Application of a Continuous Underway Sampling Device for Microplastic Assessment in Surface Offshore Waters of South Korea (East Sea)

Microplastics (MPs) are an emerging concern in marine environments due to their widespread distribution, ingestion by marine organisms, and role as sources and carriers of hazardous chemicals. This study examined MP levels, spatial distribution, and contamination characteristics in surface waters of the eastern offshore region of South Korea (East Sea). Traditional grab sampling provides limited spatial coverage and may not sufficiently capture large-scale MP patterns. To address this drawback, we utilized a custom-designed Surface Water Underway Sampler (SWUS), which enables continuous collection of surface waters, including the air–sea interface layer, from a fast-moving vessel. In this study, seawater samples (>20㎛) were collected along 15 transect lines in East Sea using the SWUS aboard the R/V Onnuri in April 2023. MPs were detected in all surface water samples collected across the East Sea, with concentrations ranging from 14.8-315.5 n/m³ (mean: 90.0 ± 79.5 n/m³). Notably, 79% of the MPs were smaller than 200 μm. Fragment-type MPs were the most dominant shape (74.8%), followed by fibers (24.8%) and films (0.4%). The predominant polymer types were polyester/polyethylene terephthalate (PES/PET, 29.3%), polypropylene (PP, 28.9%), and alkyd (10.5%), followed by polyethylene vinyl acetate (PEVA, 6.1%), polyamide (PA, 5.9%), and polyethylene (PE, 4.5%). Overall, high-density polymers (> 1 g/cm³) accounted for 58.7% of the total, indicating a relatively higher proportion compared to low-density polymers such as PP, PE, and PEVA. Higher MP abundance was observed in the central regions of the East Sea despite the lower human activity and industrial facilities, suggesting that physical oceanographic processes may play an important role in the transport and distribution of MPs in the region. To our knowledge, this is the first study to examine the distribution of microplastics (>20 µm) in surface waters of the East Sea using the SWUS. This method enhances the representativeness of MP data and provides new insights into large-scale variability in surface MP concentrations. Our findings demonstrate the utility of SWUS as an effective tool for high-resolution, spatially extensive monitoring of microplastics in marine environments.