Simulation of seasonal transport of microplastics and influencing factors in the China Seas based on the ROMS model

Elucidating the mechanisms governing microplastic transport and spatial distribution in offshore waters is essential to microplastic control. However, current research on microplastic transport in the China Seas is largely restricted to small-scale investigations, which do not provide a comprehensive result. Therefore, in this study, we used the Regional Ocean Modeling System (ROMS) combined with the Lagrangian Transport (LTRANS v.2) model to investigate how microplastics are transported around the China Seas during different seasons and under climatological river discharge. Our findings showed that the microplastic pathways and spatial distributions exhibit marked seasonal variations controlled by circulation patterns in the China Seas, river discharge values, and the characteristics of the microplastic materials. Floating microplastics exhibited the longest transport distance in summer, when microplastics from the Pearl River could be transported up to 1375.8 km through the Tokara and Tsushima straits. The heavy pollution areas in summer were located in the South Yellow Sea and East China Sea, mainly resulting from the contribution of the Yangtze River (>66%). In autumn and winter, more than three-quarters of the microplastics beached off the south-central Chinese coast. In addition, simulating the vertical velocity of the water prolonged the time required for microplastics to reach the open ocean, thereby reducing the amount of microplastics entering the Pacific Ocean by 6% compared to the simulation without the vertical velocity of the water in summer. Microplastics with higher densities were generally transported shorter distances. The transmission distances of PET and PS were two orders of magnitude smaller than that of PE. This study enhances knowledge of the sources and fates of offshore microplastics and provides scientific support for offshore microplastic control.