Daphnia's challenge: survival and reproduction when calcium and food are limiting

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Calcium levels have declined in boreal lakes in North America and Europe due to soil mineral leaching, logging and climate change. Crustacean zooplankton species with high-calcium demand such as Daphnia, are particularly vulnerable to calcium-related stress. In a factorial design, we tested the effects of three calcium concentrations (2.5, 1.0 and 0.5 mg/L Ca) and two food levels (high = 1.67 mg/L C and low = 0.16 mg/L C) on second-generation calcium-stressed Daphnia pulex × pulicaria. Calcium limitation affected reproduction, molting and population growth, but food quantity was also relevant to how Daphnia dealt with the lack of calcium. When adequate levels of calcium were available (2.5 mg/L Ca), population growth was similar at high and low food, however, individual Daphnia produced fewer neonates at low food. Under high food and low calcium, Daphnia's life-history strategy focused on reproduction, with a negative effect on survivorship due to calcium limitation. Alternatively, under low food and low calcium, their strategy was survival and somatic maintenance, minimizing reproduction. Boreal lakes with modest levels of calcium may support Daphnia populations during periods with sufficient food, but if food quantity or quality is too low to mitigate the effects of calcium limitation, Daphnia populations could disappear due to low reproductive output.

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