Primary production in the northwestern Mediterranean

SUMMARY: The Mediterranean is, globally considered, an oligotrophic sea. However, in spite of its small extension, it presents considerable heterogeneity and, specially in the Western sub-basin, a number of hydrographic features contribute to increase its potential fertility. Phosphorus appears to be the most important limiting nutrient in the Mediterranean, although it is closely followed by nitrogen in this limiting role. The basic mechanisms of nutrient enrichment in the Mediterranean photic zone include vertical mixing in winter, coastal upwelling and the input of Atlantic waters through Gibraltar. River runoff is important, specially 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 the Gibraltar Strait and receives an excess of superficial, nutrient-poor Atlantic water which compensates for the deep water outflow and the evaporation losses 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 waters become partially mixed 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 associated upwelling near the coast of the Spanish side. Besides the general fertilization mechanisms mentioned above, the Western Mediterranean presents a series of mesoscale structures which represent sites of enhanced nutrient inputs to the photic layers. These structures include the shelf-slope fronts along the continental and insular coasts and the central divergence zones of the Liguro-Provencal and Catalano-Balearic Seas, 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 the Mediterranean and other oligotrophic marine areas, are sites in which significant pulses of new production may take place, specially above the divergences, in which the nutrient-rich waters are closer to the surface. The occurrence of several mechanisms of fertilization, responding in different ways to environmental forcing, helps to enhance primary production levels throughout parts of the year including the stratification period. In the Catalan Sea, for example, the shelf/slope front and the central divergence located mid-way between the continental coast and the Balearic Islands appear to vary their relative contributions from spring to summer, with the divergence becoming more important later during the stratification period. The optimisation of sampling strategies linked to a better knowledge of the productive hydrographic structures and the improvement of methodology have lead to higher estimates of the primary production in the northwestern Mediterranean than where previously accepted. However, even with these increased estimates, the present level of total fish catches appears to be close to what could be the predicted limit according to some simple assumptions.