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

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, Karenia brevis , on competing phytoplankton species. Exposure to waterborne compounds from natural 9;A. brevis blooms resulted in growth inhibition or death for four of five co‐occurring species tested, whereas compounds exuded by 9A. brevis cultures suppressed three of these same competitors (the diatoms Asterionellopsis glacialis and Skeletonema costatum and the dinoflagellate Prorocentrum minimum ) plus one additional species (the dinoflagellate Akashiwo cf. sanguinea ) that was unaffected by bloom exudates. 9A. brevis 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, 9A. brevis exudes potent allelopathic compounds, competitors vary in their susceptibility to 9A. brevis allelopathy, and 9A. brevis 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.