Rivers as microplastics gatekeepers: Linking terrestrial inputs to coastal sediment sinks and sources

Coastal sediments are widely recognized as a critical barrier regulating the transfer of contaminants, particularly microplastics (MPs), from terrestrial environments to marine ecosystems. However, whether these sediments function primarily as long-term sinks or as episodic sources of MPs, and the mechanisms governing this functional transition, remain poorly constrained. This study investigated the key drivers controlling the seasonal and spatial distribution of MPs in coastal sediments within a densely populated urban river-to-sea system. The average MP abundance in riverine sediments was 55.63 ± 38.11 items/kg, approximately 2.7 times higher than that observed in estuarine sediments at 20.93 ± 13.08 items/kg, indicating that riverine inputs constitute the dominant source of MPs to the estuarine and coastal environment. Polyethylene and polypropylene - were the dominant plastic types in both water and sediment samples. Seasonal variations in MP accumulation within estuarine sediments were closely associated with changes in river discharge, sediment resuspension, and local hydrodynamic conditions. The lowest MP abundance occurred in autumn following a typhoon-induced extreme discharge event, suggesting that high-energy hydrological forcing mobilized and exported previously accumulated MPs from riverine and outfall-related sediments, thereby shifting sediments from temporary sinks to active sources. These findings demonstrate that sedimentary MP inventories are highly dynamic rather than static, responding sensitively to episodic hydrological disturbances and coastal engineering structures. By explicitly linking terrestrial inputs, current directions, extreme discharge events, and sediment dynamics, this work advances mechanistic understanding of when and how coastal sediments alternate between acting as sinks and sources of MPs in river-to-sea continua.