A series of experiments examined the direct and indirect effects of metazooplankton and suspended clays on planktonic ciliate assemblages. The presence of suspended fine (particle size <1 mm) and coarse (particle size < 2 um) clay suppressed the reproductive rate of the ciliate Strobilidium gyrans by up to 69 % but had no effect on three other common ciliates.

In laboratory experiments with excess food, four of six ciliates were just as susceptible to the small cladoceran Bosmina longirostris as to the much larger Daphnia pulex. The jumping response of the ciliate S. gyrans appeared to be an effective defense against B. longirostris. Thus, cladocerans can have significant, negative, direct effects on ciliate species.

Natural microzooplankton communities were significantly suppressed by Daphnia in laboratory experiments. Most rotifer species were suppressed by D. pulex but not by the smaller daphniids. Small ciliates (< 30 um), were adversely affected by all of the cladoceran treatments, while several larger ciliates (> 81 um) were unaffected in all such treatments. In most cases, the suppression of ciliates and rotifers was attributable to direct effects of the cladocerans.

Bottle experiments were performed at intervals from early June to late November in two lakes to assess the effect of temporal changes in metazooplankton community structure on ciliate assemblages. Throughout the study period rotifers had strong positive indirect effects on larger (> 40 um) ciliates with escape responses and slight negative indirect effects on small ciliates when crustacean zooplankton were also present. The copepods had strong negative direct effects on all but the largest ciliates. Large cladocerans had substantial direct effects on ciliates, while smaller cladocerans sometimes had measurable indirect effects. These experiments show that the importance of top-down effects on ciliates varies with the changing structure of metazooplankton community.