High-resolution measurements of elemental mercury in surface water for an improved quantitative understanding of the Baltic Sea as a source of atmospheric mercury

Abstract

Abstract. Marginal seas are directly subjected to anthropogenic and natural influences from land in addition to receiving inputs from the atmosphere and open ocean. Together these lead to pronounced gradients and strong dynamic changes. However, in the case of mercury emissions from these seas, estimates often fail to adequately account for the spatial and temporal variability of the elemental mercury concentration in surface water (Hg 0 wat ). In this study, a method to measure Hg 0 wat at high resolution was devised and subsequently validated. The better-resolved Hg 0 wat dataset, consisting of about one measurement per nautical mile, yielded insight into the sea's small-scale variability and thus improved the quantification of the sea's Hg 0 emission. This is important because global marine Hg 0 emissions constitute a major source of atmospheric mercury.Research campaigns in the Baltic Sea were carried out between 2011 and 2015 during which Hg 0 both in surface water and in ambient air were measured. For the former, two types of equilibrators were used. A membrane equilibrator enabled continuous equilibration and a bottle equilibrator assured that equilibrium was reached for validation. The measurements were combined with data obtained in the Baltic Sea in 2006 from a bottle equilibrator only. The Hg 0 sea-air flux was newly calculated with the combined dataset based on current knowledge of the Hg 0 Schmidt number, Henry's law constant, and a widely used gas exchange transfer velocity parameterization. By using a newly developed pump-CTD with increased pumping capability in the Hg 0 equilibrator measurements, Hg 0 wat could also be characterized in deeper water layers. A process study carried out near the Swedish island Øland in August 2015 showed that the upwelling of Hg 0 -depleted water contributed to Hg 0 emissions of the Baltic Sea. However, a delay of a few days after contact between the upwelled water and light was apparently necessary before the biotic and abiotic transformations of ionic to volatile Hg 0 produced a distinct sea-air Hg 0 concentration gradient.This study clearly showed spatial, seasonal, and interannual variability in the Hg 0 sea-air flux of the Baltic Sea. The average annual Hg 0 emission was 0.90 ± 0.18 Mg for the Baltic proper and extrapolated to 1.73 ± 0.32 Mg for the entire Baltic Sea, which is about half the amount entrained by atmospheric deposition. A comparison of our results with the Hg 0 sea-air fluxes determined in the Mediterranean Sea and in marginal seas in East Asia were to some extent similar but they partly differed in terms of the deviations in the amount and seasonality of the flux.

Abundance and expression ofhgcABgenes and mercury availability jointly explain methylmercury formation in stratified brackish waters

“…Overall, the HgT and MeHg concentrations and MeHg/HgT molar ratio observed in our study were in the same ranges as previous observations for redox-stratified waters in the Baltic Sea [23, 26], although Soerensen et al [26] observed maximum HgT concentrations that exceeded the levels measured in our study. Differences in depth of maximum MeHg concentration and/or MeHg/HgT ratio have been reported among or within studies on redox stratified coastal systems, with maxima located either at the interface between the redox transition (or hypoxic) zone and the euxinic (or anoxic) zone [22–24, 29] or in deeper euxinic water layers with H 2 S concentrations in the μM range [23–26]. Notably, the MeHg concentration and MeHg/HgT ratio, which are the parameters normally determined in studies on MeHg in coastal seas, are proxies for net formation of MeHg [30].…”

“…Higher MeHg concentrations and molar ratios of MeHg to total Hg (HgT) have generally been observed in oxygen depleted waters of redox-stratified coastal seas but contrasting vertical MeHg profiles in anoxic/euxinic zones have been reported. Notably, MeHg concentration maxima have been observed at the hypoxic-anoxic interface [22][23][24] or in deeper euxinic water layers [23][24][25][26]. The reasons for these differences in profiles are not clear and there are still significant uncertainties regarding factors and processes that control MeHg formation in oxygen depleted coastal waters.…”

Speciated atmospheric mercury and sea–air exchange of gaseous mercury in the South China Sea

“…The sea-air flux of Hg was calculated using a thin film gas exchange model developed by Liss and Slater (1974) and Wanninkhof (1992). The detailed calculation processes of Hg 0 flux have been reported in recent studies (Ci et al, 2011;Kuss, 2014;Wang et al, 2016c;Kuss et al, 2018).…”

Deltaproteobacteria and Spirochaetes-Like Bacteria Are Abundant Putative Mercury Methylators in Oxygen-Deficient Water and Marine Particles in the Baltic Sea

“…As such, the Baltic Sea represents a model for the expansion of coastal ecosystems influenced by anoxia. Elevated MeHg concentrations in the Baltic Sea have been observed in anoxic water (>1000 fM) compared to hypoxic and normoxic water (Kuss et al, 2018;Soerensen et al, 2018). Soerensen et al (2018) demonstrated that this was caused by increased rates of Hg methylation in the oxygen deficient water zones.…”