RESPONSES OF DAPHNIA-PULEX POPULATIONS TO TOXIC CYANOBACTERIA

1. We studied the effects of toxic cyanobacteria, Microcystis aerugino sa, on Daphnia pulex populations. The experiment was performed at room temperature in laboratory microcosms to which we added toxic Microcys tis in two pulses. Green alga, Scenedesmus obtusiusculus, was provided in two concentrations. 2. Microcystis exposure resulted in a decrease d population density of Daphnia. The proportion of adolescents was hig her in the Microcystis treatment than in the control, while the propor tion of newborns did not differ significantly from the control. This i ndicates delayed maturation of Daphnia exposed to Microcystis. We foun d no significant impacts of cyanobacteria, food level or clonal origin on population variability after correcting for differences in populat ion size. 3. Juveniles of the two clones studied showed different sens itivities to toxic Microcystis in the acute test. However, in the popu lation experiment we did not find clonal differences between Microcyst is and control treatments in the relative proportion of juveniles. 4. The number of ephippia produced per adult was highest in the treatment s with Microcystis. This indicates that cyanobacterial toxins may be o ne of several factors inducing ephippia formation, acting directly or through inhibition of feeding.