Spatiotemporal Variations in Dissolved Elemental Mercury in the River-Dominated and Monsoon-Influenced East China Sea: Drivers, Budgets, Implications.
Distinct spatiotemporal distributions of sea surface dissolved elemental mercury (DEM) and its air-sea exchange flux were observed in the river-dominated and monsoon-influenced East China Sea (ECS). Spatially, DEM concentrations were higher in nearshore Changjiang Diluted Water (90±20 - 260±40 fM) than in offshore Kuroshio Water (60±10 - 160±40 fM) and correlated with salinity and total Hg concentrations, suggesting total Hg discharged from Changjiang River is a controlling factor. In summer, monsoon-driven, coastal upwelling formed a transient nearshore water mass with very elevated DEM concentrations (290±20 - 320±70 fM). Seasonally, DEM concentrations in all water masses were highest in summer (120±30 - 320±70 fM). Estimated rate coefficients for DEM production varied seasonally and strongly correlated with sea surface temperature (SST). Hg0 evasion fluxes also peaked in summer (670±380 pmol m-2 d-1), while in winter DEM was close to equilibrium with gaseous elemental mercury in the atmosphere. Based on the air-sea Hg fluxes for all four seasons from this study, and regional atmospheric deposition fluxes from others, we conclude that the ECS is a net sink of Hg annually, but a source of Hg to the atmosphere in summer. Moreover, the contribution of the ECS to Hg evasion may increase as a result of flood-associated high Changjiang discharge and rising SST.