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 forci ngs in the Mediterranean is investigated by means of a three-dimensional co upled hydrodynamical-ecological model, when the lower trophic dynamics are described using an NPD aggregated model. The seasonal cycle of the ecosyste m response shows marked spatial differences according to the prevailing cyc lonic or anticyclonic regimes and to the zonal and meridional permanent tro phic gradients. The interplay between the mixed layer dynamics and the irra diance 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 se asonal biochemical cycles are resolved with the proposed approach. Moreover the orders of magnitude of obtained results, both for the inorganic nitrog en evolution and for the phytoplankton growth, are in keeping with the data considered. The main conclusion is that even a simplified trophodynamic mo del coupled with open sea hydrodynamics can satisfactory reproduce the main basin scale features and their seasonal evolution when the first trophic l evel is energy limited. (C) 1999 Elsevier Science B.V. All rights reserved.