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Microplankton annual cycles in the Bay of Villefranche, Ligurian Sea, NW Mediterranean Sea

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The Mediterranean Sea is an oligotrophic semi-enclosed basin with an eastward decrease in productivity (Sournia, 1973). However, the concentrations of nitrate and phosphate are increasing (Béthoux et al., 1998ª) and the cultural eutrophication due to the rising demographic impact is becoming an important ecological issue for the Mediterranean coastal zones (UNESCO, 1988; Turley, 1999), along with the effects of the climatic change on the Mediterranean ecosystem (Francour et al., 1994; Bianchi and Morri, 2000).

The trophic character of Villefranche Bay varies from oligotrophic [based on chlorophyll or primary production values (Bustillos-Guzmán et al., 1995; Lacroix and Nival, 1998)] to mesotrophic [based on the nitrogen dynamics and plankton composition (Selmer et al., 1993)]. The food web is predominantly based on the microbial loop(Rassoulzadegan and Sheldon, 1986; Dolan et al., 1995;

Thingstad et al., 1998). The waters are dominated by picoand nanoplankton, mainly consisting of cyanobacteria (Synechococcus sp.), nanoflagellates (responsible for the major part of the primary production) and heterotrophic bacteria (Lins da Silva, 1991; Zweifel et al., 1993; Jacquet et al., 1998). Even during the spring and autumn maxima, the abundance of microphytoplankton is low (Hagström et al., 1988; Lins da Silva, 1991; Ferrier-Pagès and Rassoulzadegan, 1994) in comparison to the Gulf of Lions ( Jacques, 1969) and to the Gulf of Marseille (Travers, 1971) where river input and wind forcing are more important.

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