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.

Tributary inflows enhance the microplastic load in the estuary: A case from the Qiantang River.

Microplastics (MPs) in the surface water of the Qiantang River and its tributaries were investigated in this study, to evaluate the contribution of riverine discharge on emerging pollution load in the Hangzhou Bay. The abundance of MPs (mean 1183 ± 269 particles/m3) showed spatially and temporally heterogeneous in the surface water. Polyamide, polyester, and polyethylene teraphalate were the major components, accounting for 77.4% of all polymer types. Fiber was the most common shape, indicating the potential anthropogenic sources of MPs. The MPs flux from the Qiantang River to the Hangzhou Bay was estimated to be 2831 tons/year, implying the importance of riverine discharge in an estuarine bay. The inputs from the complicated tributaries system led to a 20-fold increase of MPs flux, and would significantly improve their budget of downstream and estuary, so we suggested the control and management on plastic pollution should be strengthened in all aspects.