Frequency Distributions of Phytoplankton Single-Cell Fluorescence and Vertical Mixing in the Surface Ocean

Because of photoacclimation, frequency distributions of phytoplankton single-cell fluorescence reflect variation in light histories among individual cells. As a result, phytoplankton can be used as biological tracers for vertical mixing. A time series of Prochlorococcus single-cell fluorescence distributions in the Sargasso Sea revealed changes that were qualitatively consistent with changes in water-column physical dynamics. Applying a photoacclimation-diffusion model to Prochlorococcus fluorescence from the time series yields a vertical diffusivity near 30 cm2s-1for a remnant mixed layer beneath a diurnal thermocline, which is consistent with estimates obtained by other means. This estimate results from an integrated view of how Prochlorococcus responds to changes in light intensity induced by vertical mixing and therefore, may be an appropriate measure of vertical diffusion rates for productivity models that include photoacclimation dynamics. This approach provides a novel way to examine vertical mixing on time scales relevant to phytoplankton physiological responses.