Résumé
This study investigated the dormancy length, germination patterns and cyst progeny fate of Alexandrium catenella and A. tamarense found in Thau lagoon, France. In laboratory-produced cysts, the dormancy period was estimated to be less than 2 weeks. A vernalization period was not required prior to germination. However, after 1 month of dark-storage at 6°C, excystment was obtained 2-3 days from re-exposure to favourable conditions thereby suggesting that germination was synchronized. Germination patterns and germling cell viability in laboratory-produced cysts were similar to those exhibited in natural cysts. The following conditions optimized germination: temperature between 14 and 26°C, salinity between 30 and 38 psu and light exposure for >1 h at 100 μmoles photons m−2 s−1. Similar conditions prevailed at the commencement of and during Alexandrium blooms in Thau lagoon, suggesting that cysts may germinate shortly after being recruited from sediment re-suspensions. Accordingly, cyst bank germinations contribute to the initial seeding of blooms, and subsequently germinations of newly formed cysts can help to maintain blooms. In cyst banks, the high germination capabilities seem, however, to be counter-balanced by the low viability of cyst progeny, as estimated from the realized seeding ratio. This ratio is proposed as a means of quantifying the potential for cyst banks to regenerate planktonic populations.