This study investigated small microplastic particle (SMP, 350 mu m) contamination in the surface seabed sediment of the inner part of Tokyo Bay. The SMP concentration during the rainy season (May) was 100.3 +/- 45.8 pieces g-1 dry weight and higher in the inner side of the bay, whereas that of the dry season (January) was 147.6 +/- 19.5 pieces g-1 dry weight. There was no seasonal difference. The main plastic types found in the rainy season were polypropylene (PP), polyethylene (PE), and polyamide (PA). In the dry season (January), the concentration of PP decreased while that of PA increased. The mean SMP diameter did not change by site or season. Our results indicate that the seabed sediments in the inner part of Tokyo Bay are contaminated with relatively high concentrations of SMPs. Furthermore, the deposition rate of SMPs in the inner part of the bay was 12.4 mg cm-2 y-1.
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