| As an object of study in colonial Microcystis and Daphnia species fromLake Chaohu, the influence of phosphorus concentration and colonial Microcystis sizeand concentration on the growth and reproduction of Daphnia species were studiedfrom the perspectives of individual, population and interspecies competition. Theresults as follow:1. The phosphorus content in S. obliquus cells increased with the increase ofphosphorus concentration. Compared with low phosphorus content in food, the bodylength at the first pregnancy and number of offspring at first reproduction of six D.similoides clones increased under high phosphorus content, while time to maturationshorted. The effects of phosphorus concentration on the time to maturation, bodylength at first pregnancy and number of offspring at first reproduction of D. similoideswere all significant (P<0.01). Six D. similoides clones were all difficult to adapt at lowphosphorus concentration and low temperature, and their responses were different. Theinhibitory effects of clone3and clone6were obvious, while clone2had relative goodfitness. The results suggested that phosphorus-deficiency in food could restrain thegrowth and reproduction of D. similoides, and the inhibitory effect of low temperatureon the growth and reproduction of D. similoides was stronger underphosphorus-deficiency.2. The influences of colonial Microcystis size, concentration and theircombinations on the life history traits of D. similoides were studied. In all experimentalgroups, the survival rate of D. similoides was100%within8days. Then, this valuedeclined obviously in the large-colony groups and in the small-colony andhigh-concentration group of Microcystis. The maximum lifespan (49d) of D.similoides was observed in the small-colony and low-concentration group of Microcystis, whereas the minimum (22d) occurred in the large-colony andhigh-concentration group. Time to maturation and body length at the first pregnancy ofD. similoides were significantly lower in all groups that contained Microcystis than thatin the control group. The generation time of D. similoides in the low-concentrationgroups of Microcystis was significantly higher than those in the high-concentrationgroups at the same Microcystis size. The number of offspring at first reproduction perfemale in the small-colony and high-concentration group of Microcystis wassignificantly higher than those of all other groups. The maximum cumulative offspringnumbers (163.0±8.1ind.) produced per female appeared in the small-colony andlow-concentration group of Microcystis, whereas the minimum (12.8±2.9ind.)occurred in the large-colony and high-concentration group. Despite the Microcystisconcentration, the innate capacity of increase in D. similoides in small-colony groupsof Microcystis was significantly higher than those in the large-colony groups. Theresults suggested that the combined effects of colonial Microcystis size andconcentration on time to maturation, innate capacity to increase, and average life spanof D. similoides were all significant.3. Two colonial Microcystis sizes (L: large-colony≥112μm, S: small-colony<112μm) and two concentrations (H: high20mg/L, L: low5mg/L) were employed tostudy the influence of Microcystis on population dynamics and interspecificcompetition of three Daphnia species (e. g. D. similoides, D. galeata and D. pulex)from Lake Chaohu.Compared with other competitive combinations, smaller numbers of offspring atfirst reproduction of both D. similoides and D. galeata were observed in theircombination. Population densities of three Daphnia species in the control group werelower than in the groups containing Microcystis. D. pulex could not compete with D.similoides while it dominated over D. galeata in spite of Microcystis size andconcentration. In the interaction between D. similoides and D. galeata, maximumpopulation growth rates of both D. similoides and D. galeata were respectively lowerthan in competition with other Daphnia species. Maximum population growth rates ofD. similoides in the HS group of Microcystis were higher than in other groups in competition with D. galeata and D. pulex. Fewer cumulative ephippia numbers of D.galeata were observed in competition with D. similoides or D. pulex, and theproportions of ephippia containing resting eggs were lower in the presence ofMicrocystis. The maximum cumulative ephippia numbers (385±14.8ind.) of D.similoides appeared in the HL group of Microcystis in competition with D. pulex.Combined effect of Microcystis size and concentration on the cumulative ephippianumber of both D. pulex and D. galeata were significant under competition condition.In general, the responses of different Daphnia species to colonial Microcystiswere intricate under competition condition, depending on Microcystis size andconcentration. The presence of Microcystis strengthened competitive outcomes of threeDaphnia species. Our results will contribute the further understanding on communitysuccession of Daphnia in Microcystis-dominated lakes. |
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