Increasing Hg Concentration Research Articles

Sediment records of highly variable mercury inputs to mountain lakes in Patagonia during the past millennium

Abstract. High Hg levels in the pristine lacustrine ecosystems of the Nahuel Huapi National Park, a protected zone situated in the Andes of Northern Patagonia, Argentina, have initiated further investigations on Hg cycling and source identification. Here we report Hg records in sedimentary sequences to identify atmospheric sources during the past millennium. In addition to global transport and deposition, a potential atmospheric Hg source to be considered is the local emissions associated with volcanic activity, because the Park is situated in the Southern Volcanic Zone. Two sediment cores were extracted from Lake Tonček, a small, high-altitude system reflecting mainly direct inputs associated with atmospheric contributions, and Lake Moreno Oeste, a much larger and deeper lake having an extended watershed covered mostly by native forest. The sedimentary sequences were dated based on both 210Pb and 137Cs profiles. In addition, tephra layers were identified and geochemically characterized for chronological application and to investigate any association of volcanic eruptions with Hg records. Hg concentrations in sediments were measured along with 32 other elements, as well as organic matter, subfossil chironomids, and biogenic silica. Observed background Hg concentrations, determined from the sequence domains with lower values, ranged from 50 to 100 ng g−1 dry weight (DW), whereas the surficial layers reached 200 to 500 ng g−1 DW. In addition to this traditional pattern, however, two deep domains in both sequences showed dramatically increased Hg levels reaching 400 to 650 ng g−1 DW; the upper dated to the 18th to 19th centuries, and the lower around the 13th century. These concentrations are not only elevated in the present profiles but also many-fold above the background values determined in other fresh water sediments, as were also the Hg fluxes, reaching 120 to 150 μg m−2 y−1 in Lake Tonček . No correlation was observed between Hg concentrations and the contents of organic matter, subfossil chironomids, biogenic silica, or the other elements determined. However, distinctly increased Hg concentrations were observed immediately above some tephra layers, suggesting a link to volcanic events. Extended fires might be another potential atmospheric source because the earlier Hg peaks coincide with reported charcoal peaks, whereas the upper Hg peaks coincide with evidences of extended forest fires from tree-ring data and historical records.

Antioxidative response of Atriplex codonocarpa to mercury

Seedlings of Atriplex codonocarpa were grown for 4 weeks in hydroponic culture containing 0, 0.05, 0.1 and 1 mg L −1 Hg. Mercury concentrations increased in both shoots and roots with increasing Hg concentration in the medium. The greatest accumulation (6001 mg kg −1 dry weight, DW) was in roots as a result of the 1 mg L −1 Hg treatment. Mercury inhibited plant growth, mainly in roots. Membrane integrity, measured as solute leakage from roots, increased with the increase in external Hg concentration whereas total glutathione decreased in both shoots and roots. In roots of plants treated with 0.1 mg L −1 Hg, total ascorbate (AsA + DHA) was 4.5-fold higher than in the control. The AsA/DHA ratio in shoots was found to increase proportionally with the increase in the external Hg concentration while in roots this ratio was much lower than in shoots and decreased up to 0.1 mg L −1 Hg after which it slightly increased. Glutathione reductase (GR) activity was inhibited by 65% in roots and by 20–30% in shoots after 1 mg L −1 Hg treatment. However, mercury enhanced the activity of ascorbate peroxidase (APX) in both shoots and roots with the maximum activity in roots at 0.05 mg L −1 Hg treatment more than doubling. A significant Hg induced increase in the superoxide dismutase (SOD) activity in roots of A. codonocarpa seedlings was observed which peaked at 0.1 mg L −1 Hg. In shoots, SOD activity increased gradually and levelled off at 0.1 mg L −1 Hg. Two isoforms of SOD were detected in both shoots and roots of A. codonocarpa under Hg stress and they were identified as CuZn-SODs.

Effects of Diet Composition and Trophic Structure on Mercury Bioaccumulation in Temperate Flatfishes

The summer flounder Paralichthys dentatus and winter flounder Pseudopleuronectes americanus support valuable fisheries along the northeastern United States. The importance of these flatfish as a human dietary resource indicates they are potential sources of mercury (Hg) to fish-consuming citizens. In this study, summer flounder (SF) and winter flounder (WF) were collected from the Narragansett Bay (Rhode Island, USA) and were measured for total Hg burden in whole-body or dorsal muscle tissue. Interspecies differences in Hg contamination were analyzed relative to flounder body size, age, and Hg content of preferred prey. Stable isotope signatures were also used to elucidate the effect of trophic processes on Hg accumulation in the estuarine food web. The mean Hg content of SF exceeded concentrations measured in WF across multiple life-history stages (0.039-0.100 and 0.016-0.029 mg Hg/kg wet weight for SF and WF, respectively), and observed values for both species were lower than the US Environmental Protection Agency regulatory threshold of 0.3 mg Hg/kg wet weight. Total Hg concentrations were also positively correlated with flounder age and length, verifying that both flatfish bioaccumulate Hg. SF accumulate Hg at an accelerated rate, however, owing to this species consuming Hg-enriched prey (teleosts, squid, and macrocrustaceans; mean Hg content = 0.023 mg Hg/kg wet weight), whereas WF feed on prey with low Hg levels (amphipods and polychaetes; mean Hg content = 0.013 mg Hg/kg wet weight). The positive correlation observed between mean biota Hg content and stable nitrogen (delta(15)N) isotope signatures further indicates that Hg is trophically transferred through the food web, and higher trophic level organisms (i.e., enriched delta(15)N) have increased Hg concentrations. Therefore, results from this study suggest that dietary preference and trophic structure are the main factors affecting Hg bioaccumulation in the estuary. Total Hg concentrations of flatfish from the Narragansett Bay, however, do not necessarily reflect coastwide contamination patterns. This reinforces the importance of having research conducted at sufficiently small spatial scales, including the local assessment of Hg contamination for the purpose of issuing state consumption advisories.

Mercury Concentrations in Surface Water and Harvested Waterfowl from the Prairie Pothole Region of Saskatchewan

Mercury cycling in prairie ecosystems is poorly understood. We examined methylmercury (MeHg) concentrations in whole water from 49 diverse prairie wetlands and lakes in Saskatchewan. We also determined total Hg (THg) concentrations in waterfowl harvested by hunters for consumption. Average whole water MeHg concentrations ranged from 0.02 to over 4 ng L(-1) and were higher in water from wetland ponds compared to those in lakes. High MeHg concentrations in prairie wetlands present the possibility of increased Hg concentrations in biota inhabiting these and other similar systems. We therefore measured THg in 72 birds representing 13 species of waterfowl that commonly use prairie aquatic habitats. A large range in THg concentrations was observed among individual birds, with values ranging from below the detection limit to over 435 ng g(-1). When waterfowl were classified according to diet, we observed clear evidence of THg biomagnification with increasing proportion of animal prey consumed. THg concentrations in waterfowl collected by hunters did not exceed consumption guidelines of 0.5 mg kg(-1) developed for fish. This is the first study that has reported MeHg concentrations in water from the prairie pothole region of southern Saskatchewan.

Low-Level Mercury Speciation in Freshwaters by Isotope Dilution GC-ICP-MS

Atmospheric deposition of anthropogenic Hg has led to increased Hg concentrations in many ecosystems. Modeling is an effective method for predicting the complex dynamics of Hg transport in watersheds. Such models require accurate concentrations of water column methylmercury, CH3Hg+, for input parameters, yet these concentrations are very difficult to measure precisely because they are so low. We developed a method for aqueous CH3Hg+ quantification in Lake Champlain, VT, where ambient CH3Hg+ concentrations are < 0.04 ng L(-1). The analysis utilized species-specific isotope dilution, purge and trap gas chromatography inductively coupled plasma mass spectrometry and provided instrument detection limits of about 0.3 fM (0.06 pg L(-1)) and method detection limits of 15 fM (0.003 ng L(-1)) for CH3Hg+, which are among the lowest reported. Artifactual methylation of inorganic Hg(2+) was shown to be minor, and the precision of the isotope dilution method was generally <5% relative standard deviation, which is much lower than would have been the case for an external calibration approach. The method is accurate even at low concentrations of 0.025 ng L(-1). This combination of precision, accuracy, and low detection allows for quantification of significant differences in CH3Hg+ concentration between bays and over time within bays of Lake Champlain, where mean CH3Hg+ concentrations differ by only 0.006 ng L(-1) at concentrations as low as 0.014 ng L(-1).

VERTICAL STABILITY OF MERCURY IN HISTORIC AND PREHISTORIC SEDIMENTS FROM CLEAR LAKE, CALIFORNIA

Clear Lake, California, USA, is the site of the Sulphur Bank Mercury Mine, now a U.S. Environmental Protection Agency Superfund Site. Intermittent mining from 1873 to 1957 resulted in approximately 100 Mg of mercury (Hg) being deposited into the lake's ecosystem. Sediment cores to approximately 2.5 m depth (dated using 210Pb and 14C) represent approximately 3000 years of sedimentation. Clear Lake sediments have experienced Hg deposition from anthropogenic sources (mining) during historic times (to the mid-1900s) and geologic sources during prehistoric times (prior to the mid-1800s). This provides a unique opportunity to evaluate hypotheses relating to (1) the influence of the mine on Hg loading to the lake and (2) the potential upward mobilization of Hg by diagenetic processes proposed by some as an alternative explanation for increased Hg concentrations at the surface of the sediment column believed to be caused by increased global atmospheric deposition. Although Hg mining began in 1873, no significant evidence of anthropogenic Hg loading was detected in cores prior to open-pit mining ca. 1927, which also involved bulldozing mine waste rock and tailings into the lake. Exponential increases in total Hg (TotHg) and methylmercury (MeHg) were observed above the 1927 horizon, where estimated sedimentation rates were 2.2-20.4 mm/yr and peaks of both forms of Hg maintained vertical stability within the sediment column. Below the 1927 horizon, a slow increase in both TotHg and MeHg with depth was observed from approximately 1000 to 3000 years before present, where sedimentation rates ranged from approximately 0.6 to 2.0 mm/yr and elevated Hg profiles appear stable. Vertical stability of Hg in the shallow and deep sediment column suggests that both TotHg and MeHg do not undergo diagenetic upward mobilization within the sediment column under rapid or slow sedimentation rates. Because (1) these data were collected at a site with known anthropogenic and geologic sources and (2) regions of elevated Hg concentrations from both sources remain stable within the sediment column under very different sedimentation regimes, these results also support the hypothesis that elevated Hg at the surface of cores in other worldwide locations likely represents global atmospheric deposition rather than upward diagenetic mobilization.

Impact of human activities on the central Mediterranean offshore: Evidence from Hg distribution in box-core sediments from the Ionian Sea

Total Hg concentrations have been measured for five box-core sediments collected seawards of the Augusta industrial area (SE Sicily). In more coastal sediments, upcore increasing Hg concentrations, exceeding the Hg background concentration estimated for the Strait of Sicily, indicate Hg contamination over time due to the industrial area development. Strong correlation between total organic C (TOC) and Hg concentrations was found only for core BX2, that displays organic C to total N (C/N) ratios indicative of autochthonous organic matter. For other sediments, high Hg enrichment factors with respect to TOC indicate, in addition to Hg trapping by TOC, other factors as responsible for Hg accumulation. In the presence of some contribution of detrital organic matter, Hg is mainly adsorbed onto the mineral component of the bottom sediments probably because TOC is saturated by Hg excess. Contaminant impact affected also the open sea environment. Main drivers of Hg flux towards the offshore were dredged materials, which repeatedly discharged sediment, resulting in substantial increases in TOC contents and high C/N ratios. Consistent with the geochemistry of recent turbidites, these anomalous sedimentary inputs induced sediment redox environment modifications, constrained by Mn peaks, which affected Hg distribution.

Genotypic variation in grain mercury accumulation of lowland rice

AbstractA pot experiment with 38 commonly cultivated rice cultivars from the Yangtze River Delta was conducted in a greenhouse to study the effect of mercury (Hg) contamination of a paddy soil (4.7 mg Hg [kg soil]–1) on crop growth and Hg accumulation in the grains. Mercury contamination differentially affected growth, grain yield, and Hg accumulation in brown rice of the tested cultivars. The average Hg concentration in the grains was significantly higher (p &lt; 0.01) when plants were grown in the Hg‐contaminated compared to the control soil. Averaged over cultivars, the Hg concentration in brown rice exceeded the maximum permissible limit of 20 μg Hg kg–1. Increasing Hg concentrations were associated with grain‐yield declines up to 70%. This yield decline was mainly due to a reduction in the number of panicles. Japonica cultivars tended to be generally less affected by Hg than indica cultivars. The two japonica cultivars Jiahua and Chunjiang 026 showed both low Hg concentrations and no Hg‐induced grain‐yield reduction. These cultivars may be preferred candidates for cultivation in Hg‐contaminated soils or for studies on possible Hg‐exclusion mechanisms.

Impact of mining activities on soils and sediments at the historical mining area in Podljubelj, NW Slovenia

The abandoned Hg mine in Podljubelj was in operation with interceptions from 1557 to 1902. The entire operating period yielded about 110 000 tons of ore, from which 360 tons of Hg was produced. The objective of the research project was to establish the contents and spatial distribution of Hg in soils and stream sediments in the vicinity of the mine. On an area of 88 ha the soil was sampled in a 100 × 100 m grid. Two soil horizons (0–5 cm and 20–30 cm) were sampled in order to distinguish between geogenic and anthropogenic Hg sources. It was established that on an area of about 9 ha Hg content in soil exceeds The New Dutchlist action value for Hg (10 mg/kg). Total Hg concentrations in soil samples vary between 0.17 and 719 mg/kg, with a mean of 3.0 mg/kg. Mercury contents in stream sediments range from 0.065 to 1.4 mg/kg, with a mean of 0.64 mg/kg. The highest determined value in soils was found in the area around the former roasting furnace, where the ore was processed. Increased Hg concentrations were also found on the mine waste dump (108 mg/kg). Mercury contents in soils generally decrease with soil profile depth and with the distance from the mine and from the roasting furnace location. Mercury also appears in higher concentrations along the road that runs through the valley, which results from the use of Hg-bearing ore residues in road construction. The average enrichment factor (EF) of Hg in topsoil with respect to subsoil is 3.3. Calculated enrichment factors show higher values also for Cd (3.2), Pb (2.7), Ca (2.4) and P (1.9). The average enrichment factor of Hg in topsoil with regard to the established Slovenian soil averages (EF slo) is 19. EF slo of other determined chemical elements do not exceed 3.0.

The Island of Chios (east Mediterranean), Citrus Plantations and the Mercury Nightmare

Mercury poisoning, as a result of environmental pollution, has become a problem of current interest on a global scale. Natural emissions of mercury form two-thirds of the input; manmade releases form about one-third (Patra and Sarma 2000). Accumulation, toxicity response and mercury distribution seem to affect the plants and differ between plant species. It has been observed that the chlorophyll, protein and nitrogen, phosphorus, potassium contents decrease significantly with increasing Hg concentrations in young plants of Vallisneria spiralis (Gupta and Chandra 1998) while the lower the nutrient strength in the culture medium is, the higher the metal concentrations accumulated in the different plant parts (Gothberg et al. 2004). In plants of Bacopa monieri grown in high concentrations of Hg, the metal accumulation in the root tissues was about five times more than in the shoot (Sinha et al. 1996) while tobacco plants (Nicotiana tabaccum) are less tolerant to heavy metals, including Hg, than sunflower (Helianthus annuus) (Ruso et al. 2001). Salix species investigated for heavy metal uptake were found to absorb Cd and Zn at a rate higher than they absorb Ni, Hg, Cu and Pb (Labrecque et al. 1995). Chios is a Greek island globally known for the mastic trees (Pistacia lentiscus var. chia) and their products. It is located at the north eastern part of the Aegean Sea, just three and a half miles from the coast of Asia minor. The bed of the main water basin extending below the farmlands of the island, is composed of cinnabar (a mineral containing Hg). During the summer months, excessive pumping for plant irrigation lowers the underground water level by 12–15 cm daily! In a few weeks water starts to turn salty since seawater intrudes the basin. This change induces the release of mercury in the water through the reaction of NaCl with the cinnabar substrate. It is common that mercury concentration, in the basin’s water, raises to 7.5 lg Hg/L while the upper limit posed by the European Union directive is 1 lg Hg/L. It seems that the local authorities and the islanders have to deal with a serious problem. Given the water shortage in most Mediterranean islands and the extensive cultures of citrus fruits established on Chios island, a serious approach to the mercury problem has to be made. Although there is a tendency for mercury to accumulate in roots, indicating that roots serve as a barrier to mercury uptake (Patra and Sarma 2000), only after a thorough investigation can leaves and, mainly, the fruits of citrus trees be considered free of mercury contamination.

Influence of Fluctuating Water Levels on Mercury Concentrations in Adult Walleye

Atmospheric deposition of mercury (Hg) has been shown to be a significant source of Hg on the landscape (Rada et al. 1989; Swain et al. 1992) and is believed to contribute to increased Hg concentrations in aquatic food webs (Sorensen et al. 1994; Edwards et al. 1999). Methylmercury (MeHg) is the organic, bioavailable form of Hg that accumulates to toxic levels in top-level predators in aquatic systems (Suedel et al. 1994). Although limnological conditions in lakes and rivers can affect Hg methylation and concentrations in fishes, these relationships often vary among water bodies and fish species (McMurtry et al. 1989; Bodaly et al. 1993). Thus, regional studies are needed to identify mechanisms of local MeHg production and factors associated with Hg contamination in fishes. The Prairie Pothole Region of North America has a unique variety of natural wetlands and glacial lakes that are important for fish, shorebirds, waterfowl, and humans. Cyclical climate, characterized by extended wet–dry periods, cause high variation in water surface area within the region (Rosenberry 2003). For example, consecutive years of high precipitation during the mid-1990s caused dramatic surface area increases in many glacial lakes and wetlands of eastern South Dakota. After lake levels increased, several fish populations were found to contain elevated Hg concentrations (>1 lg/g), prompting local officials to post fish consumption advisories (South Dakota Game, Fish and Parks, 2006). The discovery of elevated Hg concentrations in fishes was surprising because (1) there were no apparent pointsource inputs of Hg, and (2) the limnological conditions of most of these lakes (eutrophic, high pH) generally do not favor Hg methylation or bioaccumulation (Grieb et al. 1990; Pickhardt et al. 2002). Moreover, lakes that experienced large increases in surface area generally contained fast growing fish populations, a situation that usually lowers Hg concentration due to growth dilution (Rodgers and Qadri 1982; MacCrimmon et al. 1983). In this study, we document changes in lake surface area for glacial lakes in the Prairie Pothole Region and relate this to Hg concentration in adult walleye (Sander vitreus). Although a similar phenomenon is known to occur in reservoirs with fluctuating water levels (Jackson 1988; Snodgrass et al. 2000; Sorensen et al. 2005), widespread effects of surface area changes on Hg concentrations in fishes have not been documented in natural, glacial lakes. We hypothesized that increases in lake surface area enhanced T. M. Selch (&) S. R. Chipps U.S. Geological Survey South Dakota Cooperative Fish and Wildlife Research Unit, Department of Wildlife and Fisheries Sciences, South Dakota State University, Box 2140B, Brookings, SD 57007-1696, USA e-mail: Trevor.Selch@sdstate.edu

Atmospheric Monitoring at Abandoned Mercury Mine Sites in Asturias (NW Spain)

Mercury concentrations are usually significant in historic Hg mining districts all over the world, so the atmospheric environment is potentially affected. In Asturias, northern Spain, past mining operations have left a legacy of ruins and Hg-rich wastes, soils and sediments in abandoned sites. Total Hg concentrations in the ambient air of these abandoned mine sites have been investigated to evaluate the impact of the Hg emissions. This paper presents the synthesis of current knowledge about atmospheric Hg contents in the area of the abandoned Hg mining and smelting works at 'La Peña-El Terronal' and La Soterraña, located in Mieres and Pola de Lena districts, respectively, both within the Caudal River basin. It was found that average atmospheric Hg concentrations are higher than the background level in the area (0.1 microg Nm(-3)), reaching up to 203.7 microg Nm(-3) at 0.2 m above the ground level, close to the old smelting chimney at El Terronal mine site. Data suggest that past Hg mining activities have big influences on the increased Hg concentrations around abandoned sites and that atmospheric transfer is a major pathway for Hg cycling in these environments.

Environmental biogeochemistry of mercury in Antarctic ecosystems

Polar regions are recognized as important sinks for long-range transport and deposition of Hg derived from natural and anthropogenic sources at lower latitudes. In previous studies we found enhanced Hg accumulation in soils, mosses and lichens from ice-free areas of Victoria Land facing the Terra Nova Bay coastal polynya. This study extends research to the distribution of organic C, total N, S, Hg, Al and Fe in surface soils, cyanobacterial mats and short sediment cores from four lacustrine ecosystems, each with different environmental characteristics and varying distances from the polynya. Results show that planktonic and benthic mats from lakes, along with mosses in the watershed, are the main sinks for Hg in summer meltwater. The C-normalized Hg concentrations in short sediment cores were higher in samples from lakes more exposed to marine aerosols from the coastal polynya. Reactive halogens in the aerosol promote the oxidation and deposition of atmospheric Hg in coastal ecosystems. The analysis of sediment cores did not reveal increasing Hg concentrations in recent sediments, except in the Lake 14 at Edmonson Point. The latter ice-free area is unaffected by the polynya and the increase in Hg concentrations in surface sediments could be due to local changes in lake water level and S biogeochemistry. Although change in sea ice coverage may enhance the role of Antarctic coastal ecosystems as sink in the global Hg cycle, our results seem to exclude possible risks for Antarctic terrestrial and freshwater organisms.

Fish mercury increase in Lago Manso, a new hydroelectric reservoir in tropical Brazil

It has been frequently demonstrated that mercury (Hg) concentrations in fish rise in newly constructed hydroelectric reservoirs in the Northern Hemisphere. In the present work, we studied whether similar effects take place also in a tropical upland reservoir during impoundment and discuss possible causes and implications. Total Hg concentrations in fish and several soil and water parameters were determined before and after flooding at Rio Manso hydroelectric power plant in western Brazil. The Hg concentrations in soil and sediment were within the background levels in the region (22–35 ng g −1 dry weight). There was a strong positive correlation between Hg and carbon and sulphur in sediment. Predatory fish had total Hg concentrations ranging between 70 and 210 ng g −1 f.w. 7 years before flooding and between 72 and 755 ng g −1 f.w. during flooding, but increased to between 216 and 938 ng g −1 f.w. in the piscivorous and carnivorous species Pseudoplatystoma fasciatum, cachara, and Salminus brasiliensis, dourado, 3 years after flooding. At the same time, concentrations of organic carbon in the water increased and oxygen concentrations decreased, indicating increased decomposition and anoxia as contributing to the increased Hg concentrations in fish. The present fish Hg concentrations in commonly consumed piscivorous species are a threat to the health of the population dependent on fishing in the dam and downstream river for sustenance. Mercury exposure can be reduced by following fish consumption recommendations until fish Hg concentrations decrease to a safe level.

Gene Expression Changes Related to Endocrine Function and Decline in Reproduction in Fathead Minnow ( Pimephales promelas ) after Dietary Methylmercury Exposure

BackgroundMethylmercury (MeHg) is a known neurotoxic agent, but the mechanisms by which MeHg may act on reproductive pathways are relatively unknown. Several studies have indicated potential changes in hormone levels as well as declines in vertebrates with increasing dietary MeHg exposure.ObjectivesThe purpose of this study was to identify alterations in gene expression associated with MeHg exposure, specifically those associated with previously observed changes in reproduction and reproductive biomarkers. Fathead minnows, Pimephales promelas, were fed one of three diets that were similar to documented concentrations of MeHg in the diets of wild invertivorous and piscivorous fish. We used a commercial macroarray in conjunction with quantitative polymerase chain reaction to examine gene expression in fish in relation to exposure to these environmentally relevant doses of MeHg.ResultsExpression of genes commonly associated with endocrine disruption was altered with Hg exposure. Specifically, we observed a marked up-regulation in vitellogenin mRNA in individual Hg-exposed males and a significant decline in vitellogenin gene expression in female fish with increasing Hg concentrations. Other genes identified by the macroarray experiment included those associated with egg fertilization and development, sugar metabolism, apoptosis, and electron transport. We also observed differences in expression patterns between male and female fish not related to genes specifically associated with reproduction, indicating a potential physiological difference in the reaction of males and females to MeHg.ConclusionGene expression data may provide insight into the mechanisms by which MeHg affects reproduction in fish and indicate how MeHg differs in its effect from other heavy metals and endocrine-disrupting compounds.

Bioaccumulation and physiological effects of mercury in Sesbania drummondii

The accumulation of mercury and its effect on growth, photosynthesis and antioxidative responses were studied in Sesbania drummondii seedlings. Mercury concentration in shoots as well as in the roots increased with increasing Hg concentrations in the growth solution. The accumulation of Hg was more in roots than shoots. At 100 mg l −1 Hg concentration, shoots accumulated 998 mg Hg kg −1 dry weight (dw) while roots accumulated 41 403 mg Hg kg −1 dw. Seedlings growth was not significantly affected at lower concentrations of Hg. A concentration of 100 mg l −1 Hg inhibited growth by 36.8%, with respect to control. Photosynthetic activity was assessed by measuring chlorophyll a fluorescence by determination of F v/ F m and F v/ F o values. Photosynthetic integrity was not affected up to 50 mg l −1 Hg concentration, however, concentrations higher than 50 mg l −1 affected photosynthetic integrity. Sesbania responded to Hg induced oxidative stress by modulating non-enzymatic antioxidants [glutathione (GSH) and non-protein thiols (NPSH)] and enzymatic antioxidants: superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR). Glutathione content and GSH/GSSG ratio increased up to a concentration of 50 mg l −1 while slight down at 100 mg l −1 Hg. The content of NPSH significantly increased with increasing Hg concentrations in the growth medium. The activities of antioxidative enzymes, SOD, APX and GR followed the same trends as antioxidants first increased up to a concentration of 50 mg l −1 Hg and then slight decreased. The results of present study suggest that Sesbania plants were able to accumulate and tolerate Hg induced stress using an effective antioxidative defense mechanisms.

Formation of dissolved gaseous mercury in a tropical lake (Petit-Saut reservoir, French Guiana)

Formation of dissolved gaseous mercury (DGM) and its volatilization from aquatic systems can be considered as a natural attenuation process, which limits the methylation of mercury (Hg) and Hg accumulation in fish. Although gold-mining activity and erosion of tropical soils lead to increased Hg concentration in aquatic systems, little is known about DGM production and distribution in tropical aqueous systems. Our work explores the vertical distribution and principal sources of DGM in a meromictic Amazonian reservoir. Dissolved gaseous mercury measurements carried out in Petit-Saut reservoir (French Guiana, South America) revealed DGM increase in the surface waters and at the bottom layers of the reservoir during the dry season. As in arctic and temperate lakes, high DGM concentrations in surface waters were attributed to sunlight-mediated photochemical processes. Dissolved gaseous mercury concentrations in the anaerobic hypolimnion were larger than in temperate or arctic lakes. In order to elucidate Hg(II) reduction pathways in the bottom layer of tropical reservoir, laboratory Hg(II) reduction experiments were performed with anoxic aqueous suspensions of surface sediments either untreated or treated by γ-ray and NaN 3. Our results indicated that DGM production at the bottom layer of Petit-Saut reservoir was biologically mediated. Dissolved gaseous mercury formation rates in the surface sediment suspensions were of the same order of magnitude as formation rates in freshwater lakes reported in literature.

Long-term Atmospheric Mercury Wet Deposition at Underhill, Vermont

Section 112(m) of the 1990 Clean Air Act Amendments, referred to as the Great Waters Program, mandated an assessment of atmospheric deposition of hazardous air pollutants (HAPs) to Lake Champlain. Mercury (Hg) was listed as a priority HAP and has continued to be a high priority for a number of national and international programs. An assessment of the magnitude and seasonal variation of atmospheric Hg levels and deposition in the Lake Champlain basin was initiated in December 1992 which included event precipitation collection, as well as collection of vapor and particle phase Hg in ambient air. Sampling was performed at the Proctor Maple Research Center in Underhill Center, VT. The range in the annual volume-weighted mean concentration for Hg in precipitation was 7.8-10.5 ng/l for the 11-year sampling period and the average amount of Hg deposited with each precipitation event was 0.10 microg/m2. The average amount of Hg deposited through precipitation each year from 1993 to 2003 was 9.7 microg/m2/yr. A seasonal pattern for Hg in precipitation is clearly evident, with increased Hg concentrations and deposition observed during spring and summer months. While a clear trend in the 11-year event deposition record at Underhill was not observed, a significant decrease in the event max-to-monthly ratio was observed suggesting that a major source influence was controlled over time. Discrete precipitation events were responsible for significant fractions of the monthly and annual loading of Hg to the forested ecosystem in Vermont. Monthly-averaged temperatures were found to be moderately correlated with monthly volume-weighted mean Hg concentrations (r2 = 0.61) and Hg deposition (r2 = 0.67) recorded at the Vermont site. Meteorological analysis indicated the highest levels of Hg in precipitation were associated with regional transport from the west, southwest, and south during the warmer months.

Uptake of mercury (Hg) by seedlings of rice ( Oryza sativa L.) grown in solution culture and interactions with arsenate uptake

We investigated the mercury (Hg) uptake by seedlings of rice ( Oryza sativa L.) grown in solution and interactions between Hg and arsenate uptake. The results showed that increasing Hg 2+ concentrations in the nutrient solution decreased both root and shoot biomass. Hg 2+ at concentrations of 1.0 and 2.5 mg L −1 caused 50% reduction in root biomass. A 50% reduction in shoot biomass occurred at Hg 2+ concentrations of around 0.5 mg L −1. Nevertheless, 0.5 mg As L −1 has no significant effect on plant yield. Hg accumulated in rice roots, and the Hg concentration factor in roots reached nearly 1900 at 2.5 mg Hg L −1. The addition of As slightly increased the Hg concentration in the roots. However, As concentrations in the roots decreased significantly with increasing Hg concentration in the growth solution to 1.0 or 2.5 mg Hg L −1. Shoot As concentrations decreased with increasing Hg concentrations in the growth solution, but increased again with further increase in Hg concentration to 2.5 mg L −1. Possible mechanisms of Hg uptake and interactions between Hg and As in the uptake process are also discussed.

Mercury accumulation in the fish community of a sub‐Arctic lake in relation to trophic position and carbon sources

Summary 1 Stable isotope analysis has improved understanding of trophic relationships among biota. Coupled with contaminant analysis, stable isotope analysis has also been used for tracing the pattern and extent of biomagnification of contaminants in aquatic food webs. 2 Combined analysis of nitrogen (δ 15 N) and carbon (δ 13 C) isotopes from fish species in a sub-Arctic lake were related to tissue mercury (Hg) concentrations to assess whether carbon sources influenced Hg accumulation in fish, in addition to trophic position. 3 Statistical models were used to estimate Hg biomagnification and uptake, to elucidate Hg accumulation dynamics and to appraise the relative importance of Hg exposure routes for the fish species. 4 Species Hg contamination increased as a function of trophic position (δ 15 N) and was inversely related to the δ 13 C signature. Species connected to the benthic food chain had lower Hg concentrations than species connected to the pelagic food chain. Species undergoing ontogenetic dietary shifts with increasing size, e.g. lake trout Salvelinus namaycush , also showed increased Hg concentrations with increasing reliance on pelagic fish as prey. 5 The results indicate that both vertical (trophic) and horizontal (habitat) food web structure influence Hg concentrations in fish tissue. 6 The biomagnification and uptake models indicated that contamination at the base of the food chain in the lake exceeded estimates for more southerly environments, thereby demonstrating the importance of dietary and water column Hg exposure routes in the sub-Arctic for determining Hg concentrations in fish. 7 Overall, the data reported here demonstrate how a combination of ecological concepts (food webs), developing ecological methods (stable isotopes) and environmental geochemistry can combine profitably to indicate the risks of exposure to environmental contaminants. Additional studies of the dynamics of Hg accumulation in the food webs of sub-Arctic lakes are needed, particularly in the light of the estimated high biomagnification rates and the heavy reliance of Inuit communities on subsistence fish harvests.