Determination of dissolved gaseous mercury in seawater of Minamata Bay and estimation for mercury exchange across air–sea interface

• Our study presents the first observational data of dissolved gaseous mercury and mercury evasion flux throughout the year at a site in Japan.
• DGM in the surface seawater of Minamata Bay had spatial distribution with a higher concentration at the coastal sites than at more open sea sites
• DGM concentration was higher in summer than in other seasons.
• In the case of Minamata Bay, Hg evasion flux from the sea surface is comparable with the direct atmospheric deposition flux of Hg.

Dissolved gaseous Hg (DGM) in seawater and atmospheric gaseous Hg were measured at six sites in Minamata Bay to investigate mercury (Hg) evasion flux from the sea surface. Minamata Bay was severely polluted with mono-methyl Hg (MMHg). Total Hg and MMHg, seawater characteristics such as water temperature and salinity, and meteorological parameters were also observed to estimate the air–sea exchange of Hg. The mean concentration of DGM was 116 ± 76 pg L− 1 (N = 75), ranging from 19 to 442 pg L− 1, and the concentrations were higher in summer than in other seasons. DGM concentration showed a significantly positive correlation with solar radiation, and air and water temperatures. Inversely, DGM showed a significantly negative correlation with salinity and redox potential (ORP). Hg evasion fluxes from the sea surface of the bay were calculated using a two-layer gas exchange model and ranged between 0.11 and 33 ng m− 2 h− 1 (mean, 5.4 ± 6.3 ng m− 2 h− 1). The estimated flux was slightly higher in the spring and fall when wind speed increased because the gas exchange coefficient used for estimating Hg evasion flux strongly depends on wind speed. The annual evasion flux of Hg from the sea surface of Minamata Bay was estimated to be 47 ± 56 μg m− 2, which was on the same order of magnitude as the direct atmospheric deposition flux of Hg (24 μg m− 2) during the observation period. Therefore, Hg evasion from the sea surface likely plays an important role in the Hg cycle of Minamata Bay.