Temperature effects of interspecies competition between cladoceran species in experimental conditions

In experiments with mono- and polycultures the hypothesis that small-size cladoceran species (Diaphanosoma brachyurum Lievin and Ceriodaphnia reticulata Jurine) are more successful at higher temperatures whereas large-size species (Daphnia magna Straus and Simocephalus vetulus O.F. Müller) under low temperatures was tested. The biomass of Simocephalus vetulus exceeded the biomass of the other species both in mono- and polycultures. An increase of temperature from 18 up to 25°C led to reduction of the biomass of all species. The largest species, D. magna, responded to higher temperature by increased mortality. We assume that the reduction of the biomass of the large species allowed the small C. reticulata to develop successfully in mixed cultures at higher temperature; however it was completely eliminated at lower temperature. Apparently, the survival strategy of large cladocerans involves delayed juvenile development at lower temperatures. However, at higher temperatures, juvenile development of large species accelerates, and their mortality increases. Individual strategies of the studied species to survive at different temperatures are analyzed. On the whole the increase of temperature can result in decrease of the biomass of cladocerans and prevailence of small-size species over large-size ones.