Distribution of dissolved gaseous mercury and reactive Hg and their controlling factors in the East China Sea in summer

Dissolved gaseous mercury (DGM), reactive Hg (RHg), total Hg (THg) and dissolved Hg (DHg) concentrations were analyzed in the seawater of the East China Sea (ECS) during a cruise in June 2018. The distribution patterns of DGM and RHg and their controlling factors in the East China Sea (ECS) were investigated. The concentrations of DGM and RHg in the seawater were determined to be (151.3±75.9) pg/L and (0.8±0.7) ng/L, respectively. The ratios of DGM/THg, DGM/RHg and RHg/THg were calculated to be (4.5±2.5)%, (26.7±15.0)% and(21.6±14.8)%, respectively. DGM and RHg concentrations in the ECS water were significantly higher than that in most open oceans, but lower than or comparable to that reported in most coastal seas. Both DGM and RHg in the seawater presented a complicated spatial distribution pattern, with high concentration areas located in both nearshore shallow seawater, and offshore shallow and deep seawater. These results indicate that both species of Hg may be controlled by both the terrestrial input and in situ production/removal processes. As for the vertical distribution, the concentrations of DGM and RHg in the bottom seawater were observed to be the lowest, while comparable concentrations were observed at all the other sampling layers. Although highest THg concentration was observed in the bottom seawater of the ECS, DHg concentrations in the bottom seawater were slightly lower than those at the other layers. This may be a reasonable explanation of the lower RHg and DGM concentrations in the bottom water. Spearman’s correlation and multiple regression analyses suggest that RHg and dissolved oxygen (DO) concentrations may be the major controlling factors for DGM in the seawater, while RHg in the seawater may be controlled by DO.