Spatial-temporal Variations Of Functional Groups In Benthic Algae And Theirs Driving Factors In The Tributaries Of The Three Gorges Reservoir

A large area of backwater has been formed in the tributaries due to the construction of the Three Gorges Reservoir.Water storage brings some problems,such as a rise of the water level in the backwater area,a width of water surface,a decrease of flow velocity and self-purification ability of the water body,which makes the sediment,nitrogen and phosphorus brought by the upstream water settling and accumulating in large quantities.At the same time,due to the reservoir's storage function and the disturbance of water flow,the nitrogen and phosphorus in the sediment return to the water,resulting in a significant difference in the hydrological situation and the basin environment of tributaries in contrast to the natural rivers.Notable changes in tributary environment affect the community structure and function of benthic algae enormously.Benthic algae showed that the characteristics of widely distributed,easily to collect,rapidly reproduction,short growth cycle and quickly response to changes in the physical and chemical environment of the water body,therefore,they are often used as an important biological indicators to monitor water environmental.Functional groups,based on plant morphology,physiology and ecological characteristics,classify similar environmental adaptability into a group,which create correspondence between plants and habitats and can achieve the goal of biological indicators of the environment.Therefore,a better understanding of the correlation between benthic algae functional characteristics and environmental factors,using it in the assessment of aquatic ecosystem and water health will have enormous application value.However,few studies about benthic algae and its functional groups were implemented in the Three Gorges Reservoir.Therefore,based on the characteristics of different hydrological periods of Meixi River,a tributary of the Three Gorges Reservoir,physical and chemical parameters of benthic algae were sampled and monitored and benthic algae functional groups characteristics of different hydrological periods were analyzed in normal water period?March 2017?,flood period?August 2016?and drought period?November 2016?.As well as,the relationship between functional groups of benthic algae and environmental factors in Meixi River were also discussed by a mathematical statistics.The results were showed as follows:1)106 species?including varieties?of benthic algae belonging to 47 genera and 5phyla were detected at 26 sampling sites in three hydrological periods.The cell density of benthic algae ranged from 262.1 to 5.3 x 10⁵/cm².The species belonging to Bacillariophyta are an absolute predominance in the three hydrological stages,followed by the species belonging to Cyanophyta and Chlorophyta,and lowest number was found in Gymnophyta and Cryptophyta.The composition of benthic algae abundance had strong correlation with seasonal change,and the overall abundance showed as drought period>normal period>flood period.While the diversity of benthic algae in different periods was found as flood period>drought period>normal period.In the three hydrological periods,21 functional groups were found in benthic algae,including C,D,E,F,H₁,H₂,J,Lo,M,M_P,N,P,S₁,T,T_B,Tc,T_D,W₁,X₁,X₃ and Y groups.Among them,the functional groups of Y,E,M_P,X₃ and Lo were found in oligotrophic water bodies,the functional groups of D,H₂,T_D and Lo were occurred in medium nutrient water bodies,and the functional groups of X₁,C,J,H₁,M,T_C were determined in eutrophic water bodies.2)The water temperature of Meixi River ranges from 7.59 to 28.97?in the three hydrological periods,the water temperature of every sample point in the high water period is obviously higher than that of the other two hydrological periods,and there is little difference between the water temperature of each sample point in the low water period and the flat water period;the change of dissolved oxygen in the drought period>the horizontal period>the flood period,the dissolved oxygen in the backwater area is obviously lower than that in the natural river section in the low water period;the electrical conductivity has little difference in these 3 hydrological periods,and has an overall increasing trend from upstream to downstream gradually,what more,the backwater area has a larger parameter than natural river section;the flow velocity in each period is shown as the natural river section>the backwater area;most of the sample points in the 3 periods are below 100 NTU,sample turbidity of S4 and S13 in the plain period is much greater than that of other sample points,with 2800 NTU.The average concentrations of TN and TP in the 3 hydrological periods were 1.68 mg/L and0.221 mg/L respectively,thus the eutrophication of Meixi River was serious from the point of nutrient content.3)Through multivariate stepwise regression analysis and RDA analysis,it is found that the most important environmental factors affecting the functional group density of the Meixi River during the wet season are V,WT and PO4;the most important environmental factors affecting the functional group density in the dry season are TN,pH,SPC and V;The main environmental factors affecting the density of functional groups in the flat water period are SPC,V,WT and TN.The functional groups in the three periods all changed with the environment.Among them,TN,WT,SPC and V had significant effects on the functional groups in the three periods.Therefore,these environmental factors should be concerned in the water quality management process of Meixi River.