Spatio-temporal behaviour of five picophytoplankton populations in Tyrrhe- nian Sea: Model and data

Abstract

Recent works presented detailed analyses of spatio-temporal dynamics in marine ecosystems, reproducing real vertical distributions of phytoplankton biomass. These study however do not take into account the changes in environmental variables. On the contrary, seasonal variations can influence considerably the primary production, i.e. phytoplankton biomass, in marine ecosystems, determining significative consequences in the whole food chain, in particular fish species, whose growth is mainly explained by seasonal changes in the chlorophyll concentration, a marker of phytoplankton species. Here we present a one-dimensional reaction-diffusion-taxis model to describe the spatio-temporal dynamics, along a water column, of five picophytoplankton populations in a marine ecosystem. In particular, the effects of seasonal changes of light intensity, vertical turbulent diffusivity, depth of thermocline, and thickness of the upper mixed layer close to water surface are considered to get a more realistic analysis of the picophytoplankton dynamics. By numerically integrating the equations of the model, the vertical distributions of biomass concentration, expressed in cell/m^3, of each picophytoplankton population are obtained at different times, i.e. four different days chosen in different seasons of the solar year. The numerical results, converted in total concentration of chlorophyll a (chl a) and divinil-chlorophyll a (Dvchl a), allow to obtain the chlorophyll distributions along the water column. These theoretical profiles are compared with experimental data for the chlorophyll concentration collected in a site of the Tyrrhenian Sea. In particular, the results of the chi^2 goodness-of-fot test indicate a good agreement between theoretical and experimental chlorophyll distributions for all periods (seasons) investigated. Our findings confirm that the primary production, i.e. phytoplankton biomass, is strongly dependent on the light intensity oscillations and spatio-temporal behaviour of the vertical turbulent diffusivity. These results could be useful to analyze phytoplankton dynamics in different marine ecosystems and predict future changes in primary production, contributing to devise strategies able to prevent the reduction of phytoplankton biomass and consequent decrease of fish species.