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Date Permissions Signed


Date of Award


Document Type

Masters Thesis

Degree Name

Master of Science (MS)


Environmental Sciences

First Advisor

Bingham, Brian L., 1960-

Second Advisor

Strom, Suzanne L., 1959-

Third Advisor

Muller-Parker, Gisele


Heterosigma akashiwo is one of the most ichthyotoxic species of phytoplankton, severely impacting marine ecosystems and economies worldwide. Microzooplankton may play a role in regulating blooms of this alga. This study tested the effects of H. akashiwo, when part of a mixed-prey assemblage, on the growth and feeding of microzooplankton. A saturating prey concentration of 200 μg C l-1 was determined for three ciliate species: Favella sp., Strombidinopsis acuminatum, and Metacylis sp. This was used as the total prey concentration for dual-prey experiments in which the three ciliate species were exposed to reciprocal concentrations of H. akashiwo and a beneficial prey species, as well as a starved control. The beneficial prey, defined as prey producing a relatively high growth rate, were Heterocapsa triquetra for Favella sp. and S. acuminatum and Isochrysis galbana for Metacylis sp. Toxicity was defined as grazer growth below that of the starved control. Favella sp. and Metacylis sp. exhibited a toxic response to H. akashiwo when it was the sole prey species; however, the presence of beneficial prey reduced this toxicity in the mixed-prey treatments. 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 subsurface seawater from East Sound, Orcas Island in September and October, 2007, were exposed to bloom-level concentrations of H. akashiwo. Ingestion of H. akashiwo was observed by epifluorescence microscopy and abundance and biomass of the major microzooplankton types were measured. Overall structure of the natural planktonic communities was unaffected by H. akashiwo, although slight changes in grazer size structure did occur. Bloom-concentrations of H. akashiwo were harmful to the smallest grazers and beneficial to larger Gyrodinium/Gymnodinium dinoflagellates that were able to ingest and grow on the alga. An aloricate ciliate and a round dinoflagellate also measurably ingested H. akashiwo; however, the alga was not consumed by the majority of grazers. Mixed-prey assemblages offer alternative feeding opportunities to grazers and can reduce the toxicity of H. akashiwo that is observed in unialgal exposures.





Western Washington University

OCLC Number


Subject – LCSH

Marine zooplankton--Food--Washington (State)--East Sound (Bay); Marine zooplankton--Washington (State)--East Sound (Bay)--Growth; Algae--Control--Washington (State)--East Sound (Bay); Algal blooms--Washington (State)--East Sound (Bay)

Geographic Coverage

East Sound (Wash. : Bay)




masters theses




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