Effects Of Microcystis Aeruginosa On Population Dynamics And Sexual Reproduction Of Large-bodied Cladocerans

Eutrophication of water body and cyanobacterial blooms has become one of the serious environmental problems in the world now. Many efforts have been done to control algal blooms by biomanipulation since 1970. Large-bodied cladocerans (such as Daphnia) may be effective to control density and biomass of phytoplankton, so it was likely to control algal blooms in eutrophic water body by keeping higher population density of Daphnia in certain extent. However, the difference of experimental results was obvious when using zooplankton to control phytoplankton among different Daphnia species. Cyanobaterial blooms occurred often in summer-autumn seasons in eutrophic water body. During the outbreak of cyanobacterial blooms, the dominance of large-bodied cladocerans (such as Daphnia) was replaced by small-bodied cladocerans, even the midsummer decline or disappearence of large-bodied cladocerans (such as Daphnia) was observed.On the one hand, the community succession of crustacean zooplankton and population dynamics of Daphnia during spring and summer (March-August) in Lake Chaohu were investigated using field investigations and experimental enclosure methods in the paper. On the other hand, the effect of Microcystis aeruginosa on population dynamics and change of reproduction mode of sevearl Daphnia species was studied. It was expected that the mechanisms of the outbreak of cyanobaterial blooms and the decline or disapperarence of large-bodied cladocerans (such as Daphnia) were clarified. The main results and conclusions are as follows:1. Higher biomass of M. aeruginosa is an important factor to influence on the community succession of crustacean zooplankton in Lake Chaohu. In July and June, the average biomass of M. aeruginosa in enclosure and Lake Chaohu was 0.1 mg L^-1 and 0.03 mg L^-1, 3.0mg L^-1 and 6.2 mg L^-1, the percentage among total algae biomass were respectively 24 % and 5 %, 50 % and 64 %. During the enclosure experiment, the biomass of Microcystis was in a lower level, and the density of large-bodied cladocerans was higher than small-bodied ones, with obvious competition advantage. While the biomass of colonial Microcystis was higher in Lake Chaohu, small-bodied cladocerans (such as Bosmina and C. cornuta) was dominant among crustacean zooplankton. Higher population density of large-bodied cladocerans (such as Daphnia) occurred only in April of Lake Chaohu, with the average of 10.4 ind. L^-1.2. During the enclosure experiment, maximal population density of D. hyalina and D. pulex were respectively 84 ind. L^-1 and 98 ind. L^-1. The percentage of pregnancy Daphnia was small, just with 1 to 5 eggs, and the percentage of larvaes was 64%. The drop of adult pregnant eggs and higher larvae mortality rate of Daphnia were the important reasons to the decline of Daphnia population in enclosures.The higher population density of Daphnia caused food shortage and crowded, which the males appeared and promoted ephippia(or resting eggs) production .The percentage of maximal male density (52 ind. L^-1) of D. pulex was 53%, and there was a positive relationship between male and population densities (r=0.997, n =5, p<0.01). During the enclosure experiment, the percentage of empty ephippia of D. pulex and D. hyalina was respectively 84% and 67%.The results indicated that except for fish predation and mechanical interference of colonial Microcystis, food quality and its population density were also an important influence factor to population dynamics of Daphnia in Lake Chaohu.3,Both 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 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 male and population densities of D. pulex (r=0.863, n=60, p=0.000) and D. carinata (r=0.793, n=156, p=0.000).Colonial Microcystis promoted ephippia production of D. pulex (r=0.811, n=12, p=0.001), but evidently restrained that of D. carinata (r=-0.798, n=12, p=0.002). The percentage of ephippia containing one or two resting eggs was relatively high under lower Microcystis biomass, while percentage of empty ephippia (containing no resting eggs) was relatively large under higher Microcystis biomass. The results suggest that production of ephippia and formation of resting eggs were two intricate processes in different Daphnia species, and that it might be difficult for sexual females to mate with males when percentage of males was low, consequently leading to 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.