We investigated the allelopathic effects of Skeletonema spp. on growth and interspecific competition of the co–occurring flagellates Akashiwo sanguinea, Chattonella spp., and Heterosigma akashiwo, by reviewing field data on fluctuations in their cell densities and through laboratory experiments. From 29 June to 3 September, 2007, three dense blooms were observed in the inner part of the Ariake Sea, Japan. In all the 4 stations, Skeletonema spp. were generally detected and contributed to blooms from 20 to 23 July and from 6 to 13 August. Chattonella spp. gradually grew from mid–July and formed mixed blooms with Skeletonema spp. from 6 to 13 August, and then caused an almost mono–specific bloom in the station 2 and 3 from 20 to 24 August. However, H. akashiwo and A. sanguinea generally maintained low cell densities. Laboratory experiments showed that when grown in re–enriched culture filtrates of Skeletonema sp. (NIES–324) and cultured with Skeletonema sp. under non–contact conditions, A. sanguinea suffered the highest growth inhibitory effect, followed by H. akashiwo, while C. antiqua was the most resistant species. In tri–algal culture of these flagellates, the allelopathic effects of Skeletonema sp. increased the proportion of C. antiqua cells and decreased those of H. akashiwo and A. sanguinea, perhaps giving C. antiqua a competitive advantage over the other 2 flagellates. Our results suggest that allelopathy of Skeletonema spp. has potential to influence interspecific competition of these flagellates owing to species–specific growth inhibitory levels, and sometimes may be propitious to bloom formation of Chattonella spp. in the field
