Can Daphnia behavioral endpoints be used as tool for ecotoxicological assessment of wastewater effluents ?

Abstract : Toxic stress can induce rapid behavioural changes in exposed organisms at concentration below acutely toxic levels. Analysis of behavioural parameters has therefore been proposed as tool for detection of potential toxicity in wastewater effluents. However, it is currently unclear how to derive robust ecotoxicological endpoints from observation of organism behaviour and how to quantify the sensitivity of these endpoints. The aim of the study is (i) to assess how behavioural responses can be used as ecotoxicological endpoints and (ii) to compare the sensitivity of standard versus behaviour endpoints in order to assess their usefulness for ecotoxicological assessment of wastewater effluents. As daphnid behavior shows a natural variability, different exposure concentrations and replicate testing is required in order to allow an appropriate statistical analysis of the behaviour parameters. However, commercially available image analysis systems have a limited test capacity (small number of measuring cells). A new image analysis system has been designed allowing the simultaneous tracking of up to 200 Daphnia magna distributed in 20 exposure chambers (10 daphnids per chamber). Twelve substances covering a wide range of different modes of action were tested at different concentrations with replicates. Two behavioural parameters (swimming speed and number of active organisms) were continuously recorded during 48 hours. Time courses of the two parameters were compared to control responses. Overall, significant behaviour responses are observed in the first hours of the experiment well below the acute EC50 (based on immobility at 48h) for most tested compounds. The behaviour endpoint “swimming speed” shows higher sensitivity than the number of active organisms. Differences between tested compounds are observed in the time of effect onset, duration and intensity of effects on swimming speed depending of their mode of action. Narcotics shows an intense increase peak of the swimming speed from the first hour of the experiment followed by a gradual decreased while neurotoxic chemicals induced a slightly but significant increase at different times depending on their mode of action. The exposition in the new image analysis system provides early and sensitive information compared to standard test endpoints and may be a great contribution to wastewater and effluent quality assessment.