A seasonal three-dimensional study of the nitrogen cycle in the Mediterranean Sea:: Part II. Verification of the energy constrained trophic model

In this study the lower trophic level variability induced by physical forcings in the Mediterranean is investigated by means of a three-dimensional coupled hydrodynamical–ecological model, where the lower trophic dynamics are described using an NPD aggregated model. The seasonal cycle of the ecosystem response shows marked spatial differences according to the prevailing cyclonic or anticyclonic regimes and to the zonal and meridional permanent trophic gradients. The interplay between the mixed layer dynamics and the irradiance penetration explains the gross ecosystem vertical dynamics while the general circulation contributes to establish the prevailing trophic regime in the sub-basins. Comparison of simulated chlorophyll concentrations with corresponding CZCS images shows remarkable similarities both in gradients and fields in different seasons. The main spatial characteristics of the seasonal biochemical cycles are resolved with the proposed approach. Moreover the orders of magnitude of obtained results, both for the inorganic nitrogen evolution and for the phytoplankton growth, are in keeping with the data considered. The main conclusion is that even a simplified trophodynamic model coupled with open sea hydrodynamics can satisfactory reproduce the main basin scale features and their seasonal evolution when the first trophic level is energy limited.