Modelling the time dependent photoadaptation of phytoplankton to fluctuating light

Phytoplankton in the pycnocline and in the upper mixing layer of the ocean are subjected to vertical displacements over a range of time scales characteristic of fluid motions resulting from internal waves and turbulent processes. Because underwater light decreases approximately exponentially with depth, the phytoplankton thus experience variations in irradiance over the same range of time scales. In general, because of photoadaptation, the instantaneous rate of photosynthesis does not depend only on the instantaneous irradiance but rather on some function of the cells1 light history. We present a linear response model for photoadaptation of the rate of photosynthesis to varying irradiance. Results of laboratory experiments under controlled lighting are used to determine time scales for photoadaptation and to evaluate functional relationships and coefficients for the model. The model is used to simulate the generation of photosynthesisirradiance curves from incubation experiments, and it is tested with data from a culture grown in a greenhouse under natural sunlight. For an idealized case, the frequency response function for adaptation to internal wave displacements is calculated. Waves with periods equal to or shorter than the adaptive response time allow little adaptation.