AME 39:121-134 (2005)  -  doi:10.3354/ame039121

ABSTRACT: Heterosigma akashiwo is a fish-killing raphidophyte capable of forming dense blooms. Since microzooplankton consume a large fraction of coastal phytoplankton production and readily ingest cells in the H. akashiwo size range, the formation of such blooms indicates that grazing has been inhibited. Growth rates of 7 cultured protist grazer species were measured in the presence of 2 H. akashiwo strains. Both strains were toxic to the 3 largest ciliates tested; all experienced higher mortality in the presence of H. akashiwo than when starved. Addition of Rhodomonas sp., a high-quality prey species for many protists, sometimes ameliorated H. akashiwo toxicity but in other instances did not. Two smaller ciliate species and 2 heterotrophic dinoflagellates were either unaffected by H. akashiwo or were able to survive and grow on it. For grazers susceptible to the toxicity, higher H. akashiwo cell concentrations and older H. akashiwo cultures were both associated with higher mortality. No toxicity was associated with filtrate from H. akashiwo culture or from the presence of H. akashiwo cells that were not ingested, so the toxic effect of this raphidophyte to large ciliates seems to depend on ingestion of the cells. Mortality of large ciliates, often a major component of coastal microzooplankton communities, is likely an important element of H. akashiwo’s capability to form persistent blooms.

KEY WORDS: Microzooplankton · Growth · Ingestion · Toxicity · Raphidophyte