Dynamics of microplastic transport and accumulation in a rural coastal area: Insights from the Otsuchi Bay, a small ria in Sanriku, Japan

Microplastic (MP) pollution in coastal areas has received increasing attention recently. However, studies focused on MP pollution in rural coastal areas remain limited compared to those in metropolitan coastal areas. This study observed MP particles accumulated on the seafloor of the Otsuchi Bay, a small ria bay located on the Pacific coast, Sanriku, Japan. The MP concentrations in the sediment ranged from 2.6 ± 0.3 to 13.6 ± 9.8 pcs g−1 dry weight (DW) and 2.6 ± 1.4 to 5.1 ± 1.2 pcs g−1 DW in March and September 2021, respectively. No significant difference in MP concentrations was detected between March and September. The MP concentration in the Otsuchi Bay was lower than that observed in other highly populated coastal areas but was relatively high considering the population size of the catchment area. MP particles smaller than 1000μm were the most prevalent, accounting for 96.3% of all MP samples. MP size at the bay head was smaller than that at the central bay for high-density MPs; however, the relationship was reversed for low-density MPs. Analysis of the MP distribution pattern using a two-dimensional numerical model suggests that the horizontal distribution of MPs in the Otsuchi Bay depends on the size and density of MP particles. It is also strongly influenced by both the tidal oscillating currents characteristic to the bay and vertical terminal velocity of MP particles. Sedimented MP distributions in a bay with a small catchment population with limited MP sources shed light on our understanding of MP transport dynamics.