A biogeochemical model for phosphorus and nitrogen cycling in the Eastern Mediterranean Sea: Part 1: Model development, initialization and sensitivity

The Eastern Mediterranean Sea (EMS) is the largest marine basin whose annual primary productivity is limited by phosphorus (P) rather than nitrogen (N). The basin is nearly entirely land-locked and receives substantial external nutrient fluxes, comparable for instance to those of the Baltic Sea. The biological productivity of the EMS, however, is among the lowest observed in the oceans. The water column exhibits very low P and N concentrations with N:P ratios in excess of the Redfield value. These unique biogeochemical features are analyzed using a mass balance model of the coupled P and N cycles in the EMS. The present paper describes the conceptual basis, quantitative implementation and sensitivity of the model. The model is initialized for the year 1950, that is, prior to the large increase in anthropogenic nutrient loading experienced by the EMS during the second half of the 20th century. In the companion paper, the model is used to simulate the P and N cycles during the period 1950-2000. The 1950 model set-up and sensitivity analyses support the following conclusions.(1)Phosphorus-limited primary production in the EMS is most sensitive to the P exchanges with the Western Mediterranean Sea (WMS) associated with the anti-estuarine circulation of the EMS. The supply of P through the Straits of Sicily is mainly under the form of dissolved organic P (DOP), while dissolved inorganic P (PO4) is primarily exported to the WMS. The efficient export of PO4 to the WMS maintains the EMS in its ultra-oligotrophic state.(2)Inorganic molar N:P ratios in excess of the 16:1 Redfield value observed in the water column reflect higher-than-Redfield N:P ratios of the external inputs, combined with negligible denitrification. Model simulations imply that the denitrification flux would have to increase by at least a factor of 14, relative to the 1950 flux, in order for the inorganic N:P ratio of the deep waters to approach the Redfield value.(3)The higher-than-Redfield N:P ratios of dissolved and particulate organic matter in the EMS further imply the preferential regeneration of P relative to N during organic matter decomposition.