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

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Abstract
We compared the composition and photosynthetic activity of two size fractions of phytoplankton during a cruise from the Gulf of Maine to the Sargasso Sea in August 1983. At every station, and at every depth, ultraplankton (defined here as cells passing through 3 μm pores in filters) made a major contribution to both the standing crop of chlorophyll and the rate of primary production. Ultraphytoplankton assemblages were dominated by phycoerythrin-rich cyanobacteria. Overall, the ultraplankton contribution to total primary production was greatest at low photon fluxes: (i) at the beginning and end of the photoperiod; (ii) with increasing depth in the euphotic zone; and (iii) when daily irradiance was low. The composition of ultraphytoplankton varied with depth. Surface (2 m) ultraphytoplankton assemblages were almost exclusively composed of phycoerythrin-rich cyanobacteria with smaller (0.2–0.8 μm) cyanobacteria predominating. Below the surface mixed-layer, the proportion of larger (0.8–3 0 μm) to smaller cyanobacteria increased and the eukaryotic component of the ultraphytoplankton often became important. At two Sargasso Sea stations, the greatest numbers of cyanobacteria were below the mixed layer at the 1% light level, while the maximum numbers of eukaryotic ultraphytoplankters occurred deeper still, at the 0.5% light level, coincident with the chlorophyll maximum. At the bottom of the euphotic layer in the Sargasso Sea. eukaryotes numerically dominated the ultraphytoplankton and made a major contribution to the chlorophyll maximum.