A Cautionary Note: Ceriodaphnia dubia Inter-Laboratory Test Variability

An inter-laboratory comparison of the three-brood survival and reproduction Ceriodaphnia dubia toxicity test was conducted involving three toxicity testing laboratories. This comparison was initiated due to sporadic toxicity with this test related to discharge of a mine effluent, which some regulators believed indicated adverse effects in the receiving environment. One mine effluent and two receiving water samples were evaluated. There were no adverse effects on C. dubia survival in any tests. However, sublethal effects on reproduction (i.e., the IC25; the concentration that causes a 25 % inhibitory effect in the measured sublethal endpoint) ranged from 35 % to >100 % among all test treatments. All laboratories were certified for this test and followed established procedures. Dilute mineral water appears to be the best medium for culturing, dilution, and control water. Variability in the C. dubia laboratory toxicity test should not be mistaken for adverse effects in receiving environments.