Limnetica 33

The effects of temperature on cladoceran community structure are not well understood. We assessed the dynamics of cladoceran communities in three meso-eutrophic Polish lakes within the same region but with different thermal regimes. In an artificially (power plant) heated lake, Daphnia were absent during the warmest period in July. In the two other lakes, one of which was secondarily heated and one that was non-heated, Daphnia accounted for 19-36 % of the total biomass of the cladoceran communities. Species richness was also higher in the non-heated and secondarily heated lakes than in the heated lake. Phytoplankton analysis suggested that cyanobacteria did not suppress Daphnia, as the highest relative abundance of Daphnia was observed in the non-heated lake, which also presented the highest relative abundance of cyanobacteria. Additionally, there was no indication that food depletion was greater in the heated lake than in the other two lakes. A contribution analysis to estimate bottom-up and top-down effects showed that predation was more important in controlling cladoceran birth rates in the non-heated lake compared to the other two lakes. Daphnia abundance declined with heating, while the abundance of smaller-bodied species, including Ceriodaphnia and Diaphanosoma, increased. We suggest that increased mortality due to the increased temperature in the heated lake may have been the cause of Daphnia suppression compared to the other two study lakes.