In order to understand microplastic pollution in the coastal areas of Kanagawa Prefecture, we surveyed beached microplastics on the coast at 4 sites in Sagami Bay and 1 site in Tokyo Bay. We also examined spatial and temporal variations in microplastic distribution and characteristics.

Spatially, the characteristics of microplastics that accumulated at the high tide line differed between the coastal sites. We presume that this difference was caused by the influence of microplastics derived from inland sources only, not by microplastics from the open ocean. Some of the beached microplastics at the high tide line were transferred inland by the sea breeze, and this tendency was observed in the distribution of resin pellets. In addition, we found that the proportion of large-size microplastics was higher at the high tide line than in microplastics drifting at the sea surface near the coast.

Temporally, we found that the amount of beached microplastics greatly increased when weather conditions allowed strong onshore winds to persist for a long time. The amount of microplastics deposited during typhoon conditions was three times the amount deposited during normal weather conditions. Seasonally, we found that the density of drifting microplastics in coastal areas increased as a result of prevailing seasonal winds transporting drifting microplastics from offshore to the coasts. The amount of beached microplastics around Sagami Bay increased during the spring, however the composition of beached microplastics did not change significantly during this period.

By observing small expanded polystyrene spherules beached along 2 coasts, we determined that these spherules were likely micro-foaming beads used as cushioning material in packaging. We suggest that proper treatment and disposal of such products is important for controlling coastal microplastic pollution.