Dynamic Characteristics Of Cyanobacteria Bloom In Chaohu Lake And Evaluation Of Its Control Techniques

Chaohu Lake is the largest lake in Anhui Province.Because of serious eutrophication,cyanobacteria blooms occur frequently.In recent years,under the vigorous control of the national and local governments,the water quality of Chaohu Lake continues to improve,but the water bloom area is still at a high level.There are new changes in the cyanobacteria bloom from the dominant species to the temporal and spatial distribution.Moreover,the existing control technology of cyanobacteria bloom can not fully meet the treatment needs during the early warning period of cyanobacteria bloom.In order to study the dynamic characteristics of cyanobacteria bloom in Chaohu Lake,field sampling and indoor testing were carried out for one year(August 2019 to July 2020).The physical and chemical indexes of water quality,algal density,chlorophyll-a,algae species,pseudovacuole content,vertical migration rate and vertical percentage of 12 spots in the lake were monitored,and the historical data were collected and sorted out to analyze the dynamic changes of cyanobacteria bloom and the influencing factors.This paper evaluates the effect of the new in-situ deep well pressure algae control technology,and evaluates the technical,environmental and economic benefits of the complete set of in-situ deep well pressure algae control equipment through four months of monitoring.The main results are as follows(1)From August 2019 to July 2020,the results of phytoplankton classification at12 sites in the East and west half of Chaohu Lake showed that cyanobacteria were in the absolute dominant species status in the whole lake.From the eastern and Western Lake areas,the dominant species in the western half of the lake showed seasonal changes,and Microcystis was the dominant species in the high incidence period of cyanobacteria bloom.Anabaena Flos was the dominant species of cyanobacteria in winter and spring from January to May.Except for Microcystis in July and September,the other dominant species were Anabaena aquatica,which had no obvious regularity.It was possible that the dominant cyanobacteria population in the middle and eastern Lake area was still not stable.The results of RDA analysis showed that the algae in Chaohu Lake were significantly affected by physical and chemical factors,TN,TP,water temperature and transparency were the main external factors affecting the cyanobacteria population in Chaohu Lake.(2)Algae biomass can be characterized by algae density and chlorophyll-a,which have the same trend.In terms of seasonal variation,summer and autumn are higher than eastern and spring seasons.The seasonal variation range of chlorophyll-a and algae density were 38.4μg/L-140.3μg/L and 53000-224000 cells/m L,respectively.In winter,the east half lake is higher than the west half lake,and in summer and autumn,the west half lake is much higher than the east half lake.Especially in July,the chlorophyll a is10 times higher than that of the east half lake,and the algae density is close to 50 times.During the period from August 2019 to January 2020,the pseudovacuole volume and algal density had a similar trend,with a maximum value of 23.1μm~3/cell in September.The results show that there is a strong positive correlation between cyanobacteria bloom and pseudovacuole volume.The larger the pseudovacuole volume is,the stronger the cyanobacteria floating ability is,and the easier cyanobacteria are to float and gather to form water bloom.The results of RDA analysis showed that total phosphorus was the most important factor affecting the density of pseudovacuoles and pseudovacuoles.(3)Based on the analysis of water quality and cyanobacteria bloom distribution in Chaohu Lake from 2000 to 2019,the main water quality indicators,total phosphorus and total nitrogen,showed a fluctuating downward trend after 2010,but the water quality remained between the fourth and fifth categories.The average monthly and annual area of algal blooms showed an upward trend.Generally,cyanobacteria blooms began to spread to the whole lake from the nearshore of the western half of the lake.In August and September,Chaohu Lake had the most serious cyanobacteria blooms,and the duration of the blooms mainly fluctuated in 150-200 days.The annual and monthly average of cyanobacteria bloom were analyzed by RDA.The results showed that WT and TP were the main factors affecting the area of cyanobacteria bloom,and do was negatively correlated with the bloom area.(4)The evaluation results are as follows.For cyanobacteria above 0.5 m,the sedimentation ratio can reach 99.7%,and the treated cyanobacteria are evenly distributed in the water column below 0.5 m,and the water quality of the outlet after treatment will not be affected,and it is better than that of the water inlet.The estimated cost is 0.0383 yuan/m3,the operation and maintenance cost is low,and the algae control effect is good.It can be used in the lakeside and nearshore zone of the west half lake where cyanobacteria bloom is high.