Research On Spatial And Temporal Variation Characteristics Of Algae And In Situ Algae Control Study In A Southern Water Source Reservoir

In recent years,under the influence of climate warming and extreme weather,lake and reservoir eutrophication has become one of the most serious water environment problems worldwide,even posing a serious threat to water ecosystems and public health safety.The climate in the southern tropical region is mild all year round and the seasons are not distinct.In order to understand the spatial and temporal dynamics of algae and environmental factors in a water reservoir in this region and the relationship between algae and environmental factors,phytoplankton and environmental factor monitoring was conducted at several sampling sites in a southern reservoir from December 2021 to June2022 in a tropical region of South Asia with a mild climate all year round and indistinct seasons.Affected by algae overgrowth,the Water Lifting Aerators system was turned on in mid-June 2022,and its operating conditions were divided into intermittent operation（8 h/d）and full load（24 h/d）operation.The changes in environmental factors and algae were monitored before and after the operation of the system control area to investigate the effect of in situ algae control under different working conditions.The main findings of this paper were as follows:（1）Spatial and temporal characteristics of phytoplankton.There were some differences in the structure of the algal community in different seasons,but cyanobacteria were always the dominant phylum and the spring cyanobacteria were at the absolute dominant level.The functional groups in different seasons had obvious succession characteristics,which showed that the functional group S1 in autumn changed to the functional group SN in spring and summer,but the relative density of functional groups in different sampling points of the reservoir was not significant.The density of algae generally showed a decreasing trend from the east（not directly affected by the water diversion area）to the west（directly affected by the water diversion area）.The ecological health index（Q index）indicated that the water body condition was better in autumn than in spring and summer,which was consistent with the diverse characteristics of different seasons.（2）The seasonal variations of WT,DO and Chl a were greater in spring and summer than in autumn,while TN and TP were greater in spring>autumn>summer.WT,DO and Chl a were greater in the non-directly diverted area than in the directly diverted area,especially in spring and summer,but TN,NO₃^--N and TP were smaller in the non-directly diverted area than in the directly diverted area.NH₄⁺-N,unlike other indicators,did not have significant spatio-temporal variation characteristics.The results showed that WT influenced the significant environmental factors of spatial and temporal variation of algae,followed by DO,p H and nitrogen.The trophic level index（TLI index）indicated that the reservoir was in a mesotrophic state in autumn,a mildly eutrophic state in spring,and changed from a mildly eutrophic to a mesotrophic state in summer.（3）The algae control effect and the effect on the algal community structure of the Water Lifting Aerators system under different operating conditions.Cyanobacteria,green algae and diatom densities were significantly reduced during intermittent（8 h/d）and continuous（24 h/d）operation,with the greatest reduction in filamentous cyanobacteria dominated by P Pseudanabaena,Cylindrospermopsis raciborskii,and Raphidiopsis curvata.The reduction rates of total algae,cyanobacteria,green algae and diatoms were37.8%,39.3%,38.6%and 17.2%for the 8 h/d operation and 93.4%,96.8%,82.0%and 81.3%for the 24 h/d operation,respectively.After the operation of the system,the algal community structure of the reservoir did not change significantly during the high algae period,which always showed cyanobacteria>green algae≈diatoms,but showed a trend of cyanobacteria to diatom succession during the low algae period,and the genus level showed a gradual succession of the genus Raphidiopsis curvata/Pseudanabaena to the genus Melosira sp.and Cyclotella sp..Therefore,the effect of the Water Lifting Aerators system on the algal community structure in the low algae scenario was more significant than that in the high algae scenario.