Changes in species composition of cladoceran community in Lake Kasumigaura during 1986-1989: Occurrence of Daphnia galeata and its effect on algal biomass.

Abstract

Seasonal changes of cladoceran community in Takahamairi Bay and the center region of Lake Kasumigaura were investigated from 1986 to 1989. The species composition of the cladoceran community was stable in summer, when Diaphanosoma brachyurum and Bosmina fatalis predominated. In contrast, during late fall-spring, it differed greatly among the years. This might be related to the predation by mysids, whose density also differed from year to year. The most marked change in species composition was the occurrence of Daphnia galeata, which had never been found in the lake before 1986. The grazing by D. galeata seemed to greatly reduce the phytoplan-kton biomass in Lake Kasumigaura, since the chlorophyll a concentration declined to very low levels when the daphnids became abundant. The birth rates of cladocerans were highest in summer. They tended to be higher in the shallower region than in the deeper one, indicating that the former was more productive for zooplankton.