Long-Term Changes in the Abundance, Size, and Morphotype of Marine Plastics in the North Pacific

Understanding the spatiotemporal dynamics of microplastics on the ocean surface is crucial for assessing their impact on marine ecosystems and human health; however, long-term fluctuations have not been extensively studied. We present a long-term empirical data set on floating marine plastic debris collected from 1949 to 2020 around Japan in the western North Pacific. We observed three phases: 1) a period of increase (0-10⁴ pieces/km²) from the early 1950s to the late 1970s; 2) a stagnation period, with high abundance (10⁴-10⁵ pieces/km²), from the 1980s to the early 2010s; and 3) a period of reincrease (>10⁵ pieces/km²) from the mid-2010s to the present. The shift from film to fragmented plastic in the 1980s and the continuous downsizing may have caused the expansion of the offshore polluted area, resulting in a stagnation period by enhancing removal. The removal is most likely caused by sedimentation with phytoplankton, as the abundance of the plastic debris during this period was significantly related to the winter Pacific Decadal Oscillation, an index of annual primary productivity. The recent increase in microplastics suggests that plastic discharge is outpacing its removal capacity, suggesting that the impact of pollution on ocean surface biota is becoming increasingly evident.