Spatial patterns of phytoplankton community and biomass along the Kuroshio Extension and adjacent waters in late spring

The Kuroshio Extension (KE) is a natural boundary region characterized by complex hydrodynamic processes in the western North Pacific Ocean that fuel spring phytoplankton blooms in high latitudes. This study looked at the spatial patterns and diversity of phytoplankton community and biomass associated with oceanographic profiles across the KE from the north to south in June 2011. Results showed that upper-ocean gradients were strongly correlated with observed spatial variability in phytoplankton communities (> 10 mu m). Phytoplankton abundance, species richness, and Chl a biomass were generally higher in the Kuroshio-Oyashio transition region than in the KE axis and the southern KE. No bloom event was recorded in late spring. A total of 81 phytoplankton taxa belonging to 45 genera of four phyla were identified. Dominant taxa in both abundance and occurrence included the diatoms Chaetoceros radicans, C. curvisetus, C. convolutes, C. deblis, C. peruvianus, Pseudonitzchia delicatissima, Coscinodiscus asteromphalus, Thalassionema nitzschioides, T. frauenfeldii, Thalassiothrix longissima, the dinoflagellates Protoperidinium pyrifoprmus, and the Cyanobacterium Trichodesmium thiebautii. Depth-averaged abundance and Chl a were 2076 +/- 3,615 cells L-1 and 0.20 +/- 0.10 mg m(-3), respectively. Subsurface Chl a maxima (SCM) was mainly found at 50 m, while the abundance maximum differed regionally in the water column. A cluster analysis grouped phytoplankton communities into three assemblages related to geographic locations influenced by Oyashio and Kuroshio currents. Nutrient limitation rather than light led to low Chl a and abundance during late spring. Redundancy correspondence and spearman's correlation further suggest that dissolved inorganic nitrogen was the major factor regulating phytoplankton distribution north of KE, while phosphate limitation was more prominent in the KE axis and the southern KE. The Oyashio and Kuroshio interaction caused evident variability in the hydrodynamic environment including the MLD and Z(eu) of the region, and changes in chemical profiles such as nutrients, pH, and DO. The observed distributional shift in the spatial pattern of phytoplankton was attributed to upper-ocean environmental gradients within the KE axis and its northern and southern sections.