Seasonal variability of picophytoplankton and bacteria in the western subarctic Pacific Ocean at station KNOT

Picoplankton community structure in the upper 200 m at Station KNOT in the western subarctic North Pacific was studied by flow cytometric analysis of the time-series samples collected from June 1998 through June 2000. Phototrophic picoplankton, Synechococcus and picoeukaryotes, displayed a clear seasonal cycle in population abundance during the 2-yr period, where chlorophyll a concentration remained relatively low and constant throughout the year, except in spring when the diatom bloom occurred. High picoplankton abundance occurred after the spring diatom bloom when the water column became more stratified. The maximum abundance of Synechococcus occurred in late June to early August, whereas the peak in picoeukaryotes abundance appeared to take place earlier in May and remained high through the summer. Heterotrophic bacteria abundance also displayed a seasonal variation pattern that was closely related to the picoplankton biomass, but not total chlorophyll biomass.