PRODUCTION AND LOSS OF DISSOLVED GASEOUS MERCURY IN COASTAL SEAWATER

The formation of dissolved gaseous mercury (DGM, mainly composed of el emental mercury, Hg-0) in the surface ocean and it 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 produce d, DGM was rapidly lost from solution [with a first order rate constan t of k = 0.1 h(-1)], apparently as a result of oxidation. Furthermore, laboratory experiments showed that dissolved gaseous Hg-0 could be ra pidly oxidized in the presence of chloride. In the field, most DGM pro duction (about 60%) was associated with the dissolved nad colloidal Hg [11] phases. Spiking of samples with inorganic Hg[11] prior to in situ incubation greatly increased DGM production rates, suggesting that ph otoreducible Hg[11] complexes were limiting DGM production. Diurnally, DGM seems to be formed through photoproduction in the morning; DGM pr oduction halts when substrate is exhausted, and DGM levels decrease af terwards, presumably by oxidation of Hg-0.