Differences in acute toxicity test results of three reference toxicants on Daphnia at two temperatures

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Abstract
Ten 48‐h static acute toxicity tests were conducted with Daphnia magna Straus and Daphnia pulex Leydig with each of the three reference toxicants — sodium dodecyl sulfate (SDS), sodium pentachlorophenate (NaPCP), and cadmium (Cd) (as CdCl2) — at 20 and 26°C to investigate the effect of temperature on test results at the temperature extremes usually encountered during field studies. The mean 48‐h LC50 value for SDS with Daphnia pulex at 26°C was lower (10.2 mg/L) than at 20°C (12.6 mg/L), but the difference was not significant (α = 0.05). For Daphnia magna the mean LC50 was significantly (α = 0.05) lower at 26°C (10.8 mg/L) than at 20°C (13.5 mg/L). Sensitivity of either species to NaPCP was not significantly different (α = 0.05) at the two temperatures. The data show a fourfold increase in sensitivity to Cd at the higher temperature with Daphnia magna (mean LC50 = 0.038 mg/L at 20°C and 0.009 mg/L at 26°C) and a sevenfold increase with Daphnia pulex (mean LC50 = 0.042 mg/L at 20°C and 0.006 mg/L at 26°C). These data indicate that toxicity tests conducted at 20 and 26°C may give significantly different results with Daphnia. Because test precision, control survival, and the likelihood of obtaining valid tests are better at 20°C, that temperature is recommended for acute toxicity tests with Daphnia.