Estuaries as fragmentation hotspots and transport regulators of microplastics

Estuaries serve as critical transition zones for land-derived microplastics, where strong hydrodynamic and sedimentary interactions can substantially reshape their abundance, size, and transport behavior. However, the environmental processes governing microplastics in estuarine environment remain poorly understood. Here we combined field observations and laboratory simulations to investigate the characteristics of microplastics across different seasons and regions in the Yangtze River Estuary. Results show that microplastic number concentrations decreased seaward, with higher values in the inner estuary (8-42 items/m³) nearly twice those in offshore regions (4-35 items/m³). Pronounced seasonal variations were observed, primarily driven by river runoff and salinity gradients; concentrations were lowest in summer and markedly higher in autumn and winter. Correspondingly, mass concentrations estimated by empirical formulas followed a similar pattern, being relatively higher in the inner (median 90 μg/m³) and lower in the outer (median 30 μg/m³) estuary. A clear size-reduction trend was also identified along the land-to-sea gradient: the microplastic length decreased from approximately 2000 μm nearshore to 1500 μm offshore, a 7-31% reduction across seasons. This downsizing likely results from mechanical abrasion and sediment-induced fragmentation within the turbidity maximum zone. However, this does not alter the overall seaward decreasing trend in microplastic concentration, as the sources may not have changed obviously and the water surface area has increased substantially. Overall, this study provides both the number and mass dataset to elucidate estuarine microplastic transport dynamics. The findings highlight the dual role of estuaries as both fragmentation hotspots and transport regulators, emphasizing the importance of incorporating sediment dynamics into microplastic monitoring and management.