David Point

Diamondback terrapins, Malaclemys terrapin, as a sentinel species for monitoring mercury pollution of estuarine systems in South Carolina and Georgia, USA

Total mercury concentrations were measured in diamondback terrapin blood and scutes collected from four sites in South Carolina, USA, and at a superfund site in Brunswick, Georgia, USA. There was a strong correlation between mercury concentrations in the two terrapin body compartments (Kendalls tau = 0.79, p < 0.001). Mercury concentrations in terrapin scute and blood and in salt marsh periwinkles, Littoraria irrorata, were significantly higher in Brunswick (scute x = 3810.2 ng/g, blood x = 746.2 ng/g) than from all other sites (scute x = 309.5 ng/g, blood x = 43.2 ng/g, p < 0.001). Seasonal fluctuations of total mercury in the blood and scutes of terrapins collected in the Ashley River, South Carolina, were significantly lower in August than in April, June, or October in blood (p < 0.001); however, scute concentrations did not vary seasonally. Overall, we found higher concentrations of mercury in the scutes of females than males (n = 32, p < 0.05). Larger females may preferentially prey on larger food items, like large periwinkles, which had significantly higher mercury levels in their body tissues than smaller periwinkles (p < 0.001). Methylmercury levels in terrapin scutes were measured, revealing that 90% of the total mercury stored in this compartment was in the organic form. A methylmercury biomagnification factor of 173.5 was calculated from snails to terrapin scutes, and we found that mercury levels in scutes were representative of the mercury levels in other compartments of the ecosystem. These findings show that terrapin scutes are good predictors of mercury pollution and that this species could be used as a bioindicator for assessing mercury contamination of estuarine systems.

Geographical Origin of Amazonian Freshwater Fishes Fingerprinted by 87Sr/86Sr Ratios on Fish Otoliths and Scales

Calcified structures such as otoliths and scales grow continuously throughout the lifetime of fishes. The geochemical variations present in these biogenic structures are particularly relevant for studying fish migration and origin. In order to investigate the potential of the (87)Sr/(86)Sr ratio as a precise biogeochemical tag in Amazonian fishes, we compared this ratio between the water and fish otoliths and scales of two commercial fish species, Hoplias malabaricus and Schizodon fasciatus, from three major drainage basins of the Amazon: the Madeira, Solimões, and Tapajós rivers, displaying contrasted (87)Sr/(86)Sr ratios. A comparison of the (87)Sr/(86)Sr ratios between the otoliths and scales of the same individuals revealed similar values and were very close to the Sr isotopic composition of the local river where they were captured. This indicates, first, the absence of Sr isotopic fractionation during biological uptake and incorporation into calcified structures and, second, that scales may represent an interesting nonlethal alternative for (87)Sr/(86)Sr ratio measurements in comparison to otoliths. Considering the wide range of (87)Sr/(86)Sr variations that exist across Amazonian rivers, we used variations of (87)Sr/(86)Sr to discriminate fish origin at the basin level, as well as at the sub-basin level between the river and savannah lakes of the Beni River (Madeira basin).

Consideration and influence of complexed forms of mercury species on the reactivity patterns determined by speciated isotope dilution model approaches: A case for natural biological reference materials

The processes driving the inadvertent transformations of inorganic mercury (iHg) and methylmercury (MeHg) in cryogenically homogenized fresh-frozen (FF) and freeze-dried (FD) biological standard reference materials (SRM) were investigated using alkaline digestion, derivatization and gas chromatography inductively coupled plasma mass spectrometry (GC-ICP-MS). Labile inorganic mercury (201iHg) and methylmercury (Me202Hg) isotopic standards and their cysteine-complexed analogs (201Hg(Cys)2 and Me202HgCys) were used in a double-spike speciated isotope dilution (SID) model to document the influence of complexing ligands on the equilibration, the reactivity and the transformation processes between isotopic mercury species and their endogenous analogs. Cysteine-complexed and labile Me202Hg spiking standards displayed similar equilibration processes in both classes of materials, leading to accurate MeHg determinations with negligible methylation transformations. Labile 201iHg standards provided accurate iHg concentration results in both material series, although an apparent demethylation reaction specific of FF materials was detected, with a negligible effect in FD materials. Cysteine-complexed 201iHg standards led to higher but inaccurate iHg concentrations and higher demethylation yields in both materials. This comparison illustrated a significant influence of complexing ligands on the equilibration processes between labile and/or complexed 201iHg species and their endogenous analogs. The derivatization step was found to catalyze these non-equilibrium conditions with different derivatization yields between labile and complexed iHg species, whereas no differences were observed for MeHg species. When complexation disparities existed in solution between 201iHg species and their endogenous analogs, this process specifically affected the determination of 200/201iHg ratios relative to 200/202iHg, 200/201MeHg and 200/202iHg ratios, which are all involved in the double-spike SID model to establish transformation yields and mercury species concentrations. This process and the influence of demethylating agents were presumably responsible for the apparent demethylation reaction only observed in FF materials. This result gave rise to several questions on the influence of freeze-drying procedures on the lability/complexation patterns of mercury species and/or on the activity of demethylating agents. A complementary analytical step consisting of maximizing the equilibrium conditions between isotopic and endogenous mercury species together with inhibiting the activity of demethylating agent is proposed. When applying these conditions, the two classes of materials displayed negligible transformation reactions, improving their commutability for the simultaneous determination of iHg and MeHg. The approach proposed in this work allowed for the first time documentation of the potential and limitations of speciated isotope dilution procedures to handle the problem of lability/complexation and ligand interactions in natural biological matrices when considering multiple-spike SID approaches.

Hg Stable Isotope Time Trend in Ringed Seals Registers Decreasing Sea Ice Cover in the Alaskan Arctic.

Decadal time trends of mercury (Hg) concentrations in Arctic biota suggest that anthropogenic Hg is not the single dominant factor modulating Hg exposure to Arctic wildlife. Here, we present Hg speciation (monomethyl-Hg) and stable isotopic composition (C, N, Hg) of 53 Alaskan ringed seal liver samples covering a period of 14 years (1988-2002). In vivo metabolic effects and foraging ecology explain most of the observed 1.6 ‰ variation in liver δ(202)Hg, but not Δ(199)Hg. Ringed seal habitat use and migration were the most likely factors explaining Δ(199)Hg variations. Average Δ(199)Hg in ringed seal liver samples from Barrow increased significantly from +0.38 ± 0.08‰ (±SE, n = 5) in 1988 to +0.59 ± 0.07‰ (±SE, n = 7) in 2002 (4.1 ± 1.2% per year, p < 0.001). Δ(199)Hg in marine biological tissues is thought to reflect marine Hg photochemistry before biouptake and bioaccumulation. A spatiotemporal analysis of sea ice cover that accounts for the habitat of ringed seals suggests that the observed increase in Δ(199)Hg may have been caused by the progressive summer sea ice disappearance between 1988 and 2002. While changes in seal liver Δ(199)Hg values suggests a mild sea ice control on marine MMHg breakdown, the effect is not large enough to induce measurable HgT changes in biota. This suggests that Hg trends in biota in the context of a warming Arctic are likely controlled by other processes.

Mercury isotope signatures of methylmercury in rice samples from the Wanshan mercury mining area, China: environmental implications

Rice consumption is the primary pathway of methylmercury (MeHg) exposure for residents in mercury-mining areas of Guizhou Province, China. In this study, compound-specific stable isotope analysis (CSIA) of MeHg was performed on rice samples collected in the Wanshan mercury mining area. An enrichment of 2.25‰ in total Hg (THg) δ202Hg was observed between rice and human hair, and THg Δ199Hg in hair was 0.12‰ higher than the value in rice. Rice and human hair samples in this study show distinct Hg isotope signatures compared to those of fish and human hair of fish consumers collected in China and other areas. Distinct Hg isotope signatures were observed between IHg and MeHg in rice samples (in mean ± standard deviation: δ202HgIHg at -2.30‰ ± 0.49‰, Δ199HgIHg at -0.08‰ ± 0.04‰, n = 7; δ202HgMeHg at -0.80‰ ± 0.25‰, Δ199HgMeHg at 0.08‰ ± 0.04‰, n = 7). Using a binary mixing model, it is estimated that the atmospheric Hg contributed 31% ± 16% of IHg and 17% ± 11% of THg in the rice samples and the IHg in soil caused by past mining activities contributed to the remaining Hg. This study demonstrated that Hg stable isotopes are good tracers of human MeHg exposure to fish and rice consumption, and the isotope data can be used for identifying the sources of IHg and MeHg in rice.