Nitroalboran Project.

The processes related to the nitrogen biogeochemical cycle will be studied in the Alboran sea in order to determine the limitation degree of the primary production by nitrate availability. This will allow us to determine the potential response of the phytoplankton communities to the nutrient enrichment which is occurring in the Mediterranean sea due to the increase of terrestrial discharges. For this proposal, an oceanographic survey will be performed covering the main hydrological structures identified in the Alboran sea: the upwelling areas and the anticyclonic gyres located respectively in the northern and central parts of the basin. Fifteen stations will be sampled distributed in 3 transects perpendicular to the coast line which go across the different hydrological structures. At each station, profiles of temperature, salinity and photosynthetic active radiation will be performed. Nutrient concentrations and elemental composition of the organic matter (including data of d15N) will be also obtained. Moreover, the phytoplankton communities will be characterized from the values of total and size-fractioned chlorophyll a, taxonomic composition and size spectra. Based on the chlorophyll a concentrations, stations of high and low productivity will be selected for in situ experiments, in order to study the nitrogen cycle processes which deal with the nitrate supply to the euphotic zone. New and regenerated primary production will be estimated from different 15N uptake experiments. In addition, the N2 fixation rates will be assessed because this process could contribute significantly to the new production in the Mediterranean sea. The nitrate supply by vertical turbulence will be inferred from the hydrological data. Nitrification and ammonium excretion by zooplankton will be also estimated. The N metabolism of the identified phytoplankton communities will be assessed from this set of data. In order to link the N-metabolism types to the role of the community in the export of carbon from the euphotic layer, the primary production and the respiratory activity will be estimated in the water column. Finally, incubation experiments with addition of different nutrient sources will be performed in order to determine the potential response of the phytoplankton communities to the nutrient enrichment.