The planktonic dinoflagellate Dinophysis is a major source of concern in various coastal ecosystems worldwide, where it regularly contaminates edible shellfish with diarrhetic toxins. Dinophysis is also a peculiar biological model, that relies on specific kleptoplastidy (i.e. acquisition of plastids from a specific prey) for photosynthetic growth.

Using data from decades-long observation networks, we studied the phenology of Dinophysis in a diverse array of coastal sites along the French coastline, from the English Channel to the Mediterranean Sea. Based on cell counts from systematic microscope observation, we used Generalized Additive Models (GAMs) to model seasonal dynamics of Dinophysis and, when data was available, of its prey Mesodinium rubrum (a photosynthetic, kleptoplastidic ciliate). These analyses revealed that Dinophysis and Mesodinium abundances and growth exhibit clear, region-specific seasonal patterns. We compared these patterns to those of temperature and stratification of the water column, two hydrological parameters that shape the pelagic ecosystem. Moreover, in the two distinct regions for which M. rubrum observation data was available, we observed that periods of Dinophysis growth were concomitant with periods of high prey abundance, corroborated by the detection of high-biomass M. rubrum blooms on satellite images. This finding points towards a significant trophic link between Dinophysis and M. rubrum in the wild, for which concrete evidence is rather scarce in published research conducted in natural environments.