Pollution, degradation, and risk assessment of microplastic (> 30 μm) in subsurface (5 m) seawaters along Tokyo-Bangkok shipping route

In this study, environmental microplastic samples (>30 μm) were collected from subsurface seawater (5 m depth) along a major Asia-Pacific shipping route from Tokyo to Bangkok. The samples were characterized, ecological risk was assessed, and results were also compared with surface water samples. The results showed spatial variation in microplastic concentrations along the route, with the highest concentration (5904 pieces/m³) observed in the South China Sea and the lowest concentration (3272 pieces/m³) in the offshore Tokai region. Subsurface microplastic concentrations were generally higher than surface concentrations. Polyethylene (PE) was the dominant polymer type, and polymer diversity was lower in the subsurface than in surface waters. Subsurface microplastics were found to be smaller in size while less degraded compared to surface microplastics. Despite higher concentrations, ecological risk levels at the subsurface were comparable to those in the surface due to the presence of less toxic polymer types in subsurface samples. Further evaluation indicated that estimated ecological risk levels are strongly influenced by mesh selectivity and the spatial coverage of sampling stations. This study highlights the spatial heterogeneity of subsurface microplastics and reveals important differences from surface counterparts. In addition, relying solely on surface data may result in biased estimation of total microplastic exposure and ecological impact. The findings from this study provide valuable reference data for future investigations, and can help the design of monitoring programs, the development of environmental policies, and the formulation of mitigation strategies.