Comprehensive Assessment of Distribution and Contamination Characteristics of Microplastics in Coastal and Offshore Sediment

Abstract

The seafloor is considered the ultimate sink of microplastics in the environment. Sediment can accumulate microplastics over a long period due to low rates of degradation under poor light and oxygen-deficient conditions. We conducted an intensive assessment survey to identify the levels, distribution profiles, and contamination characteristics of microplastic in the seafloor of coastal and offshore areas. The study focused on a coastal bay (Masan Bay) and a offshore area (East Sea) as study areas. Surface sediment samples were collected using Van Veen Grab and box core samplers in Masan Bay (n=27) and the East Sea (n=47), respectively, from 2022 to 2023. Microplastics were detected in all sediment samples collected from Masan Bay and the East Sea, with concentrations range of 3,189-85,618 n/kg and 991-46,015 n/kg, respectively. The overall microplastic abundance was higher in the coastal bay than in the offshore area, and a negative gradient in microplastic concentration was observed from inside to outside of the bay. Among the various polymers, polyethylene, polypropylene, polyester, and polyethylene terephthalate exhibited concentration gradients. This result indicates that land-based human activities are a significant source of microplastic in the marine environment. Fragment-type was predominant in both the coastal bay and offshore area, while fiber was relatively abundant in the coastal bay compared to the offshore area. Low-density polymers such as polyethylene, polypropylene, and polyethylene vinyl acetate were relatively abundant in the coastal region, while high-density small microplastics were relatively abundant in the offshore area. At a dumping site and its surrounding areas, exceptionally high microplastic abundances were found, and particularly spherical polystyrene were abundantly observed. We applied a Lagrangian particle-tracking model to estimate the spatial extent of the impact of ocean dumping on the microplastic distribution on the seafloor of the East Sea, and compared it with in-situ spherical polystyrene data.