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The Mediterranean Sea (MS) is an oligotrophic basin whose offshore water column exhibits low dissolved inorganic phosphorus (P) and nitrogen (N) concentrations, unusually high nitrate (NO3) to phosphate (PO4) ratios, and distinct biogeochemical differences between the Western Mediterranean Sea (WMS) and Eastern Mediterranean Sea (EMS). A new mass balance model of P and N cycling in the WMS is coupled to a pre-existing EMS model to understand these biogeochemical features. Estimated land-derived inputs of reactive P and N to the WMS and EMS are similar per unit surface area, but marine inputs are 4 to 5 times greater for the WMS, which helps explain the approximately 3 times higher primary productivity of the WMS. The lateral inputs of marine sourced inorganic and organic P support significant fractions of new production in the WMS and EMS, similar to subtropical gyres. The mass balance calculations imply that the MS is net heterotrophic: dissolved organic P and N entering the WMS and EMS, primarily via the Straits of Gibraltar and Sicily, are mineralized to PO4 and NO3 and subsequently exported out of the basin by the prevailing anti-estuarine circulation. The high deepwater (DW) molar NO3:PO4 ratios reflect the high reactive N:P ratio of inputs to the WMS and EMS, combined with low denitrification rates. The lower DW NO3:PO4 ratio of the WMS (21) compared to the EMS (28) reflects lower reactive N:P ratios of inputs to the WMS, including the relatively low N:P ratio of Atlantic surface water flowing into the WMS.

Plain Language Summary The Mediterranean Sea (MS) is a marine desert: it exhibits extremely low biological productivity despite being almost entirely surrounded by land with high nutrient loadings from a large coastal population. To explain this paradox, we analyze the sources and fate of the two main nutrient elements that support the production of marine biomass, phosphorus (P), and nitrogen (N). We find that the main source of P and N to the MS is inflow of surface water from the Atlantic Ocean via the Strait of Gibraltar, not land-derived sources. This inflow is balanced by a return to the Atlantic Ocean of deeper Mediterranean water enriched in the biologically most active forms of P and N, phosphate and nitrate. The very low productivity of the MS therefore reflects a switch from less bioavailable chemical forms of P and N entering the MS to more bioavailable forms leaving the MS. Computer simulations reproduce these chemical differences when coupling the biological utilization and recycling of P and N to the circulation of the MS, which drives the water exchanges across the Strait of Gibraltar. These simulations also reproduce the differences in productivity and nutrient distributions between the western and eastern basins of the MS.