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

Blooms of blue-green algae are often associated with declinesin populations of large-bodied cladocerans and increased importanceof small cladocerans, copepods, and rotifers. We conducted toxicityand herbivory experiments, using a wide range of herbivore taxa,to test the hypothesis that the blue-green alga Microcystisaeruginosa most strongly inhibits large cladocerans. For a varietyof herbivore taxa, M. aeruginosa was toxic or non-nutritious,and inhibited feeding on co-occurring nutritious food. The rotiferBrachionus calyciflorus was unique in several respects: it wasunaffected by M. aeruginosa toxins, it showed some ability togrow and reproduce on a diet of M. aeruginosa, and it maintainedhigh feeding rates on co-occurring nutritious food in the presenceof bloom densities of M. aeruginosa. There was a strong relationbetween the toxicity of M. aeruginosa and its inhibitory effecton herbivore feeding rates. Copepods strongly avoided consumingM. aeruginosa, but all cladocerans and rotifers tested filteredunicellular M. aeruginosa at rates similar to or higher thannutritious Chlamydomonas reinhardi. Our results indicate thatthere are a variety of mechanisms whereby herbivorous zooplanktoncan coexist with blooms of M. aeruginosa, including resistanceto toxic chemicals (B. calyciflorus), and avoidance of consumptionof M. aeruginosa by chemosensory means (copepods), or by theinability to consume large colonies (some small cladocerans). ¹Present Address: Department of Biology, George Mason University,4400 University Drive, Fairfax, VA 22030, USA     

Growth and Reproductive Responses of Daphnia to Cyanobacterial Blooms on the Potomac River

We examined reproductive responses of the cladoceran Daphnia parvula to algal assemblages from the Potomac River, during and after Cyanobacterial blooms. Algal assemblages were manipulated by filtration, dilution, or additions of nutritious algae, in order to determine causes for growth inhibition. There was no evidence for lethal toxic effects or food limitation of population growth rates during the algal bloom, although growth was food limited later in the year. However, growth rates were lower in the experiment conducted during the Cyanobacteria bloom than in later experiments, and the results suggest an inhibitory factor associated with the particulate material. Results from filtered treatments indicate that picoplankton present in hypereutrophic waters can support Daphnia growth and reproduction.