Planktivory by the predacious cladoceran Bythotrephes longimanus: effects on zooplankton size structure and abundance

In order to study density-dependent effects of invertebrate planktivory, fo ur different densities of Bythotrephes longimanus were inoculated into meso cosm enclosures with a mixed zooplankton community. Changes in size structu re and abundance of zooplankton and phytoplankton communities were recorded over a period of 3 weeks. High densities of Bythotrephes were able to redu ce total zooplankton abundance, which was mainly due to a decrease in the d ensity of the relatively large species Holopedium gibberum. The density of the smaller species Bosmina longirostris was also reduced with increasing d ensities of Bythotrephes, whereas rotifer abundance remained largely unaffe cted. The mean size of Holopedium increased with increasing densities of By thotrephes. Despite the decrease in total zooplankton biomass in high-Bytho trephes treatments, no effect of Bythotrephes density on primary producers was observed. Our experiment shows that predacious cladocerans may reduce m acrozooplankton biomass, large as well as small species. Predation from inv ertebrate planktivores results in a zooplankton community consisting of lar ger individuals. Comparing our experimental densities with densities of Byt hotrephes found in natural systems suggests that invertebrate planktivores may influence size structure and abundance of zooplankton communities even in lakes with planktivorous fish.