Abstract:The formation and germination of cysts of the bloom causative dinoflagellate Scrippsiella trochoidea were studied from January 2001 to January 2002 in Daya Bay, South China Sea, as well as the dynamic of S. trochoidea vegetative population. Cysts were monthly collected by sediment traps and TFO gravity corer. S. trochoidea vegetative cells, water temperature, salinity, dissolved oxygen were measured throughout the annual cycle. In situ formation and germination of the cysts were assessed by cyst flux settled in sediment traps and the fraction of empty cysts in surface sediments, respectively. The annual average fluxes of calcified cyst and non-calcified cyst were 1.11×104 cysts m-2d-1 and 2.13×105 cysts m-2d-1, respectively. Calcified cysts largely formed in winter, while non-calcified cysts did in summer. Excystment occurred often in spring and autumn, less in summer, and seldom in winter. The highest proportions of empty cysts in May and October were followed by the peak abundance of vegetative cells, after which cyst concentrations in surface sediments and cyst fluxes increased rapidly. It is suggested that cyst germination inoculates sufficient vegetative cells into the water column and high cell densities contribute to the formation of cyst, which led to the next onset of algal bloom.