Tide-driven microplastics transport in an elongated semi-closed bay: A case study in Xiangshan Bay, China

Coastal bays are important containers for plastic wastes before they enter the ocean. Based on field samples, this study presents the main characteristics of microplastics and uses a numerical model to study the distribution and movement of microplastics as they are driven by tidal flows in an extended semi-closed bay in Xiangshan Bay, China. The laboratory analyses of microplastic samples from 27 pollutant source samples collected in three batches provided fundamental data on microplastics. Our results show that the local microplastics are prevalent (mean abundance: 890.6 ± 419.4 particles/m³) in the water. A higher quantity of fibre- and fragment-type microplastics was identified and compared to other plastic types. The detected microplastics varied in colour and composition. The simulation suggests that the bay can trap microplastics inside it, with only 16.92 % discharged into the open ocean. A series of single-source numerical tests at nine typical observation sites were conducted to examine tide-driven microplastic transport. Our results suggest that the release location is crucial to microplastic distribution. Specifically, the microplastics tended to accumulate near the bay mouth and the Tie inlet; the microplastics released from the north shore generally evacuated the bay more easily; and the inner harbour tended to accumulate microplastics during spring tide, as opposed to the departure of microplastics at the outer bay, while the effect was reversed during neap tide. We further considered the deposit effect, which significantly reduces the discharging rate to 0.04 % with a settling velocity of 0.05 mm/s. These results may have great importance to decision-making, management, and control of microplastic pollution.