A stochastic reaction-diffusion-taxis model for picophytoplankton dynamics

The dynamics of picophytoplankton communities in marine environment is studied by a stochastic reaction-difiusion-taxis model, analyzing the time evolution of the biomass concentration along a water column. The model is based on two stochastic difierential equations, where the random fluctuations of the environmental variables are considered by inserting two multiplicative noise terms. Speciflcally, the model describes the dynamics of difiusion of picophytoplankton biomass and nutrient concentrations. In the proposed model the marine environment is characterized by poorly mixed waters and picophytoplankton is subject to intraspeciflc competition for light and nutrients. By numerically solving the system equations, we obtain the spatio-temporal dynamics of phytoplankton biomass, nutrients and light along the water column at difierent depths. The results indicate that the distributions of the picophytoplankton biomass concentration along the water column are characterized by a peak. The comparison with experimental data show that height and localization of these peaks are in a good agreement with experimental maxima obtained from data collected in a real marine ecosystem. Finally, we consider the efiect of seasonal variations of temperature by studying the picophytoplankton dynamics in the presence of a periodical driving force. The model proposed represents an improvement of previous deterministic models for phytoplankton dynamics and is able to reproduce the spatial distributions of picophytoplankton concentration observed in real marine ecosystems.