Under The Influence Of Hydropower Cascade Development In Heihe River Basin Study On Retention Characteristics Of Nitrogen And Phosphorus

River water quality is the basic condition to maintain water ecological health,but also an important indicator to reflect river water environment.As an artificial system for regulating,storing and controlling water resources,reservoir is an effective way to realize rational development and utilization of water resources.However,the"artificial lake"formed by the reservoir will affect the biogeochemical behavior of biogenic elements in the river（including the transport concentration,morphological composition and transformation mode of biogenic elements）.With the increasing accumulation of sedimentary environment,a large amount of nitrogen（N）and phosphorus（P）nutrients will result in the convergent transformation of sediment deposition,resulting in water eutrophication,affecting the water quality of regional rivers,leading to industrial and agricultural water and drinking water pollution.Due to the influence of altitude and different location,the rivers in the alpine region will show different phenological changes along the river,which makes the transport law of source factors along the river more complicated than that in the low altitude region.Heihe River Basin is located in the Qinghai-Tibet Plateau,and a large number of cascade power stations have been built in its upper main stream,which will change the migration and transformation of nutrients N and P in the reservoir while regulating and utilizing hydropower resources,resulting in the retention and accumulation of a large number of nutrients N and P in the reservoir.This study investigated and studied the water quality in the typical cascade reservoir area of the main stream of Heihe River in different seasons,analyzed the seasonal variation of N and P of water and sediment and their retention characteristics,and provided certain reference value for mastering the water quality variation in this region.The main research results are as follows:（1）The water quality of the typical reservoir area in the upper reaches of Heihe River was classⅡ～Ⅲ,and the water quality was good.The total phosphorus（TP）content in each reservoir was in the order of winter>summer>spring,with the maximum in winter and the average value between 0.146 and 0.353mg/L,and the minimum in spring and the average value between 0.015 and 0.048mg/L.The content of total nitrogen（TN）was also the highest in winter（3.595mg/L）,but the lowest in summer（0.004mg/L）.Ammonia nitrogen（NH₃-N）was different,and the content was the highest in spring,with an average value between 0.218and 0.566mg/L,while the content was similar in summer and winter.The content of nitrite nitrogen（NO₂-N）in different reservoirs in different seasons was relatively low,and all of them were less than 0.100mg/L.The imbalance of nitrogen and phosphorus ratio in each year was 2017>2021>2020>2019>2018.The results of variance analysis showed that the variation of water quality in different seasons was mainly affected by temperature,precipitation and hydrological conditions.There was a strong correlation between N and P nutrients and environmental temperature,and water temperature（WT）was the main factor affecting N and P nutrients in reservoir water.（2）The content of nitrogen and phosphorus in sediment from different reservoir areas in the upper reaches of Heihe River was significantly different,and the spatial difference increased along the river.TP content in reservoir area was characterized by transport and accumulation from upstream to downstream,and its content was related to storage capacity and reservoir regulation.The different nitrogen contents in reservoir sediment was TN>NO₃-N>NH₃-N.The dissolved inorganic nitrogen in the sediment was mainly NO₃-N.The interannual variation trend of TN was increasing year by year,and the reservoir area played a certain role in the retention of N in water.From the perspective of coefficient of variation,N and P indexes were in the medium spatial coefficient of variation.Correlation analysis showed that there was a strong correlation between total nitrogen（TN（S））in sediment and ammonia nitrogen（NH₃-N（S））in sediment,indicating that nitrogen in water and sediment mainly came from the decomposition of organic matter in the river bed.TP had some correlation with DO,p H,WT and SAL.This is consistent with the correlation analysis results of reservoir water,indicating that TP has similar pollution sources in reservoir sediment and water.The gradient change of nitrogen and phosphorus nutrient concentration in reservoir area had little relationship with the diffusion effect of nutrient concentration between water and sediment,and the gradient diffusion is not the controlling factor of nutrient migration and transformation at the sediment-water interface in the upstream dammed reach.（3）The construction of reservoir has a significant retention effect on nitrogen and phosphorus nutrients in reservoir water.On the whole,nitrogen and phosphorus nutrient flux in the reservoir area changed more obviously in winter than in summer.TP ranged from-1.53kg/month to 6.97kg/month.TN ranged from-37.27kg/month to 10.67kg/month,and TN changed more significantly than TP along the river.NO₂-N fluctuated slightly in summer,while NH₃-N changed little in spring and winter.The average annual retention rate of TP in typical reservoir area of the upper reaches of Heihe River was between-169.43%and 37.81%.The average annual retention rate of TN in reservoir area ranged from-74.32%to 36.31%.The analysis results showed that environmental factors such as WT,DO,p H and SAL in overburden water will affect the"sink"and"source"retention of N and P nutrients in the sediment.There were obvious differences in the output of different types of nutrients in both time and space.In the high altitude area,the storage capacity is large,the water depth is small,the WT content is low and the DO content is high,which accelerates the adsorption of phosphorus by zooplankton and aquatic plants,resulting in a low concentration retention rate of TP.For nitrogen,nitrification in surface sediments plays an important role and is influenced by DO and WT values.