PHYTOPLANKTON AND PRIMARY PRODUCTION CHARACTERISTICS AT SELECTED SITES IN THE GEOSTROPHIC ALMERIA-ORAN FRONT SYSTEM (SW MEDITERRANEAN-SEA)

Several phytoplankton-related characteristics were measured at six selected sites of the frontal system of the Alboran Sea during May 1991 (''Almofront-1'' cruise, JGofs-France). As compared to the surrounding oligotrophic waters, the frontal jet exhibited higher (though moderate) diatom abundance, higher nannoplankton abundance, higher chlorophyll content and higher primary production. Flow cytometry and fractionated filtration indicated the predominance of the smaller phytoplankton size classes, except on the right border of the jet. In the frontal area, the deep chlorophyll maximum (DCM) reached up to 2 mug chl. a 1-1 and extended over a 50-meter thickness with a frequent subdivision into two maxima. Photosynthesis/irradiance curves measured in surface and in deeper water were strikingly different in the oligotrophic sites and similar or identical in the frontal sites, suggesting different hydrodynamic patterns. Application of the Falkowski' model resulted in displacement times of about 1-2 d between the surface and DCM in the frontal jet, and isopycnal transports of about 10 cm s-1. Altogether, phytoplankton data (size and species composition, chlorophyll content, production and photosynthetic tests) as well as isopycnals and nitracline slopes, are consistent with other data collected during ''Almofront-1'' as regards (1) a front-induced nutrient enrichment and (2) a subsequent downwelling of plant biomass. Although the (geostrophic) Almeria-Oran front is basically a permanent one, seasonal and shorter-scale temporal variations must be taken into account.