Study On The Ecological Stoichiometry Of Nitrogen And Phosphorus And The Characteristics Of Phytoplankton Community In Hanfeng Lake During The Operation Period

After the official operation of the Hanfeng Lake in the Three Gorges Reservoir,the water body changed from a special form with the characteristics of lakes and rivers to a lake form,resulting in a large change in the biogeochemical process of the water body.Hanfeng Lake located at the confluence of the Southeast River in the upper reaches of the Xiaojiang River Basin in the Three Gorges Reservoir area.It was built to reduce the impact of the large-scale fading zone on the water quality of the Three Gorges Reservoir area.It's hydrological conditions and ecological environment characteristics were very special.In 2017,the Hanfeng Lake was officially impounded.When the low water level of the Three Gorges Reservoir was operated,the water level of Hanfeng Lake was controlled at 170.8 m?Wusong Elevation?.The dynamic conditions and hydraulic retention time have changed significantly compared with before the storage operation.As the first application of Pre-dam in the Three Gorges Reservoir area,coupled with its huge agricultural non-point source pollution and urban life pollution pressure.Studing on the stoichiometry and indicator phytoplankton after the official operation of Hanfeng Lake,clarified the key factors influencing the nutritional status of the water body was of great reference significance to the water environment protection of Hanfeng Lake and even urban lakes and reservoir banks with similar characteristics in the Three Gorges Reservoir area.To this end,seven sampling points set on the surface of Hanfeng Lake,and the points of Hanfeng Lake sampled in layers from January 2017 to December 2018.This article was based on the physical and chemical parameters monitored by Hanfeng Lake:water depth?H?,temperature?T?,pH,transparency?SD?,dissolved oxygen?DO?total nitrogen?TN?,total phosphorus?TP?,permanganate index(COD_Mn)and other aquatic ecological indicators such as phytoplankton species,cell density and biomass,etc.So we studied the characteristics of nutrient distribution,the characteristics of ecological stoichiometry and the dynamics indicating phytoplankton since the official storage operation of Hanfeng Lake.The main findings as follows:?1?The results of nutrient distribution characteristics showed that the vertical mixing of the upper,middle and lower layers of water in Hanfeng Lake was uniform,and the difference in nutrient concentration wasn't significant?P>0.05?.After the official operation,the concentration of TN and DTN increased slightly in 2018compared with 2017,with increased of 6.1%,9.3%,NO₃^--N concentration decreased by 9.3%,TP concentration increased by 38.5%,and other nutrient concentrations didn't change significantly.The phosphurus of Hanfeng Lake mainly came from the upstream Nanhe and Taoxi Rivers,generally showed a decreasing trend from the upstream Zhendong to the downstream regulating dam.Among the all nutrients,TP was the decisive factor for algae growth in Hanfeng Lake.?2?The research results of indicative phytoplankton indicated that,the phytoplankton which had the highest correlation with the nutritional status of water body gradually changed from cryptoalgae and Euglena at the initial stage of operation to cryptoalgae,cyanobacteria,chlorella and Euglena.cyanobacteria and chlorella had gradually became the dominant algae after the official operation.TP and NH₄⁺-N had the highest contribution to the dominant algae community.?3?The results of the research on ecological stoichiometry of nitrogen and phosphorus showed that:in space,DIN:SRP,NO₃^--N:SRP,NH₄⁺-N:SRP,TN:TP,DTN:DTP of HF3 all reached the highest in 2017,while the various nitrogen and phosphorus stoichiometry most reached the lowest in HF6.the two main rivers into the lake,nitrogen and phosphorus stoichiometry of Nanhe river showed a trend that downstream slightly higher than upstream.nitrogen and phosphorus stoichiometry of Donghe river showed the same trend.In 2018,DIN:SRP,NO₃^--N:SRP,NH₄⁺-N:SRP reached the highest level in HF5,and those stoichiometry in the other points were significantly lower than HF5.TN:TP and DTN:DTP were higher in the lake control section than that in the lake section.The stoichiometry from upstream to downstream generally increased.Observed from the time scale,DIN:SRP and NO₃^--N:SRP of2017 was dynamic throughout the year,and the measurement was relatively stable.NH₄⁺-N:SRP maintains around 1 throughout the year except for the abnormalities in July and August.TN:TP and volatility decreased from January to August,increased rapidly from September to December,and the year-end average was higher than the rest.2018 DIN:SRP and NO₃^--N:SRP increased sharply from January to March,and then the fluctuation decreases.NH₄⁺-N:SRP in addition to the obvious highlight in July,the fluctuations in other months were relatively low at the end of the year.The fluctuations of TN:TP and DTN:DTP decreased from January to October,and gradually increased from October to December.The environmental factors that can regulate the stoichiometry of nitrogen and phosphorus in 2018 were significantly lower than that in 2017.The effect of H on the stoichiometry of nitrogen and phosphorus is greatly reduced.The influence of conductivity and T on the stoichiometry was relatively large.Acidity and alkalinity had greater influence on the stoichiometry of soluble inorganic nitrogen and phosphorus.After the official operation period,Hanfeng Lake greatly weakened the water level fluctuations,and the hydrodynamic conditions were stable.The total amount of nitrogen salts increased slightly,the concentration of soluble inorganic nitrogen salts decreased slightly,and the total amount of phosphorus salts increased significantly.The eutrophication index of the water body increased with the running time.The eutrophic indicator algae showed a tendency to change from Euglena to Cyanobacteria and Chlorella,and these algae in Hanfeng Lake were sensitive to TP and NH₄⁺-N concentrations.As the official operating running,the environmental factors driving each nitrogen-phosphorus stoichiometry gradually decreased.The influence of H on the nitrogen-phosphorus stoichiometry was greatly reduced.The conductivity and T had relatively impact on these stoichiometries.Acidity and alkalinity had a greater influence on the ratio of soluble inorganic nitrogen and phosphorus.