A coccolithophorid bloom, dominated by Emiliania huxleyi, was detected by sea viewing wide field of view sensor (SeaWiFS) images on the French continental shelf break in April 1998. Concentrations of up to 3.2×106 coccospheres l−1 and up to 8.6×107 coccoliths l−1 were measured by microscope countings of samples taken during the first days of the bloom. Moderate chlorophyll a concentrations (range: 0.8–1.1 μg l−1) characterised the study area. Chlorophyll and carotenoid pigments, analysed by high performance liquid chromatography (HPLC), confirmed the dominance of Prymnesiophytes in the bloom area. The bloom was not monospecific and diatoms, mainly belonging to the genus Rhizosolenia, as well as silicoflagellates were observed in the phytoplankton. Outside of the bloom area, “green algae” and cryptophytes dominated the phytoplankton. Diatoms were a dominant group of the Vilaine plume community and dinoflagellates were dominant in the southern part of the study area. The development of the dissipative phase of coccolithophorid bloom and its persistence for at least 4 weeks is explained by the conjunction of water mass preconditioning by river inputs on the continental shelf, increasing PAR during spring, and internal wave formation at the shelf break during spring tides. Partial dissolving of coccoliths and lack of horizontal displacement of the bloom, during the 4 weeks, are interpreted in terms of rapid settling of coccoliths due to packaging by grazers as well as ongoing production maintained by nutrient injection via the action of internal waves.