Phytoplankton dynamics and primary production under late summer conditions in the NW Mediterranean Sea

Phytoplankton pigments were analyzed at high frequency (4–8 profiles every 24 h) during the DYNAPROC II cruise in the DYFAMED zone, NW Mediterranean Sea, in summer/fall 2004. This period is characterized by a strong stratification, which was modified by hydrological and meteorological events. Drastic changes in nutrient distribution observed in the course of the DYFAMED time-series experiment were not present during the DYNAPROC II cruise. The phytoplankton biomass (TChla) was exceptionally high at the beginning of the cruise (35–40 mg m−2) and decreased during the cruise to 20–25 mg m−2 during the second leg. Primary production was restricted to the upper 40 m and was higher at the beginning of the cruise than at the end. A short-term (day–night) variability of the taxonomic pigments was noticed, and the evaluation of biomass through chlorophyll a measurements could display a variation of nearly 40% in 24 h. Significant changes during the cruise in both the distribution of pigments and the proportion of phytoplankton classes deduced from pigment data by application of the CHEMTAX program were observed. The biomass was dominated by pico- and nanophytoplankton (around 87%) in surface waters (0–60 m), with the exception of the very beginning of the cruise, when diatoms (fucoxanthin) were abundant in deeper (60–80 m) waters. Microphytoplankton was relatively constant (around 10%) and composed of dinoflagellates and diatoms after the first phase, when diatoms were predominant. Prymnesiophytes, pelagophytes and prasinophytes were relatively stable and of approximately equal abundance. Picoplankton components showed more significant variations: synechococcus and prochlorococcus underwent rapid opposite evolutions. Observed phytoplankton dynamics are discussed in relation to wind events, nutrient availability and changes in hydrological structure. At the end of the summer period, the principal effect of wind events or hydrological changes was an increase of synechococcus in the surface layer and of prochlorophyte biomass in deeper layers.