This study shows that different species of the same genus can behave differently in relation to a salinity gradient and suggests that a halocline can act as a barrier to the diel vertical migration (DVM) of dinoflagellate species. In a laboratory study, we found that the dinoflagellates Ceratium furca, C. tripos and Dinophysis acuta did not cross a salinity gradient of 5 psu, but instead exhibited DVM exclusively below and above the halocline. Furthermore, C. tripos aggregated in the halocline during the night and migrated to the surface at midday, whereas C. furca aggregated in the halocline at midday and migrated downwards to the bottom during the dark period. Dinophysis acuta was found exclusively above the halocline, with the highest density in the surface water at midday. The present results suggest that different dinoflagellate species display unique DVM behaviours in stratified waters. The species-specific DVM strategies revealed in this study are of central importance for understanding the ecology of these taxa and to further elucidate both the ecological significance and the mechanisms of DVM.
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