Production and loss of dissolved gaseous mercury in coastal seawater

The formation of dissolved gaseous mercury (DGM, mainly composed of elemental mercury, Hg⁰) in the surface ocean and its subsequent removal through volatilization is an important component of the global mercury (Hg) cycle. We studied DGM production and loss in the coastal waters of the Gulf of Mexico using 4-26 h in situ incubation experiments. DGM production was only induced in the presence of sunlight. Once produced, DGM was rapidly lost from solution (with a first order rate constant of k = 0.1 h^-1), apparently as a result oxidation. Furthermore, laboratory experiments showed that dissolved gaseous Hg⁰ could be rapidly oxidized in the presence of chloride. In the field, most DGM production (about 60%) was associated with the dissolved and colloidal Hg(II) phases. Striking of samples with inorganic Hg(II) prior to in situ incubation greatly increased DGM production rates, suggesting that photoreducible Hg(II) complexes were limiting DGM production. Diurnally, DGM seems to be formed through photoproduction in the morning; DGM production halts when substrate is exhausted, and DGM levels decrease afterwards, presumably by oxidation of Hg⁰.