Floristic and physiological differences between the shallow and the deep nanophytoplankton community in the euphotic zone of the open tropical Atlantic revealed by HPLC analysis of pigments

Suspended matter sampled in 1982 in the North Equatorial Current, in the open Atlantic to the west of West Africa, was analyzed by high performance liquid chromatography. The pigment fingerprint of samples taken in the surface mixed layer was dominated by zeaxanthin and chlorophyll a, in agreement with observed dominance of coccoid cyanobacteria. Near the bottom of the euphotic zone the fingerprint was more complicated, with a sharp transition at the depth of the deep chlorophyll maximum layer to dominance of chlorophyll b, 19-hexanoyloxyfucoxanthin and an unknown fucoxanthin derivative in the lower part of this layer; this fingerprint suggests dominance of eukaryotes (green algae, Prymnesiophyceae and Chrysophyceae) at depth. Up to 90% of the chl a was contained in particles smaller than 8 m, and in the surface mixed layer even more than 50% in particles smaller than 1 m. The high concentration of zeaxanthin relative to chl a near the surface suggests adaptation of the cyanobacteria to exposure to high irradiance. Evidence of this adaptation was the very high specific phytoplankton growth rate between sunrise and sunset (=0.16 h^-1), measured by recording ¹⁴C incorporation into organic carbon and into chl a carbon after isolation of the latter by HPLC. The high concentration of chl b relative to chl a at depth was possibly caused by shade-adapted green algae containing more chl b than chl a. The specific growth rate of the deep shade community was low (<0.04 h^-1), yet net primary production, calculated on the basis of chl a increase during incubation, was greatest at depth.