Contributions of phytoplankton light scattering and cell concentration changes to diel variations in beam attenuation in the equatorial Pacific from flow cytometric measurements of pico-, ultra- and nanoplankton

The phytoplankton biomass in the equatorial Pacific is dominated by picophytoplankton (Prochlorococcus, < 1 μm, and Synechococcus, ∼1 μm diameter) and by mixed populations of ultraphytoplankton (1–2 μm diameter) and nanophytoplankton (2–20 μm, mostly 2–3 μm). The mean forward light scatter of each of these populations, measured on individual cells by flow cytometry, increased during the day and decreased during the night. This pattern was similar to that of bulk red beam attenuation due to particles. In contrast, cell concentration changes did not correspond to the patterns in beam attenuation. a combination of empirical calibrations relating beam attenuation to flow cytometric measurements of pure cultures of phytoplankton in the laboratory and Mie theory, we estimated the contributions of different groups of phytoplankton to the diel variations in beam attenuation observed. Our 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 most instances nanophytoplankton were the largest contributor to total beam attenuation due to phytoplankton, but ultraphytoplankton usually were more important in determining the diel variations in this property. Prochlorococcus were a smaller but appreciable contributor to beam attenuation changes, and Synechococcus were much less important.