Diurnal variations in photosynthetic rates: comparisons of ultraphytoplankton with a larger phytoplankton size fraction

We compared the composition and photosynthetic activity of twosize fractions of phytoplankton during a cruise from the Gulfof Maine to the Sargasso Sea in August 1983. At every station,and at every depth, ultraplankton (defined here as cells passingthrough 3 µm pores in filters) made a major contributionto both the standing crop of chlorophyll and the rate of primaryproduction. Ultraphytoplankton assemblages were dominated byphycoerythrin-rich cyanobacteria. Overall, the ultraplanktoncontribution to total primary production was greatest at lowphoton fluxes: (i) at the beginning and end of the photoperiod;(ii) with increasing depth in the euphotic zone; and (iii) whendaily irradiance was low. The composition of ultraphytoplanktonvaried with depth. Surface (2 m) ultraphytoplankton assemblageswere almost exclusively composed of phycoerythrin-rich cyanobacteriawith smaller (0.2–0.8 µm) cyanobacteria predominating.Below the surface mixed-layer, the proportion of larger (0.8–30 µm) to smaller cyanobacteria increased and the eukaryoticcomponent of the ultraphytoplankton often became important.At two Sargasso Sea stations, the greatest numbers of cyanobacteriawere below the mixed layer at the 1% light level, while themaximum numbers of eukaryotic ultraphytoplankters occurred deeperstill, at the 0.5% light level, coincident with the chlorophyllmaximum. At the bottom of the euphotic layer in the SargassoSea. eukaryotes numerically dominated the ultraphytoplanktonand made a major contribution to the chlorophyll maximum.