Measurements of photosynthetic p~g m e n t s , nutnents, spectral Irradiance and vanous physical parameters were performed in the western Sargasso Sea (35"N, 70°W) to ~n v e s t~g a t e the factors affecting phytoplankton blomass dlstribut~ons Algal plgment concentrations and c o m p o s~t~o n s measured during spring 1985 showed cons~derable time-dependent v a n a t~o n s which were cons~stent with those documented by dlrect mlcroscoplc observation D u r~n g early Apnl, 2-fold Increases In chlorophyll a and fucoxanthln were measured on a relatively short tlme scale of days The presence of a d~atom-dominated community malnly specles of the genera Rhlzosolen~a and Chaetoceros, suggested that we were witnessing a stage of the spring bloom Upon return to thls location 2 wk later, the diatom bloom was replaced by a considerably more diverse phytoplankton assemblage consisting of prymnes~ophytes, cyanobacter~a, d~noflagellates, green algae (~ncludlng praslnophytes) and d~a t o m s The vert~cal structures d~splayed by ~n d i v~d u a l accessory plgments dunng Apnl were markedly simllar and suggest that the major phytoplankton taxa were not un~formly distributed In the upper 200 m The phytoplankton were d~s t r~b u t e d as broadly overlapp~ng layers, 1~1th cyanobacterla and diatoms most abundant In the m~x e d layer, prymnesiophytes at lntermed~ate depths, and green algae (Including prasinophytes) deeper In the water column Results p r o v~d e descnptive evidence for a rapid succession of chromatically-adapted phytoplankton during springtime in the Sargasso Sea