Picoplankton community structure on the Atlantic Meridional Transect: a comparison between seasons

Samples collected from 10 depths at 25 stations in September-October 1996 a nd 12 depths at 28 stations in April-May 1997 on an Atlantic Meridional Tra nsect between the British Isles and the Falklnnd Islands were analysed by f low cytometry to determine the numbers and biomass of four categories of pi coplankton: Prochlorococcus Spp, Synechococcus Spp, picoeuckaryotic phytopl ankton and heterotrophic bacteria. The composition of the picoplankton comm unities confirmed earlier findings (Zubkov, Sleigh, Tarran, Burkill & Leake y, 1998) about distinctive regions along the transect and indicated that th e stations should be grouped into five provinces: northern temperate, north ern Atlantic gyre, equatorial, southern Atlantic gyre and southern temperat e, with an intrusion of upwelling water off the coast of Mauritania between the northern Atlantic Eyre and equatorial waters. Prochlorococcus was the most numerous phototrophic organism in waters of both northern and southern gyres and in the equatorial region, at concentrations in excess of 0.1x10( 6)ml(-1); it also dominated plant biomass ill the gyres, but the biomass of the larger picoeukaryotic algae equalled that of Prochlorococcus in the eq uatorial region, higher standing stocks of both Prochlorococcus and picoeuk aryotes were present in spring than in autumn in waters of both gyres. In t emperate waters at both ends of the transect the numbers and biomass of pic oeukaryotes and, more locally, of Synechococcus increased, and the Synechoc occus, particularly, were more numerous in spring than in autumn. There was a pronounced southward shift of the main populations of both Synechococcus and Prochlorococcus in April-May in comparison to those of September-Octob er, associated with seasonal changes in solar radiation, the abundance of P rochlorococcus dropping sharply near the 17 degrees C contour, while Synech ococcus was still present at temperatures below 10 degrees C. Picoeukaryote s were more tolerant of low temperatures and lower light levels. often bein g more abundant in samples from greater depths, where they contributed to t he deep chlorophyll maximum. Heterotrophic bacterial numbers: and biomass t ended to be highest in those samples where phototrophic biomass was greates t, with peaks in temperate and equatorial waters, which were shifted southw ards in April-May compared with September-October. (C) 2000 Elsevier Scienc e Ltd. All rights reserved.