PRIMARY PRODUCTION IN THE NORTHWESTERN MEDITERRANEAN

The Mediterranean is, globally considered, an oligotrophic sea. Howeve r, in spite of its small extension, it presents considerable heterogen eity and, specially in the Western sub-basin, a number of hydrographic features contribute to increase its potential fertility. Phosphorus a ppears to be the most important limiting nutrient in the Mediterranean , although it is closely followed by nitrogen in this limiting role. T he basic mechanisms of nutrient enrichment in the Mediterranean photic zone include vertical mixing in winter, coastal upwelling and the inp ut of Atlantic waters through Gibraltar. River runoff is important, sp ecially in the Western basin. One of the main causes of Mediterranean oligotrophy may be the water exchange at Gibraltar. The Mediterranean losses deep, relatively nutrient-rich water to the Atlantic through th e Gibraltar Strait and receives an excess of superficial, nutrient-poo r Atlantic water which compensates for the deep water outflow and the evaporation loses in the Mediterranean basin. However, the water flows at Gibraltar contribute to the fertility of the area through several mechanisms. One of them is due to the relatively shallow depth of the Gibraltar sill; in this zone, the entering wafers become partially mix ed with the richer outflowing waters and entrain additional nutrients into the Alboran Sea. Another enrichment mechanism is linked with the gyres induced by the Atlantic jet in the Alboran Sea and the associate d upwelling near the coast of the Spanish side. Besides the general fe rtilization mechanisms mentioned above, the Western Mediterranean pres ents a series of mesoscale structures which represent sites of enhance d nutrient inputs to the photic layers. These structures include the s helf-slope fronts along the continental and insular coasts and the cen tral divergence zones of the Liguro-Provencal and Catalano-Balearic Se as, which appear to be parts of a continuum. The relevance of some of these features for plankton production remained unrecognized until the last decades. In the vertical dimension, it has become apparent that the deep chlorophyll maxima, typical of the stratification season in t he Mediterranean and other oligotrophic marine areas, are sites in whi ch significant pulses of new production may take place, specially abov e the divergences, in which the nutrient-rich waters are closer to the surface. The occurrence of several mechanisms of fertilization, respo nding in different ways to environmental forcing, helps to enhance pri mary production levels through out parts of the year including the str atification period. In the Catalan Sea, for example, the shelf/slope f ront and the central divergence located mid-way between the continenta l coast and the Balearic Islands appear to vary their relative contrib utions from spring to summer, with the divergence becoming more import ant later during the stratification period. The optimisation of sampli ng strategies linked to a better knowledge of the productive hydrograp hic structures and the improvement of methodology have lead to higher estimates of the primary production in the northwestern Mediterranean than where previously accepted. How ever, even with these increased es timates: the present level of total fish catches appears to be close t o what could be the predicted limit according to some simple assumptio ns.