Dinophysis acuminata, the main cause of shellfish harvesting bans in Europe, blooms in the Galician Rias (NW Spain) throughout the upwelling season (ca. March to September). Here we illustrate rapid changes in vertical and across ria-shelf distributions of diatoms and dinoflagellates (including D. acuminata vegetative and small cells) in Ria de Pontevedra (RP) and Ria de Vigo (RV) during transitions from spin-down to spin-up phases of upwelling cycles. A subniche approach based on a Within Outlying Mean Index (WitOMI) showed that under the transient environmental conditions met during the cruise, both vegetative and small cells of D. acuminata colonized the Ria and Mid-shelf subniches, exhibiting good tolerance and extremely high marginality, in particular the small cells. Bottom-up (abiotic) control overwhelmed biological constraints, and shelf waters became a more favourable environment than the Rias. Contrasting higher biotic constraints inside the Rias were found for the small cells, with a subniche possibly controlled by unsuitable physiological status (notwithstanding the higher density) of the vegetative cell population. Results here on behaviour (vertical positioning) and physiological traits (high tolerance but very specialized niche) of D. acuminata give new insights into the ability of this species to remain in the upwelling circulation system. Higher shelf-ria exchanges in the Ria (RP) with more dense and persistent D. acuminata blooms reveal the relevance of transient event-scales and species- and site-specific characteristics to the fate of these blooms. Earlier statements about simple linear relationships between average upwelling intensities and the recurrence of Harmful algae bloom (HAB) events in the Galician Rias Baixas are questioned.
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