Contrasting biogeochemical controls on the spatial distribution and zooplankton bioaccumulation of total mercury versus methylmercury in a large river-dominated estuary.

River-dominated estuaries are particularly vulnerable to mercury (Hg) pollution, yet field evidence on how the large estuarine gradients modulate the spatial heterogeneity of Hg distribution and bioaccumulation in zooplankton remains limited. Based on a comprehensive field investigation along a transect from the Yangtze River estuary (YRE) to the East China Sea (ECS) inner shelf, this study reveals distinct spatial patterns of total Hg (THg, mainly as inorganic Hg) and methylmercury (MeHg) distribution in water columns and zooplankton bioaccumulation. Dissolved THg in surface and bottom waters fluctuated along the transect, whereas dissolved MeHg and the proportion of MeHg to THg (%MeHg) were notably elevated within the estuarine turbidity maximum zone. Particulate THg and partitioning coefficients of THg decreased seaward, mainly driven by salinity-induced desorption. THg concentrations were higher in small-sized zooplankton owing to greater inorganic Hg adsorption, and decreased at marine sites due to salinity-induced inhibition on Hg bioavailability via speciation modulation. Conversely, MeHg concentrations were higher in larger zooplankton due to biomagnification, and MeHg bioaccumulation was elevated within the estuarine mixing zone, likely driven by site-specific MeHg production and dietary shifts toward bacterial food sources as evidenced by fatty acid biomarkers. Our findings demonstrate contrasting biogeochemical controls on the fate and ecological risks of inorganic Hg versus MeHg, providing crucial insights for Hg pollution risk assessment in large river-dominated estuarine systems.