The spatial distribution of phytoplankton was investigated during the summer of 2014 in two different regions of the Pacific western boundary current, namely the Warm Pool near the equator and the subtropical Kuroshio south area. Traditional approaches (size-fractionated chlorophyll-a (Chl-a) and microscopic analyses) combined with single-cell analysis (using a flow cytometer) were used to analyse the whole range of phytoplankton. Flow cytometry analysis resolved five clusters, two belonging to the pico-size fraction and three belonging to the nano-size fraction. Microscopy analysis revealed that the genera Coscinodiscus, Rhizosolenia, Chaetoceros and Ceratium were numerically dominant in the region studied. The lowest values of Chl-a, phytoplankton abundance and carbon biomass were found in the Kuroshio south area. Both Chl-a concentration data and flow cytometry analysis revealed that picophytoplankton were the predominant contributors to phytoplankton in the Pacific western boundary currents. Along the three transects during the summer cruise, Synechococcus and nanocyanobacteria-like organisms numerically dominated in surface waters with higher temperature. In contrast, eukaryotes were primarily distributed in subsurface waters with higher nutrients, especially in the eddy upwelling region mainly associated with the North Equatorial Counter Current. The vertical distribution of phytoplankton over the epipelagic layer reinforces the important role of currents in the north-western Pacific during summer.
From the western boundary currents to the Pacific Equatorial Undercurrent: Modeled pathways and water mass evolutions
