Toxic and inhibitory effects of the blue-green alga <i>Microcystis aeruginosa</i> on herbivorous zooplankton

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).