The “Dreissena-Monitor” is a biological early warning system used in the continuous monitoring of water quality. The system is based on a computer assessment of valve movements in two groups of up to 42 zebra mussels (Dreissena polymorpha). Every 5 minutes, the percentage of open mussels and the average number of valve movements per mussel are calculated to describe the activity behaviour of the mussels. Rapid changes in both parameters normally reflect the onset of stress. More than 5½ years of experience in the continuous monitoring of water quality have revealed (1) that the “Dreissena-Monitor” is suited to large rivers like the Rhine as well as small rivers like the Erft or Ruhr, (2) that a lack of technical interruptions guarantees reliable, unattended operation, (3) that normally less than 3 hours of weekly maintenance are required, (4) that the validity check of the primary measurements was capable of identifying dead mussels and poorly aligned measurement components, (5) that the system can operate under unfavourable conditions (e.g. temperatures below 5° C), (6) that mortality rates were low and correlated positively with temperature, and (7) that the alarm thresholds provided a high degree of statistical security.

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