Population dynamics of Daphnia longispina (O.F. Müller, 1785) and Diaphanosoma brachyurum (Lievin, 1848) (Crustacea, Cladocera) under stable and graded temperature regimes

Laboratory experiments have been performed to study the effects of different temperature regimes-constant (16.3 +/- 0.8, 20.3 +/- 0.8, and 24.6 +/- 0.7 degrees C) and graded, with temperatures changing at intervals of 4.4 or 8.8 degrees C within the range of 15.1-25.4 degrees C--on the population dynamics of two dominant species of littoral zooplankton, Daphnia longispina and Diaphanosoma brachyurum. The results show that aperiodic stepwise changes in temperature within the ranges of 16.3-20.4, 20.4-25.1, and 16.3-25.1 degrees C, which may take place in water bodies of the temperate zone in the summer period, can exert either stimulating (direct or delayed) or inhibitory effect on the natural development of D. longispina populations. The development of D. brachyurum, a more stenothermic thermophile, is stimulated only by a stable elevated temperature (24.5 +/- 0.9 degrees C). All temperature changes within the above range, except for rapid heating from 16.3 to 25.1 degrees C, either inhibit the development of D. brachyurum populations or have no significant effect on them.