Vertical structure of picophytoplankton at different trophic sites of the tropical northeastern Atlantic Ocean

The vertical structure and chlorophyllous pigments of picophytoplanktonic populations of the northeastern Atlantic Ocean were studied by flow cytometry and spectrofluorometry. Three sites (EU, MESO and OLIGO) on a hydrological gradient from near coastal, eutrophic waters to offshore, oligotrophic waters were occupied in October 1991 (except EU), June 1992 and December 1992. The population structure of the EU site (20°32′N 18°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 NO 2+NO 3). The latter favored the blooming of diatoms, but picophytoplanktonic populations remained low. In December the upwelling was less developed, and there was a dramatic increase of the cell abundances of both prokaryotic ( Prochlorococcus and Synechococcus) and picoeukaryotic populations at the EU site. Cells of all groups were concentrated and homogeneously distributed in the upper, 30–35 m thick, mixed layer. Similar population structures were observed in June and December at the MESO site (18°29′N 21°05′W). In these cases, both Synechococcus cyanobacteria (with concentrations of up to 5 × 10 5 cells ml −1 during winter) and picoeukaryotes (typically 1–2 × 10 4 cells ml −1) made significant contributions to the integrated picophytoplanktonic biomass in terms of carbon (166–333 μg C cm −2 and 92–155 μg C cm −2, respectively). Prochlorococcus made a smaller contribution (6–48 μg C cm −2), as also indicated by a low ratio of divinyl-chlorophyll a to total chlorophyll a (≤22%). The population structure observed in October at the MESO site was much more variable, even at the time-scale of hours. At the OLIGO site (21°02′N 31°08′W), the relative contribution of Prochlorococcus to picophytoplankton carbon and total chlorophyll standing stocks increased dramatically (> 50%), mainly as a result of a sharp decrease of both Synechococcus and picoeukaryotes cell concentrations down to a few thousands cells per ml. There was little seasonal change in the vertical structure of any of the three populations at this site. From analyses of cell cycle distributions during a 31 h time-series, growth rates were estimated for Prochlorococcus as 0.41 day −1 at mid-depth (80 m) and 0.39 day −1 in the deep chlorophyll maximum.