Kystes modernes de Dinoflagellés en Baie de Vilaine-Bretagne Sud (France)

A preliminary study of modern marine dinoflagellate cysts has been performed in Vilaine Bay through winters 1986, 1987 and 1989. Direct observations of muddy sand samples and most probable number (MPN) method applied to cysts have evidenced a low win ter species diversity with Spiniferites spp. and Scrippsiella sp. cysts as predominant species. Change in relative importance of maximum species diversity area from ycar to year suggests a possible action of bottom currents. A study of dormant stages of toxie dinoflagellates, genus Alexandrium and Dinophysis, has corroborated the occurrence of A. minutum cysts (IO to 30 cysts.g-I sediment) at low amounts and absence of Dinophysis sp. resting stages. More generally, comparison of cysts in sediments with summer free swimming stages shows a certain discrepancy, probably due to different living cycles and to unsteady, species dependant, encystment/excystment period.