Ecological Threshold For Prevention And Control Of Cyanobacteria Blooms In Baiyangdian Lake

The eutrophication of lakes and the accompanying algae blooms are important environmental issues of general concern in the world.Cyanobacteria blooms are the most widely distributed and frequency-occurring algal blooms in China.The current management countermeasures for eutrophication and cyanobacteria blooms are mainly to control sources and intercept pollution and reduce the concentration of endogenous nutrients.However,practice has shown that the control method of limiting the growth of algae by controlling the concentration of nutrients generally fails.Some studies have pointed out that biomanipulation technology can more effectively limit the overgrowth of algae.The core goal of this technology is to limit the overgrowth of algae through the top-down effects of the food chain.Among them,predation by zooplankton is one of the most concerned top-down effects.Although many literature have analyzed the negative correlation between the biomass or density of zooplankton and algae,few literature have done in-depth research on the nonlinear relationship between the two and the interaction of other factors on the two.Baiyangdian Lake is the largest freshwater lake in North China Plain and is known as the"Kidney of North China".However,in recent decades,Baiyangdian Lake has experienced serious water shortages.In addition,the upstream industrial wastewater and domestic sewage around the sub-lakes have caused serious eutrophication and cyanobacteria blooms in Baiyangdian Lake.After the establishment of the Xiong’an New Area,the management for eutrophication and cyanobacteria blooms in Baiyangdian Lake have received unprecedented attention.Based on the theory and method of ecological threshold,this paper detects the possible ecological threshold for control of cyanobacteria in Baiyangdian Lake,and proposes the potential method of early warning of cyanobacteria blooms based on the threshold.It also analyzes the interaction of other main interference factors on the threshold,evaluates the impact of interference on the effect of zooplankton on algae control,and finally discusses the effective ways to prevent and control cyanobacteria blooms based on biological manipulation technology in Baiyangdian Lake.All of the contents and results are as follows:（1）The threshold detection and analysis of cladocera density and cyanobacteria density was carried out.It was proved that cladocera have the most significant non-linear relationship with cyanobacteria,and cladocera are the most effective zooplankton group to control the density of cyanobacteria.It was obtained that the ecological threshold for control of cyanobacteria in Baiyangdian Lake is 9 ind·L^-1（cladocera）,that is,when the cladocera density is greater than 9 ind·L^-1,the cyanobacteria density can be stably controlled at a low level due to the predation of cladocera.The validity of the method and the reliability of the Baiyangdian Lake detection results are verified by the analysis of other sample sites in North China.（2）An early warning analysis of the risk of cyanobacteria blooms in Baiyangdian Lake was carried out.Through the spatial interpolation analysis of the cladocera density,it was found that the water areas with the cladohorn density close to or below the threshold in 2019 are concentrated in the southern area of Baiyangdian Lake.Therefore,the current focus area of Baiyangdian Lake for the prevention and control of cyanobacteria blooms is the southern waters.（3）Carried out the ecological threshold detection and quantitative analysis between macrozoobenthos and cyanobacteria.It was found that there is an obvious threshold for control of density of cyanobacteria by Radix swinhoei,which confirms the effective control effect of certain macrozoobenthos on cyanobacteria in the wild ecosystem.The threshold for the density of Radix swinhoei and cyanobacteria is 2ind·m^-2（Radix swinhoei）.It was found that the threshold phenomenon between cladocera and cyanobacteria seemed to disappear when the radish snails were widespread.Therefore,this article believes that after the water quality gradient disappears,the threshold for control of cyanobacteria by Radix swinhoei provides another way that may be suitable for early warning of the risk of cyanobacteria blooms.（4）The possibility of the existence of food webs among Radix swinhoei,cladocera and cyanobacteria in Baiyangdian Lake was evaluated.It was found that there is a negative correlation（P>0.05）between Radix swinhoei and cladocera.It was speculated that there are food webs of Radix swinhoei,cladocera,and cyanobacteria in the water ecosystem of Baiyangdian Lake.However,in view of the grazing effect of snails on submerged macrophytes,this article believes that the use of Radix swinhoei is not suitable as an effective way to control the density of cyanobacteria.（5）The synergistic effects of submerged macrophytes,total phosphorus,Radix swinhoei,and temperature on the cyanobacteria control effect of cladocera were analyzed.The results of synergistic effect analysis shows that increasing the biomass of submerged macrophytes,reducing the total phosphorus concentration,reducing the density of Radix swinhoei,and lowering the temperature will reduce the density of cladocera required for effective control of cyanobacteria,that is,the control effect of cladocera on cyanobacteria will be promoted.（6）The application potential of the treatment strategy with the main goal of reducing the total phosphorus concentration in Baiyangdian Lake was evaluated.The evaluation results show that the continued reduction of total phosphorus concentration has a very limited contribution to algae control.Therefore,it is necessary to consider the application of biomanipulation technology based on strengthening the algae control effect of zooplankton in eutrophication treatment and cyanobacteria blooms prevention and control in Baiyangdian Lake.This study determined the threshold for control density of cyanobacteria by cladocera in Baiyangdian Lake,and proposed an effective method for early warning of the risk of cyanobacteria blooms.The results of the quantitative analysis between Radix swinhoei,zooplankton and phytoplankton provide new evidence for enriching and comprehensively understanding the food web of aquatic ecosystems.The synergistic effect analysis of interference on the target pressure-response relationship provide a basis for understanding the complex relationships and dynamics of the system,and provide a scientific reference for the effective management of eutrophication and cyanobacteria blooms.