Size-specific distribution, morphology, and polymer composition of microplastic particles in surface water and sediments of Tokyo Bay

Abstract Microplastic particles (MPs) are ubiquitous in marine environments; however, comprehensive studies comparing their characteristics across environmental matrices and size ranges remain limited. This study investigated MPs in surface water and sediment samples collected from eight stations in Tokyo Bay during October–November 2023. The samples were analyzed for MP concentration, size distribution, morphology, and polymer composition. The results showed that small MPs (20–100 μm) numerically dominated in both surface water and sediment, accounting for more than 70% of total MP concentration. However, medium and large MPs (100–350 μm and ≥ 350 μm), although less abundant, contributed substantially to total estimated MP mass in both matrices. Morphologically, fragmentary MPs were dominant in most samples, particularly among small and medium MPs, whereas fibrous MPs became more prevalent with increasing particle size in both matrices. In terms of polymer types, PMMA, PE, and PP were abundant in surface water, while PE and PP predominated in sediments, with PMMA occurring at a smaller proportion. In the large-size fraction (≥ 350 μm) of both matrices, PET was the most abundant polymer in both number- and mass-based compositions. By targeting a broad size range of MPs (20–5000 μm), this study demonstrates that key MP characteristics, particularly morphology and polymer composition, vary systematically across size classes and environmental matrices. These findings suggest that both particle size and the selected quantification metric (number- or mass-based) play an important role in the assessment of MP pollution. The dataset provides valuable size-resolved baseline information for long-term monitoring and supports future ecological risk assessments in urbanized coastal environments.