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This study investigated microplastic distribution characteristics by collecting surface seawater from sea areas to the south of Jeju Island in August 2020. The average microplastic abundance was 0.46 ± 0.27 particles/L (n = 23), and PE had a high ratio, averaging 53%. The levels of fragments and fibers were observed to be 69% and 31% on average, respectively. The most common size of the microplastics was on average 0.02–0.30 mm at a level of 69%. We found a higher abundance of microplastics in the study area than in other open waters such as the Arctic Central Basin and the Atlantic Ocean, whereas the abundance was lower than that in previous studies on coastal areas. We studied an area of open sea connecting China, Japan, and the Pacific Ocean, and, in this region, the microplastic distribution varies depending on sea currents in the surrounding areas. In the summer, the western and central regions of the study sea area have low salinity levels due to discharge from China’s Yangtze River. This generally indicates that high-density plastic deposits are found in the Yangtze River estuary, and low-density plastics are found in the study area. Furthermore, this implies that low- and high-density plastics are transported in water for long periods of time due to the Taiwan Warm Current and because the eastern sea area has high salinity.

The microplastics in the surface seawater at 23 fixed points in the southwestern waters of Jeju Island were analyzed to conduct a study on the distribution characteristics of microplastics. The abundance of microplastics was shown to be 0.17 ~ 1.37 (average 0.46) n/L, the materials were mainly PP and PE, and the shape of fragments was dominant in the 0.02–0.3 mm group in terms of sizes and shapes. Alkyd and polyester were dominant in the shape of fibers in the 1.0–5.0 mm group. The correlations between the abundance of microplastics, water temperatures, and salinity were compared and as a result, water temperatures, PE, and the 0.02–0.3 mm size group, and the shape of fragments showed negative correlations. Therefore, it is judged that the for PE, which are in the form of 0.02 ~ 0.3 mm fragments to flow into the study area through the Yellow Sea coastal current and the Korean coastal current that show tendencies of relatively low water temperatures. However, since salinity and PP and polyester showed positive correlations, it is judged the possibility for polyester, which are in the form of 1.0 ~ 5.0 mm fibers, to flow into the study area through the Taiwan warm current and Tsushima warm current. In conclusion, it could be judged that in the complex open waters where continental and oceanic waters intersect, the sources of inflow may be different depending on the components of microplastics.