Temporal And Spatial Distribution Characteristics Of Water-sediment Nutrients And Its Influencing Factors In Heihe River

The income and expenditure,level characteristics and circulation process of nitrogen and phosphorus nutrients in rivers play a key role in the operation of watershed aquatic ecosystem.However,with the urgent demand of social and economic development for water resources and energy,human activities(hydropower development,land use,industrial and agricultural activities,etc.)have largely affected the health level of river water ecosystems,and have produced a series of negative effects on the watershed,such as the destruction of river connectivity,river bed erosion,eutrophication,etc.How to improve the quality of water environment has become an urgent problem to be solved in the current comprehensive management of watershed.At present,a large number of studies have been carried out on the temporal and spatial distribution of water quality and its influencing factors at home and abroad.However,few scholars directly pay attention to the characteristics of nitrogen and phosphorus distribution and its influencing mechanism under the combined action of land use change and cascade dam construction in inland areas.Therefore,this paper takes the surface water environment and pollutant sedimentary environment of Heihe River as the research object,and studies the relationship between the spatial and temporal distribution characteristics of nitrogen and phosphorus in water and sediment and human activities(land use,dam storage),in order to provide scientific theoretical support for the protection of water environment in Heihe River Basin.The following are the main results:(1)The nitrogen and phosphorus content of the upper and middle reaches of the Heihe River basically meets the Class Ⅲ water quality standards,and the water quality is generally good,and the nitrogen content is slightly higher than the phosphorus content.The TLI of water body ranged from 38.69 to 49.26,which is at a mesotrophic level,and the TLI of sediments ranged from 0.19 to 2.2,which is at a mild pollution level.In terms of the contribution degree of different water environmental quality indexes to eutrophication,the contribution rate of total nitrogen(TN)is the largest,which is 29.89%,followed by the contribution rate of total phosphorus(TP)and ammonia nitrogen(NH3-N),which is 28.11%and 28.45%,respectively,and the contribution rate of potassium permanganate index(CODMn)is the lowest,which is 13.55%.(2)The nitrogen and phosphorus levels in water and sediment have obvious spatial and temporal differentiations.The upper reaches of damming rivers have higher nitrogen content in sediments and lower water nitrogen contents.The natural river sections in the middle reaches have higher nitrogen and phosphorus contents.The total nitrogen(TN)in the water and the total phosphorus(TP)in the sediments had significant spatial differences simultaneously.And total nitrogen(TN),ammonia nitrogen(NH3-N),total phosphorus(TP)and ammonia nitrogen(NH3-N)were the main contribution factors of water eutrophication in the damming reaches of the upper reaches in the dry and wet seasons respectively.The total nitrogen(TN)and ammonia nitrogen(NH3-N)were the major contributors to the eutrophication in the natural reaches of the middle reaches.In addition,the correlation between water temperature(WT),salinity(SAL),dissolved oxygen(DO)and nitrogen and phosphorus nutrients also has a certain spatial heterogeneity.(3)Land use has a high degree of explanation for spatial and temporal variation of nitrogen and phosphorus nutrients in water and sediment.Specifically,land use had the highest degree of explanation for water TN(R2=0.67),followed by CODMn(R2=0.33).Grassland,arable land and construction land were the main land use types that affected the nutrient distribution of water body.The highest interpretation degree of TP was found in the sediments(R2=0.46).The distribution of P in the sediments was affected by cultivated land,woodland and grassland.The explanatory degree of TN and TOC was the second(R2=0.30 and 0.36).The main land use types affecting the distribution of nitrogen and organic matter in sediments were cultivated land,grassland and construction land.At the spatial scale,the land use pattern in the large scale buffer zone(2km～10km)was more significant in explaining the spatiotemporal variation of nitrogen and phosphorus nutrients,and different land use types played a leading role in different scales.Among them,cultivated land,grassland and construction land had a higher interpretation rate of nutrients in the large-scale buffer zone,while water area had a higher interpretation rate of nutrients in the small-scale buffer zone.(4)Dam impoundment has obvious retention effect on water pollutants,but the retention effect of nutrients varies due to the different storage capacity,water depth and construction time of each reservoir.At the same time,the hypoxia at the water-sediment interface caused by water temperature stratification changes the nutrient environment and weakens the retention effect of cascade reservoirs to a certain extent.From the perspective of release flux,the reservoir with large storage capacity and short construction time has obvious deposition effect on TN,TP,NH3-N and COD,which is manifested as "sink",while the reservoir with small storage capacity and long construction time has obvious release effect on TN,TP,NH3-N,which is manifested as "source".