Cyst-based toxicity tests XIII--development of a short chronic sediment toxicity test with the ostracod crustacean Heterocypris incongruens: methodology and precision.

Experiments were carried out with neonates of the freshwater ostracod Heterocypris incongruens to verify and complete previous choices of test parameters for a new culture/maintenance-free solid-phase microbiotest for freshwater sediments. From trials with increasing volumes of reference sediment, it was concluded that 300 microL was the most appropriate amount of substrate to be put in 12-cup multiwell plates with 2 mL of standard freshwater. Tests in 3-9 replicates eventually showed that six parallels were needed to have good assay precision (repeatability). Application of the final test protocol to oil-contaminated sediments from the St. Lawrence River in Canada revealed that the 6-day chronic ostracod microbiotest had less variation in repeated tests than did the 10-day contact assay with Hyalella azteca and hence can be considered more precise. Based on the 95% confidence intervals for mortality and growth of the ostracods in the controls (reference sediment) of the 56 tests carried out for the Canadian project, a validity threshold of 20% for mortality was eventually selected, in analogy with the acceptability limits applied in many chronic bioassays. A minimum length of 600 microm in the control sediment after 6 days' exposure was also taken as the threshold for good health of the test organisms and for reliable test conditions. The new microbiotest has been tailored in a handy and user-friendly new toxkit, the Ostracodtoxkit, which is particularly suited for cost-effective routine monitoring.