The dynamic of coastal marine microphytoplankton communities is characterized by successions of species assemblages displaying typical schemes of spatio-temporal variability. Numerous studies of phytoplankton spatio-temporal variability have concerned holistic variables such as total biomass or primary production. Such studies deal with the role of primary producers in the global functioning of the pelagic ecosystem, but have little relevance to other problematics. For instance, knowledge of the taxonomic composition of phytoplankton communities and of their spatio-temporal patterns is necessary to understand the mechanisms favouring of the predominance of a precisely identified population within phytoplankton communities. In particular, the identification of the factors controlling the dynamics and the structure of phytoplankton communities is necessary to understand the development of harmful or toxic species.
Along French coasts, data collected within the context of the Phytoplankton and Phycotoxins Monitoring Network (REPHY), performed by the French Research Institute for Exploitation of the Sea (Ifremer), constitute a relevant basis to study the structure and the dynamics of phytoplankton communities at large temporal (1987/2002) and geographical (the whole French coasts) scales. The present study used these data to identify the main variability patterns of the phytoplankton communities observed along the French coasts. The temporal variability patterns of the phytoplankton populations were examined on various geographical scales. Once, the analysis of the dynamics of phytoplankton populations on a large geographical scale (on the whole French coasts) allowed to identify the temporal variability component of phytoplankton populations shared by all sampled sites. Multivariate statistical methods were used to determine seasonal and inter-annual variability. The expected temporal pattern common to all sites was identified and the seasonal cycle of the most frequently observed phytoplankton communities along French coasts was described. After the extraction of this common temporal component, a between-site analysis, using multitable comparison methods, allowed the identification of three large areas (Western English Channel, Bay of Biscay and Mediterranean Sea) according to the temporal variability patterns of microphytoplankton populations. The determination of homogeneous areas resulted in considering an analysis at the "regional" geographical scale, and in identifying the specific phytoplankton communities of each area. The knowledge of the ecological characteristics of these communities confirmed the importance of the hydrodynamic factors on the geographical structure of the phytoplankton communities along the French coasts. The ultimate step of this study was interested in dynamics of a toxic phytoplankton population on a local scale. The objective was to look for the hydroclimatic factors (using synthetic descriptors from hydrodynamical modelling) favourable to the regular development of a toxic population, the genus Dinophysis spp., in the Mor-bras (South Brittany).