The dynamic sink: How settling-resuspension cycles drive offshore transport of high-density microplastics

Marine sediments represent a major sink for microplastics (MPs), yet the hydrodynamic processes controlling their remobilization and redistribution remain insufficiently understood. By integrating field observations with a high-resolution numerical model that resolves the complete settling-resuspension cycle, this study investigates the transport of high-density MPs in the dynamic Yangtze Estuary-Hangzhou Bay system. Our results demonstrate that settling-resuspension cycles facilitate the extensive along-shelf transport, with pathways regulated by particle discharging and hydrodynamic temporal structure. We further introduce and validate three key metrics-settling flux (F_s), resuspension period (T_resus), and mean residence time (τ_resi)-to quantify bed shear stress controls on MP transport. F_s correlates negatively with surface-sediment MP abundance, challenging the conventional link that high settling flux leads to substantial deposition accumulation. The mean residence time (τ_resi) and its governing hydrodynamic proxy, the exceedance time proportion ( [Formula: see text] ), together provide a robust, scalable indicator for MP accumulation, as supported by global sediment data. This study proposes a "dynamic sink" framework that advances the traditional density-dependent transport paradigm, providing a temporally explicit mechanistic basis for predicting MP fate in coastal environments worldwide.