Temporal And Spatial Variations Of Typical Biogenic Factors Ecological Stoichiometry And Their Effects On Phytoplankton In Hanfeng Lake

The Three Gorges Reservoir adopts the operation scheme of"storing clear water and discharging turbidity"and the operation mode of"low water level in summer and high water level in winter",so that the reservoir area along the river has formed a water-leveling zone with the maximum water level drop of 30m,which leads to the increasing pressure on the ecological environment of the reservoir area,the reduction of environmental carrying capacity,and the contradiction between man and nature more prominent.The water environment in the reservoir area is more fragile than before.Some reservoir bays and tributaries appear bloom phenomenon for many times,and eutrophication is aggravated,which has become the focus of social attention.Hanfeng Lake is located in the upper reaches of Xiaojiang Basin,which is the largest pre-reservoir in the Three Gorges Reservoir area.During the operation of low water level in the Three Gorges Reservoir area,the water level of Hanfeng Lake is controlled at170.8m,which can reduce the influence of large area fluctuation zone on the water quality in the Three Gorges Reservoir area.The regulation of Hanfeng Lake is of great significance to the improvement of water environment in the reservoir area.However,in recent years,the phenomenon of water bloom has occurred several times and the water body has a trend of eutrophication.The change of water quality has been widely concerned by scholars at home and abroad.Based on this,this study took Hanfeng Lake,a pre-reservoir in the Three Gorges Reservoir area,as the research object from July 2021 to June 2022,and set up 7sampling points on the lake surface to explore the concentration of typical biogenic factors in Hanfeng Lake basin and the spatial and temporal distribution characteristics of the measurement and their mutual relations,and analyze their effects on the growth of phytoplankton.In order to provide scientific basis for the improvement of water environment quality,water pollution treatment and water ecological protection of Hanfeng Lake basin,and provide scientific theoretical support and decision-making reference for the green ecological development and comprehensive treatment of Hanfeng Lake basin.The main research results are as follows:（1）The results showed that the concentration of DOC in Hanfeng Lake was significantly higher in summer than in spring and winter.DSi concentration in spring and winter was significantly higher than that in summer and autumn.TN concentration in spring and summer was significantly higher than that in autumn and winter.TP concentration was significantly higher in summer than in other seasons.The concentrations of NH₄⁺-N and DO are low in summer,while the concentration of NO₃^--N is stable.The concentration of Chl-a in spring was significantly higher than that in other seasons.In terms of spatial variation,DOC concentration was the lowest at section5#and the highest at section 2#.The concentration of DSi at 3#section was significantly lower than that at 4#section,5#section and 6#section.TN concentration at 6#section is significantly higher than that at 1#section,3#section and 7#section.TP concentration was the lowest in section 1#and the highest in section 6#.The concentration of Chl-a was the lowest in section 7#and the highest in section 2#.The concentration of NH₄⁺-N was the lowest in section 7#and the highest in section 4#.The concentration of NO₃^--N was the lowest in section 3#and the highest in section 6#.The DO concentration was the lowest in section 5#and the highest in section 4#.（2）The spatial-temporal variation of stoichiometric ratios showed that the Hanfeng Lake DOC:DSi:TN:TP was smaller than the optimal growth element ratio of aquatic organisms proposed by Redfield 106:16:16:1.Both DOC:DSi and DOC:TN were higher in summer and autumn,and they were significantly positively correlated.The nutrient absorption intensity of algae is high,so the nutrient is not easy to accumulate,and the phytoplankton synthesizes DOC after absorbing nutrients,resulting in the change of metrology ratio.DSi:TN is greater than 1:1,which is the most suitable for the growth of diatoms.The Si element in Hanfeng Lake is abundant.Compared with Si element,it is mainly N element that restricts the growth of diatoms.The overall ratio of TN:TP to Redfield is close,indicating that Hanfeng Lake is suitable for algae growth in most of the year,and the P element fluctuation is the main influencing factor leading to the change of TN:TP.（3）Temporal and spatial changes of phytoplankton showed that:Seven species of algae were detected in Hanfeng Lake,including cyanobacteria,green algae,diatom,cryptophyta,dinoflagellate,nude algae and chrysoflagellate.The total biomass of algae was higher in spring and winter and lower in summer and autumn.Dinoflagellate and diatom were the two dominant algae in Hanfeng Lake,dinoflagellate was absolutely dominant in spring and winter,while diatom was the dominant species in summer and autumn.The environmental factor that caused the change of the dominant position of the two algae was the nitrogen and phosphorus ratio.The nitrogen and phosphorus ratio of diatom,green algae,cryptoalgae and other algae was between 12:1 and 16:1,while the nitrogen and phosphorus ratio of dinoflagellate was between 16:1 and 22:1.Step linear regression analysis showed that NO₃^--N concentration had an effect on the biomass of green algae,diatoms,dinoflagellates and eukaryotes,among which NO₃^--N concentration had a negative effect on green algae,diatoms and eukaryotes,and a positive effect on dinoflagellates.NO₃^--N is a controlling nutrient,and its concentration fluctuation would change the nitrogen and phosphorus ratio in water and further affect the composition of algae community.The total biomass of phytoplankton was the highest in section 6 and the lowest in section 7,because the Nanhe River,where section6 is located,was the most seriously polluted by agricultural non-point sources and had the highest degree of eutrophication.Except for diatoms,other algae had the highest biomass at section 6#,while diatoms had the highest biomass at section 3#.The reason for this phenomenon may be that the concentration of NO₃^--N at section 6#was the lowest,and too high concentration of NO₃^--N would have a negative effect on the growth of diatoms,and NO₃^--N had a greater negative effect on diatoms than that of green algae and nude algae.In conclusion,the water environment of Hanfeng Lake is suitable for the growth of phytoplankton,and the biomass of phytoplankton is the highest when the high water level is running,and the dominant species is dinoflagellates.NO₃^--N is the controlling factor affecting the growth of phytoplankton,and the increase of its concentration will have a positive effect on the growth of dinoflagellates.