Spreading Of Cyanobacteria Blooms From Lake To Connected River And Its Water Quality Effects

Lake eutrophication has become a global water pollution problem,which seriously affects the balance of aquatic ecosystem.River-lake connectivity is a common engineering method to protect the ecological health of urban water,which can effectively alleviate the degree of cyanobacteria bloom in eutrophic lakes.However,the algae particles carried by the outflow of the lake will have a significant impact on the water quality.In order to analyze the impact of water diversion from eutrophic lakes on the water quality of their connected rivers,an important research area will be selected from Taihu Lake.Taking Liangxi River,the connected River in Meiliang Bay,as an example,high frequency monitoring of algae particle flux and water quality will be carried out daily from June to August in 2019,The variation of particle flux of cyanobacteria bloom and its influence on river water quality were analyzed.On this basis,in order to meet the demand of real-time control of algal particle flux in the river connected with heavy cyanobacterial bloom lakes,combined with the above high-frequency algal particle flux survey,based on the horizontal and vertical distribution survey of algal particles in Liangxi river,a high-frequency automatic monitoring method of algal particle flux in the river was developed by using the shore based automatic water quality monitoring system real time on-line monitoring of particle flux.In addition,the differences of phytoplankton community structure and water quality in different water of Wangyu River and Western Lake in Gonghu bay were studied from September to December 2020.The main results are as follows:（1）The results show that the river is U-shaped,and the spatial distribution of algae particles is characterized by high surface layer,low bottom layer and slightly high coastal zone.The chlorophyll a concentration obtained by the automatic monitoring system of near shore surface sampling is 30%higher than the average value of the whole section.During the monitoring,the change of algal particle flux of automatic and manual monitoring was consistent on the whole,and the total amount was the same,the ratio was 1.07,which accurately reflected the change rule of algal particle flux of the whole river section.Using this system,it was revealed that the algal particle flux from Meiliang Bay to Liangxi River was mainly controlled by hydrological factors,and the diversion flow,temperature and nutrients were the main influencing factors of algal particle flux.（2）The improvement of the algal cyanobacteria bloom in Meiliang Bay and the operation of the pressure-controlled algae well at the entrance of the river significantly reduced the concentration of chlorophyll a and the flux of algae particles in the Liangxi River.The average value of the concentration of chlorophyll a and the flux of algae particles in the river in summer2019 were 54.34μg·L^-1 And 84.7 t·d^-1,significantly lower than 2017;The water diversion has a significant effect on improving the water quality of the receiving water.Except for DTP,the nitrogen and phosphorus concentrations of the remaining forms of the Liangxi River showed a downward trend from 2017 to 2019,indicating that the water quality of Liangxi river has been improved after water transfer;A large amount of cyanobacterial blooms entering the channel significantly increased the particulate nitrogen and phosphorus content of the water.From 2017to 2019,the nitrogen and phosphorus in the Liangxi River were mainly PN and PP,accounting for 62.5%and 70.8%of TN and TP,respectively;The water quality of the Beijing-Hangzhou Grand Canal and other connected rivers has not been affected by the algal particles in Meiliang Bay.In August 2019,the chlorophyll a in the canal water decreased by 65%compared with June,indicating that cyanobacterial bloom particles have not accumulated in the Grand Canal;On the premise that the cyanobacteria bloom in the lake has not been effectively improved,the algae particles carried by the water diversion will have an impact on the water quality and landscape of the local reach connecting the river.（3）The study of algae community structure and water quality in different waters showed that there were 65 species of phytoplankton in 7 phyla from Gonghu bay to Wanshan lake.The algal biomass and chlorophyll-a of different water bodies along the line showed a decreasing trend on the whole in space.The algal biomass of Wanshan lake was 44%lower than that of Gonghu Bay,and the proportion of Cyanophyta was also decreased on the whole.The water quality of Wanshan lake was different due to different pollution sources.The water body of Wanshan lake was in eutrophication,and the nitrogen and phosphorus indexes were higher than that of Gonghu Bay Chl.a concentration was positively correlated with WT,DO,TP and permanganate index,and negatively correlated with SD,DTN and DTP.RDA analysis showed that 13 dominant species were significantly correlated with 11 environmental factors.Water temperature,p H,suspended matter,permanganate index,ammonia nitrogen,nitrate nitrogen and phosphate were the main environmental variables affecting the succession of phytoplankton community structure.Under the condition that the degree of cyanobacteria bloom in Taihu Lake has not been effectively improved,the automatic monitoring system of water quality connecting Liangxi River to monitor the algae particle flux found that the nutrient and chlorophyll-a concentration of Liangxi river was greatly affected by the water from Meiliang Bay.In addition,the water quality and algal community structure of Wangyu River and its west lake connected with Taihu Lake are obviously different.Different pollution sources lead to different water quality and different contribution to different algae.