Monitoring the response of microcontaminants by dynamic Daphnia magna and Leuciscus idus assays in the Rhine Delta: biological early warning as a useful supplement.

Following some severe upstream calamities in the eighties, biological early warning systems with fish and waterfleas were installed at monitoring stations in the Rhine and the Meuse. Laboratory tests suggest that these whole-organism monitors are likely to respond at concentrations close to lethal levels. Current average and peak concentrations of compounds that can be identified and of which information on toxicity is available are often below these levels. Nevertheless, several alerts were registered in recent years. This may be attributed to the combined effect of known and unknown compounds under prevailing field conditions. Results are compared to experiences at other locations and some prospects on development in the future are given.