Nutrients cycling in Mediterranean basins: the role of the biological pump in the trophic regime

The Mediterranean Sea is generally characterized by the presence of oligotrophic conditions, which are more pronounced in its eastern part. The effects of physical and biological fluxes on the nitrogen and phosphorus distributions are here addressed by means of a box model. Numerical simulations obtained with realistic fluxes, as well as the analysis of the asymptotic behavior of the model support the hypothesis that inverse estuarine circulation is not sufficient to explain the oligotrophic regime. Conversely, it appears that the downward fluxes of organic matter play a major role in sustaining and stabilizing the oligotrophy as found in the observations. Indeed the trophic gradients between the western and the eastern Mediterranean arise as a consequence of the unbalance of the loads emitted in the basins, with greater inputs into the western, but, in any case, they are maintained in the long run by the biological pump.