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 (43degrees25'N, 7degrees52'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. The seasonal 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. Seasonal 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 facoxanthin, is maximal in January or February and is followed 1 month later by 19'-hexanoyloxyfiicoxanthin-containing nano flagellates. 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. There 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. (C) 2002 Published by Elsevier Science Ltd.