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

Abstract

Ultraplankton (0.2 to 5 pm) provided ~88 % of euphotic layer chlorophyll (Chl) at 2 Sargasso Sea stations in July/August 1986. Communities were further charactehed to quantify the separate abundances of phycoerythrin4luorescing cyanobactezia Syz~ecIzococc~~ spp. and ChLfluores~ cing algae in 0.2 to 0.6, 0.6 to 1 and 1 to 5 m size fractions. Throughout the water-column at both stations, the majority of Spe&ococcUs cells were consistently found in the 0.6 to 1 Sargasso Sea WIT?803 serogroup was not a dominant component of . trations# and @gae were the main contributors to Chl maxima at 1 to 3 % &,. Above th& depth, a pigment maximum in phycoery&rin (PE) l-l occurred at the nitracline, coin&dent with a peak in primary productivity. PE maxima were due to an increase in PX! content of Spdocm ceils and not to an increase in their abundance. The 2 stations did however exhibit significant dUferences. High surface productivity at Stn 1 was supported by nanomolar changes in nitrate concentrations, which selectively and rapidly induced a Spe&ococm bloom. In contrast, the water-column characteristics of depth. Highest numbers of Syne&txoccus cells were always in the surface isotherm4 layer, where they accounted for >95 % of all ultraphytoplankton cells. At the base of-tie euphotic layer total numbers of _ photoautotrophs were low* but numbers of algae increw The v-g distribution of the 2 ultraphyto- plankton componena with depth was also reflected in the& separate contributions to Chl cozen- SynecI~ococc~~ populations at any relatively stable, SJVXX&XX~~~ spp. were less abundant throughout tie water-column, while algae were twice as abundant at the Chl maximum and 0.5 % light level, where most were of picoplankton size (?9 to 89 % of algae were in < 1 w fractions). Data suggest that there is a considerable variability in the abundance, composition, and productivity of stratified oceanic communities. Stn 2 were titraphytoplankton degree of m fraction; the