Phytoplankton Growth and Diel Variations in Beam Attenuation through IndividualCell Analysis

摘要

In order to understand diel variations in the bulk water inherent opticalproperty beam attenuation, mean forward light scatter of different populations of phytoplankton, from flow cytometric analysis of individual cells, was determined. Forward light scatter showed diel patterns similar to those of beam attenuation, with a minimum near dawn and a maximum near dusk, for populations of prokaryotic picophytoplankton (Prochlorococcus and Synechococcus) and eukaryotic nanophytoplankton measured in the equatorial Pacific and the Sargasso Sea. The cell concentration patterns do not generally correspond to those of beam attenuation. Using a combination of empirical calibrations relating beam attenuation to flow cytometric measurements of pure cultures of phytoplankton in the laboratory, and Mie theory, the contributions of different groups of phytoplankton to the diel variations in beam attenuation were determined. The results indicate that the phytoplankton assemblage measured by flow cytometry can account for essentially all of the diel variation in the beam attenuation signal. In addition, flow cytometric measurements of phytoplankton light scattering and cell concentration over the diel light cycle were used to estimate in situ phytoplankton group-specific growth and loss rates in the equatorial Pacific and the Sargasso Sea.