Field assessments and evidence of impact of calcium decline on Daphnia (Crustacea, Anomopoda) in Canadian shield lakes

Many Ca-demanding freshwater crustaceans are likely experiencing chronic metabolic stress, because of widespread recent decline in aqueous calcium (Ca) in freshwater ecosystems. A survival threshold of 0.5 mg of Ca/L has been identified for well-fed Daphnia pulex in the laboratory. However, I documented reduced occurrence of five daphniid species at 1.3-1.7 mg of Ca/L, and increased sensitivity to Ca in unstratified waterbodies across 304 Ontario lakes. Discrepancies between laboratory-defined and field-defined survival thresholds were explained by in situ exposures of D. pulex to natural food (NF) levels and high food (HF) levels in four lakes across a Ca gradient from 1.1-2.3 mg/L. I identified a survival threshold of 1.3 mg/L in natural waters, and observed increased survival in HF treatments. I hypothesize that daphniid energetic expenditure associated with low Ca is inflated in high temperature environments of unstratified lakes, and may be masked by HF quality and quantity.