Growth and grazing of microzooplankton in response to the harmful alga Heterosigma akashiwo in prey mixtures

Abstract

Microzooplankton grazers may play a role in regulating blooms of the ichthyotoxic alga Heterosigma akashiwo. This study tested the effects of H. akashiwo, when part of a mixed-prey assemblage, on the growth and feeding of microzooplankton. Laboratory cultures of 3 ciliates, Favella sp., Metacylis sp., and Strombidinopsis acuminatum, were exposed to reciprocal concentra- tions of H. akashiwo and a non-toxic prey, at saturating prey concentrations. Heterosigma akashiwo was toxic to Favella sp. and Metacylis sp. when H. akashiwo was the sole prey species; however, this toxicity was eliminated in the mixed-prey treatments, likely because of avoidance of H. akashiwo and selective feeding on non-toxic prey. In contrast, the growth rate of S. acuminatum was unaffected by H. akashiwo. Both Favella sp. and S. acuminatum ingested H. akashiwo but selected against the alga when other prey was available. In addition, natural planktonic communities, collected from East Sound, Orcas Island, northern Puget Sound, in September and October 2007, were exposed to bloom- level concentrations of H. akashiwo. Abundances of the major microzooplankton types, primarily cil- iates and Gyrodinium/Gymnodinium dinoflagellates, were unaffected by H. akashiwo, although slight changes in grazer size structure did occur. Heterosigma akashiwo was harmful to the smallest grazers, mainly aloricate ciliates and small Gyrodinium/Gymnodinium dinoflagellates, and beneficial to larger Gyrodinium/Gymnodinium dinoflagellates that were able to ingest and grow on the alga. The alga was not consumed by the majority of grazers. Preferential feeding on alternate prey reduces toxic effects of H. akashiwo on microzooplankton. Avoidance of H. akashiwo by a major group of grazers would promote bloom formation by reducing H. akashiwo mortality and focusing community grazing pressure on potential competitor species.