Contrôle du phytoplancton du bassin ligure par le front liguro-provençal (secteur corse)

The evolution of the phytoplankton distribution was followed along the Calvi - Nice axis, from Calvi to 30 miles offshore, in relation with the hydrological features of the Liguro-Provençal front (Corsican sector) and the nutrients disponibility. Oceanographic cruises repeated between 1982 and 1991 have emphasized the permanence of the front and the interannual stability of its hydrological characteristics. Moreover, the front and the associated vertical circulation influenced highly, and in a reproducible way, the algal distribution within distances of few kilometres.

In March, within the Ligurian current (on the coastal side), the front and the central zone (on the open sea side), stabilization of upper layers of water column, induced by a beginning of vertical stratification, is the main factor controlling the starting of the bloom. As soon as phytoplankton development is initiated, highest values of chlorophyll a and primary production are observed in the frontal area. During spring, a significant amount of the phytoplankton produced in the surface layer was carried down along the isopycnals in relation with the frontal convergence. As a consequence, the chlorophyll a distribution was typically like a plume, extending from the surface to at least 100 m deep.

In all zones, during the stratified period, the position of the chlorophyll a maximum in the water column is determined by the opposing gradients of light and nitrate. In May, the maximum of phytoplankton was permanently observed in the frontal area where it was strictly correlated with the nitracline. A sharp core of high chlorophyll a concentrations was also observed in the central zone while the Ligurian current was characterized by very low pigment levels. During that period, the ecological role of the front seems to be particularly important because it delimits the unique area of enhanced phytoplanktonic biomass in an oligotrophic or very oligotrophic environment. In June, a well-developed deep chlorophyll a maximum was observed everywhere, from the central zone to the coast. The contribution of the front to the enhancement of the biomass appears to be less important than during spring.