Medium-Term Oceanic Transport of River-Derived Microplastics in the South China Sea Analyzed with a Particle Tracking Model

Plastic waste is currently one of the biggest global environmental issues. To gain the comprehensive understanding of oceanic microplastic contamination as a key global environmental problem, Lagrangian particle tracking experiments were conducted to evaluate the transport of microplastics (MPs) derived from the four major rivers that have been known to discharge large amounts of plastic waste into the South China Sea (SCS). We carried out two types of experiments using a pre-computed 3D current climatological oceanic model: (1) 2D tracking of MP particles placed at the surface to represent positively buoyant (light) MPs, and (2) fully 3D tracking of neutrally buoyant MP particles that are passively transported by ambient current. The seasonally varying monsoons in the SCS were found to provoke strong seasonal variability in the river-derived MP transport. It was found that a large number of MPs, especially from south China, are transported to the East China Sea in the seasons when the southwesterly monsoon prevails. Moreover, the difference in the density of MPs substantially affects their oceanic transport patterns. The buoyant MPs accumulated near the surface tend to be transported toward nearshore areas by wind-driven Ekman currents.