In a conceptual model proposed before, the onset of the upwelling season and availability of Mesodinium rubrum prey are essential conditions for the initiation of Dinophysis acuminata blooms in the Galician Rías Baixas (NW Spain). Distribution of sexual cysts from the previous year in the top sediment layer is an important parameter in forecasting models of cyst-forming species (e.g. Alexandrium spp.). But the existence of sexual cysts of Dinophysis spp. has not been confirmed, Dinophysis planozygotes can divide and produce vegetative cells directly, and it is possible that species of this genus do not rely on sexual cysts but on some kind of overwintering planktonic form as a seeding mechanism. Here we examine winter conditions—persistence or absence of overwintering cells of Dinophysis within the rías and the adjacent shelf and time of initiation of the upwelling season—from observations in the last 20 years. Our ultimate goal was to explain the abundance and early initiation in April 2012 of an extraordinary bloom of D. acuminata, in what appeared to be a mesoscale event affecting other regions in Western Europe. The dynamics of D. acuminata populations appeared tightly coupled to the upwelling season. Our results suggest that anomalous upwelling patterns (predominance of upwelling in winter) combined with the presence of overwintering (inoculum) cells lead to early initiation of the Dinophysis acuminata growth seasonEn prens
