| Due to the eutrophication of water bodies caused by human activities and global warming,cyanobacterial blooms are increasing in frequency and intensity globally,especially,a large number of filamentous cyanobacteria have been frequently detected in recent years.Pseudanabaena sp.,as common haramful filamentous cyanobacteria,can produce odor substances,deteriorate water quality,and threaten human and aquatic ecological health.However,current research on the filamentous cyanobacteria Pseudanabaena sp.mainly focuses on the environmental factors that affect its abnormal proliferation,the pathways for synthesizing taste and odor compounds,and the methods for efficient removal,but the occurrence mechanism of different degrees of Pseudanabaena sp.bloom in lakes and its influence on phytoplankton community structure are still poorly understood.Elucidating the occurrence mechanism of Pseudanabaena sp.blooms at different levels and clarifying its influence on the structure of phytoplankton communities is the key to ensure the health of water ecology,therefore,the study of the occurrence mechanism of Pseudanabaena sp.blooms and its influence on the structure of planktonic algal communities is of great significance.Luoma Lake,with the dominant cyanobacteria Pseudanabaena sp.,is located on the East Route of the South-to-North Water Transfer Project.The water transfer project has indeed solved the problem of water shortage and uneven distribution of water resources in arid areas,but they also disturb the water environment by transferring water between isolated water bodies.The water quality of lakes involved will change due to water mixing and water quantity increase.Phytoplankton community is the first to respond to the external environment change,and it responds through a rapid growth rate and short life cycle.Therefore,scientists all over the world are very concerned about the impact of water transfer on the phytoplankton community.But so far,few studies have revealed the effects of water transfer projects on phytoplankton communities through long-term monitoring.Given the sensitivity of phytoplankton to water mixing,it is increasingly urgent to study the impact of water transfer projects on water ecology.Given the lack of research on patterns of inter-annual variability in the dominant cyanobacterial genus Pseudanabaena sp.and interrelationships between Pseudanabaena sp.and other algae.Based on the long-term data(March 2011～December 2021)of phytoplankton at six sampling points in Luoma Lake,this paper utilized various ecological methods,such as the annual trend analysis,niche width analysis to identify the change of Pseudanabaena sp.,and refers to the bloom threshold(cyanobacterial density over 200×106 cells/L is the high level cyanobacterial bloom;cyanobacterial density between 100 and 200×106 cells/L is the medium level cyanobacterial bloom;and cyanobacterial density between 20 and 100×1l06 cells/L is the low level cyanobacterial bloom)to divide the level of Pseudanabaena sp.bloom.The results show that non-filamentous cyanobacteria had statistically significant decreases(p<0.05)in monitoring years,especially in the case of Microcystis,Chroococcus,and Merismopedia;Microcystis had an average decline of 1.5 percent per year,and the density of both Chroococcus,and Merismopedia decreased by more than 1%.Surprisingly,most filamentous cyanobacteria had statistically significant increases(p<0.05),Among them,the growth of Pseudanabaena sp.is particularly prominent,and its density increases by about 2%every year.From the niche breadth of dominant cyanobacteria genus,it is found that Pseudanabaena sp.has the largest niche breadth,and compared with other cyanobacteria genus,Pseudanabaena sp.has better environmental adaptability and stronger competitiveness.In addition,the maximum Pseudanabaena sp.density in summer and autumn is greater than 20×106 cells/L,and different degrees of Pseudanabaena sp.blooms occur in Luoma Lake every year.To elucidate the occurrence mechanism of different levels of Pseudanabaena sp.blooms,and to propose prevention and control strategies for Pseudanabaena sp.bloom in Luoma Lake.The contribution rates of the environmental drivers to the occurrence of different levels of Pseudanabaena sp.blooms were calculated and suitable environmental conditions for the occurrence of different levels of Pseudanabaena sp.blooms were obtained by nonlinear regression model.The following conclusions were drawn:when the water body of Luoma Lake is eutrophic(TSI>50),there is a potential to occur low-level Pseudanabaena sp.blooms.When the water body is eutrophic(TSI>50)and low-level bloom occurs,with TN ranging from 1 to 2 mg/L,coupled with suitable water temperatures(WT>25℃),we should be alerted to the occurrence of medium-level Pseudanabaena sp.blooms.In addition,the occurrence conditions for high-level Pseudanabaena sp.bloom are more stringent.When the water body is eutrophic(TSI>50)and medium-level bloom occurs,with TN ranging from 1-2 mg/L,coupled with suitable water temperatures(20℃<WT<30℃)and water depths(2～4 m),we should be vigilant about the occurrence of highlevel Pseudanabaena sp.bloom.Prevention and control measures for Pseudanabaena sp.blooms in Luoma Lake:control the input of external nitrogen and phosphorus nutrients as well as the removal of existing nitrogen and phosphorus nutrients in the lake;in addition,as a storage lake,Luoma Lake has hydraulic conditions different from those of other lakes,and it is also possible to control the occurrence of Pseudanabaena sp.blooms by adjusting the water depth through water transfers.Furthermore,we used network analysis and non-metric multidimensional scale analysis to study the effects of different levels of Pseudanabaena sp.blooms on the phytoplankton community structure,and found that diversity and evenness indices of algal communities decreased significantly during high bloom levels but did not change significantly under low and medium levels of bloom;moderate Pseudanabaena sp.blooms stabilized communities more than low levels of blooms,in contrast to high levels that may destabilize phytoplankton community structure.Moreover,in addition to the variation of cyanobacterial density,and the community recovery time at different Pseudanabaena sp.blooms level was also different.The community under low-level bloom usually recovered in October every year,while the phytoplankton community was disturbed by medium-level and high-level bloom usually recovered in November or even December of that year.In addition to facing the problem of Pseudanabaena sp.blooms in summer and autumn every year,Luoma lake,as an important storage lake in the East Route of the South-to-North Water Transfer Project,was officially put into operation in October 2013.The effect of the water environment changed under the pressure of water transfer on the phytoplankton community need to be elucidated.Based on the water quality and algal data before and after water diversion,we used community stability analysis,niche breadth analysis,community similarity analysis and redundancy analysis to conclude that the the first decrease and then increase of nitrogen nutrient and increase of phosphorus nutrient after water diversion.Total algal density and community diversity remained unchanged,while the duration of high cyanobacterial density shortened,and phytoplankton community composition changed with the change of water environment.More compact phytoplankton communities appeared before water transfer,revealing that water mixing does have negative effects on the compactness of community structure.The stability of the algal community decreased in the initial stage of water transfer but began to recover in 2019.Moreover,water transfer also has an effect on the phytoplankton niche width,water diversion caused a significant decrease in the niche width of Cyanobacteria,while the niche of Euglenozoa increased significantly.In addition,the changes in community structure were due to the interference of the external environment.The results of the Mental test showed that the change of water environment under the pressure of water transfer significantly affected the community structure of phytoplankton,and the nitrogen and phosphorus nutrients also played an important role in determining the phytoplankton community structure.Furthermore,the forms of nitrogen salts that affect phytoplankton changes are different before and after water transfer,NH4-N were dominant variables related to variations in phytoplanoton community composition before water diversion,while NO3-N had a stronger effect on phytoplankton composition after water diversion. |
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