The hypothesis is examined that the recent light history of phytoplankton contains information about vertical mixing processes in the euphotic zone. Chlorophyll/P700 ratios are used to estimate the degree of light or shade adaptation in natural phytoplankton communities. Along with information about the time- and light-dependent rates of change of chlorophyll/P700 ratios, a model is presented to estimate how recently populations at the surface were at the 1% light depth and vice versa. The model is based on first-order kinetics and employs a temperature correction. The model is used to estimate vertical displacement rates (i.e., piston velocities) on Georges Bank, in the New York Bight, and off the coast of Hawaii. The results suggest that vertical displacement rates vary by about two orders of magnitude (from ca 3.8 × 10−3 cm/sec to 1.1 × 10−1 cm/sec). These values are in general agreement with theoretical calculations based on physical parameters.