Intra-Specific Variation in Sensitivity to Low Calcium in Daphnia Pulex
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Freshwater systems experience many stressors that can affect aquatic communities; one such stressor is calcium decline. Cation depletion of surrounding soils and watersheds of many North American and Scandinavian lakes is in part due to the legacy effect of logging and acid deposition. An important freshwater grazer, Daphnia, is especially sensitive to calcium decline as it has a heavily calcified chitin carapace that is frequently moulted, resulting in the need to uptake large amounts of calcium from the surrounding water. Studies have documented variation among populations of Daphnia spp. in response to predators, food concentrations, and cyanobacteria, however only one study has assessed intra-specific variation in Daphnia spp. to calcium decline. We predicted that calcium sensitivity of individual Daphnia iso-female lines would depend on the lake calcium concentration of their lake of origin, if the historical exposure to low calcium allowed for a fitness advantage (higher reproduction and survival). Alternatively, variation in calcium tolerance among iso-female lines may exist independent of historical exposure. We tested the calcium sensitivity of 10 Daphnia pulex sensu lato iso-female lines in a life-history experiment under low food and soft water conditions that are associated with the Muskoka, Canada region. Daphnia were exposed to one of five calcium concentrations (0.7, 1.0, 1.5, 2.5, 7mg Ca L-1) for a 21-day period and monitored daily for survival and reproduction and one size measurement was taken at day six. We found that the calcium sensitivity of Daphnia varied among iso-female lines for three responses; total reproduction, size at day six, and population growth rate, but this variation was not explained by source-lake calcium. Overall, our results highlight the existence of intra-specific variation in Daphnia sensitivity to calcium decline, and the need for future experiments to include multiple genotypes to fully understand the impacts this stressor has on Daphnia communities.