Mercury (Hg) in the “Skin” of the Ocean: Dissolved Gaseous Hg, Total Hg, and Hg Redox Chemistry in Sea Surface Microlayer and Implication for Air–Sea Hg Exchange

The sea surface microlayer (SML) is the uppermost ∼1000 μm of the surface of the ocean. With distinct physicochemical properties and position relative to the adjacent subsurface waters (SSWs), the ubiquitous distribution and high dynamics of the SML greatly regulate the global air–sea gas exchange and biogeochemistry. Mercury (Hg) redox chemistry in surface seawaters and air–sea exchange of gaseous Hg (mainly Hg(0)) fundamentally control the global oceanic Hg cycle. However, the occurrence and transformation of Hg in the SML have been poorly quantified. Here we optimize the traditional SML sampling system to make it more suitable for dissolved gaseous Hg (DGM, mainly Hg(0)) sampling. We then assess the temporal and spatial variability of DGM, total Hg, dissolved organic carbon (DOC), and Hg redox chemistry in the SML and SSWs of diverse marine environments. Our data suggest a general DGM, total Hg, and DOC enrichment in the SML relative to the SSWs but with complex variability in time and space. The incubation experiments further reveal the complex characteristics of Hg redox chemistry between the SML and SSWs. We discuss important implications of the SML Hg cycle on air–sea Hg exchange and suggest wider investigations of the SML Hg cycle in the global hydrosphere

Mercury Redox Chemistry in Waters of the Eastern Asian Seas: From Polluted Coast to Clean Open Ocean

We performed incubation experiments using seawaters from representative marine environments of the eastern Asian seas to determine the mercury (Hg) available for photoreduction (Hgr­(II)), to investigate the Hg redox reaction kinetics, and to explore the effect of environmental factors and water chemistry on the Hg redox chemistry. Results show that Hgr­(II) accounted for a considerable fraction of total Hg (THg) (%Hgr­(II)/THg: 24.90 ± 10.55%, <i>n</i> = 27) and positively correlated with THg. Filtration decreased the Hgr­(II) pool of waters with high suspended particulate matter (SPM). The positive linear relationships were found between pseudo-first order rate constants of gross Hg­(II) photoreduction (<i>k</i>_r) and gross Hg(0) photo-oxidation (<i>k</i>_o) with photosynthetically active radiation (PAR). Under the condition of PAR of 1 m mol m^–2 s^–1, the <i>k</i>_r were significantly (<i>p</i> < 0.05) lower than <i>k</i>_o (<i>k</i>_r/<i>k</i>_o: 0.86 ± 0.22). The Hg(0) dark oxidation were significantly higher than the Hg­(II) dark reduction. The Hg­(II) dark reduction was positively correlated to THg, and the anaerobic condition favored the Hg­(II) dark reduction. Filtration significantly influenced the Hg photoredox chemistry of waters with high SPM. UVB radiation was important for both Hg­(II) photoreduction and Hg(0) photo-oxidation, and the role of other wavebands in photoinduced transformations of Hg varied with the water chemistry