Dynamics Of Cyanobacterial Dominant Populations And Its Controlling Experiments In Situ In A Typical Reservoir, South China

In a large reservoir,located in South China,floating plant rafts and cutoff wall for inflow control were applied against cynaobacterial blooms in a outlet zone where water was pumped for drinking water supply.In order to understand the dynamics of cyanobacteria abundance and biomass and to evaluate the effects of floating plant rafts and cutoff wall,the phytoplankton near the pumping point and the dam,nutrients at the two sides of the water cutoff wall were sampled and measured.A large-scale enclosure experiment was conducted in the reservoirs to know the application of floating plant rafts against cyanobacteria blooms. The phytoplankton was sampled near the outlet and the dam every month(except in February) in 2008.The phytoplankton and nutrients were sampled at the depth of 0.5m,5m and10m at the two sides of the cutoff wall on November 21,and December 9 in 2008.We conducted the large enclosure experiment from May to July,2008.Three treatments and one control were designed,three repeats for each treatment,three treatments responds to the coverage:10%, 20%,30%of the floating plant rafts,respectively.In total,the water was sampled four times in each enclosure.This reservoir was a eutrophic water body,the phytoplankton was dominated by cyanobacteria in the whole observation year.The predominant three species of caynobacteria showed a remarkable seasonal pattern,Anabaena flosaquae dominated from January to June, Cylindrosperopsis raciborskii from July to September,Microcystis flos-aquae from October to December.The annual phytoplankton abundance ranged from 1.85×10⁶ to 20.05×10⁶ cells·L^-1,the annual phytoplankton biomass ranged from 0.211 to 10.525 mg·L^-1 in the outlet.The annual phytoplankton abundance ranged from 0.94×10⁶ to 20.9×10⁶ cells·L^-1,the annual phytoplankton biomass ranged from 0.157 to 6.035 mg·L^-1 near the dam.There are no significant difference between the sampled zone for both phytoplankton abundance and biomass It strongly suggests that the accumulation of phytoplankton biomass was reduced largely by the floating plant rafts.To evaluate the measurement of water inflow control by the cutoff wall,Correlation of the nutrient concentrations at the each depth in the twosides near the cutoff wall showed,that the inflow water to the outlet was mostly from the depth of 5m and 10m.It indicated that the cutoff wall can function in controlling the water inflow.In the enclosure experiment,the phytoplankton abundance was reduced by 55%in the second sampling period in the treatment with 10%coverage,by 67.8%in the treatment with 20%coverage,and by 67.9%in the treatment with 30%coverage,while it increased by 56.8%in the control.The main contributor of the reduced abundance was from cyanobacteria that was reduced by 93.2%in the treatment with 30%coverage of floating plant rafts.The phytoplankton abundance was reduced until the third sampling.However,the phytoplankton abundance in each treatment increased in the fourth sampling,the recover of phytoplankton abundance may be influenced by the high precipitation in the experimental period.