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

The Mediterranean Sea is generally characterized by the presence of oligotr ophic conditions, which are more pronounced in its eastern part. The effect s of physical and biological fluxes on the nitrogen and phosphorus distribu tions are here addressed by means of a box model. Numerical simulations obt ained with realistic fluxes, as well as the analysis of the asymptotic beha vior of the model support the hypothesis that inverse estuarine circulation is not sufficient to explain the oligotrophic regime. Conversely, it appea rs that the downward fluxes of organic matter play a major role in sustaini ng 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 t he long run by the biological pump. (C) 2001 Elsevier Science B.V. All righ ts reserved.