ORIGIN OF HIGH PHYTOPLANKTON CONCENTRATION IN DEEP CHLOROPHYLL MAXIMUM (DCM) IN A FRONTAL REGION OF THE SOUTHWESTERN MEDITERRANEAN-SEA (ALGERIAN CURRENT)

In June 1990, we investigated the Atlantic flow along the Algerian coast in the Western Mediterranean. Intensive and fine resolution sampling along 13 longitudinal transects, consisting of 4-12 stations (3-5 nautical miles apart), was carried out with physical, chemical and biological measurements. This sampling enabled us to describe quasi-synoptically the spatial distribution over a large region between 1-degrees and 4-degrees-E. Two oligotrophic systems, where the integrated chlorophyll was lower than 30 mg m-2, were separated by an almost continuous rich band characterized by integrated chlorophyll higher than 60 mg m-2. This chlorophyll-rich band, with concentrations greater than 2 mg m-3 (up to 15.8 mg m-3), was found along the boundary of the Atlantic flow, indicating that the phytoplanktonic biomass distribution is determined by hydrodynamics. The patchiness of phytoplankton, both in chlorophyll concentration and in species abundance, along a density surface resulted from interactions between Atlantic and Mediterranean waters. Also, a first assessment of the biological response to the mesoscale hydrodynamical features can be deduced from the comparative analysis of the physical, chemical and biological parameters. Large anomalies in nutrient and chlorophyll distributions indicated strong vertical advections along the front, either following density surface (isopycnal mixing) or crossing it (diapycnal mixing). Important isopycnal mixing of nutrients were evidenced on the inshore side of the current, while diapycnal mixing was observed only on the seaward side. These two processes seem to bring nutrients into the photic zone, leading to an enhanced in situ growth. A cross-front circulation is proposed to explain (i) the vertical distribution of chemical parameters near the front and (ii) the associated phytoplankton patchiness.