The memorable message from both of these elaborate dilution studies is that
microzooplankton grazing is a major fraction of phytoplankton growth, especially in
oligotrophic but even in nutrient-rich, substantially eutrophic pelagic ecosystems.
Similar studies worldwide have similar results: protists primarily, plus some
metazoans <200 μm, usually consume half or more of primary production. Relatively
greater consumption by macrozooplankton than microzooplankton sometimes occurs
in eutrophic ecosystems, because large phytoplankton are often a dominant part of
autotrophy there.
The Importance of Protist Grazers on Larger
Phytoplankton
(^) Sherr and Sherr (2007) have provided a literature review (including some new
observations) establishing that microplanktonic (20 to 200 μm), heterotrophic and
mixotrophic dinoflagellates, most prominently Gymnodinium and related forms, are
significant and at times the dominant grazers of microplanktonic phytoplankton,
particularly including diatoms. They credit Evelyn Lessard (1991) for early
recognition of that. Some of that feeding is accomplished using peduncles (Fig. 2.14)
inserted through the cell membranes of prey. However, other dinoflagellates can
ingest remarkably large cells, even diatom chains, using a pallium (Fig. 2.13) or
simply by stretching around them to greater dimensions than their unfed size (Fig.
9.8). Heterotrophic dinoflagellates (Hdinos) increase in parallel with diatom blooms
(Fig. 9.9), although their growth does not accelerate until diatom increase progresses
to provide enough food. Thus, diatom stocks do, under quite consistent conditions,