Zooplankton of a small eutrophic lake: impact of a biomanipulation attempt.

In a 3-year experiment carried out in the small eutrophic Plusssee (North Germany), we tested the hypothesis that large grazers as well as invertebrate predators are favoured by biomanipulation. The effect of piscivorous fish addition and removal of planktivores on zooplankton was estimated by ARIMA-modelling. The intervention analysis of zooplankton abundance data sets from a 13-year period revealed that invertebrate predators (Chaoborus and Leptodora) were more abundant during the biomanipulation period. However, large grazers of the genus Daphnia were not favoured. The small cladoceran Diaphanosoma occurred more frequently whereas the rotifer Asplanchna was reduced. Biomanipulation success was limited because most piscivores could not be successfully established. Biomanipulation to a certain extent reduced perch which fed on multiple prey but left roach as the most important planktivore on large grazers uncontrolled