Study On The Causes Of Algal Bloom In Typical Reservoirs Of Lake Taihu Basin

The algal bloom caused by abnormal proliferation of algae is one of the major ecological disasters that threatens the security of many drinking-water resource reservoirs in China.Exploring the driving factors of algae proliferation in drinking-water resource reservoirs is the premise of preventing and controlling algal bloom and water quality problems.In this thesis,Fushi Reservoir and Laoshikan Reservoir and Shahe Reservoir,which are three typical water-source reservoirs in Lake Taihu basin,were selected to investigate the community structure of phytoplankton and associated environmental factors.Combined with historical data,the seasonal abnormal proliferation characteristics of harmful algae in the source reservoir of the Lake Taihu basin and its water quality risk were analyzed.The impact mechanism and threshold conditions of meteorological,hydrological,and water environmental factors on algal blooms were explored.The results are as follows:(1)The annual variation characteristics of harmful algae community structure and water quality risk in typical mesotrophic reservoirs in the Lake Taihu basin were found.By analyzing the annual algae and water quality data of Fushi Reservoir and Laoshikan Reservoi,it is found that the nutrient status of the two reservoirs is generally at the mesotrophic level,which is the most common nutrient status in the water-source reservoirs in the subtropical region of China,but there is also the risk of abnormal proliferation of harmful algae in both reservoirs.The main dominant phyla of phytoplankton in reservoirs are cyanobacteria and diatom.The dominant genera of phytoplankton in Fushi Reservoir are Microcystis,Aphanizomenon,Dolichospermum,Merismopedia,while the dominant genera of phytoplankton in Laoshikan Reservoir is Achnanthes.According to the ecological assessment of phytoplankton diversity,both reservoirs are in moderate pollution state and have risk of eutrophication.Analysis shows that in medium nutrient reservoirs,the ecosystem structure is simple and vulnerable,which is prone to have water quality risk due to environmental changes.(2)It has been revealed that meteorological and hydrological conditions,as well as nutrient conditions,jointly affect the sudden abnormal proliferation process of algae in mesotrophic water-source reservoirs.During the annual observation period,both Fushi Reservoir and Laoshikan Reservoir experienced periodic abnormal proliferation of cyanobacteria and diatom.A serious diatom bloom dominated by Achnanthes occurred in May in Laoshikan Reservoir and the corresponding cell density of diatom reached 6232×10~4 cells/L,while cyanobacterial blooms occurred in June and September respectively in Fushi Reservoir.Result of redundancy analysis indicates that total phosphorus,water temperature and water level are key factors affecting the variation of phytoplankton community structure in reservoirs.A concentration of total phosphorus higher than 0.024 mg/L could promote algae proliferation.The risk of cyanobacteria and diatom blooms increased distinctly when the water temperature was higher than 20 and16℃,respectively,while the best water temperature for cyanobacteria and diatom blooms was30 and 22℃,respectively.The absolute value and amplitude of water level both have an impact on the occurrence of algal bloom.Low or drastic fluctuation of water level could create advantages for algal bloom.During the observation period of two reservoirs,different abnormal algal proliferation characteristics were observed under the same meteorological conditions,indicating the complexity and multi factor synergy of algal bloom risk inducing conditions in mesotrophic water-source reservoirs.Therefore,attention should be paid to water quality risk prevention and control in water source reservoirs.(3)The important impact of nutrient conditions and climate change on the interannual algal regime change and algal bloom risk of water-source reservoirs in the Lake Taihu basin is expounded.Based on the annual observation of algae and environmental conditions in the Shahe Reservoir with the analysis of long-term reservoir water environment observation data from2010 to 2021,it is found that there was an interannual and seasonal succession in the phytoplankton community structure of Shahe Reservoir:from 2010 to 2017,the whole year was dominated by cyanophyta,supplemented by diatoms,chlorophyta and cryptophyta.From 2018to 2021,the summer and autumn were dominated by cyanophyta,winter and spring were dominated by cyanophyta,diatoms and chlorophyta.Then to 2022,the summer and autumn were dominated by cyanophyta,and the winter and spring were dominated by diatoms and cryptophyta.Dominant genera of phytoplankton included Aphanizomenon,Pseudoanabaena,Raphidiopsis,Merismopedia,Achnanthes,Synedra,Cyclotella,Chroomonas,Scenedesmus.The concentration of nitrogen and phosphorus nutrients,temperature and rainfall are the main factors affecting the changes of phytoplankton community structure in Shahe Reservoir.The increase of temperature and phosphorus concentration synergistically promote the proliferation of cyanophyta and diatoms in spring and summer.The decrease in nitrogen concentration has driven the transformation of the dominant genera of cyanophyta from Pseudoanabaena to Aphanizomenon.The impact of rainfall on the proliferation of algae in the Shahe Reservoir is mainly promoting,and drought may lead to a decrease in the number of algae.In brief,typical water-source reservoirs in the Lake Taihu basin are generally mesotrophic,but still have risk of cyanobacteria and diatom blooms.The abnormal proliferation of algae is an important factor affecting the water quality safety of such reservoirs.The abnormal proliferation of phytoplankton or algal bloom is affected by nutrient,hydrological and meteorological factors in mesotrophic drinking-water resource reservoirs.The synergistic promotion of multiple factors is a common mechanism for algae disasters in this type of reservoir.In reservoir analysis and prevention and control,high-frequency monitoring and condition identification of the comprehensive process of multiple factors should be strengthened to improve the water quality safety of the reservoir.