Picophytoplankton dynamics in the equatorial Pacific: diel cycling from flow-cytometer observations

The French JGOFS cruise FLUPAC was conducted in October-November 1994 in the western and central equatorial Pacific Ocean. During a 7-day time series at 150 degrees W, in addition to conventional sampling (four times per day from discrete depths between 0 and 150 m), a high frequency (hourly) experiment was performed by continuously pumping water at 5 m below the surface over 24 h. Flow cytometric measurements allowed us to recognise and to follow separately the two major components of equatorial picophytoplankton, the Prochlorococcus and the picoeukaryotes. The hourly surface experiment confirmed the synchrony of Prochlorococcus cell division and showed that picoeukaryotes exhibited a similar behaviour. The main consequence is that the maximum potential growth rate of picophytoplankton is one doubling per day for both cell groups. The vertical profiles indicated that the diel cycling extends throughout the surface layer for both algal groups. The cells were observed to divide daily from late afternoon to the middle of the night, and then to disappear, probably as a result of grazing. In the surface layer variations of abundance allowed us to estimate a growth rate of about 0.6 day(-1). Mean cell light scattering <(FS)over bar> as measured by the cytometer indicated a decrease in cell size concurrent with cell division and an increase during the photosynthetic growth phase.