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 sel ected 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 fractionate d filtration indicated the predominance of the smaller phytoplankton s ize classes, except on the right border of the jet. In the frontal are a, the deep chlorophyll maximum (DCM) reached up to 2 mug chl. a 1-1 a nd 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 an d similar or identical in the frontal sites, suggesting different hydr odynamic patterns. Application of the Falkowski' model resulted in dis placement times of about 1-2 d between the surface and DCM in the fron tal jet, and isopycnal transports of about 10 cm s-1. Altogether, phyt oplankton data (size and species composition, chlorophyll content, pro duction 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 subseq uent downwelling of plant biomass. Although the (geostrophic) Almeria- Oran front is basically a permanent one, seasonal and shorter-scale te mporal variations must be taken into account.