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

TLDR

Flagellates, comprising 60–80 harmful phytoplankton species, mainly dinoflagellates, use a swim strategy versus diatoms’ sink strategy. The study examines how turbulence affects HAB taxa’s photoadaptive strategies, growth rates, and nutrient uptake affinity. Flagellate HAB taxa offset low nutrient affinity through adaptations such as nutrient‑retrieval migrations, mixotrophy, allelochemically enhanced competition, and antipredation defenses, while their motility behaviors—phototaxis, vertical migration, pattern swimming, and aggregation—support nutrient retrieval, metal detoxification, antipredation, depth‑keeping, and turbulence avoidance. Flagellate HAB taxa have lower nutrient uptake affinity than diatoms, lack a distinct physiological syndrome separating them from nonharmful flagellates, yet differ ecophysiologically from diatoms by being more vulnerable to turbulence, more dependent on stratification, more mixotrophic, more allelochemically competitive, and exhibiting motility‑driven behaviors.

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 ( K s ) 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.