Picophytoplankton show promise as dynamic tracers for vertical mixing if an appropriate index of photoacclimative state and the kinetics of change in that index can be determined. To this end, several light shift incubations were carried out with laboratory cultures of Prochlorococcus MED4 and natural populations of Prochlorococcus at sea. These time series revealed systematic changes in chlorophyll fluorescence in response to changes in incident light intensity. Prochlorococcus also exhibited strong diel patterns in fluorescence and light scattering resulting from the phasing of cell division to the daily photoperiod, In order to obtain a parameter that was insensitive to the daily photoperiod, Prochlorococcus red fluorescence was normalized to the 0.33 power of forward angle light scattering, an empirically derived normalization factor. We propose that this normalized fluorescence could be an appropriate measure of photoacclimation in Prochlorococcus, suitable for use in inverse modeling of mixing dynamics. A logistic model for photoacclimation was found to fit the experimental data well, although there was some systematic deviation from the data for populations shifted to high irradiances. Estimates of photoacclimative rates ranged from 0.9 to 2 d(-1). (C) 2001 Elsevier Science Ltd. All rights reserved.