Toxic and inhibitory effects of the blue-green alga Microcystis aeruginosa on herbivorous zooplankton

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Abstract
Blooms of blue-green algae are often associated with declines in populations of large-bodied cladocerans and increased importance of small cladocerans, copepods, and rotifers. We conducted toxicity and herbivory experiments, using a wide range of herbivore taxa, to test the hypothesis that the blue-green alga Microcystis aeruginosa most strongly inhibits large cladocerans. For a variety of herbivore taxa, M. aeruginosa was toxic or non-nutritious, and inhibited feeding on co-occurring nutritious food. The rotifer Brachionus calyciflorus was unique in several respects: it was unaffected by M. aeruginosa toxins, it showed some ability to grow and reproduce on a diet of M. aeruginosa, and it maintained high feeding rates on co-occurring nutritious food in the presence of bloom densities of M. aeruginosa. There was a strong relation between the toxicity of M. aeruginosa and its inhibitory effect on herbivore feeding rates. Copepods strongly avoided consuming M. aeruginosa, but all cladocerans and rotifers tested filtered unicellular M. aeruginosa at rates similar to or higher than nutritious Chlamydomonas reinhardi. Our results indicate that there are a variety of mechanisms whereby herbivorous zooplankton can coexist with blooms of M. aeruginosa, including resistance to toxic chemicals (B. calyciflorus), and avoidance of consumption of M. aeruginosa by chemosensory means (copepods), or by the inability to consume large colonies (some small cladocerans).

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