Microzooplankton grazing on harmful dinoflagellate blooms: Are ciliates or heterotrophic dinoflagellates important?

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Microzooplankton grazing can be important in regulating growth of dinoflagellate populations, including species responsible for harmful algal blooms. In the Chesapeake Bay region, microzooplankton community grazing coefficients on small cell‐size dinoflagellates are often greater than potential gross growth coefficients of dinoflagellates, and thus grazing may prevent bloom formation. Who are the major microzooplankton grazers on small dinoflagellates? Ciliates or other dinoflagellates? Data from Chesapeake Bay and its tributaries indicate both; sometimes ciliates and sometimes dinoflagellates are the major grazers. The importance of ciliates and heterotrophic dinoflagellates varies with season and location, but often one group dominates the microzooplankton assemblage. Specific clearance and division rates of ciliates are higher than that of heterotrophic dinoflagellates, thus it could be expected that ciliates would be the dominant microzooplankton grazers. However, during summer, small heterotrophic dinoflagellates are often the dominant grazers on small dinoflagelllates in the mesohaline Bay. Differential predation by copepods on ciliates may be responsible for this pattern. When microzooplankton community grazing is less than dinoflagellate cell division, red tides may result. Thus, it is important to understand the factors controlling both ciliate and heterotrophic dinoflagellate populations and their grazing impacts.