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 ti me series at 150 degrees W, in addition to conventional sampling (four times per day from discrete depths between 0 and 150 m), a high frequ ency (hourly) experiment was performed by continuously pumping water a t 5 m below the surface over 24 h. Flow cytometric measurements allowe d us to recognise and to follow separately the two major components of equatorial picophytoplankton, the Prochlorococcus and the picoeukaryo tes. The hourly surface experiment confirmed the synchrony of Prochlor ococcus cell division and showed that picoeukaryotes exhibited a simil ar behaviour. The main consequence is that the maximum potential growt h rate of picophytoplankton is one doubling per day for both cell grou ps. The vertical profiles indicated that the diel cycling extends thro ughout the surface layer for both algal groups. The cells were observe d 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 lay er variations of abundance allowed us to estimate a growth rate of abo ut 0.6 day(-1). Mean cell light scattering <(FS)over bar> as measured by the cytometer indicated a decrease in cell size concurrent with cel l division and an increase during the photosynthetic growth phase.