Distribution Characteristics and Transport Processes of Biodegradable Microplastics in the Seto Inland Sea, Japan

Abstract

Microplastic (MP) pollution is a prevalent environmental problem that affects ecosystems globally. In the Seto Inland Sea (SIS), the fate of MPs remains unknown. 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) SIS. 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 MP concentration in seawater. However, large upward and downward velocities outside this range do not result in pronounced changes. Using biodegradable MPs allows their concentration in sediments to stabilize rather than continue increasing and reduces the amount of their outflow from the SIS, which consequently mitigates MP pollution in the marine environment.

Keywords: Microplastics, Spatiotemporal distribution, Settling velocity, Biodegradation, Seto Inland Sea