Role of temporary cysts in the population dynamics of Alexandrium taylori (Dinophyceae)

Although temporary cyst stages are common in dinoflagellates,their role remains unclear. Every year Alexandrium taylori (Dinophyceae)forms dense patches (10⁶ cells l^-1) along La Fosca beach (Spain,northwest Mediterranean), which last for 2 months (July, August).One of the characteristics of the life history of A. tayloriis the shift from a vegetative motile stage to non-motile temporarycysts. Here we present the temporal changes in the abundanceof temporary cysts in sediments and their in situ encystmentand excystment rates. The in situ encystment rate of temporarycysts from the water column to the sediment ranged from 1.8x 10⁶ to 4.4 x 10⁶ cysts m^-2 day^-1, whereas the excystment ratewas between 0.9 x 10⁶ to 2.7 x 10⁶ cysts m^-2 day^-1 during thebloom period. Some of the temporary cysts in the sediment tookmore than 1 day to produce vegetative cells and remained viablefor at least 4 days. We propose that temporary cyst formationin this species is a tool for reducing population losses. Theproduction of temporary cysts can be an advantage since partof the population is stored in the sediments.