The Influences Of Nitrogen,Temperature And Illumination On Interactions Among Daphnia Magna,Ceratophyllum Demersum And Microcystis Aeruginosa

Eutrophication has become a main water quality problem in most of the freshwater and offshore marine ecosystems in the world,and causes algae bloom.Microcystis aeruginosa,a common dominant species in the outbreak of algae bloom,belongs to non-nitrogen-fixing cyanobacteria.Ecological restoration is the best way to effectively control algae blooms.The restoration and reconstruction of aquatic vegetation and biological manipulation are two common methods of ecological restoration,but their algae control effects are limited by the concentration of nitrogen in the water,and they are also affected by temperature,light and other ecological factors.In order to elucidate the effect of nitrogen,temperature and light on the algae control,we have constructed a microscopic cosmos model of D.magna,C.demersum and M.aeruginosa,and conducted relevant research to provide an important basis for the management of algae bloom.Daphnia magna,Ceratophyllum demersum and Microcystis aeruginosa were selected as the representative of zooplankton,submerged macrophyte and phytoplankton respectively.When two or three of them were cultured together,the changes of growth rate and removal rate of nitrogen and phosphorus were studied with five kinds of solution with different nitrogen concentrations.At the same time,the effects of different temperature,light intensity and light-dark ratio on the algae-controlling effect of zooplankton and submerged macrophytes were studied.The results were as follows:(1)Under the condition of 25?,light intensity 2600 lx,light-dark ratio 14 h:10 h,phosphorus concentration 1.5 mg/L and nitrogen concentration gradient 0.5,2,4,8 and 16 mg/L:(1)When M.aeruginosa was cultured in BG-11 culture solution singly with different nitrogen concentrations,the nitrogen concentration could be controlled below 1.97 mg/L,which could effectively reduce the growth rate of algae in culture solution.At the end of the experiment(15 d),the cell density of M.aeruginosa remained below 1.86×10~7 cells/mL,and the algae could not continue to grow.(2)When D.magna and M.aeruginosa were co-cultured,the nitrogen concentrations which D. magna could effectively control algae were from 0.5 to 4 mg/L.At this time,the growth rate of M.aeruginosa was significantly lower than that of other nitrogen concentration treatment groups,whereas the growth rate of D.magna was the opposite.At the end of the experiment(15 d),the growth rate of M.aeruginosa slowed down and the density of algae cells remained below 2.98×10~6 cells/mL.(3)When C.demersum and M.aeruginosa were co-cultured,the effect of algae control was better.The nitrogen concentrations which C.demersum could effectively control algae were from 0.5 to 2 mg/L.When the nitrogen concentration was respectively at 0.5 and 2 mg/L,the cell densities of M.aeruginosa were 23.89%and 21.51%of that when D.magna and M.aeruginosa were co-cultured at the last of the experiment(15 d).(4)When the three of D.magna,C.demersum and M.aeruginosa were co-cultured,the effect of algae control was the best.When the nitrogen concentrations were from 0.5 to 16 mg/L,the algae could be effectively controlled,and especially when the nitrogen concentrations were from 0.5 and 4 mg/L,the growth rates of M.aeruginosa were all negative and significantly lower than those of D.magna and C.demersum.In the co-culture system of D.magna and M.aeruginosa,with the addition of large submerged macrophyte,the effect of algae control could be effectively enhanced and the nitrogen and phosphorus in water could be decreased,which can effectively improve the water quality over the long term.(2)Experiments on the co-culture of D.magna,C.demersum and M.aeruginosa under the conditions of light intensity 2600 lx,light-dark ratio 14 h:10 h and concentration of nitrogen and phosphorus were the same as(1),the temperature gradient was 15,20,25 and 30?:When the temperature was 15 to 25?and the nitrogen concentration was 0.5 to 4 mg/L,the growth rate of M.aeruginosa was lower than the growth rate of D.magna and C.demersum,and the control effect on algae was remarkable.However,when the temperature was 15 to 20?,the reduction rate of nitrogen and phosphorus in the culture medium was small.When the temperature was 30?,the growth rate of D.magna was less than150%and the growth of D.magna was poor;when the concentration of nitrogen was 4 to 16 mg/L,M.aeruginosa proliferated explosively and the water body was blue-green;at this time,the growth of algae could not be controlled.(3)Experiments on the co-culture of D.magna,C.demersum and M.aeruginosa under the conditions of 25?,light-dark ratio 14 h:10 h and concentration of nitrogen and phosphorus were the same as(1),the light intensity gradient was 1600,2600,3600 and 5000 lx:When the light intensity was 1600 to3600 lx and the nitrogen concentration was 0.5 to 4 mg/L,the growth rate of M.aeruginosa was lower than the growth rate of D.magna and C.demersum,and the control effect on algae was good.However,when the light intensity was 1600 lx,the reduction of nitrogen and phosphorus in the culture medium was relatively low.When the light intensity was 5000 lx,the growth rate of M.aeruginosa increased with the increase of nitrogen concentration.While the growth rate of C.demersum was the opposite.The water body was turbid and the algae control effect was not effective.(4)Experiments on the co-culture of D.magna,C.demersum and M.aeruginosa under the conditions of 25?,light intensity 2600 lx and concentration of nitrogen and phosphorus were the same as(1),the light-dark ratio gradient was 10:14,12:12,14:10 and 16:8:When the light-dark ratio was 10 h:14 h,C.demersum could not grow well and its growth rate was less than 15%,and it was not good for the reduction of nitrogen and phosphorus in the medium.When the light-dark ratio was 12 h:12 h and 14 h:10h and the nitrogen concentration was 0.5 to 4 mg/L,the growth rate of M.aeruginosa was lower than that of D.magna and C.demersum,and the control effect on algae was significant.When the light-dark ratio was 16 h:8 h and the nitrogen concentration was 2 to 16 mg/L,there was no significant difference among the growth rate of the three organisms,and the inhibitory effect on the algae was bad.