The occurrence and distribution of microplastics in epipelagic zone of the western Indian Ocean

This study investigated the occurrence and distribution of microplastics utilizing zooplankton samples collected in the Western Indian Ocean because there is no information concerning epipelagic zone in the open ocean. We collected microplastics from three water layers [surface mixed layer(SML), middle layer(ML), lower layer(LL)] within 200 m using a Multiple Opening/Closing Net and Environmental Sensing (opening: 1 ㎡) at 22 stations of 1 degree interval between 5°N and 16°S along the 67°E of Western Indian Ocean in 2017. The microplastics were consistently found in almost all samples and the microplastic abundance ranged between 0.00-2.01 particles/㎥ from the 3 layers. And the average microplastic abundance was highest in the lower layer (0.30±0.09 particles/㎥) and lowest in middle layer (0.26±0.08 particles/㎥). The percentage of fiber was highest in the SML (55.7%) and the LL (45.9%), and the percentage of film was highest in ML (46.8%). The microplastic abundance in the size of 1.0-5.0 ㎜ was highest in SML (42.0%), while the abundance in the size of 0.2-0.5 ㎜ was highest in ML(56.8%) and LL(54.5%). The stations can be divided into four sections including upwelling characterized by Seychelles-Chagos Thermal Ridge (SCTR) based on the 20℃-isotherm depth (D20). The average microplastic abundance was the highest in SML (0.23±0.06 particles/㎥) in 1°S~5°S, and in LL (0.50±0.25 particles/㎥) at latitudes of 10°S~16°S and in LL (0.32±0.16 particles/㎥) at latitudes between 5°N~EQ. However, the average microplastic abundance at latitudes of 6°S ~9°S corresponding to the upwelling zone was highest in the ML (0.65±0.38 particles/㎥) with the high percentage of film (68.7%). Cluster analysis by microplastics occurred in each water layers showed that the stations were divided into 3 groups in each layer. Groups in SML and LL were mainly clustered by fiber, whereas groups in ML was mainly clustered by film, which was associated with the upwelled region of Seychelles-Chagos Thermal Ridge (SCTR). Fourier transform infrared spectroscopy analysis showed that the main types of microplastics were dominated by fiber (40.6%) and film (73.2%) characterized by polycarbonate. Present results showed that meridional and vertical distribution of microplastics in the epipelagic zone varied with the physical characteristics of upwelling zone characterized by Seychelles-Chagos Thermal Ridge (SCTR) in the Western Indian Ocean.