NITRIFICATION RATES, NITRITE AND NITRATE DISTRIBUTION IN THE ALMERIA-ORAN FRONTAL SYSTEMS (EASTERN ALBORAN SEA)

In the western Mediterranean Sea, the Almeria-Oran frontal zone constitutes the eastern boundary of the Alboran Sea. During the ''Almofront-1'' cruise (April-May 1991) physical, chemical and biological features of the front and surrounding Atlantic and Mediterranean Waters were investigated. In the present study, the nitrite and nitrate concentrations and nitrification rates were measured. The nitracline was close to the surface in the north, close to the Spanish coast and in the south on the left side of the jet. The highest concentration of nitrite (0.25 muM) was located in the incoming Atlantic Waters. The ammonium oxidation rates were from 0 to 16 nmol l-1 d-1, except in Atlantic Waters in which the rates reached 28-42 nmol l-1 d-1. For all types of superficial water, i.e. Atlantic, jet core, Mediterranean, the nitrite produced by nitrification could account for the in situ concentrations in less time than the ages of the water masses. In contrast, to reach the ambient concentration, the production of nitrate would need more time than the lifetime of the water masses, except for the Atlantic Waters. In these Atlantic Waters, showing an increased rate of nitrate production, the in situ concentration of nitrate would be reached within its lifetime. The contribution of the nitrate produced by nitrification to the nitrate uptake by primary producers was estimated to range from 7% (Mediterranean Waters) to 40% (Atlantic Waters).