Effects of harmful algal blooms on competitors: Allelopathic mechanisms of the red tide dinoflagellate Karenia brevis

<jats:p>Because competitive interactions may have led to adaptations enabling bloom‐forming phytoplankton to dominate pelagic communities, we explored the allelopathic effects of one red tide dinoflagellate, <jats:italic>Karenia brevis</jats:italic>, on competing phytoplankton species. Exposure to waterborne compounds from natural <jats:italic>9;A. brevis</jats:italic> blooms resulted in growth inhibition or death for four of five co‐occurring species tested, whereas compounds exuded by <jats:italic>9A. brevis</jats:italic> cultures suppressed three of these same competitors (the diatoms <jats:italic>Asterionellopsis glacialis</jats:italic> and <jats:italic>Skeletonema costatum</jats:italic> and the dinoflagellate <jats:italic>Prorocentrum minimum</jats:italic>) plus one additional species (the dinoflagellate <jats:italic>Akashiwo</jats:italic> cf. <jats:italic>sanguinea</jats:italic>) that was unaffected by bloom exudates. <jats:italic>9A. brevis</jats:italic> exudates lowered photosynthetic efficiency and damaged cell membranes of competing phytoplankton, but had no effect on competitor esterase activity, nor did they limit competitor access to iron. Overall, during blooms, <jats:italic>9A. brevis</jats:italic> exudes potent allelopathic compounds, competitors vary in their susceptibility to <jats:italic>9A. brevis</jats:italic> allelopathy, and <jats:italic>9A. brevis</jats:italic> may achieve nearly monospecific blooms by lowering the photosynthetic efficiency of competitor species and increasing competitor membrane permeability, eventually resulting in competitor growth suppression or death.</jats:p>