Plastic waste that reaches the ocean via rivers degrades and breaks down into microplastics (MPs). Detection in seafloor sediment cores suggests that even materials with a lower specific gravity than seawater sink to the seafloor through biological processes such as biofilm formation. In multi-layered seawater samples taken from the subsurface layer of the North Pacific, microscopic MPs with major axes of less than several hundred micrometers were detected at rates on the order of several thousand particles per cubic meter. It appears that those whose specific gravity increases through biological processes and become nearly neutrally buoyant remain floating in the ocean for long periods of time.

"Setouchi Oceans X," a project between the Nippon Foundation and the four Setouchi prefectures, has developed practical, evidence-based marine debris countermeasures and accumulated knowledge in one of Japan's largest enclosed coastal seas. This process transcends administrative silos and overcomes physical, economic, and institutional challenges through technical expertise and local collaboration, and offers effective solutions to marine and coastal issues that require comprehensive management. It is expected to provide a clear path toward resolving the challenges facing the world's oceans.

International negotiations on rules to curb plastic pollution have progressed slowly, raising concerns that easing countermeasures will shift the burden onto future generations. Effective countermeasures require a combination of (1) reducing consumption, (2) controlling leakage into the environment, and (3) promoting material substitution, and policy and technology are particularly important for advancing (2) and (3). To ensure effective progress on the more challenging (1), we outline the approach of "measure, recognize, reduce, and collaborate."