Water outflow as a cause of changes in trophic conditions for zooplanktivorous fish in reservoirs

Abstract

The seasonal changes and vertical distribution of zooplankton, the export of zooplankters by outflow through water management installations, and the feeding of zooplanktivorous fish were studied in two Bulgarian reservoirs. In a bottom‐draining reservoir, planktonic crustacea > 1.0 mm were found to be selectively exported as a result of their diurnal vertical migrations. Increases in water outflow for irrigation resulted in a decrease in total zooplankton abundance, as well as changes in zooplankton size and species composition at the beginning of the summer. At the same time, major changes occurred in the feeding patterns of bleak, Alburnus alburnus (L.), and juvenile pikeperch, Stizostedion lucioperca (L.), the principal zooplanktivores in this reservoir. It is easy for bleak to switch to feeding on an alternative food, but a lack of available large prey organisms is suggested as the main cause of the downstream movement of pikeperch juveniles. In a surface‐draining reservoir, the pattern of zooplankton export corresponded to those in natural lakes. Discharged water carried with it mainly small‐sized epilimnetic zooplankton from the most numerous groups (i.e. rotifers, juvenile copepods and cladocerans) in the reservoir. Furthermore, outflow exports represent a relatively small part of total zooplankton number, and thus, do not disturb the natural zooplankton succession, and changes in abundance and composition Coincidental changes in the trophic responses for zooplanktivores (mainly juvenile cyprinids and percids) were not found.