Zooplankton of a middle-sized dimictic valley reservoir:: Dubnik II

To help improve the water quality of a flood control reservoir (Dubnik II reservoir near Stara Tura, Slovakia) we studied its zooplankton assemblage. The reservoir has an area of 21.6 ha, a volume of about 1.1 million m(3), a maximum depth of 10.6 m and a 40-day retention period. The reservoir is also used for fish extensive culture and for recreation. It is dimictic and surface water temperatures reached 21-23 degreesC in summer. Secchi disc transparency was generally low (60-140 cm) during the study. In general, except for July 1997, higher pH, higher dissolved oxygen content and lower transparency was observed at the surface in summer, whereas lower pH, lower dissolved oxygen content and higher transparency were observed in autumn. Anoxia occurred near the bottom in September and October of both years. As for water quality, based on zooplankton structure, BOD5, and O-2 content the reservoir is of the better betamesosaprobic degree. Rotifers predominated in the zooplankton both in density and biomass, followed by copepods. The least represented group were cladocerans. Crustaceans were represented mostly by small-bodied species: Bosmina longirostris, Daphnia cucullata, Thermocyclops oithonoides and Eudiaptomus gracilis. The biomass (wet weight) and abundance of zooplankton were low: in 1996, 1.9 mg.L-1 and 1,111 ind.L-1; in 1997, 1.7 mg.L-1 and 1,168 ind.L-1. The high dominance of rotifers, developmental stages of copepods, the small average size of cladocerans, and the absence of the "clear-water phase" indicated that the density of zooplanktivorous fish stocks is to high and these fish populations need to be reduced in density and kept in check through introduction of predators, such as pike-perch (Sander lucioperca). This would increase the economic value of the fish stocks and promote a switch to clean-water state.