Methylmercury production in sediments of Chesapeake Bay and the mid-Atlantic continental margin

Methylmercury (MeHg) concentration and production rates were studied in bottom sediments along the mainstem of Chesapeake Bay and on the adjoining continental shelf and slope. Our objectives were to 1) observe spatial and temporal changes in total mercury (HgT) and MeHg concentrations in the mid-Atlantic coastal region, 2) investigate biogeochemical factors that affect MeHg production, and 3) examine the potential of these sediments as sources of MeHg to coastal and open waters. Estuarine, shelf and slope sediments contained on average 0.5 to 1.5% Hg as MeHg (% MeHg), which increased significantly with salinity across our study site, with weak seasonal trends. Methylation rate constants (k_meth), estimated using enriched stable mercury isotope spikes to intact cores, showed a similar, but weaker, salinity trend, but strong seasonality, and was highly correlated with % MeHg. Together, these patterns suggest that some fraction of MeHg is preserved thru seasons, as found by others [Orihel, D.M., Paterson, M.J., Blanchfield, P.J., Bodaly, R.A., Gilmour, C.C., Hintelmann, H., 2008. Temporal changes in the distribution, methylation, and bioaccumulation of newly deposited mercury in an aquatic ecosystem. Environmental Pollution 154, 77] Similar to other ecosystems, methylation was most favored in sediment depth horizons where sulfate was available, but sulfide concentrations were low (between 0.1 and 10 μM). MeHg production was maximal at the sediment surface in the organic sediments of the upper and mid Bay where oxygen penetration was small, but was found at increasingly deeper depths, and across a wider vertical range, as salinity increased, where oxygen penetration was deeper. Vertical trends in MeHg production mirrored the deeper, vertically expanded redox boundary layers in these offshore sediments. The organic content of the sediments had a strong impact on the sediment:water partitioning of Hg, and therefore, on methylation rates. However, the HgT distribution coefficient (K_D) normalized to organic matter varied by more than an order of magnitude across the study area, suggesting an important role of organic matter quality in Hg sequestration. We hypothesize that the lower sulfur content organic matter of shelf and slope sediments has a lower binding capacity for Hg resulting in higher MeHg production, relative to sediments in the estuary. Substantially higher MeHg concentrations in pore water relative to the water column indicate all sites are sources of MeHg to the water column throughout the seasons studied. Calculated diffusional fluxes for MeHg averaged  1 pmol m^− 2 day^− 1. It is likely that the total MeHg flux in sediments of the lower Bay and continental margin are significantly higher than their estimated diffusive fluxes due to enhanced MeHg mobilization by biological and/or physical processes. Our flux estimates across the full salinity gradient of Chesapeake Bay and its adjacent slope and shelf strongly suggest that the flux from coastal sediments is of the same order as other sources and contributes substantially to the coastal MeHg budget.     

Total and methylmercury in a Brazilian estuary,Rio de Janeiro

Guanabara Bay, in Rio de Janeiro state, is impacted by organic matter, oil and heavy metals. The present study evaluated the total mercury (THg) and methylmercury (MeHg) concentrations and the MeHg to THg ratio (%MeHg) in water samples from different points of the bay and in 245 organisms of three different trophic levels sampled between 1990 and 2000. Dissolved mercury concentration in estuarine water samples ranged from 0.72 to 5.23 ngl(-1). THg and MeHg in mussel, ranging from 11.6 to 53.5 microg THg kg(-1) wet wt. and 4.5-21.0 microg MeHg kg(-1) wet wt., varied according to sampling point and water quality. Planktivorous fish and mussel presented similar MeHg concentrations, meanwhile THg in planktivorous fish were lower than in mussel. Carnivorous fish showed higher THg and MeHg concentrations (199.5 +/- 119.3, 194.7 +/- 112.7 microg kg(-1) wet wt. respectively) than organisms from other feeding habits and lower trophic levels. There was a significant difference in the %MeHg among trophic levels: mussel presented lower MeHg percentage (33%) than planktivorous fish (54%) and carnivorous fish (98%).     

Speciation and Determination of Inorganic Mercury and Methylmercury by Headspace Single Drop Microextraction and Electrothermal Atomic Absorption Spectrometry in Water and Fish

In this study, headspace single drop microextraction (HS‐SDME) method in combination with electrothermal atomic absorption spectrometry (ETAAS) method was developed and validated for the speciation and determination of inorganic mercury (iHg) and methylmercury (MeHg). MeHg and iHg species were reduced to volatile methylmercury hydride (CH₃HgH) and elemental mercury, respectively, in the presence of NaBH₄ and trapped onto a drop of acceptor phase in the tip of a microsyringe. Thiourea and ammonium pyrrolydinedithiocarbamate (APDC) were tested as the acceptor phase. The experimental parameters of the method such as microextraction time, temperature, NaBH₄ concentration, acceptor phase concentration, and pH of the medium were investigated to obtain distinctive conditions for mercury species. Possible interference effects have also been investigated. In order to validation of the method, analytical figures of merits such as accuracy, precision, limit of detection (LOD), limit of quantitation (LOQ), and linear working range have been evaluated. Accuracy of the method has been verified by analyzing certified reference materials (BCR 453 Tuna fish) and spiked samples. The proposed method was applied for the speciation and determination of mercury species in water and fish samples. Mercury species (MeHg and iHg) have been determined in the real samples with a relative error less than 10%.     

Difference of mercury bioaccumulation in red mullets from the north-western Mediterranean and Black seas

The relationships between total mercury (Hg) concentration and stable nitrogen isotope ratio (δ¹⁵N) were evaluated in Mullus barbatus barbatus and M. surmuletus from the Mediterranean Sea and M. barbatus ponticus from the Black Sea. Mercury concentration in fish muscle was six times higher in the two Mediterranean species than in the Black Sea one for similar sized animals. A positive correlation between Hg concentration and δ¹⁵N occurred in all species. Increase in Hg concentration with δ¹⁵N was high and similar in the two Mediterranean fishes and much lower in the Black Sea species. Since this was neither related to trophic level difference between species nor to methylmercury (MeHg) concentration differences between the north-western Mediterranean and the Black Sea waters, we suggested that the higher primary production of the Black Sea induced a dilution of MeHg concentration at the base of the food webs.     

A comparison of anthropogenic mercury pollution in Kastela Bay (Croatia) with pristine estuaries in Ore (Sweden) and Krka (Croatia)

Anthropogenic mercury pollution was studied in Kastela Bay (Croatia), 10 years after chloralkaline plant (PVC) stopped production. The concentration of total mercury determined in sediments which are composed mainly of calcite and quartz, are in the range 14,280-30,400 ng/g. The values are higher than reported in the literature for Elbe and Seine estuaries and also above 25,000 ng/g used for criterion in remediation project in Minamata Bay. The concentration of methylmercury 6.05-36.74 ng/g are typical for slightly to highly contaminated estuarine sediments. The low ratio of methylmercury to total mercury found in sediments of Kastela Bay is in the range 0.04-0.18%. It can be explained that in this region predominate conditions which do not promote in-situ methylation. Sediments were found to be highly anoxic. Concentrations of total mercury in unfiltered surface waters are in the range 69-145 ng/l and in unfiltered bottom waters in the range 230-1,418 ngl(-1). High concentrations found in bottom waters suggest that either resuspension or partial dissolution of sediments takes place. An experiment performed on filtered and unfiltered waters showed that about 85% of total mercury in surface water and almost 100% in bottom water was retained on 45 microm filters. To demonstrate contrasts, two pristine estuaries from norths and south Europe were studied. Silicious sediments of Ore estuary (Sweden) and calcareous sediments of Krka estuary (Croatia) have total mercury concentrations close to accepted background level. The ratio of methylmercury to total mercury is < or = 1% in all samples with one exception. The highest observed ratio (2.70%) was in the surface sediment from E2 station in Krka estuary measured in March 2000. This location is suitable for studying methylmercury formation in pristine environment.     

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.     

Total and organic mercury concentrations in the muscles of Pacific albacore (Thunnus alalunga) and bigeye tuna (Thunnus obesus)

Muscles of 115 North Pacific albacore (ALB, Thunnus alalunga) and 75 Pacific bigeye tuna (BET, Thunnus obesus), collected from 2001 to 2006, were analyzed. No ALB, but 13 large BET had organic mercury (OHg) concentrations exceeding 1 μg g⁻¹ wet weight. For both ALB and BET, total mercury (THg) and OHg concentrations were significantly and positively correlated with fork length (FL) and body weight. The muscle Hg bioaccumulation rates of BET were higher than those of ALB, particularly in the adult fish. Moreover, the lines had crossover points among the two species that imply the young BET (FL < 110 cm) contains lower muscle Hg concentrations than ALB of the same size. The suggested weekly dietary intake of ALB and small-BET meats is 340 g, and of BET meat it is 150 g for a 60-kg person based on the provisional tolerable weekly intake (PTWI) of methylmercury set by the WHO.     

Mercury concentrations and omega-3 fatty acids in fish and shrimp: Preferential consumption for maximum health benefits

The consumption of fish and shrimp containing omega-3 fatty acids can result in protective health effects including a reduced risk of cardiovascular disease, stroke, and diabetes. These protective effects may be decreased by the presence of mercury in the muscle tissue of fish and shellfish. Mercury can increase the risk of cardiovascular problems and impede neurological development. The objective of this project was to determine appropriate consumption amounts of selected fish species and shrimp based on mercury levels and recommended intake levels of omega-3 fatty acids. Species that are high in omega-3s and low in mercury include salmon, trout, and shrimp. Species with both high levels of mercury and omega-3 fatty acids include tuna, shark, and halibut, swordfish, and sea bass.     

The impact of resuspension on sediment mercury dynamics, and methylmercury production and fate: A mesocosm study

The objective of this study was to investigate the effects of resuspension on the fate and bioaccumulation of mercury (THg) and methylmercury (MeHg) in shallow estuarine environments, using mesocosms. Two 4-week experiments were conducted in July (Experiment 1) and October (Experiment 2) of 2001 with Baltimore Harbor sediments. Hard clams, Mercenaria mercenaria, were introduced into sediments for Experiment 2. Tidal resuspension (4 h on and 2 h off cycles) was simulated, with 3 replicate tanks for each treatment—resuspension (R) and non-resuspension (NR). Sediment cores were collected during the experiments for THg, MeHg, organic content and AVS analyses, and for the determination of methylation/demethylation using Hg stable isotopes (¹⁹⁹Hg(II) and CH₃¹⁹⁹Hg(II)). Zooplankton samples were collected once a week while clams were taken before and after Experiment 2 for THg and MeHg analyses. Our results suggest that the interplay between Hg methylation and MeHg degradation determines the overall MeHg pool in sediments. Sediment resuspension does not appear to directly impact the Hg transformations but can lead to changes in the association to Hg binding phases, influencing Hg methylation. The bioaccumulation results indicate that sediment resuspension can play an important role in transferring sediment MeHg into organisms.     

Evidence of methylmercury production and modification of the microbial community structure in estuary sediments contaminated with wastewater treatment plant effluents

The Seine’s estuary (France) waters are the receptacle of effluents originating from wastewater treatment plants (WWTP). In this estuary, mudflats are deposition zones for sediments and their associated contaminants, and play an essential role in the mercury (Hg) biogeochemical cycle mainly due to indigenous microorganisms. Microcosms were used to assess the impact of WWTP-effluents on mercury methylation by monitoring Hg species (total dissolved Hg in porewater, methylmercury and total mercury) and on microbial communities in sediments. After effluent amendment, methylmercury (MeHg) concentrations increased in relation with the total Hg and organic matter content of the WWTP-effluents. A correlation was observed between MeHg and acid-volatile-sulfides concentrations. Quantification of sulfate-reducing microorganisms involved in Hg methylation showed no increase of their abundance but their activity was probably enhanced by the organic matter supplied with the effluents. WWTP-effluent spiking modified the bacterial community fingerprint, mainly influenced by Hg contamination and the organic matter amendment.