Distribution of microplastics on sandy beaches in western and southern Japan and their fate in the oceans

The Japanese archipelago is surrounded by the Sea of Japan, the Sea of Okhotsk, the Pacific Ocean, and the East China Sea. The oceans are known to be contaminated by waste artifacts (marine litter). To investigate how these artifacts contaminate sandy beaches in Japan and their fate in the oceans, we sampled sand from 19 sandy beaches ranging from the western area of the main island of Japan (Honshu), which faces the Sea of Japan, to the westernmost island in the Japanese archipelago (Yonaguni Island), which faces the East China Sea. Almost all beaches were contaminated by marine litter. The maximum density by number of pieces was 604,600 per m3 in a 2-cm depth of sand on Yonaguni Island. Polystyrene foam was the most common artifact, and plastic fragments were second. The densities of artifacts by number and weight, and the type of artifacts, varied by location. A statistically significant positive relationship between the weights of artifacts and natural objects was observed, suggesting that they experience similar fates in the oceans. The major axis of the polystyrene foam in the Yonaguni area differed significantly from those in the Shimane, Tsushima, and Okinawa areas. The major axis in the Tsushima area significantly differed from those in all other studied areas. This study found the geological difference in the distribution and the major axis of polystyrene foams. These differences may be relevant to environmental physical factors (wind, rain, and waves), chemical alterations by ultraviolet rays, exposed time, the gyre, and local currents.