VERTICAL STRUCTURE OF PICOPHYTOPLANKTON AT DIFFERENT TROPHIC SITES OFTHE TROPICAL NORTHEASTERN ATLANTIC-OCEAN

The vertical structure and chlorophyllous pigments of picophytoplankto nic populations of the northeastern Atlantic Ocean were studied by flo w cytometry and spectrofluorometry. Three sites (EU, MESO and OLIGO) o n a hydrological gradient from near coastal, eutrophic waters to offsh ore, oligotrophic waters were occupied in October 1991 (except EU), Ju ne 1992 and December 1992. The population structure of the EU site (20 degrees 32'N 18 degrees 34'W) was greatly influenced by a permanent, wind-induced upwelling. The latter was well developed in June, and an important nutrient enrichment of surface waters ensued (>10 mM NO2+NO3 ). The latter favored the blooming of diatoms, but picophytoplanktonic populations remained low. In December the upwelling was less develope d, and there was a dramatic increase of the cell abundances of both pr okaryotic (Prochlorococcus and Synechococcus) and picoeukaryotic popul ations at the EU site. Cells of all groups were concentrated and homog eneously distributed in the upper, 30-35 m thick, mixed layer. Similar population structures were observed in June and December at the MESO site (18 degrees 29'N 21 degrees 05'W). In these cases, both Synechoco ccus cyanobacteria (with concentrations of up to 5 x 10(5) cells ml(-1 ) during winter) and picoeukaryotes (typically 1-2 x 10(4) cells ml(-1 )) made significant contributions to the integrated picophytoplanktoni c biomass in terms of carbon (166-333 mu g C cm(-2) and 92-155 mu g C cm(-2), respectively). Prochlorococcus made a smaller contribution (6- 48 mu g C cm(-2)), as also indicated by a low ratio of divinyl-chlorop hyll a to total chlorophyll a (less than or equal to 22%). The populat ion structure observed in October at the MESO site was much more varia ble, even at the time-scale of hours. At the OLIGO site (21 degrees 02 'N 31 degrees 08'W), the relative contribution of Prochlorococcus to p icophytoplankton carbon and total chlorophyll standing stocks increase d dramatically (>50%), mainly as a result of a sharp decrease of both Synechocorcus and picoeukaryotes cell concentrations down to a few tho usands cells per mi. There was little seasonal change in the vertical structure of any of the three populations al this site. From analyses of cell cycle distributions during a 31 h time-series, growth rates we re estimated for Prochlorococcus as 0.41 day(-1) at mid-depth (80 m) a nd 0.39 day(-1) in the deep chlorophyll maximum. Copyright (C) 1996 El sevier Science Ltd