Phytoplankton assemblages and characterization of a Dinophysis acuminata population during an upwelling–downwelling cycle

The distributions of Dinophysis acuminata, its potential prey Myrionecta rubra, and the microplankton populations associated with them, were studied in Ria de Pontevedra (NW Spain) during a 2 wk cruise that covered an upwelling-downwelling cycle, and during intensive sampling over 21 h at the end of the cruise. Special attention was focused on the characterization and physiological condition of D. acuminata. There was vertical segregation between a phytoplankton assemblage that was dominated by upwelling-promoted diatoms and another assemblage that was dominated by small dinoflagellates, located in the warmer surface layer (0 to 5 m) where D. acuminata was observed. High spatio-temporal variability was observed in the frequency of cells containing starch granules (indicating photosynthetic activity) and digestive vacuoles (indicating heterotrophic feeding); this pattern corresponded with the apparent availability of M rubra cells. Populations of D. acuminata and M rubra have their own niches and distinct responses to physical forcing, but both are located in common water masses and occasionally meet. Changes in vacuolation of D. acuminata were followed by a substantial increase (similar to 35 %) in cellular volume. D. acuminata did not exhibit daily migratory behaviour, and expressed a high division rate (0.51 d(-1)) under downwelling conditions. This work sheds new light on the relation between D. acuminata populations and its potential prey, and on its ecophysiology; it also questions previous assumptions about the environment that is suitable for its development.