Evidence for phytoplankton succession and chromatic adaptation in the Sargasso Sea during spring 1985

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Abstract
Measurements of photosynthetic pigments, nutrients, spectral irradiance and various physical parameters were performed in the western Sargasso Sea (35.degree.N, 70.degree.W) to investigate the factors affecting phytoplankton biomass distributions. Algal pigment concentrations and compositions measured during spring 1985 showed considerable time-dependent variations which were consistent with those documented by direct microscopic observation. During early April, 2-fold increases in chlorophyll a and fucoxanthin were measured on a relatively short time scale of days. The presence of a diatom-dominated community, mainly species of the genera Rhizosolenia and Chaetoceros, suggested that we were witnessing a stage of the spring bloom. Upon return to this location 2 wk later, the diatom bloom was replaced by a considerably more diverse phytoplankton assemblage consisting of pyrmnesiophytes, cyanobacteria, dinoflagellates, green algae (including prasinophytes) and diatoms. The vertical structures displayed by individual accessory pigments during April were markedly similar and suggest that the major phytoplankton taxa were not uniformly distributed in the upper 200 m. The phytoplankton were distributed as broadly overlapping layers, with cyanobacteria and diatoms most abundant in the mixed layer, prymnesiophytes at intermediate depths, and green algae (including prasinophytes) deeper in the water column. Results provide descriptive evidence for a rapid succession of chromatically-adapted phytoplankton during springtime in the Sargasso Sea.