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Abstract

We investigated the mechanisms behind the negative effects of cyanobacteria on zooplankton by comparing the effects of Microcystis aeruginosa single cells, colonies, and toxins on the feeding behaviour of Daphnia pulicaria in three independent experiments. The animals were fed a mixture of Scenedesmus supplemented by increasing proportions (0, 20, 50, 80, 100%) of Microcystis or concentrations (0, 50, 500, 5,000 ng ml(-1)) of purified microcystin-LR. The changes in feeding behaviour, as indicated by the appendage beat, mandible or labrum movement rates were evaluated by a direct observation method that coupled video recording and computerized image analysis. Daphnia responded in a different manner to the presence of single cells and colonies. In the case of the single cells, the mandibular movement rate (MMR) declined more than appendage beat rate (ABR), suggesting that Daphnia have the ability to discriminate between Microcystis and Scenedesmus. Colonies, on the other hand, produced a typical feeding interference response: the animals increased their labral rejection rate (LRR) and showed starvation signs. LRR increased in the presence of both unicellular and colonial Microcystis. In both cases, the changes in MMR and ABR were rapidly reversible and hence unlikely to be caused by intoxication from the presence of cell-bound microcystins. In contrast, the addition of purified microcystin-LR at the concentration of 5,000 ng ml(-1) produced a nonreversible impairment of Daphnia feeding behaviour.