Spatial And Temporal Variation Of Water Environment And Diatom Community Characteristics Under Different Water Levels In The Three Gorges Reservoir

Maintaining watershed ecosystem health and protecting aquatic ecosystem have become a great significance to the study of ecological environment in the Three Gorges Reservoir area.The main environmental problems in the Three Gorges Reservoir were eutrophication,algae blooms and excessive nitrogen and phosphorus nutrients.In order to maintain water environment safety and water ecological health in the Three Gorges reservoir area,it was necessary to understand the structure,function and process of lake and reservoir ecosystem,so as to set realistic goals of ecological environment protection and restoration.Through scientific quest of algae community succession and habitat change process,the characteristics of the habitat environment and the environmental mechanism change,it can provide scientific basis for the ecological research on the succession and habitat change of algae community in freshwater reservoir and for the management of water environment and water ecology in reservoir.Diatoms were used in some European countries to provide benchmark conditions for the restoration of good aquatic ecosystems.As primary producers,diatomswere of great significance for maintaining the structural stability of the ecosystem in the Three Gorges Reservoir.Hanfeng Lake was the water source of Pengxi River and closely connected with Gaoyang Lake.It was very important for the whole Three Gorges reservoir to maintain the biodiversity of the twolakes.In this paper,Hanfeng Lake and Gaoyang Lake were selected as the research objects.Through annual sampling of water bodies,the characteristics of overlying water environment,it compared and analyzed the temporal and spatial succession characteristics of water diatoms,the community structure characteristics of surface sediment diatoms and the driving factors of diatom community succession under different water levels.It discussed the relationship between water level changes and diatom species,the main contents and research results are summarized as follows:The analysis of the water environment of the lakes throughout a year showed that there were significant differences in water environment between the two lakes,and the nutrient concentration in Hanfeng Lake was higher than that in Gaoyang Lake,mainly because most of the nutrients in Gaoyang Lake were carried away and migrated by current except for the deposition cycle of the nutrients.Except for August,November and December,Hanfeng Lake was eutrophic at other times of the year.Gaoyang Lake was in eutrophication except in August and December.The nitrogen concentration of the two lakes was less affected by water level regulation,and exogenous input and precipitation were the main influencing factors.The phosphorus concentration in Gaoyang Lake fluctuates obviously with the water level regulation,which showed that the water level regulation had a more direct impact on the phosphorus cycle.Water level regulation,precipitation and phytoplankton growth had effects on nitrogen and phosphorus concentration in Hanfeng Lake.Water level fluctuation may be the main influencing factors of nitrogen and phosphorus nutrient concentration in Gaoyang Lake.A total of 97 species belonging to 29 genera of diatoms were identified in the water body of Hanfeng Lake,among which,73 species belonging to 26 genera in the impoundment period,62 species belonging to 20 genera in the fluctuating period,and78 species belonging to 22 genera in the drainage period.A total of 88 species belonging to 27 genera of diatoms were identified in the water body of Gaoyang Lake,among which,75 species belonging to 22 genera in the impoundment period,60 species belonging to 17 genera in the fluctuating period,and 76 species belonging to 20 genera in the drainage period.The genera and species of Melosira granulate,Melosira ambigua and Melosira alpigena are distributed in chains,which are resistant to disturbance and corrosion,and prefer water bodies with high nutrition levels.These genera and species were the dominant species in Hanfeng Lake and Gaoyang Lake throughout the year,indicating that the lake water body was disturbed frequently and the nutrition level was high,which was consistent with the results of environmental monitoring.There were 143 species belonging to 32 genera of diatoms and 99 species belonging to 33 genera of diatoms in the surface sediments of Hanfeng Lake and Gaoyang Lake,respectively.In surface sediments,the number of dominant species of planktonic species was relatively small.The dissolution of diatoms,the uptake of zooplankton,the disturbance of benthic animals and the existence of microorganisms may affect the preservation of surface sediments.The nutrients and water level changes were still the main driving factors for the distribution of diatom community.At different water levels,the main influencing factors of diatom community in Hanfeng Lake were water depth and nitrogen,while the diatom community in Gaoyang Lake was significantly affected by water depth,nitrogen and phosphorus.Water temperature,Chla,NH4⁺-N,TP and NO₃^--N were the main environmental factors affecting the diatom community in the surface sediments of Hanfeng Lake,while Chla,TOC,TN,TP and water depth were the main environmental factors affecting the diatom community in the surface sediments of Gaoyang Lake.Theresults of Non-metric Multidimensional Scaling（NMDS）showed that the distribution of dominant species of Melosira and Cyclotella was similar,and the degree of species similarity was high.The competition between different genera and species would also affect the distribution of diatom communities.The distribution of diatom species in the two lakes may be affected by light,the change of nutrient concentration,the fluctuation of water level and the competition between genera and species.Diatom had significant response to the variation of water level,and the indicator species in response to the variation of water level were different in Hanfeng Lake and Gaoyang Lake.Melosira granata and Asterionella formosa were indicators of water level fluctuation in Hanfeng Lake.Melosira alpigena and Fragilaria capucina were indicators of water level fluctuation in Gaoyang Lake.Although the ecological significance revealed by the variation of diatom community structure and the ecological mechanism driving diatom community distribution still need to be deeply excavated,this study might provide support for the scientific evaluation of the ecological impact of gate and dam construction,and had certain scientific value for understanding the potential impact of hydraulic development on ecological function of rivers and its potential impact under the background of global change.