Capacity of herbivorous protists to control initiation and development of mass phytoplankton blooms

Abstract

Microzooplankton, heterotrophic organisms 20 to 200 µm in size and mainly phagotrophic protists, are recognized as the dominant consumers of phytoplankton in all regions of the sea. During his long career, Fereidoun Rassoulzadegan has been a major contributor to the under- standing of the importance of protists in marine pelagic food webs. In the spirit of his research, we consider here one aspect of microzooplankton ecology that, in our opinion, has been misunderstood: the capacity of protist herbivores to control initiation and development of mass phytoplankton blooms, with a focus on diatom blooms. We argue that microzooplankton grazers probably cannot prevent bloom initiation and out-growth during mesotrophic to eutrophic conditions. We base our contention on the propositions that (1) mass diatom blooms do, in fact, routinely occur in the ocean at all latitudes; (2) since prey abundance is a main factor controlling protist growth, the grazing impact of herbivorous protists will be minor in the early stages of a bloom when prey abundances and, therefore, protist growth rates, are low; (3) due to food limitation during pre-bloom conditions, protist biomass will usually be low at the onset of a bloom, which serves to limit initial grazing impact; and (4) biomass of herbivorous protists is ultimately top-down controlled, thus curtailing the potential for microzooplankton to grow to sufficient abundance to graze down a developed bloom. We con- clude that herbivorous protists are not likely to be able to prevent the initiation and development of mass blooms when conditions are favorable for rapid phytoplankton growth.