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The chemical speciation of mercury (Hg), methyl mercury (MeHg), sulfur and iron was investigated in the sediment and porewater of Lake Angessjon, a boreal, shallow (maximum depth 2.5 m), oligo-/dystrophic lake in northern Sweden. The lake receives terrestrial stream runoff from surrounding coniferous forest soils and peatlands having a low pH (4.6) and high concentrations of dissolved organic matter (DOM, annual average: 45 mg L−1), Fe (60 µM), sulfate (105 µM), inorganic Hg (1200 pM) and MeHg (250 pM). Sulfur K-edge XANES and Hg LIII-edge EXAFS spectroscopic measurements were used to characterize and quantify the sulfur speciation in the lake sediment at nine occasions, covering different seasons in the years of 2007 and 2009. In the surface sediment (0–3 cm) sulfate is reduced to zero-valent S and inorganic sulfide, that in turn reacts with Fe to form FeSm (mackinawite) and FeS2 (framboidal pyrite). The latter mineral becomes increasingly dominant by depth in the sediment. Thermodynamic modeling successfully predicted measured porewater concentrations of Hg in the sediment. Metacinnabar (β-HgS) and Hg(NOM-RS)2 complexes (the latter formed as a reaction between Hg(II) and thiol groups associated with natural organic matter, NOM-RSH) were the dominant forms of Hg(II) in the solid phase of sediments and Hg(II)-polysulfides (aq) dominated in the porewater. We argue that FeSm is a key component that indirectly controlled the Hg(II) speciation in the sediment by keeping the aqueous phase concentration of inorganic sulfide in the 0.5–2 µM range throughout the year. Besides providing a pool of readily soluble inorganic sulfide for formation of β-HgS(s), as demonstrated by previously reported EXAFS experiments, we further suggest FeSm may serve as a precursor for the formation of a more crystalline (less soluble) β-HgS(s) phase than present in environments devoid of FeSm. Support for this was provided by comparing our results with previously reported thermodynamic modelling results of Hg(II) and MeHg solubility in organic soils devoid of FeSm. In more general terms, we suggest the presence or absence of FeSm, through its influence on the chemical speciation of Hg and MeHg, may be a key factor behind the variability in rates of Hg(II) and MeHg transformation processes, such as methylation, reduction and demethylation, reported for different environmental settings.

Methylmercury (MeHg) is a neurotoxin formed from inorganic divalent mercury (Hg-II) via microbial methylation, and boreal wetlands have been identified as major sources of MeHg. There is however a lack of studies investigating the relationship between the chemical speciation of Hg-II and MeHg formation in such environments, in particular regarding to role of thiol compounds. We determined Hg-II methylation potentials, k(meth), in boreal wetland soils using two Hg-II isotope tracers: Hg-198(OH)(2)(aq) and Hg-II bonded to thiol groups in natural organic matter, Hg-200(II)-NOM(ads), representing Hg-II sources with high and low availability for methylation. The Hg-198(OH)(2)(aq) tracer was consistently methylated to a 5-fold higher extent than Hg-200(II)-NOM(ads), independent of environmental conditions. This suggests that the concentration of Hg-II in porewater was a decisive factor for Hg-II methylation. A comprehensive thermodynamic speciation model (including Hg-II complexes with inorganic sulfide (H2S), polysulfides (H2Sn), thiols associated with natural organic matter (NOM-RSH) and specific low molecular mass thiols (LMM-RSH) provided new insights on the speciation of Hg-II in boreal wetland porewaters, but did not demonstrate any clear relationship between k(meth) and the calculated chemical speciation. In contrast, significant positive relationships were observed between k(meth) and the sum of LMM thiol compounds of biological origin. We suggest two possible mechanisms underlying these correlations: 1) LMM thiols kinetically control the size and composition of the HgII pool available for microbial uptake, and/or 2) LMM thiols are produced by microbes such that the correlation reflects a relation between microbial activity and MeHg formation. (C) 2020 The Author(s). Published by Elsevier B.V.

Terrestrial discharge can cause pelagic zone trophic shifts and enhance methylmercury accumulation in plankton three- to sixfold., The input of mercury (Hg) to ecosystems is estimated to have increased two- to fivefold during the industrial era, and Hg accumulates in aquatic biota as neurotoxic methylmercury (MeHg). Escalating anthropogenic land use and climate change are expected to alter the input rates of terrestrial natural organic matter (NOM) and nutrients to aquatic ecosystems. For example, climate change has been projected to induce 10 to 50% runoff increases for large coastal regions globally. A major knowledge gap is the potential effects on MeHg exposure to biota following these ecosystem changes. We monitored the fate of five enriched Hg isotope tracers added to mesocosm scale estuarine model ecosystems subjected to varying loading rates of nutrients and terrestrial NOM. We demonstrate that increased terrestrial NOM input to the pelagic zone can enhance the MeHg bioaccumulation factor in zooplankton by a factor of 2 to 7 by inducing a shift in the pelagic food web from autotrophic to heterotrophic. The terrestrial NOM input also enhanced the retention of MeHg in the water column by up to a factor of 2, resulting in further increased MeHg exposure to pelagic biota. Using mercury mass balance calculations, we predict that MeHg concentration in zooplankton can increase by a factor of 3 to 6 in coastal areas following scenarios with 15 to 30% increased terrestrial runoff. The results demonstrate the importance of incorporating the impact of climate-induced changes in food web structure on MeHg bioaccumulation in future biogeochemical cycling models and risk assessments of Hg.