A DUAL SHEATH FLOW CYTOMETER FOR SHIPBOARD ANALYSES OF PHYTOPLANKTON COMMUNITIES FROM THE OLIGOTROPHIC OCEANS

Flow cytometry has been used to study phytoplankton since the early 19 80s. Because of the wide range of cell sizes and concentrations that o ccur in natural samples, multiple instrument configurations have been required to characterize pico, ultra-, and nanophytoplankton. Lengthy changeover times between configurations have made synoptic analyses of phytoplankton communities extremely cumbersome and therefore rare. To overcome this problem, we have added dual-sheath capability and adjus table optics to a commercially available flow cytometer. These additio ns expand the instrument's capabilities during routine operation. The dual-sheath cytometer can characterize both the submicron picoplankter Prochlorococcus at sample flow rates of 10 mu l min(-1) and, after a 1-min changeover, ultraphytoplankton and nanophytoplankton at sample f low rates >1 ml min(-1). These capabilities make possible real-time an alysis of phytoplankton size spectra at sea.