Coastal bays serve as reservoirs for microplastics from East China: insights from a mass budget model based on sedimentary findings

The presence of sedimentary microplastics (MPs) remains a significant concern due to their substantial contribution to the "missing" MPs in marine environments and their crucial role in the global MP cycle. In this study, we investigated MPs in sediments from three representative bays in the Zhejiang Great Bay Area, China, and examined their sources and mass budgets within these bay systems. High spatial similarities were observed among these regions. In Hangzhou Bay, sedimentary MPs primarily originated from textile and fishing activities, while local mariculture and coastal tourism significantly influenced MP levels in Sanmen Bay. In Wenzhou Bay, packaging and agricultural sources contributed to the presence of MPs. Riverine discharges are crucial for MP mass budget in the three bays, accounting for 41.5 %-96.7 % of the total. Hydrodynamic conditions and topographic features significantly influence oceanic inflow/outflow patterns that affect both water column and sediment MPs. The estimated retention time suggests that these bay systems serve as temporary reservoirs, facilitating the migration of part of MPs from land to open sea. Importantly, the continuous transport pathways, whether through offshore-bound MPs in the water or sediment-resuspended particles, create persistent mobilization mechanisms that may enhance ecological risks in marine ecosystems. These findings emphasize the need for timely action to reduce MP pollution in bay environments and provide valuable information for future policy development regarding MPs.