Changes in marine phytoplankton communities over short-time scales have rarely been examined. We sampled nearly daily, and determined taxonomic composition and abundance of the main microplanktonic groups from both net and bottle sampling during the DYNAPROC 2 cruise (September–October 2004), in the NW Mediterranean Sea. Hydrological conditions were characterised by the existence of a highly-stratified water column and nutrient-depleted conditions. In this almost steady state situation, we found a decrease in regularity, which we attribute to competitive exclusion, leading to the dominance of species with the best fitness. While overall diversity appeared to be quite invariant, important variations occured among specific taxonomic groups. For example, within the genus Ceratium, the dominance index tended to increase throughout the study from 44% to a maximum of 73% at the end of the cruise. We also discuss the value of taxonomic studies, especially in the context of global change, as able to provide valuable information on biogeochemically-important groups of species, as well as those indicative of water masses or warming, and trophic aspects of the community that are ignored or largely underestimated with other kind of phytoplankton studies, such as pigment analysis.