A seasonal three-dimensional study of the nitrogen cycle in the Mediterranean Sea: Part I. Model implementation and numerical results

The influence of the general circulation on the lower trophic level variability in the Mediterranean is investigated by means of a three-dimensional coupled hydrodynamical ecological model. While the hydrodynamical forcing is obtained by a primitive equation seasonal model, the trophic dynamics are described using an aggregated model based on inorganic nitrogen, phytoplankton and detritus. The model results exhibit a quasi-repeating seasonal cycle and the calculated fluxes through the principal straits are of the same order as those estimated from literature data. The climatological influence of the general circulation on the nutrient distribution is evident in permanent cyclonic areas, while the anticyclonic circulation does not give a detectable signal, due to the high oligotrophy of the upper layer. The seasonal cycle shows marked spatial differences according to the different regimes induced by the surface circulation, leading to the tentative conclusion that, in the upper layer, the circulation directly or indirectly determines the distribution of the dissolved inorganic nitrogen concentration.