Abstract: With the method of population accumulation culture, the population growth and reproduction of rotifer(Brachionus calyciflorus ) were observed in pH 3.5～11.5 media with an increment of 1 pH unit. The results show that rotifer population grows faster at pH6.5～8. 5 and fastest at pH 8.5, indicating that in this pH range, the instantaneous rate(r) is fast and the density is the maximum. At pH5.5～9.5. the r value is positive. but at pH3.5～4.5 and 9.5～10.5, it is negative, i. e.. the population shows a negative growth. Rotifer can survive at pH3.5～11.5, but the optimum pH for population growth is 8.5. where the rotifer shows a maximum reproductive sped. and the absolute egg density is the highest (132 cells·ml^-1 ). The relative egg density is the highest at pH9.5, 2～4 times larger than that at other pH values.

Key words: Brachionus calyciflorus, Population growth, Instantaneous growth rate, Reproduction, Egg density, ¹³C, ¹⁵N, elevated CO₂, organic matter decomposition, nitrogen mineralization, oak