Heterotrophic dinoilagellates: a significant component of microzooplankton biomass and major grazers of diatoms in the sea

This review summarizes evidence that heterotrophic (non-pigmented, phagotrophic) dinoflagellates are often a significant component of the biomass of phagotrophic protists in the microplankton size class (20 to 200 mu m), and that heterotrophic dinoflagellates are likely to be more quantitatively significant consumers of bloom-forming diatoms than copepods and other mesozooplankton. Although it has been assumed that the microzooplankton community is usually dominated by ciliates, non-pigmented dinoflagellates, including both thecate (armored forms, e.g. Protoperidinium spp.) and athecate gymnodinoid species, frequently compose > 50% of microzooplankton biomass, and often occur at high abundance during diatom blooms. Since phagotrophic dinoflagellates appear able to grow on diverse prey taxa and to persist at low food abundance, these protists may be able to survive during non-bloom conditions and then grow up when phytoplankton blooms occur. Heterotrophic dinoflagellates are also likely to be an important food resource for mesozooplankton. Due to fundamental differences in growth and grazing rates between phagotrophic dinoflagellates and copepods, it is vital for accurate modeling of planktonic food webs to account for grazing by both of these groups of herbivores during diatom blooms.