| Popis: |
The evolution of the phytoplankton distribution was monitored along the Calvi-Nice axis, from Calvi to 30 miles offshore, in relation with the hydrological features of the Liguro-Provencal front (Corsican sector) and nutrient availability. Oceanographic cruises repeated between 1982 and 1991 have emphasized the permanence of the front and the perennial stability of its hydrological characteristics. Moreover, the front and the associated vertical circulation influenced strongly, and in a reproducible fashion, the algal distribution within distances of few kilometres. In March, stabilization of the upper layers of the water column, induced by a beginning of vertical stratification, is the main factor controlling the onset of the phytoplancton bloom in the Ligurian current (on the coastal side), the front and the central zone (on the open sea side). As soon as phytoplankton development begins, 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 is carried downwards along the isopycnals in relation with the frontal convergence. As a consequence, the chlorophyll a distribution is typically plume-shaped, 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 is permanently observed in the frontal area, where it is strictly correlated with the nitracline. A sharp core of high chlorophyll a concentrations is also observed in the central zone, while the Ligurian current is characterized by very low pigment levels. During this period, the ecological role of the front seems to be particularly important because it delimits the sole area of enhanced phytoplanktonic biomass in an oligotrophic or very oligotrophic environment. In June, a well-developed deep chlorophyll a maximum is 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. |
