Combined effects of calcium deficiency, cyanobacteria, and warming on the population fitness of three cladocerans

Aqueous calcium decline is a novel and widespread threat to soft-water lakes and usually co-occurs with other stressors. A three-way full factorial design was used to investigate the combined effects of a chemical stressor (calcium decline from 20 to 5 mg L^-1), a biological stressor (50 % Microcystis aeruginosa in diets), and a physical stressor (temperature increase from 25 to 29 °C) on the population fitness of three common cladocerans with distinct body sizes. Both Ca deficiency and increasing cyanobacteria significantly depressed fitness in the large-bodied Daphnia pulex, while warming did not. The combined effect of Ca deficiency and increasing cyanobacteria was significantly synergistic in D. pulex with a greater fitness reduction in the presence of cyanobacteria. Ca deficiency significantly depressed Simocephalus vetulus fitness only at 29 °C. Ca deficiency and warming had a synergistic effect on S. vetulus fitness, but the combined effect of warming and cyanobacteria was antagonistic. The fitness of the small-bodied B. longirostris increased with temperature, but was not influenced by Ca deficiency, cyanobacteria, or the interactions of the three stressors. These results suggest that D. pulex was most vulnerable to ongoing Ca decline. The non-additive effects of multiple stressors should be considered when predicting zooplankton responses in soft-water lakes.