Microplastics on the sea surface of the semi-closed Tokyo Bay

Microplastics (MPs) pollution surveys were conducted in Tokyo Bay using neuston nets (May 2019 and January 2020). Although the pollution level in Tokyo Bay was high (3.98 pcs/m3, May), it was lower than reported in other semi-closed bays because of differences in the Enclosed Index. It was found that polyethylene fragments dominated the retrieved MPs; the mode of MPs size was 800 μm. As MPs abundance in rivers had the same seasonality as found in the inner bay, rivers were considered the main source of MPs. The seawater residence time is shorter than the time required for the density of MPs to become greater than that of seawater; therefore, it was considered that MPs are transported out of the bay instead of sinking. MPs were aggregated into a convergence zone by residual currents (the thermohaline front) in May (January). These findings will improve understanding of MPs pollution in other bays. •Detailed information on MPs in Tokyo Bay was reported.•Waste management and geographical characteristics dominate the abundance of MPs.•Seasonality of abundance of MPs in Tokyo Bay was the same as that in rivers.•Large particles of MPs are released from Tokyo Bay into the open ocean.•Physical conditions affect the distribution of MPs in Tokyo Bay.