Abstract: To clarify the optimal water level for the growth of Hydrilla verticillata and provide theoretical basis for the successful colonization of H. verticillata, we planted H. verticillata in pots with depths of 0.5-1.4 m under water using controlled experiment to examine the influence of water depths on chlorophyll content, antioxidative enzyme activity, malondialdehyde (MDA), and root activity ofH. verticillata. With the increases of water depth, underwater light intensity significantly decreased, whereas chlorophyll a, chlorophyll b, and chlorophyll a+b contents of H. verticillata increased. No significant change was found in chlorophyll a/b, and carotenoid (Car) content increased constantly. Water depth had a significant effect on the antioxidant enzyme activity ofH. verticillata (P<0.05). The activity of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) increased first and then decreased. The membrane lipid peroxidation of H. verticillata intensified at the depth of 0.5-0.8 m, which promoted the maximum MDA content and increased the activity of antioxidant enzymes (SOD, POD). When water depth increased from 0.5 m to 1.4 m, there was no significant change of soluble protein content. Root activity increased from 0.12 to 0.51 mg·g^-1·h^-1, with significant differences among different water depths (P<0.05). Water depth affected the physiology of H.verticillata. The growth and physiology of H.verticillata in the shallow depths (0.5-0.8 m) were strongly inhibited by light availability, manifested as yellow plants, broken branches, residual leaves, reduced chlorophyll content, and increased membrane lipid peroxidation. During the ecological restoration of waters, therefore, a range of 0.9-1.4 m water depth should be considered to restore H.verticillata population, which would facilitate the restoration of aquatic ecosystems.

Key words: soil acid buffering capacity, nutrient balance, acid titration curve, soil acidification