Spatial distribution and historical trend of microplastic pollution in sediments from enclosed bays of South Korea

Seafloor sediment is an important sink for microplastics, and vertical profile of microplastic accumulation in a sediment core can provide historical pollution trend. However, microplastic pollution in coastal sediment has not been addressed well, and a few studies have investigated the trends of microplastic pollution in age-dated core sediments. In this study, the microplastics in surface sediments in urban, aquafarm and reference areas of South Korea were analyzed to evaluate the pollution characteristics of microplastic according to different sea area use patterns. In addition, the historical trend of microplastic pollution was investigated in core sediments in the urban and aquafarm areas. The abundance of microplastics in surface sediment were in order of urban area (6,887 ± 6,100 particles/kg d.w.), aquafarm area (5,121 ± 2,428 particles/kg d.w.), and reference area (2,474 ± 522 particles/kg d.w.). Polymer types were diverse in the urban area, and expanded polystyrene used for buoys was dominant in the aquafarm area. Fragment type microplastic was dominant in all three areas, and the proportion of fiber was higher in urban and aquafarm areas than in reference area. The polymer composition of fiber was different in urban (polyester 51% and polypropylene (PP) 29%) and aquafarm areas (PP 84% and polyamide 13%). These results support that the characteristics of microplastic pollution well reflect the sea area use patterns. Historical trend of microplastic pollution has increased since the 1980s and the increasing rate steeply increased around the early and mid-2000s in both the core samples. Their increasing trend reflected the influence of population or surrounding input sources (i.e. effluent discharge amount of a wastewater treatment plant). The clear increasing trend of historical microplastic pollution up to now indicates that more efforts is highly required to reduce the microplastic pollution.