A biological early warning system based on the phototactic behaviour of Daphnia magna has been evaluated in relation to the reproducibility of its response and sensitivity towards potential pollutants. In addition to the infrared detection system of the monitor, video recording and digital image processing have been used in order to obtain more information on the behaviour of the Daphnia. The biomonitor performed better with Daphnia clones selected on positive phototactic behaviour than with non selected organisms. The variations in response over a five day period were too large to allow a reliable use of the monitor. The biomonitor was not able to detect sublethal changes in behaviour.

Digital image processing of video recordings yielded more information on the behaviour of the Daphnia that the infrared detection system. Using this technique it was possible to detect sublethal changes in Daphnia behaviour as well as conceptual shortcomings in the existing behaviour.