Top-down impact by copepods on ciliate numbers and persistence depends on copepod and ciliate species composition

Abstract

Experiments with the copepods Acartia clausi, Centropages hamatus and Pseudocalanus sp. were performed to assess the species-specific effect of these copepods on the development of monospecific algae (Nephroselmis pyrifomis) and ciliate communities (Strombidium vestitum, Strombidium conicum, Strombidium sp. and Lohmanniella oviformis). It was hypothesized that potentially switching copepods like A. clausi will stabilize the algal community by switching between ciliate and algal food, in contrast to copepods with stereotypic filter feeding behaviour (Pseudocalanus sp.). In treatments with Pseudocalanus sp. and C. hamatus, all ciliate species were wiped out in 2 days, resulting in blooms of N. pyriformis. In treatments with A. clausi, two of the ciliate species were able to persist, but the combined ciliate and copepod community was not able to control the algal bloom. Ciliates became abundant in control treatments without copepods, but only S. vestitum and S. conicum seemed able to establish grazing control. Hence, when evaluating the role ofciliates in food webs, their actual numbers and species composition should be taken into account. Likewise, the species composition of copepods may be crucial; these experiments demonstrate that small filter feeding copepods may have tremendous impact on ciliate numbers.