Seasonal survey of the phytoplankton biomass, composition and production in a littoral NW Mediterranean site, with special emphasis on the picoplanktonic contribution

Over a one-year cycle, several shifts occur in the phytoplankton community of the Bay of Banyuls-sur-Mer, a French northwestern Mediterranean shallow bay. In order to determine what drives these shifts, seasonal changes in littoral phytoplankton are considered together with environmental parameters. Our methodology combined a survey of the physical structure and nutrient contents of the water column with the complete characterization of the phytoplankton community structure at 3-m and 24-m depths. The sampling did provide a detailed record of the seasonal changes and successions that occur in northwestern Mediterranean littoral waters. Intrusions of low salinity water clearly had a significant impact on phytoplankton community structure. Such water freshening events both freshened the surface water and introduced substantial amounts of nutrient. The ratio between nitrogen and phosphorus changed on these occasions and proved to be a reliable clue for the characterization of phytoplankton community structure. Picophytoplankton predominated when the N:P ratio was below 10:1 under summer warm oligotrophic situations (Synechococcus and Prochlorococcus) and at the start of water nutrient enrichment (picoeukaryotes). As the N:P ratio increased in late wintereearly spring period, eukaryotic phytoplankton was promoted at the expense of Synechococcus and Prochlorococcus. Successions in pico-, nano- and microphytoplankton communities appeared mainly to be driven by differences in the ability to acquire nutrients and by differences in species-specific growth rates. Further characterization of picophytoplankton from samples collected during summer when the N:P ratio reached 5:1 suggested the existence of two cyanobacteria subpopulations. This study clearly shows that northwestern littoral Mediterranean waters are not always well mixed and that nutrient availability in the Bay of Banyuls-sur-Mer depends primarily on low salinity water intrusions.