Production and consumption of oxygen were analysed in phytoplankton populations using a fast biomass-concentration technique. The variation of productivity with irradiance was studied using different incubation periods, from minutes to hours. The influence of the exposure time on the photosynthesis-irradiance (P/I) curve parameters was analysed. The results show that dissolved oxygen concentration in the water does not increase linearly with time and that P-max(B) (maximum biomass-specific production rate and I-opt (optimal light intensity) are time-dependent. Exponential and Linear expressions were proposed to describe P-max(B) and I-opt as a function of time. A model was built to investigate the importance of static (steady-state) vs dynamic (time-dependent) simulation in the prediction of phytoplankton production under different light intensities. The results show the importance of using dynamic expressions for the P/I relationship, especially in a mixed water column.