Mesoscale eddies have significant effects on primary production and phytoplankton community structure in the ocean, which in turn impacts the entire marine ecosystem. To better understand the interrelation between the physical process and the community structure and succession of planktonic system, larger spatial and temporal scale and higher-precision observation should be conducted. In this study, 15 stations in the open Northwest Pacific affected by the Kuroshio extension and the Oyashio were chosen for data sampling in spring 2015. The temperature, salinity, nutrients (NO₃^--N, NO₂^--N, PO₄^3--P and SiO₃^2--Si) and size-fractionated chlorophyll a concentration were measured, and the phytoplankton community structure at each station was analyzed. For the physical data, we acquired the SLA Sea level anomaly) data from AVISO and SST (Sea Surface Temperature) from OSTIA (Operational sea surface temperature and sea ice analysis). A total of 221 species (including variants) representing Bacillariophyta (diatoms; 43 genera, 132 species), Dinoflagellata (dinoflagellates; 43 genera, 75 species), Chrysophyta (7 species), Cryptophyta (4 species) and Chlorophyta (3 species) were identified in the studied area. Diatom were the most dominant phylum. We found an obvious warm eddy induced by the Kuroshio extension and the temperature, salinity, abundance and biodiversity of phytoplankton were higher in the warm eddy. Significant pycnocline separated the water mass and phytoplankton assembled above it. Phytoplankton in the warm eddy was mainly composed of the warm coastal species and warm pelagic species. The abundance of diatoms, in particular Chaetoceros, was higher than the areas nearby. The phytoplankton cells were likely transported by the Kuroshio from the southern pelagic and coastal warm waters, not due to multiplication of the local population. The western region of the studied area was affected by the Kuroshio Extension. Temperature and salinity under the thermocline and halocline dramatically decreased as the layer deepend and the nutrients concentrations were lower because of the transport of deeper water upward was interfered by the thermocline and halocline. Concentration of Chl a and phytoplankton abundance were lower and the phytoplankton community in this region was mainly composed of diatoms. We speculated that the Kuroshio Extension did not disturbed the water mass. The northern part of the studied area was affected by the Oyashio and temperature and salinity were lower and nutrients and Chl a concentration as well as the phytoplankton abundance were higher in this area. The phytoplankton were mainly composed of coastal and pelagic cold water species, with cryptophytes the most abundant phytoplankton, followed by the dinoflagellates. Furthermore, a relative high abundance of red Noctiluca scintillans (10,764 cells/m³) which is a neritic species appeared in the area affected by the Oyashio and Kuroshio extension, a phenomenon rarely reported in the oceanic surveys. The N. scintillans cells at this station were likely carried from the coastal areas by the currents. The confluence of the Oyashio and Kuroshio extensions in this area resulted in the tempestuously water exchange and caused the accumulation of N. scintillans.

Eddies; Kuroshio Extension; Oyashio; Northwestern Pacific Ocean; Phytoplankton Community Structure