Contamination de l'etang de Thau par Alexandrium tamarense. Episode de novembre a decembre 1998

A bloom of the toxic dinoflagellate Alexandrium tamarense was observed for the first time in Thau iagoon between November and December 1998. Maximum cell concentrations detected in shellfish farming areas (85 000 Cells/1) were high enough to make mussels and clams unsuitable for human consumption (max. 850 ug. STX eq.100 g'1) while toxic levels in oysters did not exceed, or even reach, the international public health threshold established for PSP toxins. This event, new to French Mediterranean waters, was investigated by both local and national veterinary services and by the Ifremer teams involved in the National Monitoring Network for Phytoplankton and Phycotoxines (Réphy). The chronological and operational description of the event was compared with data from international literature. This study also provided an opportunity to propose further improvements to monitoring systems and define research priorities : toxic algal species' origin, risks associated with transfering shellfish to safe areas and the exact role of local hydroclimatic conditions in triggering microal-gal blooms.