The Study On The Sediments Phosphorus Fractions And Its Transformation Mechanism In The Deep Reservoirs Of Lancang River

In recent years,with the vigorous development of water conservancy and hydropower in China,large and super-large reservoirs have emerged.The number of high dams and reservoirs,especially those with a water depth of more than 100 meters,is increasing.Because of their large storage capacity,deep water and strong storage capacity,these reservoirs are significantly different from those of natural rivers and shallow lakes in the water cycle and its associated biogenic material circulation evolution process.Therefore,it is of great significance to study the evolution process and mechanism of water ecological environment of deep-largescale reservoir for comprehensively understanding the environmental impact of damming rivers and deepening the ecological evolution process and mechanism of cascade reservoirs.At present,the research on the ecological evolution of deep-largescale reservoir is still in the initial stage,and the research on the spatial distribution and control mechanism of sediment biogenic elements circulation is not systematic and in-depth.As one of the most important biogenic substances,phosphorus（P）is the main element of biological cell protoplasm,and it is also the energy of biomass metabolism.The enrichment degree of phosphorus in natural water and its circulation and transportation process affect the balance of the whole aquatic ecosystem.Sediment is an important part of aquatic ecosystem and plays an important role in the geochemical cycle of phosphorus.There have been studies on phosphorus cycle,revealing the driving mechanism of phosphorus cycle in lakes and reservoirs,but the understanding of reservoirs over 100 meters deep is obviously insufficient,and there is still a lack of in-depth and systematic research.Therefore,it is of great theoretical significance and application value to study the distribution characteristics,laws,migration and transformation mechanism of phosphorus in sediments of typical deep-largescale reservoir.In this thesis,Xiaowan（XW）and Nuozhadu（NZD）,which are located in Yunnan section of Lancang River,are selected as the research objects.Through field observation,sample collection,experimental analysis and mathematical statistical analysis,the spatial distribution characteristics of phosphorus in sediments of typical deep-largescale reservoirs are studied.By using DGT（Diffusive Gradients in Thin Films）,the undisturbed sediment was observed,and the migration and transformation mechanism of phosphorus in reservoir sediments and its influencing factors were revealed.The main research results are as follows:（1）On the basis of monitoring the water environment and water quality indexes of reservoirs,the surface sediments along XW and NZD reservoirs were collected and the phosphorus fractions were analyzed in laboratory.The results showed that the total phosphorus in the surface sediments of the two reservoirs was at a low pollution level,and the residual phosphorus accounted for the highest proportion of phosphorus fractions.The contents of calcium-bound phosphorus and iron-manganese chelated phosphorus in the sediments of the two reservoirs are significantly different,and the spatial distribution along the river changes obviously at XW dam site.The phosphorus inertia of the upstream sediments is higher,and the phosphorus activity of the downstream sediments is higher.Thermal stratification in deep-water leads to vertical differentiation of water conductivity,p H value,dissolved oxygen,etc.with the depth of water.The overlying water of sediment is anoxic and shows certain reducibility,which is easy to cause the migration of sediment phosphorus,and there is a great risk of endogenous release.（2）The vertical distribution and bioavailability of phosphorus fractions in sediments of XW and NZD reservoirs were analyzed by improved chemical continuous extraction method.The analysis results of the vertical change of phosphorus fractions show that the total phosphorus,iron/aluminum phosphorus and residual phosphorus decrease with the depth,indicating that human activities have affected the phosphorus deposition process.Using one-way ANOVA,the differences of phosphorus fractions in sediments of two reservoirs were analyzed.The results show that,due to the interception effect of cascade reservoirs and the difference of non-point source input,there are significant differences in phosphorus fractions in sediments of NZD and XW reservoir.The activity of phosphorus in sediments of NZD reservoir is higher,and the average bioavailable phosphorus is three times that of XW reservoir.（3）Using DGT technology,the sediment cores of XW and NZD reservoir were synchronously observed by multi-elements,and the available P and Fe were analyzed by linear regression.The results show that Fe is still the main factor controlling the migration and transformation of sediment phosphorus in the 200-meter deep reservoir,but the control degree of sediment phosphorus in the two reservoirs is obviously different,and the supplement degree of solid active phosphorus in sediment of XW to pore water is higher.（4）The dynamic characteristics of phosphorus adsorption/desorption in XW and NZD sediments were studied by designing indoor simulation experiments of low concentration and high concentration.The results showed that the fast adsorption stage of phosphorus in the surface sediments of XW and NZD was mainly between 0～1h.The maximum adsorption rate of phosphorus by sediments occurred within 0～5min.The adsorption capacity of sediment to phosphorus can be well fitted by Langmuir model.Surface sediment of XW has a stronger capacity to receive pollutants,while surface sediment of NZD has a stronger adsorption capacity.EPC₀ in XW and NZD sediments has a significant positive correlation with Fe and Mn.