Effects of unicellular and colonial forms of toxic Microcystis aeruginosa from laboratory cultures and natural populations on tropical cladocerans

Three life-table experiments, two growth experiments and one feeding inhibition experiment, were performed to study the effects of the toxic cyanobacterium Microcystis aeruginosa on the cladocerans of a tropical lagoon (Jacarepagua Lagoon, Rio de Janeiro, Brazil). Different experimental designs were used to estimate toxic effects of both field samples and laboratory cultures of Microcystis aeruginosa on cladoceran life history parameters and juvenile growth rates. Effects of nutritional deficiency could be distinguished from toxic effects in experiments where green algae in high carbon concentration were mixed with Microcystis. Our results show that natural assemblages of Microcystis caused much less pronounced toxic effects than laboratory cultures and that unicellular forms were more toxic than colonial forms, even though both contained high concentrations of toxins. One possible explanation is that colonies were too large to be ingested by the small Moina micrura and Ceriodaphnia cornuta. Feeding inhibition by single cells and small colonies seems to be another mechanism that contributes to the harmful effects of Microcystis on cladocerans, both in the laboratory and in the field. Thus, caution is needed in extrapolating results from the laboratory to the field. We did find, however, that toxic algae in natural seston can inhibit growth and reproduction of native cladocerans populations.