Adaptations and selection of harmful and other dinoflagellate species in upwelling systems. 2. Motility and migratory behaviour

The motility and migrational behaviour of upwelling dinoflagellates as adaptations for growth in upwelling systems is evaluated. Traits considered include hydrodynamic streamlining; chain formation; motility rates of single cells and chains; adaptations to turbulence; turbulence sensing; and migrational scattering to avoid turbulence, including its role in the maintenance of indigenous populations. Motility rates are compared to vertical mixing and upwelling rates. Diverse combinations of cell shape, size and motility rates characterize the dinoflagellate species selected for growth in physically energetic upwelling systems. Specific or unique combinations of cell shape, size, propulsion system and swimming rate are not evident. The traits are shared with dinoflagellates generally, and probably reflect their swim-based ecology. Experimental evidence – primarily from Alexandrium catenella – suggests upwelling dinoflagellates can sense turbulence leading to three distinct, but coherent, adaptive responses: chain formation (in such species); increased swimming speed (including non-chain-forming species); and the capacity to re-orient swimming trajectory in response to changes in turbulence, and at time-scales appropriate to survival and growth in the turbulence field being experienced. The added swimming power that dinoflagellates gain through chain formation does not appear to be a major requirement for their selection or success in upwelling systems. Only three of the 42 most prominent dinoflagellates that bloom in eastern boundary upwelling systems form chains, a representation far below expectations. Most chain-forming dinoflagellates are excluded from those upwelling systems. The role of temperature in this exclusion is evaluated. Field and experimental evidence suggests that strong turbulence would be required to overwhelm the swimming-based ecology of the upwelling dinoflagellates and deter their blooms. The Yamazaki–Kamykowski model demonstrating that the cells within a migrating population exposed to wind-induced vertical mixing do not migrate uniformly, but scatter differentially within the mixing layer, is applicable to upwelling dinoflagellates. The adaptive value of migrational scattering is that it protects some of the cells from turbulence-induced mortality or impairment, with the survivors available as potential seed stock for subsequent blooms. It also allows an indigenous population to develop and be maintained. The physically robust features of upwelling systems do not appear to be major impediments to the survival and growth of dinoflagellates.