Concentrations and carbonyl index of microplastic in surface seawater in southeastern coastal region off Japan, Northwestern Pacific.

In this study, microplastic concentrations in the southeastern coastal regions of Japan were measured along the northward ocean current at seven stations from Okinawa to Tokai region. Concentrations ranged from 0.014 to 0.094 pieces/m, except for a station near the Bungo Channel mouth, which had 0.723 pieces/m. Polystyrene (PS) foam was most prevalent near the east side of Kyushu, suggesting origination from nearby coastal areas. Fragmentation levels were higher in the Tokai region. In addition, carbonyl index (CI) of polyethylene (PE) microplastics increased northward, indicating northward movement from southern regions. Standard PE microplastics showed chemical treatment does not significantly alter CI values. Further spectral analysis suggested potential oxidation of polypropylene (PP) and PS foam by chemical treatment. This study provides a comprehensive understanding of the abundance, distribution, and characteristics of microplastics in the southeastern coastal regions of Japan in the northwest Pacific, enhancing the understanding of environmental fate of microplastics.