Effects Of Toxic Microcystis Aeruginosa On Population Dynamics And Resting Egg Formation Of Daphnia

This paper studied the influence of Microcystis aeruginosa to zooplankton byMicrocystis aeruginosa strains and microcystin, we try to change the concentrations ofmicrocystin and Scenedesmus obliquus, which to found the relationship to population dynamics,ephippial production and resting egg formation of Daphnia. The results could contribute tocomprehend the construction and succession of zooplankton in different eutrophic lake.The mainresults were as follows:1. We examined the hypothesis that toxic effects of Microcystis aeruginosa to populationdynamics, ephippial production, and resting egg formation of Daphnia were restricted by foodquality level. Four unicellular, toxic M. aeruginosa strains with contrasting concentration ofmicrocyctin and two Daphnia species were used in this study. The number of offspring at firstreproduction, population densities, and maximum population growth rate of two Daphnia specieswere lower at the highest toxic M. aeruginosa7820treatment compared with other treatmentsunder two concentrations of S. obliquus. The maximum number of offspring at first reproductionof two Daphnia species appeared in the higher toxic M. aeruginosa526strain at the highconcentration of S. obliquus. The effects of M. aeruginosa strains, S. obliquus, and theircombination on the number of offspring at first reproduction and the maximum population growthrate of two Daphnia species were significant (P <0.001).Two Daphnia species could not reproduce in the highest toxic M7820treatment under twoconcentrations of S. obliquus. They had lower population size and maximum population growthrate in the higher toxic M526treatment at the low concentration of S. obliquus, but they werehigher at the high concentration of S. obliquus. This result suggests that high S. obliquusconcentration could relieve the toxicity of M. aeruginosa to Daphnia, and Daphnia could utilizethe lower toxic Microcystis as food. The cumulative ephippia numbers of two Daphnia specieswere more at the high concentration of S. obliquus than those at the low concentration. Thepercentage of ephippia containing no resting eggs of two Daphnia species was evidently higher atthe low concentration of S. obliquus but was lower at the high concentration of S. obliquus. Ourresults indicated that the cumulative ephippia numbers of Daphnia were population densitydependent at the high-level food, and the productions of ephippia in Daphnia were significantlycontrolled by microcystin concentration at the low-level food.2. The combined effects of single-cell toxic M. aeruginosa and S. obliquus on the growthand reproduction of D. similoides. The results showed: D. similoides could not grow andreproduce under only M. aeruginosa as food. Time to maturation of D. similoides decreased asincreased S. obliquus concentrations, while body length at maturity increased. No. offspring at first reproduction, maximal population density and maximal population growth rate increased asincreased S. obliquus concentrations. Both maximal population density and maximal populationgrowth rate appeared in the2×106cells/mL of S. obliquus concentration group,302.7ind.(200mL)-1and0.213d-1, respectively. D. similoides did not produce the ephippium under thelow S. obliquus concentration (1×105cells/mL). There were more ephippia produced under thehigher S. obliquus concentrations (1×106cells/mL), and the peak (77.3ind.) occurred in the1×106cells/mL of S. obliquus concentration group. The percent of ephippia containing one and tworesting eggs among total ephippia were obviously higher under the higher S. obliquusconcentration groups (1×106cells/mL and2×106cells/mL) than under the lower S. obliquusconcentration groups (2×105cells/mL and4×105cells/mL). It suggested that increasing S. obliquusconcentrations might relive the inhibition of single-cell toxic M. aeruginosa on the growth andreproduction of D. similoides, while the ephippium production resting-egg formation of D.similoides were influenced by population density and M. aeruginosa together.3. In M526strain, time to maturation of D. similoides increased with increased M.aeruginosa concentrations,while body length at maturity of D. similoides decreased. It was notobvious in MCH strain.In MCH strain, the number of offspring at first reproduction and maximum populationdensity of D. similoides were higher than those in the control group. It suggested that D.similoides could feed lower toxic M. aeruginosa. However, D. similoides could not grow andreproduce in the higher toxic and conentration of M526strain. In MCH strain, the cumulativeephippia numbers of D. similoides were higher than those in the controp group, and the percent ofephippia containing two resting eggs increased with increased M. aeruginosa.