Individual and combined effects of humic acid on life-history characteristics of the water flea Moina macrocopa upon whole-lifespan cadmium exposure

Investigation of the effects of natural compounds in aquatic ecosystems on the modulation of toxicity of coexisting xenobiotics is important for realistic ecological risk assessments. We investigated the individual and interactive effects of sublethal cadmium (Cd) concentrations and environmentally realistic humic acid (HA) concentrations on several life-history characteristics of Moina macrocopa. Female individuals were exposed to Cd and HA for their entire lifespan from the neonatal stage. The survival and reproductive output were recorded daily, and the growth and swimming velocity were determined upon maturity. Cd at the tested concentrations in single treatments did not significantly affect the growth and lifespan. But 5 µg/l Cd significantly decreased the swimming velocities and reproduction. Greater than 20 mg/l HA in single treatments significantly increased the individuals’ growth, reproduction, and lifespan. Swimming velocities were significantly increased at 10 mg/l HA in single treatments. In combined treatments, such beneficial effects on all life-history characteristics were still seen under the co-occurrence of 1 µg/l Cd, but with 5 µg/l Cd, those beneficial effects on life-history characteristics except for swimming velocities were not observed. These dissimilar responses of different life-history characteristics indicate energy tradeoffs for maintenance, reproduction, and longevity, upon exposure to stressors in M. macrocopa.