Temporal And Spatial Variation Of Nutrient Structure And Its Effect On Phytoplankton In Hanfeng Lake

In aquatic ecosystem,nutrient elements such as nitrogen,phosphorus and silicon,as the basic elements of energy transfer in the ecosystem,provide basic energy for plankton,bacteria,aquatic animals.When the content of nitrogen and phosphorus exceeds the limit,a series of aquatic ecological problems will occur,especially eutrophication of water body.Hanfeng Lake is one of the important water sources in the Three Gorges Reservoir area and its water monitoring plays an important role in protecting the ecosystem of the three Gorges.Therefore,it is of great significance to study the temporal and spatial variation of nutrients in Hanfeng Lake and its effect on phytoplankton characteristics for controlling water pollution,maintaining water quality and protecting water ecological environment in Hanfeng Lake.From June 2020 to May 2021,7 sampling sites were set up in Hanfeng Lake Regulation Dam（HF1）,Intersection of Southeast River（HF2）,Donghe Bridge（HF3）,Donghu County（HF4）,Tou Dao River into Lake（HF5）,Shilongchuan Bridge（HF6）and Zhendong Daqiu（HF7）to collect water samples monthly and determine hydrologic and water quality indexes.In HF1,HF2,HF3,HF7 point setting,monitoring samples of phytoplankton sampling phytoplankton species,density,biomass,such as data,has been clear about the han feng lake water body water quality index,nutritional quality,such as nitrogen,phosphorus,than as well as the spatial and temporal distribution of phytoplankton and its change rule,and explore the phytoplankton and the relationship between the nutrient structure mass ratio.The main results are as follows:（1）TN,TP and DSi ranged from 0.89-3.94 mg/L,0.04-0.40mg/L and 7.05-33.89mg/L,with mean values of 1.75 mg/L,0.14 mg/L and 19.13 mg/L,respectively.DTN,DIN and NO₃^--N accounted for 86.12%,67.24%and 56.03%of TN,respectively.PP was the main component of TP in summer and April,and DTP was the main component of TP in the rest of the year.In terms of seasons,nitrogen forms and TP showed the order of spring>winter>summer>autumn,DTP and PO₄^3--P showed the order of winter>spring>autumn>summer,PP showed higher in spring and summer than in autumn and winter,DSi showed higher in spring and autumn and lower in summer and winter.At the point,except for the east River Bridge（HF3）,the concentration of nitrogen and phosphorus increased from the middle of the lake to the upstream,downstream and tributaries.The concentrations of DSi in the middle and lower reaches（HF1 and HF4）were lower than those in the upper and tributaries.On the vertical plane,TN,DTN and DIN show the order of upper layer>lower layer>middle layer,NO₃^--N,NH₄⁺-N and NO₂^--N reach the maximum value in the lower layer,middle layer and upper layer,respectively.TP,DTP,PP and DSi had no significant difference in vertical plane,and the concentration of PO₄^3--P was significantly different in upper and lower layers and middle layers.（2）The variation trend of different N/P mass ratios is similar.DSi:DTN and DSi:DTP showed similar trends.In season,TN:TP and DTN:TP showed summer>winter>spring>autumn,DIN:TP and DTN:DTP showed summer>spring>autumn>winter,DSi:DTN showed autumn>spring>winter,DSi:DTP showed autumn>summer>spring>winter.At the point,HF3 was the largest in all nutrient mass ratios,indicating that eutrophication was the least.The TN:TP,DIN:TP and DTN:DTP of HF3and HF7 showed significant differences,while there were no significant differences at other points.DSi:DTN is significantly different between HF1 and HF6,and DSi:DTP of Donghe Bridge（HF3）is significantly different from That of Zhendongdaqiu（HF7）and Shilongchuan Bridge（HF6）.（3）During the period of monitoring,a total of 8 phyla,21 orders,38 families,91genera and 256 species of phytoplankton were identified in Hanfeng Lake.The phytoplankton community consisted of green algae and diatoms in winter and spring,and green algae dominated in summer and autumn.The phytoplankton community at the east River bridge was the most abundant,and the community at the regulating dam was the simplest.The biomass of phytoplankton was dominated by diatoms in spring and autumn,green algae in summer,and dinoflagellates in winter.HF3>HF1>HF2>HF7;The phytoplankton density was dominated by green algae in spring,blue algae in summer and autumn,and diatoms in winter.HF2>HF1>HF3>HF7.（4）Correlation analysis showed that the biomass and density of phytoplankton were affected differently by environmental factors in different seasons.In general,the total biomass of phytoplankton in Hanfeng Lake was not limited by DSi and was greatly affected by TP,but DIN:TP and DTN:DTP were better than TN:TP to predict the changes of phytoplankton species.