Behavior of mercury in the Patuxent River estuary

An overview of a comprehensive study of the behavior and fate of mercury in the estuarine Patuxent River is presented. Total Hg (Hg_T) and methylmercury (MeHg) exhibited weakly non-conservative behavior in the estuary. Total Hg concentrations ranged from 6 ng L^-1 in the upper reaches of the sub-urbanized tidal freshwater river to <0.5 ng L^-1 in the mesohaline lower estuary. Filterable (0.2 µm) Hg_T ranged from 0.2 to 1.5 ng L^-1. On average, MeHg accounted for <5% of unfiltered Hg_T and <2% of filterable Hg_T. Dissolved gaseous section Hg (DGHg) concentrations were highest (up to 150 pg L^-1) in the summer in the mesohaline, but were not well correlated with primary production or chlorophyll a, demonstrating the complex nature of Hg⁰ formation and cycling in an estuarine environment. Organic matter content appeared to control the Hg_T content of sediments, while MeHg in sediments was positively correlated with Hg_T and organic matter, and negatively correlated with sulfide. MeHg in sediments was low (0.1 to 0.5% of Hg_T). Preliminary findings suggest that net MeHg production within sediments exceeds net accumulation. Although Hg_T in pore waters increased with increasing sulfide, bulk MeHg concentrations decreased. The concentration of MeHg in sediments was not related to the concentration of Hg_T in pore waters. These observations support the hypothesis that sulfide affects the speciation and therefore bioavailability of dissolved and/or solid-phase Hg for methylation. Comparison with other ecosystems, and the negative correlation between pore water sulfide and sediment MeHg, suggest that sulfide limits production and accumulation of MeHg in this system.     

System controls on the aqueous distribution of mercury in the northern Florida Everglades

The forms and partitioning of aqueous mercury species in the canals and marshes of the Northern Florida Everglades exhibit strong spatial and temporal variability. In canals feeding Water Conservation Area (WCA) 2A, unfiltered total Hg (HgT_U) is less than 3 ng L^-1 and relatively constant. In contrast, methyl mercury (MeHg) exhibited a strong seasonal pattern, with highest levels entering WCA-2A marshes during July. Stagnation and reduced flows also lead to particle enrichment of MeHg. In the marshes of WCA-2A, 2B and 3A, HgT_U is usually <5 ng L^-1 with no consistent north–south patterns. However, for individual dates, aqueous unfiltered MeHg (MeHg_U) levels increase from north to south with generally lowest levels in the eutrophied regions of northern WCA-2A. A strong relationship between filtered Hg species and dissolved organic carbon (DOC), evident for rivers draining wetlands in Wisconsin, was not apparent in the Everglades, suggesting either differences in the binding sites of DOC between the two regions, or non-organic Hg complexation in the Everglades.     

Trophic transfer of methyl mercury in the northern Florida Everglades

There are spatial differences in methyl mercury (MeHg) concentrations in biota in Water Conservation Areas 2 and 3 in the Everglades, with higher concentrations generally found in the southern areas. Fish and hemipterans had the most MeHg on a wet weight basis, with levels exceeding 30 ng g^-1. The magnitude of MeHg accumulation in biota varies seasonally and does not always appear to be associated with changes in water column concentration. This is exemplified by periphyton, the base of the foodweb in the Everglades, at a high nutrient sampling site. Although limited in scope, MeHg concentrations presented for biota provide insight into beginning to understand the dynamic nature of Hg transfer in the Everglades foodweb on a spatial and temporal basis.     

Aspects of Bioavailability of Mercury for Methylation in Pure Cultures of Desulfobulbus propionicus (1pr3)

We have previously hypothesized that sulfide inhibits Hg methylation by decreasing its bioavailability to sulfate-reducing bacteria (SRB), the important methylators of Hg in natural sediments. With a view to designing a bioassay to test this hypothesis, we investigated a number of aspects of Hg methylation by the SRB Desulfobulbus propionicus, including (i) the relationship between cell density and methylmercury (MeHg) production, (ii) the time course of Hg methylation relative to growth stage, (iii) changes in the bioavailability of an added inorganic Hg (Hg_I) spike over time, and (iv) the dependence of methylation on the concentration of dissolved Hg_I present in the culture. We then tested the effect of sulfide on MeHg production by this microorganism. These experiments demonstrated that under conditions of equal bioavailability, per-cell MeHg production was constant through log-phase culture growth. However, the methylation rate of a new Hg spike dramatically decreased after the first 5 h. This result was seen whether methylation rate was expressed as a fraction of the total added Hg or the filtered Hg_I concentration, which suggests that Hg bioavailability decreased through both changes in Hg complexation and formation of solid phases. At low sulfide concentration, MeHg production was linearly related to the concentration of filtered Hg_I. The methylation of filtered Hg_I decreased about fourfold as sulfide concentration was increased from 10⁻⁶ to 10⁻³ M. This decline is consistent with a decrease in the bioavailability of Hg_I, possibly due to a decline in the dissolved neutral complex, HgS⁰.     

Methylmercury and methane production potentials in North Carolina Piedmont stream sediments

Methylated mercury (MeHg) can be produced by all microbes possessing the genes hgcA and hgcB, which can include sulfate-reducing bacteria (SRB), iron-reducing bacteria (FeRB), methane-producing archaea (MPA), and other anaerobic microbes. These microbial groups compete for substrates, including hydrogen and acetate. When sulfate is in excess, SRB can outcompete other anaerobic microbes. However, low concentrations of sulfate, which often occur in stream sediments, are thought to reduce the relative importance of SRB. Although SRB are regarded as the primary contributors of MeHg in many aquatic environments, their significance may not be universal, and stream sediments are poorly studied with respect to microbial Hg methylation. We evaluated suppression of methanogenesis by SRB and the potential contributions from SRB, MPA and other MeHg producing microbes (including FeRB) to the production of MeHg in stream sediments from the North Carolina Piedmont region. Lower methanogenesis rates were observed when SRB were not inhibited, however, application of a sulfate-reduction inhibitor stimulated methanogenesis. Greater MeHg production occurred when SRB were active. Other MeHg producing microbes (i.e., FeRB) contributed significantly less MeHg production than SRB. MPA produced MeHg in negligible amounts. Our results suggest that SRB are responsible for the majority of MeHg production and suppress methanogenesis in mid-order stream sediments, similar to other freshwater sediments. Further investigation is needed to evaluate the generality of these findings to streams in other regions, and to determine the mechanisms regulating sulfate and electron acceptor availability and other potential factors governing Hg methylation and methane production in stream sediments.     

Aspects of bioavailability of mercury for methylation in pure cultures of Desulfobulbus propionicus (1pr3)

We have previously hypothesized that sulfide inhibits Hg methylation by decreasing its bioavailability to sulfate-reducing bacteria (SRB), the important methylators of Hg in natural sediments. With a view to designing a bioassay to test this hypothesis, we investigated a number of aspects of Hg methylation by the SRB Desulfobulbus propionicus, including (i) the relationship between cell density and methylmercury (MeHg) production, (ii) the time course of Hg methylation relative to growth stage, (iii) changes in the bioavailability of an added inorganic Hg (Hg(I)) spike over time, and (iv) the dependence of methylation on the concentration of dissolved Hg(I) present in the culture. We then tested the effect of sulfide on MeHg production by this microorganism. These experiments demonstrated that under conditions of equal bioavailability, per-cell MeHg production was constant through log-phase culture growth. However, the methylation rate of a new Hg spike dramatically decreased after the first 5 h. This result was seen whether methylation rate was expressed as a fraction of the total added Hg or the filtered Hg(I) concentration, which suggests that Hg bioavailability decreased through both changes in Hg complexation and formation of solid phases. At low sulfide concentration, MeHg production was linearly related to the concentration of filtered Hg(I). The methylation of filtered Hg(I) decreased about fourfold as sulfide concentration was increased from 10(-6) to 10(-3) M. This decline is consistent with a decrease in the bioavailability of Hg(I), possibly due to a decline in the dissolved neutral complex, HgS(0).     

Role of Organic Carbon Sources and Sulfate in Controlling Net Methylmercury Production in Riverbank Sediments of the South River,VA (USA)

Mercury (Hg) transport and methylmercury (MeHg) production in riverbank sediments are complex processes influenced by site-specific physical and biogeochemical conditions. The South River watershed in VA, USA, contains elevated concentrations of Hg in riverbank and floodplain sediments, which has the potential to methylate. The role of specific organic carbon sources in promoting methylation reactions in natural sediments under dynamic flow conditions is not well understood. Four saturated column experiments were conducted, including a control column, which received South River water as an influent solution, and three columns that received South River water amended with: acetate (5.8 mM); lactate (5.7 mM); and lactate (5.7 mM) with SO₄^2? (10.1 mM). The amendments were selected to promote growth of different microorganisms to gain an understanding of the microbial processes, controlling rates of methylation. The column receiving lactate and SO₄^2? had the highest MeHg concentrations in the effluent and in the pore water near the effluent at 1.8 and 4.9 μg L^?1, respectively. At the cessation of the column experiments, the lactate–sulfate column sediments contained the highest populations of enumerable sulfur-reducing bacteria and the highest solid-phase MeHg at 530 ± 100 ng g^?1 dry wt. from the interval closest to the influent. The results suggest that the form and availability of electron donors and acceptors are primary factors controlling rates of methylation in the South River sediment.     

Anaerobic Microflora of Everglades Sediments: Effects of Nutrients on Population Profiles and Activities

Everglades sediments (wetland soils) near sources of agricultural runoff had low redox potentials, were blackened with sulfide, and displayed high porewater phosphorus (total) concentrations and high water column conductivities. These sediments yielded 10(sup3)- to 10(sup4)-fold-higher numbers of culturable anaerobes, including methanogens, sulfate reducers, and acetate producers, than did sediments from Everglades and Lake Okeechobee comparative control sites not as directly associated with agricultural runoff. These observations demonstrated that there was a general, rather than specific, enhancement of the anaerobic microflora in the sediments most likely influenced by agricultural runoff. Despite these differences in microfloral patterns, methylmercury and total mercury levels were similar among these contrasting sediments. Although available sulfate and phosphorus appeared to stimulate the productivity of sulfate reducers in Everglades sediments, the number of culturable sulfate reducers did not directly correspond to the concentration of sulfate and phosphorus in porewaters. Microcosms supplemented with sulfate, nitrate, and phosphate altered the initial capacities of the sediment microflora to produce acetate and methane from endogenous matter. For sediments nearest sources of agricultural runoff, phosphorus temporarily enhanced acetate formation and initially suppressed methane production, sulfate enhanced acetate formation but did not significantly alter the production of methane, and nitrate totally suppressed the initial production of both methane and acetate. In regards to the latter, microbes capable of dissimilating nitrate to ammonium were present in greater culturable numbers than denitrifiers. In microcosms, acetate was a major source of methane, and supplemental hydrogen was directed towards the synthesis of acetate via CO(inf2)-dependent acetogenesis. These findings demonstrate that Everglades sediments nearest agricultural runoff have enhanced anaerobic microbial profiles and that the anaerobic microflora are poised to respond rapidly to phosphate, sulfate, and nitrate input.     

Potential causes of enhanced transfer of mercury to St. Lawrence River Biota: implications for sediment management strategies at Cornwall, Ontario, Canada

We examined factors and pathways involved in the transfer of mercury (Hg) to the food web in St. Lawrence River embayments near Cornwall, Ontario, where natural remediation of contaminated sediments (eventual burial by settling of cleaner sediments) has been adopted as a management strategy. Yellow perch (Perca flavescens) from one of the study zones (Zone 1) along the river by Cornwall contained significantly higher total mercury (THg) concentrations than perch from other equally contaminated zones. While THg concentrations in benthic invertebrates did not vary among contaminated zones, THg concentrations in yellow perch and invertebrate prey recovered from the perch stomachs were 1.5–2.5 times higher in Zone 1 than those from other zones, suggesting that prey selection affects THg accumulation more than habitat location. No significant differences were found in THg concentrations among different prey species within Zone 1, although there were significant differences in THg concentrations in the same prey species within Zone 1. In contrast, THg concentrations among different prey species increased significantly with trophic level in other contaminated and reference zones. The lack of correspondence between trophic position and THg accumulation in Zone 1 suggests two possibilities: (1) yellow perch in Zone 1 are highly mobile and are assimilating THg from a wide range of prey across Zone 1 with variable THg concentrations and (2) there may be an important non-dietary source of THg to the Zone 1 food web. Potential waterborne Hg sources to Zone 1 were investigated. Whereas THg and MeHg values in discharges from a disused canal were similar to Zone 1 surface water values (0.97 and 0.04 ng l^?1, respectively), concentrations in storm sewer and combined sewer overflows discharging in the vicinity of Zone 1 were 19–45-fold (THg) and 2–4-fold (MeHg) higher than upstream river water. Contributions of Hg to the water column from sediment–water diffusion, estimated using a simple, well-mixed reactor model, ranged 0.05–0.1% of the surface water THg concentration and 1–2% of the MeHg concentration measured in summer months in Zone 1. Although not investigated in the other zones, a strong correlation (r ² = 0.82) was found between MeHg in porewater and amphipod concentrations in Zone 1, indicating that the sediment porewater is bioavailable and likely an important pathway for transfer of sediment Hg to the foodweb. Large areas of Zone 1 contain bark deposits and produce high rates of gas ebullition, and may not provide favourable conditions for progressive burial with clean sediments and attenuation of Hg transfer to biota through natural remediation. Careful monitoring of surface sediment concentrations and biota is required in these areas. Failure to reduce concentrations of Hg in these media would indicate alternative or additional management measures are required.     

Factors Affecting Methylmercury Levels in Surficial Tailings from Historical Nova Scotia Gold Mines

Large quantities of Hg remain in tailings dumps from historical Nova Scotian gold mines. Depth profiles of total Hg (Hg_T) and methylmercury (MeHg) were compared with geochemical and microbiological variables, to identify factors influencing MeHg levels in tailings. Hg_T and MeHg were highly variable in tailings (0.2–73.5 μ mol kg^{? 1} and < dl-56.4 nmol kg^{? 1}, respectively), and were influenced by a complex set of in situ factors. Elevated MeHg was linked with > 5 μ mol kg^?1 Hg_T, organic matter, hydrology, abundance and activity of sulfate reducing bacteria, and demethylation processes. Methylmercury levels in tailings from a wet, bog-like site appeared to undergo seasonal fluctuations, with higher concentrations measured in September and October, and lower concentrations in May. Evaluations of amalgamation tailings should examine MeHg and Hg_T transport out of low-lying, saturated tailings dumps after snowmelt and major rainfall events, and should take into account the possibility of seasonal variation in MeHg levels in northern regions.     

Heterotrophic microbial activity in shallow aquifer sediments of Long Island,New York

Bacterial numbers and activities (as estimated by glucose uptake and total thymidine incorporation) were investigated at two sites in Long Island, New York aquifer sediments. In general, bacterial activities were higher in shallow (1.5–4.5 m below the water table or BWT), oxic sediments than in deep (10–18 m BWT), anoxic sediments. The average total glucose uptake rates were 0.18 ± 0.10 ng gdw^–1 h^–1 in shallow sediments and 0.09 ± 0.11 ng gdw^–1 h^–1 in deep sediments; total thymidine incorporation rates were 0.10 ± 0.13 pmol gdw^–1 h^–1 and 0.03 ± 0.03 pmol gdw^–1 h^–1 in shallow and deep sediments, respectively. Incorporation of glucose was highly efficient, as only about 10% of added label was recovered as CO₂. Bacterial abundance (estimated from acridine orange direct counts) was 2.5 ± 2.0 × 10⁷ cells gdw^–1 and 2.0 ± 1.3 × 10⁷ cells gdw^–1 in shallow and deep sediments, respectively. These bacterial activity and abundance estimates are similar to values found in other aquifer environments, but are 10- to 1000-fold lower than values in soil or surface sediment of marine and estuarine systems. In general, cell specific microbial activities were lower in sites from Connetquot Park, a relatively pristine site, when compared to activities found in sites from Jamesport, which has had a history of aldicarb (a pesticide) contamination. To our knowledge, this is the first report of bacterial activity measurements in the shallow, sandy aquifers of Long Island, New York.Correspondence to: D.G. Capone     

A Michaelis–Menten type equation for describing methylmercury dependence on inorganic mercury in aquatic sediments

Methylation of mercury (Hg) is the crucial process that controls Hg biomagnification along the aquatic food chains. Aquatic sediments are of particular interest because they constitute an essential reservoir where inorganic divalent Hg (Hg^II) is methylated. Methylmercury (MeHg) concentrations in sediments mainly result from the balance between methylation and demethylation reactions, two opposite natural processes primarily mediated by aquatic microorganisms. Thus, Hg availability and the activity of methylating microbial communities control the MeHg abundance in sediments. Consistently, some studies have reported a significant positive correlation between MeHg and Hg^II or total Hg (Hg_T), taken as a proxy for Hg^II, in aquatic sediments using enzyme-catalyzed methylation/demethylation mechanisms. By compiling 1,442 published and unpublished Hg_T–MeHg couples from lacustrine, riverine, estuarine and marine sediments covering various environmental conditions, from deep pristine abyssal to heavily contaminated riverine sediments, we show that a Michaelis–Menten type relationship is an appropriate model to relate the two parameters: MeHg = aHg_T/(K _m  + Hg_T), with a = 0.277 ± 0.011 and K _m  = 188 ± 15 (R ² = 0.70, p < 0.001). From K _m variations, which depend on the various encountered environmental conditions, it appears that MeHg formation and accumulation are favoured in marine sediments compared to freshwater ones, and under oxic/suboxic conditions compared to anoxic ones, with redox potential and organic matter lability being the governing factors.     

Spatial and temporal dynamics of mercury in Precambrian Shield upland runoff

Methylated and total Hg, and TOC concentrations were measured in precipitation and runoff in a first order Precambrian Shield watershed, and in precipitation, throughfall, shallow groundwater and runoff in a zero Precambrian Shield watershed. Plots dominated by open lichen-covered bedrock and another containing small patches of conifer forest and thin discontinuous surficial deposits were monitored within the zero order catchment. Methyl (3–10 fold) and non-methyl (1.4–2.8 fold) Hg concentrations changed irregularly during rainfall and snowmelt runoff events in all catchments. Temporal patterns of Hg concentration in runoff included flushing and subsequent dilution as well as peak concentrations coinciding with peak or recession flow. Mercury export was highest from lichen-covered bedrock surfaces as a result of high runoff yields and minimal opportunity for physical retention and in the case of MeHg demethylation. Forest canopy and lichen/bedrock surfaces were often net sources for Hg while forest soils were mostly sinks. However, upland soils undergoing periodic reducing conditions appear to be sites for the in situ production of MeHg.     

Effect of Electron Donor to Sulfate Ratio on Mercury Methylation in Floodplain Sediments under Saturated Flow Conditions

A column transport experiment was conducted to examine the release and methylation of Hg using Hg contaminated sediment from the floodplain of the South River near Waynesboro, Virginia. Three input solutions were sequentially introduced into the column. Input 1 was unamended South River water, Input 2 was river water amended with 100 mg L^?1 SO₄ and 3600 mg L^?1 lactate, and Input 3 was river water amended with 500 mg L^?1 SO₄ and 340 mg L^?1 lactate. During the first stage of the experiment (Input 1) the effluent Hg concentration was initially 4 µg L^?1 and peaked at 21 µg L^?1 and after 21 pore volumes stabilized at 13 µg L^?1. During the second stage, at high lactate to SO₄ ratios, elevated concentrations of acetic and propionic acids were detected, indicating that fermentative bacteria were dominant. During the third stage, at high SO₄ to lactate ratios, a decrease in SO₄ and an increase in H₂S concentrations were detected in the column effluent indicating that SO₄ reduction was occurring. Concentrations of methyl Hg (MeHg) in the effluent were variable over the duration of the experiment. During the first phase, concentrations of MeHg remained <3.3 ng L^?1. During the fermentative stage, concentrations of MeHg increased to a maximum value of 32 ng L^?1, and during the sulfate-reducing stage to a maximum value of 266 ng L^?1. When the column was deconstructed both molecular and cultural techniques indicated that sulfate reducing bacteria were most dominant near the influent port. These results indicate that the formation of MeHg in the sediment is not limited by the availability of Hg and that the bacterial community that contributes to mercury methylation can respond quickly to changes in the abundances of electron donors and acceptors.     

Xylem sap as a pathway for total mercury and methylmercury transport from soils to tree canopy in the boreal forest

Conifer needles are an important link in the cycling of Total Mercury (THg) and Methylmercury (MeHg) in the boreal ecosystem due to the high THg and MeHg concentrations in litterfall. Translocation within the tree of Hg from soils to the crown canopy has been assumed to be a minor source of the Hg in litterfall. This paper, however, is the first to present direct observations of THg/MeHg transport from the soil via xylem sap. Xylem sap concentrations of THg and MeHg were measured in sap drained from different levels along the boles of freshly cut 100 year old Norway spruce (Picea abies) and Scots pine (Pinus sylvestris). The trees came from a mixed stand growing on podzolized till soils at the Svartberget Forest Research Station in N. Sweden. Soil solution concentrations of THg and MeHg at different levels in the soil profile were measured for comparison.Concentrations of THg in xylem sap ranged from 10–15 ng L^-1 in both the Scots pine and Norway spruce. Concentrations of MeHg varied from 0.03 ng L^-1to 0.16 ng L^-1, with higher values in Scots pine than Norway spruce. If these concentrations are representative of the transport from soils to needles in xylem sap at this site, then only 3% of the MeHg in litterfall (0.12 mg ha^-1 yr^-1) and 11% of the THg (26 mg ha^-1 yr^-1) can originate via this pathway. The upward transport via xylem sap is larger relative to the open field inputs (84% of THg and 17% of MeHg). Comparison of soil solution and xylem sap THg/MeHg suggested some degree of THg exclusion during water uptake in Scots pine and Norway spruce, but MeHg exclusion only in Norway spruce.     

Mercury dynamics in Tivoli South Bay, a freshwater tidal mudflat wetland in the Hudson River

The accumulation of total mercury (Hg_T) andmethylmercury (MeHg) was evaluated in sediments ofTivoli South Bay, a freshwater tidal mudflat wetlandin the Hudson River National Estuarine ResearchReserve system. Hg_T concentrations in sedimentcores were measured to evaluate the spatialvariability of Hg_T deposition, and to establisha chronology of Hg_T accumulation. Cores takenfrom the northern, middle, and southern sections ofthe bay had similar distribution patterns andconcentrations of Hg_T, suggesting a common sourceof Hg_T throughout the bay. Sedimentconcentrations ranged from 190 to 1040 ng Hg g^–1,2 to 10 times greater than concentrations expected insediments from non-anthropogenic sources. Hg_Tdeposition rates were similar in different regions ofthe bay, and increased from 200 ng Hg cm^–2yr^–1in the 1930s to a maximum of 300 ngHg cm^–2 yr^–1 in the 1960s. Deposition rateshave steadily declined since the 1970s and arecurrently at 80 ng Hg cm^–2 yr^–1. Transportof Hg_T by tidal waters from the Hudson River islikely the main source of Hg_T in the bay.Distribution patterns and absolute concentrations ofMeHg in sediment cores were similar throughout thebay, with concentrations ranging from 0.43 to 2.95ng g^–1. Maxima in MeHg concentration profilesoccurred just below the sediment-water interface andat a depth of 30 cm. The maximum at 30 cm wascoincident with maximum Hg_T concentrations. MeHgconcentrations in suspended particulate matter (SPM)from the Hudson River suggest that MeHg in the baycould be derived from riverine SPM rather than formedin situ.     

Scaling mercury biodynamics from individuals to populations: Implications of an herbivorous fish on mercury cycles in streams

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Inorganic sulfur turnover in oligohaline estuarine sediments

Inorganic sulfur turnover was examined in oligohaline (salinity < 2 g kg^-1) Chesapeake Bay sediments during the summer. Cores incubated for < 3 hr exhibited higher sulfate reduction (SR) rates (13–58 mmol m^-2 d^-1) than those incubated for 3–8 hr (3–8 mmol m^-2 d^-1). SR rates (determined with³⁵SO ₄ ^2- ) increased with depth over the top few cm to a maximum at 5 cm, just beneath the boundary between brown and black sediment. SR rates decreased below 5 cm, probably due to sulfate limitation (sulfate < 25 μM). Kinetic experiments yielded an apparent half-saturating sulfate concentration (K_s) of 34 μM, ≈ 20-fold lower than that determined for sediments from the mesohaline region of the estuary. Sulfate loss from water overlying intact cores, predicted on the basis of measured SR rates, was not observed over a 28-hr incubation period. Reduction of³⁵SO ₄ ^2- during diffusion experiments with intact core segments from 0–4 and 5–9 cm horizons was less than predicted by non-steady state diagenetic models based on³⁵SO ₄ ^2- reduction in whole core injection experiments. The results indicate that net sulfate flux into sediments was an order of magnitude lower than the gross sulfur turnover rate. Solid phase reduced inorganic sulfur concentrations were only 2–3 times less than those in sediments from the mesohaline region of the Bay, despite the fact that oligohaline bottom water sulfate concentrations were 10-fold lower. Our results demonstrate the potential for rapid SR in low salinity estuarine sediments, which are inhabited by sulfate-reducing bacteria with a high affinity for sulfate, and in which sulfide oxidation processes replenish the pore water sulfate pool on a time scale of hours.     

Microbial sulfate reduction in a brackish meromictic steppe lake

Patterns of sulfate reduction were studied in water and sediments of Lake Shira, South Siberia, Russia. The lake was characterized by a high level of sulfate (91-116 mM). The concentration of hydrogen sulfide in the anoxic waters of the lake reached 0.6 mM. In summer the sulfate reduction rate in the water column, measured by radiometric technique, varied from 0.25 to 9.81 mol sulfate l^-1 d^-1. There were two peaks of sulfate reduction activity: just below the chemocline and near the sediment surface. Sulfate reduction rate in the profundal silts ranged from 4.1 to 90.6 mol l^-1 d^-1. The zone of the most active sulfate reduction was restricted to the surface sediment layers. The acceleration of sulfate reduction rate (up to 236 mol l^-1 d^-1) and the increase of density of viable sulfate reducers (up to 2 x 10⁵ cells ml^-1) were recorded in the littoral sediments adjacent to the mouth of the Son River and sewage discharge. It was apparently caused by the input of allochthonous organic substrates and also by a high environmental temperature. On an areal basis, sulfate reduction rate in the water was approximately 8 times higher than that in the profundal sediments. Sulfate reduction was the most important process of anaerobic oxidation of organic carbon in Lake Shira. In summer in the profundal zone of the lake, sulfate reducers were able to mineralize about 67% of the daily integrated primary production of phototrophic and chemotrophic organisms.     

MMHg production and export from intertidal sediments to the water column of a tidal lagoon (Arcachon Bay,France)

Hg cycling in biologically productive coastal areas is of special importance given the potential for bioaccumulation of monomethylmercury (MMHg) into aquatic organisms. Field experiments were performed during three different seasons in Arcachon Bay, a mesotidal lagoon (SW France), to assess the variability of the water column concentrations, sediment–water exchanges and potential formation and degradation of MMHg. The objectives were to evaluate the contribution of intertidal mudflats to MMHg production and the various pathways of Hg species export. Dissolved and bulk concentrations of Hg species in the water column downstream of tidal flats were measured throughout several tidal cycles. The Hg benthic fluxes at the sediment–water interface were determined by means of benthic chambers for three different stations. Hg methylation and demethylation potentials were determined in surficial sediments and the water column using isotopic tracers. The tidal surveys demonstrated that benthic remobilization of Hg occurs primarily in association with sediment erosion and advection during ebb tide. However, elevated dissolved Hg concentrations observed at low tide were found to be caused by a combination of pore-waters seeping, benthic fluxes and methylation in the water column. Benthic fluxes were more intense during late winter conditions (median MMHg and inorganic Hg (IHg) fluxes: 64 and 179 pmol m^?2 h^?1, respectively) and subsequently decreased in spring (median 0.7 and ?5 pmol m^?2 h^?1, respectively) and fall (median ?0.4 and ?1.3 pmol m^?2 h^?1, respectively). The trends in methylation and demethylation potentials were at the opposite of the fluxes, two times lower during winter than for spring or fall conditions. In this tidal environment, MMHg production in surface sediments and its subsequent release is estimated to be the major source of MMHg to the water column during winter and spring time. However, during the more productive summer period, the Hg methylation extent in the water column may be very significant and equivalent to the sediment contribution.