TROPHIC RELATIONS BETWEEN CYCLOPOID COPEPODS AND CILIATED PROTISTS - COMPLEX INTERACTIONS LINK THE MICROBIAL AND CLASSIC FOOD WEBS

Two field experiments examined the effects of cyclopoid copepods on ci liates. The presence or absence of Cyclops abyssorum, Cyclops kolensis , and zooplankton >64 mu m was manipulated to determine the relative i mportance of direct cyclopoid predation on protists vs. indirect effec ts mediated through cyclopoid predation on other metazooplankton. In t he second experiment, presence or absence of C. abyssorum was cross-cl assified with five concentrations of the metazooplankton community. Cy clopoid effects on ciliates were dependent on predator and prey specie s and on the abundance of alternate prey for cyclopoids. A trophic cas cade was also observed, but only for two small ciliates, and only with the larger C. abyssorum. C. abyssorum had a stronger predation effect on oligotrich ciliates when metazooplankton had been removed, and thi s effect appeared at a lower metazooplankton concentration with a larg er ciliate, compared to a smaller species of the same genus. These res ults suggest that for cyclopoid-ciliate interactions, switching behavi or in the predator may be at least as important as a trophic cascade.