Impact of Chaoborus flavicans -Predation on the Zooplankton in a Mesotrophic Lake – a Three Year Study

The predatory impact of Chaoborus flavicans on zooplankton, especially on daphniids, is often regarded as a crucial factor in the propagation or repeal of top-down effects in lakes. Therefore consumption rates and prey selectivity of C. flavicans were determined in mesotrophic Lake Groser Vatersee, where cladocerans were the dominant zooplankton prey and the predatory impact of fish towards C. flavicans and cladocerans was low. Two generations of the main predatory instar IV of C. flavicans appeared in the pelagial, the first in autumn and the second in spring after having over wintered in the sediment. In discrepancy to the abundances, only the autumn generation showed high peaks of consumption. C. flavicans fed preferentially on medium-sized cladocerans, but mostly disregarded daphniids and rotifers. Copepods were only selected when cladocerans were not present. The highest losses due to Chaoborus predation were found in Bosmina longirostris, Bosmina coregoni and Chydorus sphaericus. There was no evidence that the predatory impact of C. flavicans on daphniids substantially influenced lower trophic levels. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)