Short-term dynamics of microplankton abundance and diversity in NW Mediterranean Sea during late summer conditions (DYNAPROC 2 cruise; 2004)

Abstract. Changes in marine phytoplankton communities at short-time scales have rarely been examined. As a part of the DYNAPROC 2 cruise (14 September–17 October 2004), conducted in the NW Mediterranean Sea, we daily sampled and determined taxonomic composition and abundance of the microphytoplankton and the dominant microzooplanktonic groups from both net and bottle sampling, during 4 cycles of 5 days. Hydrological conditions were characterised by the dominance of a stratified water column and nutrient-depleted conditions. However the stratification index revealed a destabilization of the water column from the beginning of the second cycle, related to a wind stress event accompanied with coastal water intrusion. This conducted to an increase of taxonomic richness and a general decrease of evenness, depicting the dominance of species with the best fitness. We also emphasised on the great interest of taxonomic studies, as able to provide valuable information on biogeochemical-important groups of species, potential water masses indicators and trophic aspects of the community that are ignored or largely underestimated with other kind of phytoplankton studies, such as pigment analysis.