Effects Of Cyanobacterial Microcystis Aeruginosa And Cylindrospermopsis Raciborskii On The Life-history Of Large Body-sized Cladocera Daphnia Sinensis

The life-history of Daphnia sinensis that was isolated from a sub-tropical pond were observed in three growth experiments. In the first experiment, D. sinensis was fed on pure diet containing Chlorella pyrenoidosa(CP), M. aeruginosa FACHB915(F915), M. aeruginosa FACHB905(F905), M. aeruginosa FACHB469(F469), C. raciborskii(N8, N10 and C510).In the second experiment, M. aeruginosa(F915, F905, F469)and C. raciborskii(N8)were mixed with edible green algae C. pyrenoidosain in different ratios, respectively. In the third experiment, M. aeruginosa F469 and C. raciborskii N8 were simultaneously mixed with C. pyrenoidosa. All the treatments have a fixed total food concentration of 1 mg C/L, and in the mixed food, the proportion of cyanobacteria were 75%, 50% and 25%. Our results were showed as follows:(1)Compared to the sole diet of C. pyrenoidosa,sole cyanobacterial diets inhibited the growth and reproduction of D. sinensis, and single-celled Microcystis has a larger negative effect on Daphnia than filamentous C. raciborskii. The lifespan and intrinsic rate of increase(r)of D. sinensis were different among three strains of C. raciborskii, which was highest in C510 and lowest in N8. D. sinensis fed with C510 had a lifespan of 15.7 d and r of 0.08. However, when fed on all three Microcystis, D. sinensis did not produce any newborn at all and their lifespan in the diet of F915 and F905 were less than 3.83 d.(2) In the second experiment, we found relatively low survival among animals in the treatments containing Microcystis F915 and F905 and the lifespan of D. sinensis were no more than 4 d. In contrast, D. sinensis was able to maintain higher growth rate and reproductive rate when the animals were fed on a mixed diet containing Microcystis F469 and Cylindrospermopsis N8. In the mixed diet containing 75% Chlorella, Daphnia had a significantly longer lifespan in the 25% F469(44.5 d) than in the 25% N8(35.5 d)(P<0.01). However, the r values were much lower(P<0.05)for the 25% F469(0.25) compared with 25% N8(0.29).(3) In the third experiment, the mixed foods containing both F469 and N8(F469+N8)had a higher inhibition effect on the growth and reproduction of D. sinensis compared to foods containing only F469 or N8. In the mixed diet containing 75% Chlorella, the lifespan and r value for F469+N8 treatment were 33.7 d and 0.214,which were much lower than for F469 treatment(44.5 d and 0.253, P<0.05)and N8 treatment(35.5 and 0.29, P<0.05).Our results showed that both M. aeruginosa and C. raciborskii can inhibit the growth and reproduction of D. sinensis, and M. aeruginosa pose a larger negative effect on Daphnia than C. raciborskii. The fact that D. sinensis grew better in the mixed diets containing F469 but its fecundity was higher in the mixed diets containing N8 indicated the responses of zooplankton to cyanobacteria were both species- and strains-specific. The coexistence of two cyanobacterial species in the food exerted a distinctly synergistic negative effect on D. sinensis compared to mono-cyanbacterial species, therefore we suggested that the influence of different cyanobacteria coexist in the waterbody on the zooplankton community structure should be taken into consideration when evaluating the ecological effect of cyanobacterial blooms.