A stochastic model for phytoplankton dynamics in the Tyrrhenian Sea

In this work we try to answer the question how relevant the random fluctuations are in the dynamics of a natural system. Specifically we ask which role the external fluctuations play in a real ecosystem, which is a typical example of complex system, since it is governed by nonlinear dynamics and subject to deterministic and random perturbations coming from the environment. For this purpose we study the spatio-temporal dynamics of phytoplankton abundances in a marine ecosystem, comparing results from the model with experimental findings. The study is based on a stochastic reaction-diffusiontaxis model, which is used to analyze the spatio-temporal dynamics of five phytoplankton groups in the middle of the Tyrrhenian Sea, inside the Modified Atlantic Water (MAW), that is the upper layer of the water column of the Mediterranean Sea (from the surface down to 200 m). The study is performed by considering the intraspecific competition of the phytoplanktonic groups for limiting factors, i.e. light intensity and nutrient concentration, and the seasonal changes of environmental variables. Moreover, we take into account the effects of the random fluctuations of the temperature and the velocity field on the phytoplankton populations by inserting terms of multiplicative noise in the differential equations of the model. In order to compare theoretical results with experimental findings, the picophytoplankton abundances obtained by the stochastic model are converted in chlorophyll a concentrations. The statistical analysis, based on the chi-square test, shows that the vertical distributions of total chlorophyll concentration are in a good agreement with experimental data acquired in the marine site investigated during four different sampling periods (seasons) of the year.