Mercury speciation and distribution in Aha Reservoir which was contaminated by coal mining activities in Guiyang, Guizhou, China

Two sampling campaigns were carried out in March and August 2005 representing dry and wet seasons, respectively, to investigate the distribution patterns of Hg species in the water column and sediment profiles at two sampling stations in Aha Reservoir located in Guiyang, Southwestern China. Aha Reservoir has been contaminated by Hg due to small scale coal mining activities. Mercury concentrations in both water and sediment were elevated. A clearly seasonal variation of dissolved Hg (DHg), particulate Hg (PHg) and total Hg (THg) concentrations in the water column was observed. The concentrations of these Hg species in the wet season were significantly higher than in the dry season. Runoff input and diffusion of Hg from sediments could be the reasons for elevated concentrations of these Hg species in the wet season. The contaminated sediment is acting as a secondary contamination source for both inorganic Hg (IHg) and methylmercury (MeHg) to the overlying water. The cycling of Mn in the sediment governs the diffusion process of IHg to the water column. In the dry season (winter and spring), Mn occurs as MnO₂ because the uppermost part of sediment is in an oxic condition and Hg ions are absorbed by MnO₂. In the wet season (summer and fall), the uppermost part of the sediment profile is in a reduced condition because of stratification of the water column and MnO₂ is reduced to Mn²⁺, which results in transformation of Hg²⁺ into porewater as Mn²⁺ became soluble. This causes a higher diffusive flux of IHg from sediment to overlying water in the wet season. Both sampling stations showed a consistent trend that THg concentrations decreased in the uppermost part of sediment cores. This demonstrated that the measures taken to reduce ADM contamination to Aha Reservoir also reduced Hg input to the reservoir. Methyl Hg diffusive fluxes from sediment to overlying water were higher in the wet season than the dry season demonstrating that high temperatures favor Hg methylation processes in sediment.     

Methylmercury Bioaccumulation in an Urban Estuary: Delaware River,USA

Spatial variation in mercury (Hg) and methylmercury (MeHg) bioaccumulation in urban coastal watersheds reflects complex interactions between Hg sources, land use, and environmental gradients. We examined MeHg concentrations in fauna from the Delaware River estuary, and related these measurements to environmental parameters and human impacts on the waterway. The sampling sites followed a north to south gradient of increasing salinity, decreasing urban influence, and increasing marsh cover. Although mean total Hg in surface sediments (top 4 cm) peaked in the urban estuarine turbidity maximum and generally decreased downstream, surface sediment MeHg concentrations showed no spatial patterns consistent with the examined environmental gradients, indicating urban influence on Hg loading to the sediment but not subsequent methylation. Surface water particulate MeHg concentration showed a positive correlation with marsh cover whereas dissolved MeHg concentrations were slightly elevated in the estuarine turbidity maximum region. Spatial patterns of MeHg bioaccumulation in resident fauna varied across taxa. Small fish showed increased MeHg concentrations in the more urban/industrial sites upstream, with concentrations generally decreasing farther downstream. Invertebrates either showed no clear spatial patterns in MeHg concentrations (blue crabs, fiddler crabs) or increasing concentrations further downstream (grass shrimp). Best-supported linear mixed models relating tissue concentration to environmental variables reflected these complex patterns, with species specific model results dominated by random site effects with a combination of particulate MeHg and landscape variables influencing bioaccumulation in some species. The data strengthen accumulating evidence that bioaccumulation in estuaries can be decoupled from sediment MeHg concentration, and that drivers of MeHg production and fate may vary within a small region.     

Environmental geochemistry of an active Hg mine in Xunyang,Shaanxi Province,China

The Xunyang Hg mine (XMM) situated in Shaanxi Province is an active Hg mine in China. Gaseous elemental Hg (GEM) concentrations in ambient air were determined to evaluate its distribution pattern as a consequence of the active mining and retorting in the region. Total Hg (HgT) and methylmercury (MeHg) concentrations in riparian soil, sediment and rice grain samples (polished) as well as Hg speciation in surface water samples were measured to show local dispersion of Hg contamination. As expected, elevated concentrations of GEM were found, ranging from 7.4 to 410 ng m⁻³. High concentrations of HgT and MeHg were also obtained in riparian soils, ranged from 5.4 to 120 mg kg⁻¹ and 1.2 to 11 μg kg⁻¹, respectively. Concentrations of HgT and MeHg in sediment samples varied widely from 0.048 to 1600 mg kg⁻¹ and 1.0 to 39 μg kg⁻¹, respectively. Surface water samples showed elevated HgT concentrations, ranging from 6.2 to 23,500 ng L⁻¹, but low MeHg concentrations, ranging from 0.022 to 3.7 ng L⁻¹. Rice samples exhibited high concentrations of 50–200 μg kg⁻¹ in HgT and of 8.2–80 μg kg⁻¹ in MeHg. The spatial distribution patterns of Hg speciation in the local environmental compartments suggest that the XMM is the source of Hg contaminations in the study area.     

Microbial cycling of mercury in contaminated pelagic and wetland sediments of San Pablo Bay,California

San Pablo Bay is an estuary, within northern San Francisco Bay, containing elevated sediment mercury (Hg) levels because of historic loading of hydraulic mining debris during the California gold-rush of the late 1800s. A preliminary investigation of benthic microbial Hg cycling was conducted in surface sediment (0-4 cm) collected from one salt-marsh and three open-water sites. A deeper profile (0-26 cm) was evaluated at one of the open-water locations. Radiolabeled model Hg-compounds were used to measure rates of both methylmercury (MeHg) production and degradation by bacteria. While all sites and depths had similar total-Hg concentrations (0.3-0.6 ppm), and geochemical signatures of mining debris (as )Nd, range: -3.08 to -4.37), in-situ MeHg was highest in the marsh (5.4Dž.5 ppb) and А.7 ppb in all open-water sites. Microbial MeHg production (potential rate) in 0-4 surface sediments was also highest in the marsh (3.1 ng g^-1 wet sediment day^-1) and below detection (<0.06 ng g^-1 wet sediment day^-1) in open-water locations. The marsh exhibited a methylation/demethylation (M/D) ratio more than 252 that of all open-water locations. Only below the surface 0-4-cm horizon was significant MeHg production potential evident in the open-water sediment profile (0.2-1.1 ng g^-1 wet sediment day^-1). In-situ Hg methylation rates, calculated from radiotracer rate constants, and in-situ inorganic Hg(II) concentrations compared well with potential rates. However, similarly calculated in-situ rates of MeHg degradation were much lower than potential rates. These preliminary data indicate that wetlands surrounding San Pablo Bay represent important zones of MeHg production, more so than similarly Hg-contaminated adjacent open-water areas. This has significant implications for this and other Hg-impacted systems, where wetland expansion is currently planned.     

Seasonal Cycling and Transport of Mercury and Methylmercury in the Turbidity Maximum of the Delaware Estuary

The Delaware River Estuary (DRE) is a cornerstone of industrialization, shipping, and urban usage, and has a long history of human impact on pollution and recovery. Mercury (Hg) is a contaminant of concern in the DRE based upon concentrations in some fish samples that were found to exceed State and Federal fish tissue criteria. Methylation of Hg often follows a seasonal pattern as its production is biologically mediated. Surveys were conducted in November 2011, April 2012, and July 2012 to assess this effect. We sampled surface and bottom water at six sites spanning the estuarine turbidity maximum (ETM) in the main channel of the river, plus three sediment sites at shallow, subtidal locations. Our results indicate there is a clear seasonal increase in both water column and sediment methylmercury (MeHg) and %MeHg concentrations in the ETM during July. Water-column-filtered total mercury (HgT), suspended particle HgT, and MeHg concentrations were found to fluctuate little with location or season in the ETM. In contrast, sediment MeHg, water-column-filtered MeHg, and pore water HgT varied seasonally. Furthermore, pore water MeHg levels were elevated in concert with increased k _meth rates in July. Estimated river input and sediment and atmospheric depositional MeHg flux were compared seasonally. River flux was more than an order of magnitude higher than sediment flux in April, coinciding with higher fluvial transport. However, during July, river flux decreases and sediment flux becomes a larger relative source. This trend has potential implications for fish and other biota residing in the DRE during summer.     

Mercury distribution and transport in a contaminated river system in Kazakhstan and associated impacts on aquatic biota

The River Nura in Central Kazakhstan has been heavily polluted by Hg originating from an acetaldehyde plant. A number of studies were undertaken to investigate the transport, fate and bioavailability of Hg in this river system. The sediments within a 20 km section of the river downstream of the effluent outfall canal are highly polluted and are acting as a strong source of surface water contamination. Mercury transport in the river is dominated by the remobilization of contaminated bed sediments and river bank erosion during the annual spring flood. Peak Hg concentrations in unfiltered surface water samples during a larger than usual flood event in 2004 were in the order of 1600–4300 ng L⁻¹. The majority of the particulate-bound Hg appears to be sedimented in the shallow Intumak reservoir 75 km downstream of the source of the pollution, leading to a drop in aqueous Hg concentrations by an order of magnitude. Nevertheless, background concentrations of Hg in surface water are not reached until at least 200 km downstream, and during the flood period Hg is also detected in the terminal wetlands of the river.Mercury concentrations in sediment cores taken from the river bed in the most contaminated section of the Nura ranged from 9.95 to 306 mg kg⁻¹. Methylmercury (MeHg) levels in shallow sediment cores were highest in surface sediments and ranged between 4.9 and 39 μg kg⁻¹, but were generally less than 0.1% of total Hg (THg). A significant inverse relationship was found between THg concentrations and the percentage of MeHg formed in the sediments, irrespective of the sampling depth. The observed relationship was confirmed by comparison with results from a different river system, indicating that it may be true also for other highly contaminated aquatic systems. It is hypothesized that at high THg levels in severely contaminated sediments, the accumulation of MeHg may be limited by increasingly efficient demethylation processes, and that this underlying trend in sediments is the reason why MeHg levels in surface water are often found to be higher at less contaminated sites compared to upstream sites.Mercury concentrations in biota in the most contaminated section of the river were 15–20 times higher than background levels. Fish were found to be impacted for more than 125 km downstream from the source, indicating significant transport of dissolved MeHg to downstream areas and/or in-situ MeHg production in less contaminated downstream reaches. There were also indications that impoundments may increase the bioavailability of Hg.     

The distribution of total mercury and methyl mercury in a shallow hypereutrophic lake (Lake Taihu) in two seasons

To understand the geochemical cycle of Hg in hypereutrophic freshwater lake, two sampling campaigns were conducted in Lake Taihu in China during May and September of 2009. The concentrations of unfiltered total Hg (unfTHg) were in the range of 6.8–83 ng L⁻¹ (28 ± 18 ng L⁻¹) in the lake water and total Hg in the sediment was 12–470 ng g⁻¹, both of which are higher than in other background lakes. The concentration of unfTHg in ∼11% of the lake water samples exceeded the second class of the Chinese environmental standards for surface water of 50 ng L⁻¹ (GB 3838-2002), indicating that a high ecological risk is posed by the Hg in Lake Taihu. However, the concentrations of unfiltered total MeHg (unfMeHg) were relatively low in the lake water (0.14 ± 0.05 ng L⁻¹, excluding two samples with 0.81 and 1.0 ng L⁻¹). Lake sediment MeHg varied from 0.2–0.96 ng g⁻¹, with generally low ratios of MeHg/THg of <1%. The low concentrations of TMeHg in the lake water may have resulted from a strong uptake by the high primary productivity and the demethylation of MeHg in oxic conditions. In addition, contrary to the results of previous research conducted in deep-water lakes and reservoirs, the low concentrations of MeHg and low ratio of MeHg/THg in the lake sediment indicates that the net methylation of Hg was not accelerated by the elevated organic matter load created by the eutrophication of Lake Taihu. The results also showed that sediments were a source of THg and MeHg in the water. Higher diffusion fluxes of THg and MeHg may be partly responsible for the higher concentrations of THg in the lake water in May, 2009.     

Concentration,distribution and bioaccumulation of mercury in the Xunyang mercury mining area,Shaanxi Province,China

Samples of surface water, soil, sediment and plants from the Xunyang Hg mining area in Shaanxi Province, China, were analyzed to assess the effects of the Hg mining activities on the environment. The results show that: (1) the mining activities are sources of Hg to the environment surrounding the mine; (2) the environment, especially riverbeds in the Xunyang area, is contaminated with Hg and (3) Hg accumulation in cabbage leaves, the main vegetable for the local residents, is high, reaching concentrations that are 6–540 times higher than the maximum Hg concentration recommended for edible plants in China. Action should be taken to remediate contaminated sites, manage waste discharges and reduce the exposure of Hg to local residents by issuing advisories against consuming agricultural products grown in the area, which are contaminated with Hg.     

Microbial mercury transformations in marine,estuarine and freshwater sediment downstream of the Idrija Mercury Mine,Slovenia

The Idrija Mine, the second largest Hg mine in the world, ceased operation in 1995, but still delivers large quantities of Hg downstream including into the northern Adriatic Sea, 100 km away. Transformation of Hg species in sediment in sites over 60 km from the mine, including marine sites in the Adriatic Sea, was measured to determine the ability of the system to transform and mobilize Hg and to produce methylmercury (MeHg). Cores from a freshwater impoundment, a brackish estuarine site, and three marine sites in the Gulf of Trieste were sectioned anaerobically, and Hg methylation and MeHg demethylation activities determined using radio-techniques (²⁰³Hg for methylation and ¹⁴C-MeHg for demethylation). Total and dissolved Hg and MeHg were determined as were other geochemical parameters. In addition, rates of SO₄ reduction were determined in marine sediment using a ³⁵S technique. Mercury was readily methylated and demethylated at all sites. Marine sediment was investigated in winter and summer with rates of Hg transformation and SO₄ reduction corresponding only in winter. Methylation of Hg in summer displayed subsurface peaks that may have been influenced by bioturbation. Total Hg and MeHg were most abundant in the freshwater, estuarine, and near-shore marine sites, but dissolved pore water Hg and MeHg were highest in the estuarine region where S cycling appeared ideal for the mobilization of Hg. The impoundment sediment also seemed to be a ‘hotspot’ of Hg transformations. MeHg demethylation occurred via the oxidative demethylation pathway (CO₂ produced from MeHg), except in surficial sediment offshore in the Gulf during winter, where sediment was more oxidizing and significant amounts of CH₄ were liberated during MeHg degradation via reductive demethylation. The CH₄ formation was likely due to an increased influence from the expression of MeHg degradative enzymes encoded by the mer detoxification bacterial genetic system. The freshwater site also liberated CH₄ from MeHg, but it appeared to be due to oxidative demethylation by methanogenic bacteria.     

Source Identification of Florida Bay's Methylmercury Problem: Mainland Runoff Versus Atmospheric Deposition and <Emphasis Type="Italic">In situ</Emphasis> Production

The first advisory to limit consumption of Florida Bay fish due to mercury was issued in 1995. Studies done by others in the late 1990s found elevated water column concentrations of both total Hg (THg) and methylmercury (MeHg) in creeks discharging from the Everglades, which had its own recognized mercury problem. To investigate the significance of allochthonous MeHg discharging from the upstream freshwater Everglades, we collected surface water and sediment along two transects from 2000 to 2002. Concentrations of THg and MeHg, ranging from 0.36 ng THg/L to 5.98 ng THg/L and from <0.02 ng MeHg/L to 1.79 ng MeHg/L, were elevated in the mangrove transition zone when compared both to upstream canals and the open waters of Florida Bay. Sediment concentrations ranged from 5.8 ng THg/g to 145.6 ng THg/g and from 0.05 ng MeHg/g to 5.4 ng MeHg/g, with MeHg as a percentage of THg occasionally elevated in the open bay. Methylation assays indicated that sediments from Florida Bay have the potential to methylate Hg. Assessment of mass loading suggests that canals delivering stormwater from the northern Everglades are not as large a source as direct atmospheric deposition and in situ methylation, especially within the mangrove transition zone.     

Hydrologic controls on water chemistry and mercury biotransformation in a closed river system: The Carson River,Nevada

The Carson River flows in a closed basin system and the total flow of the river water decreases downstream due to both evaporation and consumptive uses. This river system is fed primarily by snow pack in the Sierra Nevada during the winter, which flows down gradient following melting in spring and summer. Water loss through evaporation in the Carson River results in a downstream buildup of conservative elements such as Cl⁻ and certain oxyanion forming elements including Se, Mo and W, which are known to interfere with the transformation of Hg within the S cycle. In addition to these naturally occurring hydrologic processes and the resulting affects on water chemistry, the Carson River Basin has been historically impacted by Au and Ag mining that used Hg amalgamation techniques. Contamination of Hg in the Carson River system is now well documented and published Hg concentrations in different environmental compartments are extremely high. In this study, hydrologically driven changes in water chemistry of the river system and the resulting effects on Hg cycling were examined. Results show that periods of low water flow correspond to high water pH (up to 8.3), relatively high concentrations of oxyanion forming elements (e.g., As, Se, Mo and W), and low Hg methylation potential in sediment. In contrast, periods of high flow bring about dilution, which results in lower pH (∼7), lower concentrations of oxyanion forming elements, but higher Hg methylation potential. Overall, changes in flow regimes likely affect rates of methyl-Hg (MeHg) production through a combination of factors such as high pH, which favors MeHg demethylation, and the occurrence of relatively high concentrations of Group VI oxyanions that could interfere with microbial SO₄ reduction and MeHg production.     

Seasonal distribution of total mercury and methylmercury in sediments of the Wujiangdu Reservoir, Guizhou, China

The concentrations of total mercury and methylmercury in sediments were determined at the dam of the Wujiangdu Reservoir in different seasons. Total mercury (HgT) levels in the whole sediment profile were 254.2±47.0 ng/g in winter, 254.2±31.6 ng/g in spring, and 256.7±60.8 ng/g in summer, without significant variations in different seasons or at different depths. In contrast, the methylmercury (MeHg) compounds were most abundant at the sediment-water interface and decreased progressively with depth. MeHg contents of the sediments during different seasons are highly dependent on microbial activity, and seem to be higher when Hg (II)-methylating microorganisms are active. Thus, MeHg levels tend to rise in the loci where nutrient supplies and biological productivity are favorable. The percentage of HgT that is present as MeHg in the sediments increased gradually from December 2003 to April 2004 and to July 2004.     

Total mercury and methylmercury in sediments near offshore drilling sites in the Gulf of Mexico

Concentrations of total Hg in sediments near six drilling sites in the Gulf of Mexico were elevated well above average background values of 40–80 ng/g. The excess Hg was associated with barite from discharged drilling mud. In contrast with total Hg, concentrations of methylmercury (MeHg) in these sediments did not vary significantly at nearfield (<100 m) versus farfield (>3 km) distances from the drilling sites. Observed variability in concentrations of MeHg were related to local differences in redox state in the top 10 cm of sediment. Low to non-detectable concentrations of MeHg were found in nearfield sediments that were anoxic, highly reducing and contained abundant H₂S. At most drilling sites, nearfield samples with high concentrations of total Hg (>200 ng/g) had similar or lower amounts of MeHg than found at background (farfield) stations. Higher values of MeHg were found in a few nearfield sediments at one site where concentrations of TOC were higher and where sediments were anoxic and moderately reducing. Overall, results from this study support the conclusion that elevated concentrations of MeHg in sediments around drilling sites are not a common phenomenon in the Gulf of Mexico.     

Mercury speciation in soils of the industrialised Thur River catchment (Alsace,France)

Methylmercury (MeHg) and total Hg (THg) concentrations in soil profiles were monitored in the Thur River basin (Alsace, France), where a chlor-alkali plant has been located in the city of Vieux-Thann since the 1930s. Three soil types were studied according to their characteristics and location in the catchment: industrial soil, grassland soil and alluvial soil. Contamination of MeHg and THg in soil was important in the vicinity of the plant, especially in industrial and alluvial soil. Concentrations of MeHg reached 27 ng g⁻¹ and 29,000 ng g⁻¹ for THg, exceeding the predictable no effect concentration. Significant ecotoxicological risk exists in this area and remedial actions on several soil types are suggested. In each type of soil, MeHg concentrations were highest in topsoil, which decreased with depth. Concentrations of MeHg were negatively correlated with soil organic matter and total S, particularly when MeHg concentrations exceeded 8 ng g⁻¹. Under these conditions, MeHg concentrations in soil seemed to be influenced by THg, soil organic matter and total S concentrations. It was found that high MeHg/THg ratios (near 2%) in soil were mainly related to the combined soil environmental conditions such as low THg concentrations, low organic C/N ratios (<11) and relatively low pH (5–5.5). Nevertheless, even when the MeHg/THg ratio was low (∼0.04%), MeHg and THg concentrations were elevated, up to 13 ng g⁻¹ and to 29,000 ng g⁻¹, respectively. Thus, both THg and MeHg concentrations should be taken into account to assess potential environmental risks of Hg.     

Mercury transformations and fluxes in sediments of a riverine wetland

Porewater samples were obtained on five occasions during spring, summer and fall by in situ dialysis from three sites of a large freshwater wetland situated along the St. Lawrence River. These samples were analysed for total dissolved mercury ([Hg]_T) and methylmercury ([MeHg]) concentrations and for complementary variables including dissolved sulfate, sulfide and elemental sulfur concentrations. Sediment cores were obtained on three occasions from one of these sites for the determination of total mercury ({Hg}_T) and methylmercury ({MeHg}) concentration as well as mercury methyltransferase (HgMT) activity profiles. {MeHg} and HgMT activity varied with time and sediment depth. The porewater [Hg]_T and [MeHg] depth profiles varied with time and among sites. Modeling the porewater [MeHg] profiles with a one-dimensional reaction-transport equation allowed identification of the sediment depths where MeHg is produced or consumed, as well as an estimate of the net in situ MeHg production rates in the sediments. The model-predicted depths of MeHg production, as well as the sulfate concentration and the HgMT activity depth distributions are all consistent with the involvement of sulfate reducing bacteria in the production of MeHg.     

Mercury remobilization in Saguenay Fjord (Quebec,Canada) sediments: Insights following a mass-flow event and its capping efficiency

A mass-flow event triggered by the 1996 flood in the Saguenay region buried the mercury-contaminated indigenous sediments at the head of the Saguenay Fjord under up to 50 cm of postglacial deltaic sediments. The vertical distributions of total mercury and methyl-mercury in the sediments and pore waters were measured in box cores recovered from the Saguenay Fjord within and outside the affected area prior to and on six consecutive years after the flood. The total solid mercury (THg_s) profiles show that remobilization was limited and most of the mercury remobilized from the contaminated, indigenous sediments was trapped below or slightly above the former sediment–water interface by authigenic acid-volatile sulfides (AVS). Nonetheless, a small fraction of the remobilized mercury diffused into the flood layer, some of it was methylated and/or scavenged by organic matter and AVS. Elevated solid-phase methyl-mercury concentrations, [MeHg_s], at depth in the sediment are correlated to peak AVS and THg_s but, in the absence of elevated dissolved methyl-mercury concentrations, [MeHg_d], the higher [MeHg_s] may reflect an earlier episode of Hg methylation, the product of which was scavenged by the AVS and buried. Throughout the sediment cores, sediment–water partitioning of MeHg and Hg(II) appears to be controlled in great part by the AVS and residual organic matter content of the sediment.     

Behavior of mercury in an urban river and its accumulation in aquatic plants

The characteristics of mercury in the aquatic environment have been intensively studied in mining areas with heavy mercury pollution but little work has been conducted in urban areas, with no significant Hg source. This paper presents a study of the Haihe River, which flows through an urban area in North China. The concentrations of total mercury (THg) and methylmercury (MeHg) in the river water were 3.6–31.2 and 0.12–3.21 ng/l, and the corresponding values in river sediment were 22.9–374.8 and 0.03–0.46 μg/kg. These values are lower than the reported values from mining areas. The THg concentration in sediment samples collected from the urban areas was higher than that from the rural areas and the global background levels, indicating the influence of urbanization on mercury contamination. Samples of typical riparian and floating plants, reed and hornwort, were collected. Correlation analysis showed that sediment is the major source of THg and MeHg in reed and water is the major source of MeHg in hornwort. The higher bioaccumulation factor of reed indicates its higher potential to accumulate MeHg from the environment.     

Benthic fluxes of mercury species in a lagoon environment (Grado Lagoon,Northern Adriatic Sea,Italy)

The role of the major biogeochemical processes in Hg cycling at the sediment–water interface was investigated in the Grado Lagoon (Northern Adriatic Sea). This wetland system has been extensively contaminated from the Idrija Hg Mine (Slovenia) through the Isonzo River suspended load carried by tidal fluxes. Three approaches were used to study the sediment–water exchange of total Hg (THg), methylmercury (MeHg), reactive Hg (RHg) and dissolved gaseous Hg (DGHg): (1) estimation of diffusive fluxes from porewater and overlying water concentrations, (2) measurements of benthic fluxes using a deployed light benthic chamber in situ and (3) measurements of benthic fluxes during oxic–anoxic transition with a laboratory incubation experiment. The THg solid phase, ranging between 9.5 and 14.4 μg g⁻¹, showed slight variability with depth and time. Conversely, MeHg contents were highest (up to 21.9 ng g⁻¹) at the surface; they tended to decrease to nearly zero concentration with depth, thus suggesting that MeHg production and accumulation occur predominantly just below the sediment–water interface. Porewater MeHg concentrations (0.9–7.9 ng L⁻¹, 0.15–15% of THg) varied seasonally; higher contents were observed in the warmer period. The MeHg diffusive fluxes (up to 17 ng m⁻² day⁻¹) were similar to those in the nearby Gulf of Trieste [Covelli, S., Horvat, M., Faganeli, J., Brambati, A., 1999. Porewater distribution and benthic flux of mercury and methylmercury in the Gulf of Trieste (Northern Adriatic Sea). Estuar. Coast. Shelf Sci. 48, 415–428], although the lagoon sediments contained four-fold higher THg concentrations. Conversely, the THg diffusive fluxes in the lagoon (up to 110 ng m⁻² day⁻¹) were one- to two-fold higher than those previously estimated for the Gulf of Trieste. The diurnal MeHg benthic fluxes were highest in summer at both sites (41,000 and 33,000 ng m⁻² day⁻¹ at the fishfarm and in the open lagoon, respectively), thus indicating the influence of temperature on microbial processes. The diurnal variations of dissolved THg and especially MeHg were positively correlated with O₂ and inversely with DIC, suggesting an important influence of benthic photosynthetic activities on lagoon benthic Hg cycling, possibly through the production of organic matter promptly available for methylation. The results from the dark chamber incubated in the laboratory showed that the regeneration of dissolved THg was slightly affected by the oxic–anoxic transition. Conversely, the benthic flux of MeHg was up to 15-fold higher in sediments overlain by O₂ depleted waters. In the anoxic phase, the MeHg fluxes proceeded in parallel with Fe fluxes and the methylated form reached approximately 100% of dissolved THg. The MeHg is mostly released into overlying water (mean recycling efficiency of 89%) until the occurrence of sulphide inhibition, due to scavenging of the available Hg substrate for methylation. The results suggest that sediments in the Grado Lagoon, especially during anoxic events, should be considered as a primary source of MeHg for the water column.