified below or mixed with current O and A horizon material (Hg concentrations 122 to 401 μg kg−1). Mean mineral soil pools of Hg (13–24 mg m−2) dominated over the mean organic horizon Hg pools (2–11 mg m−2). Near cement plants and steel works, Hg concentrations and pools in organic horizons and mineral soils were within the range reported from pristine Czech forest soils. Elevated Hg concentrations in organic horizons were found near a chlor-alkali plant. Thermal decomposition analysis indicated that Hg in A horizons at all sites and dust horizons near cement plants was bound similarly to Hg in foliage.

Translocation and distribution of mercury in biomasses from subtropical forest ecosystems: evidence from stable mercury isotopes

To understand its source, distribution, storage, and translocation in the subtropical forest ecosystems, mercury (Hg) concentrations and stable isotopes in forest biomass tissues (foliage, branch, bark, and trunk) were investigated at Ailao Mountain National Nature Reserve, Southwest China. The total Hg (THg) concentrations in the samples show the following trend: mature foliage (57 ± 19 ng g−1) > bark (11 ± 4.0 ng g−1) > branch (5.4 ± 2.5 ng g−1) > trunk (1.6 ± 0.7 ng g−1). Using the measured THg concentrations and the quantity of respective biomasses, the Hg pools in the forest are: wood (60 ± 26 μg m−2) > bark (51 ± 18 μg m−2) > foliage (41 ± 11 μg m−2) > branch (26 ± 8.3 μg m−2). The tree biomasses displayed negative δ202Hg (− 1.83‰ to − 3.84‰) and ∆199Hg (− 0.18‰ to − 0.62‰). The observed ∆200Hg (− 0.08‰ to 0.04‰) is not significantly from zero. A ∆199Hg/∆201Hg ratio of 1.05 was found in tree biomasses, suggesting that mercury has undergone Hg(II) photoreduction processes. A Hg-isotope based binary mixing model suggests that Hg in the tree biomasses mainly originated from foliage uptake of atmospheric Hg0, constituting 67%, 80%, and 77% of Hg in wood, branch, and bark, respectively. Our study sheds new light on the transportation and sources of Hg in the subtropical forest ecosystems.

A tale of three cities: Mercury in urban deciduous foliage and soils across land-uses in Poughkeepsie NY, Hartford CT, and Springfield MA USA.

Mercury is a global pollutant that harms human and wildlife health through chronic exposure. The role of urban forests in Hg biogeochemistry has been understudied in cities without historical mining or current coal combustion. This study aimed to quantify total Hg concentrations and pools in urban forests to determine whether adjacent land-use impacts Hg accumulation. Three cities in the northeastern United States were studied: Hartford, Connecticut; Poughkeepsie, New York; and Springfield, Massachusetts. We identified ~20 urban forests sites in a ~10km by ~10km grid for each city and sampled foliage and soil at each site. Foliage from Populus exhibited significantly lower Hg concentrations (15.6±2.1ngg-1) than mean foliar Hg concentrations (23.7±0.6ngg-1) but most deciduous genera had comparable concentrations. Average forest floor Hg concentrations (195±21ngg-1) and Hg pools (1.9±0.5mgm-2) were similar to previous, non-urban studies in the region. Average A horizon (182±19ngg-1) and B horizon (125±14ngg-1) Hg concentrations were double those of regional forest soils. Mineral soil Hg pools for the top 30cm (49±6mgm-2) averaged two to ten times higher than rural, montane forests in the region. Soil pH, LOI, and %clay were poorly correlated with mineral soil Hg concentrations. Instead, highest foliar and soil Hg concentrations and pools were in urban forests adjacent to high and medium intensity developed areas in Springfield and Hartford. To differentiate the impact of land-uses not captured by the National Land Cover Database (NLCD) system, we implemented new land-use categories. Industrial areas had highest foliar and soil Hg concentrations and pools of any land use. Our results show increasing land-use increases Hg accumulation in urban forests.

Modelling Hg mobility in podzols: Role of soil components and environmental implications

A high-resolution soil sampling has been applied to two forest podzols (ACB-I and ACB-II) from SW Europe in order to investigate the soil components and processes influencing the content, accumulation and vertical distribution of Hg. Total Hg contents (THg) were 28.0 and 23.6 μg kg−1 in A horizons of ACB-I and ACB-II, then they strongly decreased in the E horizons and peaked in the Bhs horizons of both soils (55.3 and 63.0 μg kg−1). THg decreased again in BwC horizons to 17.0 and 39.8 μg kg−1. The Bhs horizons accounted for 46 and 38% of the total Hg stored (ACB-I and ACB-II, respectively). Principal component analysis (PCA) and principal components regression (PCR), i.e. using the extracted components as predictors, allowed to distinguish the soil components that accounted for Hg accumulation in each horizon. The obtained model accurately predicted accumulated Hg (R2 = 0.845) through four principal components (PCs). In A horizons, Hg distribution was controlled by fresh soil organic matter (PC4), whereas in E horizons the negative values of all PCs were consistent with the absence of components able to retain Hg and the corresponding very low THg concentrations. Maximum THg contents in Bhs horizons coincided with the highest peaks of reactive Fe and Al compounds (PC1 and PC2) and secondary crystalline minerals (PC3) in both soils. The THg distribution in the deepest horizons (Bw and BwC) seemed to be influenced by other pedogenetic processes than those operating in the upper part of the profile (A, E and Bhs horizons). Our findings confirm the importance of soils in the global Hg cycling, as they exhibit significant Hg pools in horizons below the uppermost O and A horizons, preventing its mobilization to other environmental compartments.

Mercury source changes and food web shifts alter contamination signatures of predatory fish from Lake Michigan

To understand the impact reduced mercury (Hg) loading and invasive species have had on methylmercury bioaccumulation in predator fish of Lake Michigan, we reconstructed bioaccumulation trends from a fish archive (1978 to 2012). By measuring fish Hg stable isotope ratios, we related temporal changes in Hg concentrations to varying Hg sources. Additionally, dietary tracers were necessary to identify food web influences. Through combined Hg, C, and N stable isotopic analyses, we were able to differentiate between a shift in Hg sources to fish and periods when energetic transitions (from dreissenid mussels) led to the assimilation of contrasting Hg pools (2000 to present). In the late 1980s, lake trout δ202Hg increased (0.4‰) from regulatory reductions in regional Hg emissions. After 2000, C and N isotopes ratios revealed altered food web pathways, resulting in a benthic energetic shift and changes to Hg bioaccumulation. Continued increases in δ202Hg indicate fish are responding to several United States mercury emission mitigation strategies that were initiated circa 1990 and continued through the 2011 promulgation of the Mercury and Air Toxics Standards rule. Unlike archives of sediments, this fish archive tracks Hg sources susceptible to bioaccumulation in Great Lakes fisheries. Analysis reveals that trends in fish Hg concentrations can be substantially affected by shifts in trophic structure and dietary preferences initiated by invasive species in the Great Lakes. This does not diminish the benefits of declining emissions over this period, as fish Hg concentrations would have been higher without these actions.

Mercury distribution in the foliage and soil profiles of a subtropical forest: Process for mercury retention in soils

Forest canopy can exert great influence on the global mercury (Hg) accumulations and soil Hg pools. However, there exists a debate that forests are net sinks or sources of atmospheric Hg. The current study aimed to study whether forest soils in the southwestern China are net sinks for atmospheric Hg and the soil residence time of Hg in the subtropical forest soils. Foliage samples of fern and pine in sixty-six plots and six soil profiles were sampled at the Tieshanping Forest Park (TFP) in southwestern China. To study the process of Hg budget, the flux of Hg input and the output from the forest soils were estimated and the mean soil Hg residence time (MRT) were calculated using a simple two-box model. The Hg concentrations in foliage (98 ng g−1) was relatively higher than remote areas because the elevated atmospheric Hg concentrations. Mercury concentrations in the understory foliage were positively correlated with altitude, which was resulted from the altitudinal distribution of increasing soil Hg concentrations. The annual Hg deposition flux via litterfall was estimated at 42 μg m−2 and the total Hg input to the forests was approximately 108 μg m−2. The annual Hg retention in forest soil was about at 76 μg m−2. The MRT was longer in surface horizon (0–10 cm, 134 ± 14 yr) than that in the mineral horizon (20–80 cm, 595 ± 34 yr). Subtropical forests in southwestern China act as net sinks for atmospheric Hg and 69% of the total Hg deposition was sequestrated in the forest soils. Short residence time of Hg in surface soil enhanced the deposited Hg output from the forest, which may be the reason for the comparable soil Hg concentration with some other remote forests with lower Hg depositions.

Mercury Isotope Fractionation in the Subsurface of a Hg(II) Chloride-Contaminated Industrial Legacy Site.

To understand the transformations of mercury (Hg) species in the subsurface of a HgCl2-contaminated former industrial site in southwest Germany, Hg isotope analysis was combined with an investigation of Hg forms by a four-step sequential extraction protocol (SEP) and pyrolytic thermodesorption. Data from two soil cores revealed that the initial HgCl2 was partly reduced to metallic Hg(0) and that Hg forms of different mobility and oxidation state coexist in the subsurface. The most contaminated sample (K2-8, 802 mg kg–1 Hg) had a bulk δ202Hg value of around −0.43 ± 0.06‰ (2SD), similar to published average values for industrial Hg sources. Other sample signatures varied significantly with depth and between SEP pools. The most Hg-rich samples contained mixtures of Hg(0) and Hg(II) phases, and the water-extractable, mobile Hg pool exhibited heavy δ202Hg values of up to +0.18‰. Sequential water extracts revealed slow dissolution kinetics of mobile Hg pools, continuously releasing isotopically heavy Hg into solution. This was further corroborated by heavy δ202Hg values of groundwater samples. Our results demonstrate that the Hg isotope signature of an industrial contamination source can be significantly altered during the transformations of Hg species in the subsurface, which complicates source tracing applications but offers the possibility of using Hg isotopes as process tracers in contaminated subsurface systems.

Spatial distribution of mercury in seawater, sediment, and seafood from the Hardangerfjord ecosystem, Norway

Hardangerfjord is one of the longest fjords in the world and has historical mercury (Hg) contamination from a zinc plant in its inner sector. In order to investigate the extent of Hg transferred to abiotic and biotic ecosystem compartments, Hg and monomethylmercury (MeHg) concentrations were measured in seawater, sediment, and seafood commonly consumed by humans. Although total mercury in seawater has been described previously, this investigation reports novel MeHg data for seawater from Norwegian fjords. Total Hg and MeHg concentrations in seawater, sediment, and biota increased towards the point source of pollution (PSP) and multiple lines of evidence show a clear PSP effect in seawater and sediment concentrations. In fish, however, similar high concentrations were found in the inner part of another branch adjacent to the PSP. We postulate that, in addition to PSP, atmospheric Hg, terrestrial run-off and hydroelectric power stations are also important sources of Hg in this fjord ecosystem. Hg contamination gradually increased towards the inner part of the fjord for most fish species and crustaceans. Since the PSP and the atmospheric Hg pools were greater towards the inner part of the fjord, it is not entirely possible to discriminate the full extent of the PSP and the atmospheric Hg contribution to the fjord food web. The European Union (EU) Hg maximum level for consumption was exceeded in demersal fish species including tusk (Brosme brosme), blue ling (Molva dypterygia) and common ling (Molva molva) from the inner fjord (1.08 to 1.89 mg kg−1 ww) and from the outer fjord (0.49 to 1.07 mg kg−1 ww). Crustaceans were less contaminated and only European lobster (Homarus gammarus) from inner fjord exceeded the EU limit (0.62 mg kg−1 ww). Selenium (Se) concentrations were also measured in seafood species and Se-Hg co-exposure dynamics are also discussed.

Tree species affects the concentration of total mercury (Hg) in forest soils: Evidence from a forest soil inventory in Poland

This study was performed to test the hypothesis that tree species significantly affects mercury (Hg) sequestration in forest soils. We analyzed the effect of seven dominant tree species (Scots pine, black alder, Norway spruce, silver birch, deciduous oak, silver fir, and European beech on the concentrations and pools of Hg in a range of forest soils in Poland. We set up 277 sample plots representing dominant tree species in Poland. Soil samples were taken and analyzed for total Hg content, soil texture, and soil C and nitrogen (N) content. Concentrations of total Hg in forest soil (organic and mineral horizons) varied by several orders of magnitude as a result of natural variations in organic matter, sand content, and altitude. Spatial analysis revealed that maximum concentrations (mg kg−1) and stocks (mg m−2) of Hg were related to mountain stands at higher elevations with loamy soils and greater accumulation of soil organic matter. The stocks of Hg in the investigated soil profiles increased in the order of: pine (12 mg m−2) ≈ birch (15 mg m−2) < oak (21 mg m−2) ≈ alder (24 mg m−2) < beech (45 mg m−2) ≈ spruce (50 mg m−2) < fir (66 mg m−2). Simple analysis of variance suggested an important effect of dominant tree species on Hg concentrations and stocks in entire soil profiles, but multiple regression analysis showed that dominant tree species had a significant effect on accumulation of Hg in soil, but only in the organic horizon; in mineral soil the Hg was content was related to C content, soil texture and altitude. The organic horizon had greater accumulation of Hg under coniferous tree species (Scots pine, silver fir and Norway spruce) and European beech when compared with deciduous oak, black alder, and silver birch.