Microplastics in the tropical Northwestern Pacific Ocean and the Indonesian seas

The abundance of microplastics fragments with sizes larger than 0.30-0.35 mm is measured during two research cruises in the tropical Northwestern Pacific Ocean and inside the Indonesian seas. The fragments are confirmed to be primarily nylon, polyester, and polyethylene etc. microplastics in the western Pacific by Fourier transform infrared spectroscopy (FT-IR), whereas dominated by polyethylene in the Indonesian seas based on Raman spectrum analyses. The abundance of the microplastics is found to be very low (<0.018 particles m-3) in the surface waters of the North Equatorial Current and the Kuroshio, suggesting no westward extension of the Great Pacific Garbage Belt and lack of microplastics accumulation due to mass convergence in the western boundary current. The abundances of the microplastics in the Makassar Strait of the Indonesian seas are found to be higher by an order of magnitude than those in the Northwestern Pacific, believed to be due to river run-offs carrying microplastics from nearby lands. The largest microplastics abundance is found in the northeastern Maluku Sea, where concurrent current measurements suggest a branch of the Indonesian Throughflow bringing accumulated microplastics from the eastern Indonesian seas into the convergent area of multiple strong currents. The study underlines the importance of ocean circulation in the microplastics distributions and transportation, and suggests the importance of high-resolution spatial mapping in the budget analysis of global microplastics distribution.