Effects of Microcystis aeruginosa on population dynamics and sexual reproduction in two Daphnia species

摘要

Laboratory experiments were conducted to evaluate effects of Microcystis aeruginosa on population variations, male and resting-eggs production of two Daphnia species, D. carinata and D. pulex. The male and population densities of the two Daphnia dropped with the increase of Microcystis biomass. Population density of the large-bodied D. carinata was more strongly restrained by mechanical interference of colonial Microcystis than the small-bodied D. pulex. Male density of the two Daphnia increased quickly at the beginning of the experiment, and then remained at a steady level or declined. There was a positive relationship between population density and density of males for both D. pulex and D. carinata. The presence of Microcystis colonies in the diet was positively associated with the production of ephippia for D. pulex but negatively associated with ephippia production for D. carinata. The percentage of ephippia containing one or two resting eggs was relatively high under lower Microcystis biomass, while the percentage of empty ephippia (containing no resting eggs) was relatively large under higher Microcystis biomass. The results suggest that the production of ephippia and formation of resting eggs were two intricate processes in the different Daphnia species, and that it might be difficult for sexual females to mate with males when the percentage of males was low, consequently leading to the presence of more empty ephippia. It is likely that mechanical interference of colonial Microcystis may accelerate the frequently observed midsummer decline of Daphnia in Microcystis-dominated lakes.