Distribution characteristics and transport processes of biodegradable microplastics in the Seto Inland Sea, Japan

Microplastics (MPs) pollution is a prevalent environmental problem that affects ecosystems globally. Despite the growing research on the environmental effects of MPs, a significant research gap remains in understanding the differences of environmental behavior and distribution patterns between biodegradable MPs and traditional MPs. Using a three-dimensional hydrodynamic model and treating MPs as tracers with vertical velocity, this study simulated the transport of positively, neutrally, and negatively buoyant biodegradable MPs from rivers. The results show that positively buoyant MPs have significant seasonal variations and are mainly distributed in the surface layer. Neutrally buoyant MPs are distributed in all water depths, with a high (low) concentration in the eastern (western) Seto Inland Sea (SIS), characterized by winter mixing and summer stratification. Negatively buoyant MPs accumulate in the sediments and exhibit lower concentrations in seawater. Positively and neutrally buoyant MPs mainly outflow from the SIS into the Pacific Ocean, whereas negatively buoyant MPs hardly leave the SIS and are primarily deposited and degraded near river mouths. A settling velocity of -10^-6 to -5×10^-5 m s^-1 (downward) greatly affects the concentration of MPs in seawater. However, large upward and downward velocities outside this range do not result in pronounced changes. Compared with traditional MPs, biodegradable MPs are less environmentally persistent by not accumulating in sediments and keeping a low concentration there, which contributes to the reduction of transport flux of MPs to the Pacific Ocean.