Steady-state single cell model simulations of photoacclimation in a vertically mixed layer: implications for biological tracer studies and primary productivity

A numerical single-cell photoacclimation–diffusion model was constructed and used to develop criteria regarding the use of individual phytoplankton cells as tracers for vertical mixing and to illustrate how rates of vertical mixing might affect phytoplankton physiology. Both first-order and logistic representations of photoacclimation kinetics were used. Steady state was assumed for simplicity and to provide a starting point for further investigations. The modeled variance and higher moments (within a phytoplankton population) of a generic photoacclimative parameter all show trends, which are diagnostic of mixing rates and/or boundary effects. This allowed the establishment of criteria by which frequency distributions of phytoplankton physiological properties (e.g., cell fluorescence) might be used as indicators of vertical mixing. The same model can be used to predict the effects of vertical mixing on phytoplankton productivity and growth. Application of the model to both photosynthesis and carbon to chlorophyll ratios suggested that a combination of vertical mixing and hysteresis (as represented in the logistic model of photoacclimation) in acclimation kinetics can enhance specific growth rates of phytoplankton. This enhanced growth occurred as a result of mixing-induced variation in carbon to chlorophyll ratios and is in contrast to chlorophyll-specific productivity, which was maximal at low mixing rates. Differential rates of photoacclimation to upward vs. downward shifts in irradiance, may enable phytoplankton cells to better survive in a turbulent environment.