Ten years of practical experience with the Dreissena-Monitor, a biological early warning system for continuous water quality monitoring

The Dreissena-Monitor is a biological early warning system for the aquatic environment. It is based on the valve movements of up to 2 x 42 specimens of the zebra mussel Dreissena polymorpha, exposed in a flow-through system of 2 experimental channels connected to the water to be tested. A computer records whether a mussel is open or closed. After an analysis period of 5 min, the percentage of open mussels and the number of valve movements are computed as running averages for each experimental channel. With consideration being given to the normal behaviour of the mussels and the response of the system during several toxicity tests, reliable alarm-thresholds were established which are based on dynamic limits. In addition to these alarm functions (including the automatically safekeeping of a water sample) and the primary measurements, the computer fulfils every function necessary for an early warning system, e.g., complete storage and documentation of all data, validity-check of the primary measurements, and graphical presentation of all results. Together with more than 20 biological early warning systems, the Dreissena-Monitor was tested and evaluated by a joint federal government/federal states project group in Germany. As one of three systems, the Dreissena-Monitor is now recommended for implementation at the measuring stations of the 'CyGerman Commission for the Protection of the Rhine Against Pollution.' Starting in 1992, the Dreissena-Monitor is now applied at 13 water control stations in Germany (Rhine, Elbe, Danube, several tributaries). The experience of the users at the different stations have revealed that the Dreissena-Monitor complies quite well with the main functional requirements of an early warning system: (1) reliable, unattended operation for at least 1 week, (2) ease of handling, (3) an average of less than 3 h of maintenance per week, and (4) automatic detection of alarm situations within 30 min. In particular, the Dreissena-Monitor detected some water alarms at different rivers that give first clues on the sources of emission.