In lake ecosystems the importance of top-down relative to bottom-up control might depend on the trophic state. Studies mainly focused on eutrophic or oligotrophic lakes. This study aimed at quantifying fish predation on zooplankton (with a focus on Daphnia spp.) in a small (12 ha) mesotrophic lake, as part of a comprehensive project on the relative importance of top-down control from predatory fish to bacteria. Four aspects were investigated: Habitat use by fish was assessed on a diel and seasonal scale, diet analysis were conducted, daily rations estimated by bioenergetics modelling as well as by direct estimates and population sizes assessed by multiple-mark-recapture experiments (age-classes > 1) and from bongo-net catches (age-class 0). Altogether, this allowed for the calculation of zooplankton consumption by fish.

Roach (Rutilus rutilus) of age-1 and older and age-0 perch (Perca fluviatilis) were the principal planktivores in Lake Grosser Vaetersee. Regular diel horizontal migrations of roach, and the habitat shift of age-0 perch from the pelagic to the littoral zone in summer strongly influenced the composition of the fishes’ diet, and were therefore key factors in understanding fish predation on pelagic zooplankton. Age-0 perch consumed large cladocerans during their initial pelagic residence. Roach stayed in the littoral zone during daytime and concentrated on littoral prey. At night they were found in the pelagic zone, and pelagic zooplankton dominated their food. Bioenergetics modelling in combination with an evacuation rate method for estimating daily rations allowed to take these changes of the roach’s feeding modes into account, and so to determine food item specific daily rations over the season. With the evacuation rate method applied on selected days, diel changes in diet compositions and feeding intensities could be quantitatively accounted for. When the 24 h integrated diet proportions were then used as an input parameter for bioenergetics modelling, food item specific consumption could be determined over the entire sampling season. The consideration of the diel diet shifts proved to be essential for the model output. If only the daytime or the nighttime diet composition (derived from one single daily sampling) was taken into account for bioenergetics modelling, severe under- or overestimations of daily rations for specific food items resulted. Daily rations of age-0 perch were determined by bioenergetics modelling alone, as their diel diet shifts were less pronounced. During their pelagic life span age-0 perch consumed similar amounts of daphnids per day as the roach stock, although their overall biomass was distinctly lower.