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SummaryShotr-term toxicity tests were performed on micro-phytobenthos from a branch of the Rhine river. The effects of copper (Cu) on photosynthesis were monitored using a PAM fluorometer. Photosynthetic efficiency (φPSII) was used to determine the effect level (EC50) for the added CuCl2. Suspended particles in the river water played an important role in lowering the copper toxicity for the microalgae; the structure of the biofilm also played a protective role against copper. Cu induced a fast degradation of chlorophyll, shown by the decrease of the fluorescence in the dark, in accordance with HPLC-determined pigments, which also showed a rapid destruction of the accessory pigments.