We investigated the effects of colonial Microcystis aeruginosa and interspecific competition on the population dynamics and resting egg formation of two cladocerans. In the single-species culture experiments, high M. aeruginosa concentrations favored the population growth of Moina irrasa but strongly inhibited that of Daphnia carinata. Small-bodied M. irrasa was predominant over large-bodied D. carinata regardless of the presence of the colonial M. aeruginosa in the mixed cultures. The per capita cumulative number of ephippia and the percentage of ephippia containing D. carinata resting eggs both increased from under 0 to 10 mg/L M. aeruginosa in the singlespecies culture but decreased in the mixed culture. However, the per capita cumulative ephippia number of M. irrasa in the mixed culture was higher than those in the singlespecies culture at the high M. aeruginosa levels. The effects of M. aeruginosa, interspecific competition, and their combination on the per capita cumulative ephippia numbers of the two cladocerans were significant. The occurrence of ephippia was related to the higher population density of the two species. Our results suggest that interspecific competition inhibits the ephippium production and resting egg formation of the large-bodied cladocerans (D. carinata) in the presence of colonial M. aeruginosa.

Laboratory experiments were conducted to evaluate the effects of food phosphorus (P) concentrations on the population dynamics, ephippium production, and resting egg formation of two cladocerans, Moina irrasa and Daphnia similoides, fed on Chlorella vulgaris that were cultured in five different P concentrations (0.05, 0.1, 1, 5, and 10 mg/L). The body length at the first pregnancy of M. irrasa and D. similoides significantly increased with higher P concentration, whereas the maturation time significantly decreased. The maximum population density of M. irrasa (mean AE SD, 6220 AE 1129 ind/L) and D. similoides (1816.5 AE 63.5 ind/L) appeared in the 5 and 10 mg P/L groups, respectively. The maximum male density of M. irrasa (150 AE 113.0 ind/L) appeared in the low P concentration of 0.05 mg/L, whereas it was in the higher P concentrations for D. similoides. The maximum cumulative ephippia number of M. irrasa (51/female) and D. similoides (6.2/female) appeared in the 0.05 and 1 mg P/L groups, respectively. The maximum percentage of ephippia containing resting eggs of M. irrasa (69.3%) appeared in the lowest P concentration. The percentage of ephippia containing resting eggs of D. similoides increased gradually with higher P concentration, and the ratio was above 70% in higher P concentrations (5-10 mg P/L). Our results suggest that low P can advance and induce the production of ephippium in two cladocerans and that higher male density promotes the resting egg formation of cladocerans.

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