Is the difference in population dynamics of Daphnia galeata in littoral and pelagic areas of a long-term biomanipulated reservoir affected by age-0 fish predation?

Abstract

We analysed population dynamics of Daphnia galeata in littoral and pelagic areas of the long-term biomanipulated Bautzen reservoir during spring and early summer of two consecutive years. Simultaneously, consumption of the age-0 fish community was estimated by means of a bioenergetics model. As a consequence of biomanipulation, the abundance of adult planktivorous fish in Bautzen reservoir is low and the interactions between underyearling percids and Daphnia are a key process in the food web of this water. In both years, density and mean size of D. galeata were markedly lower in littoral than in pelagic areas, whereas no differences were found in the size at maturity and clutch size. However, the lower proportion of adults in the littoral zone resulted in drastically reduced birth rates, which could not explain the strong fluctuations in Daphnia abundance. These findings were consistent during the whole investigation period. Daphnia consumption by age-0 fish was markedly higher in the littoral than in the pelagic zone and accounted for up to 50% and even 100% of Daphnia mortality in this habitat. However, due to gape-size limitation, age-0 fish did not feed on daphnids before June. We conclude that besides underyearling percids older planktivorous fish contribute to the suppression of D. galeata in the littoral zone of Bautzen reservoir and that recruitment of daphnids in this habitat mainly takes place via (active or passive) immigration.