Distribution and Transfer of Mercury in the Soil–Plant Systems of the Yellow River Estuary Coastal Wetland: Important Roles of the River–Sea Interaction

Coastal estuarine wetlands are vulnerable to river overflows and tidal inputs, yet their roles in mercury (Hg) cycling remain unclear, especially regarding soil Hg distribution, neurotoxic methylmercury (MeHg) production, and plant accumulation. This study investigated the spatial patterns and transfer of total Hg (THg) and MeHg in soils and plants of the Yellow River Estuary wetland (YREW). Soil THg (14.6 ± 6.09 ng/g) decreased from the river bank and coast toward wetland interiors, consistent with the weakening influence of Yellow River discharge and Bohai Sea tidal inputs. Soil MeHg (0.0837 ± 0.0521 ng/g) showed a broadly similar spatial pattern but was likely influenced by external inputs and in situ Hg methylation. Tidal-influenced soils exhibited higher MeHg/THg ratios (0.715–0.944%, commonly representing Hg methylation) compared to river-dominated soils (0.389–0.397%), probably suggesting relatively greater MeHg production and/or accumulation under stronger tidal influence. Moreover, plant THg and MeHg accumulation varied among species and broadly corresponded to soil Hg patterns, suggesting that river–sea interactions may influence plant Hg accumulation through soil-derived uptake and species-specific transfer. Suaeda salsa exhibited higher bioconcentration and transfer factors than Spartina alterniflora and Phragmites australis, indicating stronger Hg accumulation and root-to-aboveground translocation.