Phytoplankton in the subtropical Atlantic Ocean: towards a better assessment of biomass and composition

A comparative study was made of different methods to determine phytoplankton composition and biomass distribution at 5 stations in the eastern Atlantic Ocean (11° to 35°N). The methods used include plant-pigment analysis and flow cytometry (size class, chlorophyll fluorescence, and phytoplankton DNA distribution). At all 5 stations a Deep Chlorophyll Maximum (DCM) at depths ranging from 80 to 130 m. As indicated by chlorophyll-a2 content, Prochlorococcus sp. made up 25–60% of the total chlorophyll-a biomass in the euphotic zone. However, in terms of relative phytoplankton DNA and carbon content or cellular chlorophyll fluorescence, much higher values were found (45–70%) for this algal species. Prochlorococcus dominated the surface waters, but the eukaryotic phytoplankton component showed the highest biomass values at the DCM and deeper. Analysis of the accessory pigments (applying CHEMTAX) showed a clear vertical gradient at each station, but also the spatial distribution of the different taxonomic classes varied. Flow cytometric analysis of the water samples gave far less detailed information on the species/taxonomical classes present. Classification at the species level was possible only for Prochlorococcus and Synechococcus. In the case of the eukaryotes, the resolution of composition of the two main subpopulations (pico- and small eukaryotes) was enhanced after a DNA specific dye was applied. Staining the genome of the phytoplankton community revealed in most cases 5–15 separable DNA peaks, representing a similar number of different species. In general only a maximum of three DNA peaks dominated (>500 cells per ml). lankton carbon estimates based on DNA content matched those based on size fractionating filtration and conversion of cell size into carbon. lankton carbon data combined with the pigment data were used to estimate the carbon to chlorophyll ratios (θ) across the light gradient for a single species (Prochlorococcus) and the eukaryotic phytoplankton community. Because of the increase in chlorophyll-a2 content by a factor 35, the carbon to chlorophyll ratio (θ) of Prochlorococcus showed a fairly uniform pattern with a steep gradient, ranging from a maximum of 450 at the surface to 15 μg C/μg chlor. at 150 m. Furthermore, there was a south to north increase in θ by a factor of two mainly due to lower pigment values per cell of Prochlorococcus in the southern region. In contrast the θ of the collective eukaryotic phytoplankton community showed much lower values at the surface (30–80 μg C/μg chlor.), and varied with depth were far less than did θ of Prochlorococcus (only 3–7 fold variation). The reduced variability in θ of the eukaryotes can be explained by a co-variation of pigmentation and cell size. The latter can be derived from the increase in DNA content per cell with depth. Moreover, both parameters showed that the trend towards a dominance of larger species with declining growth irradiance is accompanied by a shift in community structure. This response differs greatly from that of Prochlorococcus, which tends to possess a high degree of photoacclimation enabling a single species to cover the whole euphotic zone.