IMPACTS OF NITROGEN CONCENTRATION ON THE INTERACTIONS AMONG MICROCYSTIS AERUGINOSA, DAPHNIA MAGNA AND CERATOPHYLLUM DEMERSUM

To evaluate the impacts of nitrogen concentration on both biomanipulation and the competition between algae and water grass, Microcystis aeruginosa, Daphnia magna and Ceratophyllum demersum were selected for experiments to represent phytoplankton, zooplankton and submerged macrophyte, respectively. When two or three of them were cultured together, the changes of growth rate and removal rate of nitrogen and phosphorus were observed with the phosphorous concentration of 1.5 mg/L and 5 different nitrogen concentrations (0.5, 2, 4, 8 and 16 mg/L, prepared with KNO₃ solution), under temperature of 25℃, illumination of 2600 lx, light dark ratio 14h﹕10h. The results indicated that whenM. aeruginosa was cultured separately, its growth rate effectively reduced with a nitrogen concentration less than 1.97 mg/L. When D. magna andM. aeruginosa were co-cultured, the nitrogen concentration which D. magna could effectively control algae was from 0.5 to 4 mg/L. However, the nitrogen concentration which C. demersum could effectively control algae was from 0.5 to 2 mg/L, when C. demersum and M. aeruginosa were co-cultured. When nitrogen concentrations were at 0.5 and 2 mg/L, the cell densities of M. aeruginosa were 23.89% and 21.51%, respectively, of that when D. magna and M. aeruginosa were co-cultured at the last of experiment. So the effect of algae control was better when C. demersum and M. aeruginosa were co-cultured. The algae could be effectively controlled with nitrogen concentration from 0.5 to 16 mg/L, when D. magna, C. demersum and M. aeruginosa were co-cultured, under which the effect of algae control was the best. Especially, the growth rate of M. aeruginosa was all negative and significantly lower than those of D. magna and C. demersum when nitrogen concentration was bewteen 0.5 and 4 mg/L. In the co-culture environment of D. magna and M. aeruginosa, with the addition of large submerged macrophyte, the effect of algae control could effectively enhance, and the nitrogen and phosphorus in the water could be decreased, which can effectively improve the water quality over the long term.