Picoplanktonic community structure on an Atlantic transect from 50°N to 50°S

Plankton samples were collected from 10 depths at 25 stations spaced at intervals of about 4° of latitude along a transect from the British Isles to the Falkland Islands. Four categories of picoplankton were discriminated: Synechococcus spp., Prochlorococcus spp., eukaryotic picophytoplankton and heterotrophic bacteria. The populations in each category in the samples were counted by flow cytometry and the mean size of bacterial cells was determined by fractionation through filters. Categories of phototrophic cells were discriminated by size and by the fluorescence of photosynthetic pigments; samples stained with the fluorochrome TOTO were used to enumerate heterotrophic bacteria (and Prochlorococcus in surface waters where their chlorophyll content was very small). The carbon biomass concentration of each category in each sample was calculated. Prochlorococcus was present at all stations between 47°N and 38°S, and reached peak population densities above 200,000 cells ml-1 in equatorial waters; the depth occupied by these cells increased in oligotrophic waters, where they dominated picophytoplankton biomass. Synechococcus reached high concentrations in the Mauritanian upwelling region and in the frontal region near the southern end of the transect, where they represented the largest single component of picophytoplankton biomass, but was almost absent in oligotrophic regions. Picoeukaryotes were present in low numbers at all latitudes, but they are larger cells and constituted a substantial part of the total picophytoplankton biomass at most latitudes. The depth-integrated (200 m) biomass of heterotrophic bacteria was nearly as great as that of the picophytoplankton at all latitudes, because substantial numbers of cells occurred at all depths. Numbers and biomass of these bacteria were maximal in the upwelling region and high at both ends of the transect. There was a clear contrast in the composition of the picoplankton community in both the North and South Atlantic between mesotrophic waters where Synechococcus and picoeukaryotes dominated the biomass, and oligotrophic waters where the smaller total biomass was dominated by Prochlorococcus.

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