Phytoplankton community structure at station ALOHA (22 45'N, 158W) during fall 1991

R. R. Bidigare, L. Campbell, M. E. Ondrusek, R. Letelier, D. Vaulot, D. M. Karl

Abstract

During October 1991, the phytoplankton community structure at station ALOHA was investigated by reverse- and normal-phase HPLC pigment analyses; flow cytometry; and epifluorescence and light microscopy. Resulting data were used to determine depth-dependent variations in the abundance, chlorophyll biomass and carbon biomass of the major phytoplankton groups. Prochlorococcus spp. were numerically dominant and accounted for ~50% of the euphotic zone chlorophyll. Prochlorococcus spp. carbon biomass contributions ranged from 90% in surface waters to 30% near the base of the euphotic zone. Comparative analysis of the HPLC pigment and flow cytometry data revealed the presence of two distinct, overlapping Prochlorococcus populations. Cyanobacteria accounted for ~20% of the chlorophyll in the mixed layer, of which approximately half was provided by Trichodesmium spp. Eukaryotic autotrophs accounted for ~30% of the water column chlorophyll, and 10 to 70% of the phytoplankton carbon biomass in surface and deep waters, respectively.