Accumulation of microplastics in a downstream area of a semi-enclosed bay: Implications of input from coastal currents

Coastal beaches are a vital transitional zone in which terrestrial microplastics (MPs) enter the oceans. However, little is known about the impact of coastal currents on the dispersion and accumulation of MPs, especially in semi-enclosed bays. To address this knowledge gap, we investigated the spatiotemporal variation of MPs in the coastal sediments of Haizhou Bay. The abundance of MPs ranged from 10.94 n/kg to 1309.02 n/kg. Overall, a higher amount of MPs was found in intertidal sediment than in the supratidal zone. Significant seasonal variation was observed only in the intertidal areas of Haizhou Bay, suggesting that hydrodynamics plays a critical role shaping MP abundance. Statistical analyses revealed a roughly exponential increase of MP abundance with distance from the coast, and that a relatively higher abundance of MPs would be found at offshore sites in downstream areas. These results suggest that coastal currents are important contributors of MPs. A numerical model of vertical profiles estimated the total inventory of MPs in coastal areas to be 13.83-580 trillion pieces. Our results suggest a neglected sink of MPs in the sedimentary environments of coastal regions. As a result, the amount of MPs in coastal regions in previous studies could have been underestimated.