A review of possible pathways of marine microplastics transport in the ocean

Abstract Marine microplastics pollution has been a new challenge to marine environmental protection. The research results have shown that microplastics exist everywhere in the ocean. However, understanding of the transport of microplastics in the ocean, including coastal zones, is not clear. This paper provides a holistic overview of the modelling of microplastic transportation. The transport processes are complex, including surface drifting, vertical mixing, beaching, and settling. Besides the dynamic conditions of oceans, the transportation of microplastics is influenced by their physical characteristics, such as size, shape, and density. For buoyant particles, a Lagrange track model is used to simulate the surface drift process, considering current, windage effect, and Stokes drift. It is difficult to observe the vertical mixing process of microplastics because of their small size (<5 mm), therefore the parameters of the vertical mixing process in the model are still less known. Large accumulation of microplastics in sediments may be a result of settlement and entrainment. Also, biofilm formation can increase their density and thus, deposition. Considering sedimentation of microplastics is somewhat different from sediment deposition, some primary parameters (e.g., diffusivity, Stokes-drift, settling rate, biofouling rate) are required in future studies to better understand the transport of marine microplastics.