A laboratory system for the quantification of phytoplankton photosynthesis under fluctuating light climate conditions is described. It consists of 2 temperature-controlled incubators with a variable light supply, an algal batch culture in incubation bottles with appropriate stirrers and a set of oxygen electrodes to monitor algal photosynthesis. By the rotation of special grey filters between the incubator and the light source, a regular up and down movement in the water column is simulated in up to 7 parallel bottles. Different ratios of euphotic depth to mixing depth and different velocities can be applied. Simultaneously, 8 bottles can be incubated under constant light. The system is demonstrated in experiments with Chlamydomonas sp. Further possibilities of application are proposed.