Contrasting distribution patterns and mobilization mechanisms of pore water heavy metals in riverine and coastal ecosystems

Heavy metal (HM) contamination in sediment pore water poses significant ecological risks in aquatic environments, yet the mechanisms controlling HM mobilization in coastal and riverine ecosystems remain poorly understood. Understanding these mechanisms is essential for effective pollution management and ecological risk assessment. In this study, we applied high-resolution dialysis (HR-Peeper) to quantify nine HMs (As, Co, Cr, Cu, Fe, Mn, Ni, Zn, V) in the pore water of five wetland types (tidal marshes, aquaculture ponds, mud flats, mangrove wetlands, and tidal estuaries) within Changsha Bay and the adjacent Huangjiang River, China. Results revealed that the concentrations and pollution levels of HMs were generally higher in coastal than in riverine ecosystems, with mangrove wetlands exhibiting the greatest accumulation potential. Along the river, the HM concentrations decreased from upstream to downstream, while diffusive fluxes exhibited a non-linear pattern. These spatial variations were primarily driven by hydrodynamic conditions and anthropogenic inputs. In coastal ecosystems, the concentration gradients between pore water and overlying water primarily governed HM mobilization, with pore water generally acting as a source, except in aquaculture ponds. These findings provide new insights into the transport pathways and ecological risks of HMs, highlighting key factors for managing heavy metal pollution in coastal and riverine ecosystems.