Seasonal and interannual dynamics of nutrients and phytoplankton pigments in the western Mediterranean Sea at the DYFAMED time-series station (1991–1999)

The phytoplankton dynamics in the western Mediterranean Sea has been studied at the DYFAMED (France JGOFS) time-series station from 1991 to 1999 by means of an analysis of monthly pigment profiles in the 0–200-m layer. The site (43°25′N, 7°52′E) is located in the central zone of the Ligurian Sea, NW-Mediterranean Sea, and is protected from coastal inputs by the presence of Ligurian current flowing along the coast. asonal hydrological regime varies from winter mixing (January–February) to strong thermal stratification in summer and fall. Nutrients are depleted in the surface layer during summer oligotrophic conditions and re-injected to the surface layer during winter mixing. The nitrate-to-phosphate ratio is about 20 in deep waters, which indicates a general tendency to P-limitation. Nevertheless our 9-year study indicates that N/P ratio in surface layer is always higher than 20 (up to 60) during oligotrophic period and generally lower than 20 during the rest of the year. This indicates a probable shift from N-limitation in winter to P-limitation in summer. al variations of phytoplankton dynamics have been characterized using the HPLC pigment approach, and related to hydrological conditions and distribution of nutrients. The 0–200 m integrated chlorophyll a during the 9-year study is highly variable (from 12 to 230 mg m−2). The phytoplankton biomass is dominated by 19′-hexanoyloxyfucoxanthin-containing algae all the year round, but the relative abundance of the characteristic populations with respect to total biomass indicates a seasonal succession. The contribution of diatoms to biomass, as inferred from fucoxanthin, is maximal in January or February and is followed 1 month later by 19′-hexanoyloxyfucoxanthin-containing nanoflagellates. The ratio of zeaxanthin (associated to cyanobacteria) to chlorophyll a is highest at the beginning of the stratified period, just before maximal contribution of divinyl-chlorophyll a (prochlorophytes) to biomass. was a general increase in total phytoplankton biomass during the 9-year study. This increase seems mainly due to small-sized phytoplankton as a specific response to the lengthening of the summer stratification period favoring the growth of species supporting the regenerated production.