Harmful algal blooms: Their ecophysiology and general relevance to phytoplankton blooms in the sea

Abstract

From 60 to 80 species of phytoplankton have been reported to be harmful; of these, 90% are flagellates, notably dinoflagellates. The effects of turbulence on harmful algal bloom (HAB) taxa, their photoadaptive strategies, growth rate, and nutrient uptake affinity (Ks) are considered. Flagellates, including HAB taxa, collectively have a lower nutrient uptake affinity than diatoms. Four major adaptations are suggested to have been evolved to offset the ecological disadvantages of their low nutrient affinity: nutrient retrieval migrations; mixotrophic tendencies; alle‐lelochemically enhanced interspecific competition; and allelopathic, antipredation defense mechanisms. Motility‐based behavioral features of flagellates contributing to their blooms include: phototaxis, vertical migration, pattern swimming, and aggregation, which facilitate nutrient retrieval, trace metal detoxification, antipredation, depth‐keeping, and turbulence avoidance. Neither a general physiological syndrome nor distinctive physiological profile distinguishes harmful flagellate species from nonharmful taxa. However, HAB flagellates exhibit significant ecophysiological differences when compared to diatoms, including greater biophysical vulnerability to turbulence, greater bloom dependence on water‐mass stratification, greater nutritional diversity involving mixotrophic tendencies, greater potential use of allelochemical mechanisms in interspecific competition and antipredation defenses, and unique behaviorial consequences of their motility. Flagellates use a “swim” strategy; diatoms a “sink” strategy.