Microplastic concentration in the water column of the Banggai Sea, Central Sulawesi, Indonesia

Microplastic pollution in marine environments has been found from the surface layer to the deep sea. Oceanographic phenomena in the ocean and the tendency of microplastics to sink within the water column can facilitate their horizontal and vertical transport. This study determined the concentration and distribution of microplastics in the water column from the surface layer to the deep sea in the Banggai Sea, Indonesia, an area influenced by seasonal upwelling and located along the Indonesian Throughflow route. Samples were collected using a Rosette Water Sampler attached to a conductivity-temperature-depth (CTD) devices at four different depths. The samples were filtered using filter paper and observed following strict criteria combined with the hot needle test using a microscope. The average microplastic concentrations in the surface, maximum chlorophyll, thermocline, and deep-sea layers were 20.80 ± 6.52 particles/L, 23.65 ± 8.09 particles/L, 25.94 ± 10.79 particles/L, and 21.74 ± 8.52 particles/L, respectively. No difference was found among depth layers, but the concentrations varied significantly among stations. Fiber-shaped microplastics were the most common, accounting for 92.13 % of all particles. Raman microspectroscopy identified polyethylene terephthalate, polypropylene, polyvinyl chloride, and polystyrene polymers in the microplastic samples. Grouping stations based on phytoplankton abundance as a proxy for upwelling showed that microplastic concentration in the thermocline layer was lower in upwelling areas (21.16 ± 7.07 particles/L) compared to non-upwelling areas (30.73 ± 11.09 particles/L), indicating the potential role of upwelling in redistributing microplastics vertically.