Studies have demonstrated that photosynthesis rates at any given irradiance depend critically on the light history over time scales of minutes to hours and days. However, these studies have failed to provide a predictive understanding of photosynthesis in variable light. New technology makes it possible to examine the roles of photosynthetic, photoprotective and photoinhibitory processes in controlling the instantaneous rate of photosynthesis. This technology includes fast repetition rate (FRR) fluorescence for examining photoprotection, photoinhibition and photosynthesis, and 18O mass spectrometry for measuring the rate gross photosynthesis and oxygen cycling in the light. In the laboratory, this study will calibrate FRR fluorescence-based estimate of gross photosynthesis against measured rates of gross photosynthesis obtained from 18O2 exchange and quantify the roles of photoprotection and photoinhibition in controlling the rate of photosynthesis. In field research, in situ FRR fluorescence measurements will be used to establish a two-dimensional (i.e. depth and time) description of the photochemical and non- photochemical quenching coefficients, calculate the instantaneous rate of photosynthesis and examine the roles of the photoprotection and photoinhibition in controlling phytoplankton photosynthesis in situ.
