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Ultraplankton (0.2 to 5 ~ m ) provided >88% of euphotic layer chlorophyll (Chl) at 2 Sargasso Sea stations in July/August 1986. Communities were further characterized to quantify the separate abundances of phycoerythrin-fluorescing cyanobacteria Synechococcus spp. and Chl-fluorescing algae in 0.2 to 0 6, 0.6 to 1 and 1 to 5 pm size fractions. Throughout the water-column at both stations, the majority of Synechococcus cells were consistently found in the 0.6 to 1 pm fraction; the Sargasso Sea WH7803 serogroup was not a dominant component of Synechococcus populations at any depth. Highest numbers of Synechococcus cells were always in the surface isothermal layer, where they accounted for >95 % of all ultraphytoplankton cells. At the base of the euphotic layer total numbers of photoautotrophs were low, but numbers of algae increased. The varying distribution of the 2 ultraphytoplankton components with depth was also reflected in their separate contributions to Chl concentrations, and algae were the main contributors to Chl maxima at 1 to 3 % I,. Above this depth, a pigment maximum in phycoerythrin (PE) 1-' occurred at the nitracline, coincident with a peak in primary productivity. PE maxima were due to an increase in PE content of Synechococcus cells and not to an increase in their abundance. The 2 stations did however exhibit significant differences. High surface productivity at Stn 1 was supported by nanomolar changes In nitrate concentrations, which selectively and rapidly induced a Synechococcus bloom. In contrast, the water-column characteristics of Stn 2 were relatively stable, Synechococcus spp. were less abundant throughout the water-column, while algae were twice as abundant at the Chl maximum and 0.5 O/ O llght level, where most were of picoplankton size (79 to 89 % of algae were in < 1 ,um fractions). Data suggest that there is a considerable degree of variability in the abundance, composition, and productivity of stratified oceanic ultraphytoplankton communities.

Pico- and ultraplankton Sargasso Sea communities: variability and comparative distributions of Synechococcus spp. and algae