Phytoplankton acclimation to changing light intensity in a turbulent mixed layer: A Lagrangian modelling study

A new individual-based plankton model is used to test the hypothesis that the timescale of photoacclimation of phytoplankton within the surface mixing layer of the ocean is slow relative to mixing, in which case the chlorophyll to carbon (Chl:C) ratio of individual cells shows little adjustment in response to changes in light environment driven by vertical displacement. Rates of photoacclimation are shown to be a strongly non-linear function of light intensity that depends on the balance of intrinsic chlorophyll synthesis at low irradiance versus increasing growth rate at high irradiance. Predicted photoacclimation was negligible for cells experiencing rates of turbulent mixing typical of the open ocean surface boundary layer (10−3 to 10-1 m2 s-1), in which case Chl:C is set by mean light intensity. The model was extended to incorporate a simple ecosystem of nutrient, phytoplankton, zooplankton and detritus and, using two-layer slab physics, used to study photoacclimation in a more realistic setting, the seasonal cycle of plankton dynamics at Ocean Weather Station India in the North Atlantic (59 °N, 20 °W). Results were remarkably similar when compared with an equivalent ecosystem model that used an Eulerian representation of phytoplankton, reinforcing our conclusion that mixing rates within the surface mixed layer of the ocean are typically too fast to permit photoacclimation by phytoplankton to ambient light.