Interactions between piscivores, zooplanktivores and zooplankton in submerged macrophytes: Preliminary observations from enclosure and pond experiments

Abstract

The effects of piscivores upon zooplanktivore behaviour and distribution and the impact of zooplanktivores on the abundance and distribution of zooplankton are well documented. However, the potential indirect effect of piscivores reducing the predation pressure upon grazing zooplankton through behavioral changes of zooplanktivores has received little attention, even though this may be an important mechanism in enhancing the stability of submerged macrophytes in shallow lakes. Preliminary observations from an unreplicated large-scale field enclosure experiment and a replicated pond experiment suggest that this mechanism is plausible with the set of piscivores (pike Esox lucius and perch Perca fluviatilis) and the zooplanktivores (O+roach Rutilus rutilus and perch) common in temperate Europe. The presence of piscivores typically changed the habitat use and the activity level of zooplanktivores and the presence of zooplanktivores typically changed the habitat selection of cladoceran zooplankton. In the case of piscivore/zooplanktivore interactions, the risk of predation was enough to generate clear responses even where the losses to predation were low. However, only in the enclosure experiment was an indirect impact of the presence of piscivores, enabling Daphnia spp. to utilise open water in the presence of a high density of zooplanktivorous fish observed. Whether the magnitude and direction of the effect of piscivores is sufficient to benefit zooplankton may depend on the functional group (capable of foraging within structured habitats) of the predator (both piscivore and zooplanktivore), absolute and relative densities of predator and prey and predator dietary choice.