Atlantic forcing of the Mediterranean oligotrophy

26 The Mediterranean Sea shows a peculiar anomaly in its nutrient pattern compared to the 27 global ocean, as there is decrease in nutrient concentration from west to east. This 28 feature has been attributed to the anti-estuarine circulation at the Strait of Gibraltar, 29 where an eastward flow of Atlantic nutrient-poor surface waters is compensated by a 30 westward counter-current of Mediterranean nutrient-rich deep waters. This water 31 exchange has been suggested as the ultimate cause for the oligotrophy of the 32 Mediterranean basin, even though only a few studies have accurately examined the 33 magnitude of the nutrient flux through the Strait of Gibraltar. In this work, data from the 34 Gibraltar Fixed Time series (GIFT) between 2005 to 2008 were used to assess nutrient 35 distributions. Applying a two-layer model of water mass exchange and using the 36 Mediterranean outflow recorded in situ, the net export of nutrients from the 37 Mediterranean to the Atlantic was calculated as 139 and 4.8 Gmol y -1 of nitrate and 38 phosphate, respectively. The results also demonstrated that the Atlantic inflow is not 39 nutrient depleted and in particular, contains significant levels of phosphate, which is the 40 limiting factor for biological productivity in the Eastern Mediterranean. The distribution 41 of the quasi-conservative parameter N* in the Western and Eastern basins indicated that 42 nitrate deficient surface waters are transformed into phosphate deficient bottom waters 43 by internal cycling processes. Therefore, phosphate depletion in the Mediterranean does 44 not have its origin in the entry of a phosphorus impoverished Atlantic inflow through 45 the Strait of Gibraltar. 46