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 Karenia brevis (ex Gymnodinium breve) and 12 cooccurring phytoplankters could be explained by allelopathic effects of compounds released by K. brevis and (2) whether waterborne, lipophilic molecules, including brevetoxins, are involved. Nine of 12 phytoplankton species were suppressed when grown with live K. brevis at bloom concentrations. K. brevis 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 K. brevis. Brevetoxins at ecologically reasonable waterborne concentrations accounted for the modest inhibition by K. brevis of only one competitor, Skeletonema costatum. The addition of brevetoxins also caused significant autoinhibition, reducing the maximum concentration of K. brevis. Allelopathy is one mechanism by which K. brevis appears to exhibit competitive advantage over some sympatric phytoplankters, although unidentified compounds other than brevetoxins must be involved, in most cases. K. brevis was also susceptible to competitive exclusion by several species, including Odontella aurita and Prorocentrum minimum, known to thrive during K. brevis 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.Competition is one of the dominant forces structuring communities, including marine pelagic communities (Hutchinson 1961). The production and release of compounds that inhibit competitors, a process known as allelopathy, is a mechanism of interference competition that is hypothesized to be important among phytoplankton (Smayda 1997), affecting species succession (Keating 1977), especially under eutrophic conditions (Maestrini and Bonin 1981). Allelopathy may be a successful strategy for phytoplankton species that occur in dense blooms, maximizing the concentration of allelopathic compound(s) exposed to competitors and mini-1 Corresponding author (julia.kubanek@biology.gatech.edu).
AcknowledgmentsThis research was supported by NSF grant OCE-0134843 to J.K. M.K.H. was supported as a summer undergraduate research assistant by the Camille and Henry Dreyfus Foundation Environmental Chemistry Program. Nutrient analyses were funded by NOAA MERHAB grant MER 02-627A to T.A.V., and ELISA analyses were funded by COP-NOAA MERHAB, NIEHS PO1 ES10594-03, and the CDC-FDOH U50/CCU423360-01 to J.N. We thank P. Tester, R. Pierce, M. Hay, and E. Litchman for advice in planning experiments and two anonymous reviewers for suggestions that improved the manuscript. We also thank E. Prince, A. Prusak, E. John, D. Collins, and A....