Abstract: Four Brachionus calyciflorus clones (clone A, B, C, and D) different i n biochemical and genetic characteristics were collected from Jinghu Lake in sum mer, and their life history traits were studied at 25 ℃ under the conditions of feeding with 10×106, 20×106, 40×106 and 80×106 cells·L -1 of Scenedesmus obliquus. The results showed that S. obliquus concen tration had different effects on the survival rate and fecundity of different B. calyciflorus clones. Among the four clones, clone C had the shortest gene ration time, clone B had the longest generation time, life expectancy and averag e lifespan, and clone A had the highest percentage of mictic females in its offs pring. No significant differences were observed in the net reproductive rate and individual fitness among the four clones. At 20×106 cells·L-1 of S. obliquus, the net reproductive rate of B. calyciflorus was the lowest ; at 10×106 cells·L-1 of S. obliquus, B. calyciflorus had the sh ortest average lifespan and life expectancy but the highest mictic rate in its o ffspring; at 80×106 cells·L-1 of S. obliquus, B. calyciflorus ha d the highest intrinsic rate of population increase and the longest average life span and life expectancy; and at 40 and 80×106 cells·L-1 of S. obliquus, the individual fitness of B. calyciflorus was the highest. Clone C had the smallest individual fitness at 39×106 cells·L-1 of S. obliquus and clone D had the highest one at 634×106 cells·L-1 of S. obliquus, while the individual fitness of clones A and B was not corr elated with S. obliquus concentration. After July, the disappearance of gene tically different B. calyciflorus clones from Jinghu Lake might attribute to the va riation of algal food concentration, and the similar individual fitness of the four clones might be one of the reasons that contribute to the c oexistence of B. calyciflorus clones in Jinghu Lake in summer.

Key words: Loess Plateau, mixed litter, soluble organic carbon, soluble total nitrogen.