Does the red tide dinoflagellate <i>Karenia brevis</i> use allelopathy to outcompete other phytoplankton?

<jats:p>Monospecific blooms of phytoplankton can disrupt pelagic communities and negatively affect human health and economies. Interspecific competition may play an important role in promoting blooms, and so we tested (1) whether the outcome of competition between the red tide dinoflagellate <jats:italic>Karenia brevis</jats:italic> (ex <jats:italic>Gymnodinium breve</jats:italic>) and 12 cooccurring phytoplankters could be explained by allelopathic effects of compounds released by <jats:italic>K. brevis</jats:italic> and (2) whether waterborne, lipophilic molecules, including brevetoxins, are involved. Nine of 12 phytoplankton species were suppressed when grown with live <jats:italic>K. brevis</jats:italic> at bloom concentrations. <jats:italic>K. brevis</jats:italic> extracellular filtrates or lipophilic extracts of filtrates inhibited six of these nine species, indicating allelopathy. However, these inhibitory effects were weaker than those experienced by competitors exposed to live <jats:italic>K. brevis</jats:italic>. Brevetoxins at ecologically reasonable waterborne concentrations accounted for the modest inhibition by <jats:italic>K. brevis</jats:italic> of only one competitor, <jats:italic>Skeletonema costatum</jats:italic>. The addition of brevetoxins also caused significant autoinhibition, reducing the maximum concentration of <jats:italic>K. brevis</jats:italic>. Allelopathy is one mechanism by which <jats:italic>K. brevis</jats:italic> appears to exhibit competitive advantage over some sympatric phytoplankters, although unidentified compounds other than brevetoxins must be involved, in most cases. <jats:italic>K. brevis</jats:italic> was also susceptible to competitive exclusion by several species, including <jats:italic>Odontella aurita</jats:italic> and <jats:italic>Prorocentrum minimum</jats:italic>, known to thrive during <jats:italic>K. brevis</jats:italic> blooms. Although field experiments are required to assess whether allelopathy plays a fundamental role in bloom dynamics, our results indicate that allelopathy occurs widely but with species‐specific consequences.</jats:p>