Field observations of oceanic mixed layer dynamics and picophytoplankton photoacclimation

To explore the relationship between mixing dynamics in the surface ocean and picophytoplankton optical properties, we measured the distributions of single cell fluorescence in Prochlorococcus populations throughout a diel cycle. Two time series were conducted in the N. Atlantic, and both showed a shoaling of the mixed layer, due to surface warming or a rain-formed surface layer. These dynamics, coupled with the diel cycle of solar irradiance, drove the development of a depth gradient in mean red fluorescence of Prochlorococcus, due to photoacclimation, in the newly-stratified layer. Furthermore, the frequency distribution of single-cell fluorescence within field populations appears to have responded to changing mixing and photoacclimation dynamics, with photoacclimation in the absence of strong mixing generally resulting in a reduced variance in fluorescence within sample populations. Nighttime mixing in the absence of photoacclimation reversed this process and resulted in increased variation of single-cell fluorescence. Departures from normality in observed distributions suggest vertical mixing time-scales that are slightly longer than time-scales of photoacclimation. The behavior of the mean and variance, and possibly the third and fourth moments, of single-cell optical properties is consistent with photoacclimation in response to the physical dynamics.