Transformation And Accumulation Effect Of Nitrogen And Phosphorus In Water Under Cascade Hydropower Development Of River

River is the main channel for the transport of nitrogen and phosphorus from land to sea,and it is also an important place for biogeochemical cycle.In recent decades,with the continuous advancement of the construction of river cascade water conservancy projects in Southwest China,the original natural environmental conditions of the river have been changed,which plays an important role in the downstream water environment and material migration and transformation process.There have been a lot of studies on the spatial and temporal distribution,dynamic changes and influencing factors of nitrogen and phosphorus in cascade reservoirs.However,the mechanism of exogenous input on the original nutrient transport process and the dominant factors of nutrient transformation in the reservoir are still unclear.In order to explore the accumulation and transformation mechanism of nitrogen and phosphorus in the reservoir under cascade damming,this study takes the international river Lancang River and the important tributary Jinsha River of the Yangtze River as the research objects.Through multiple water environment surveys and monitoring along the river from 2020 to 2021,combined with multi-year field monitoring data from 2016 to 2019,the GlobalNEWS2 watershed nutrition output model and the nitrogen and phosphorus speciation transformation model in the reservoir were used to compare and analyze the pollution input flux and nitrogen and phosphorus input flux in the sub-basins of the two cascade reservoirs.The main influencing factors and influencing mechanisms of nitrogen and phosphorus retention rate and speciation conversion rate in each reservoir were explored.The influence process of on-way input on nitrogen and phosphorus flux and form transformation was clarified,and the cumulative effect of river cascade hydropower development on nitrogen and phosphorus production in water body was revealed.The main results are as follows:(1)The nitrogen input along the Lancang River Basin is 293.8*102 Gmol/yr,and the phosphorus pollution load is 12.8*102 Gmol/yr.The nitrogen pollution load along the Jinsha River Basin is 98.1*102 Gmol/yr,and the phosphorus pollution load is 35.2*102 Gmol/yr.The nitrogen and phosphorus pollution loads are mainly derived from non-point source pollution such as human activity emissions and rainfall erosion.Among them,the nitrogen pollution load in the two basins was mainly DIN,and the phosphorus pollution load was mainly PP,accounting for about 50%and 60%of the total input,respectively.(2)In the cascade reservoirs of the Lancang River and Jinsha River Basin,the inflow fluxes of TN,TP,NH4+and SRP are greater than the outflow fluxes,and the inflow fluxes of NO3are less than the outflow fluxes,that is,nitrogen and phosphorus are retained and transformed in the reservoir.Among them,the average retention rates of TN and TP in Lancang River were 7%and 25%respectively,and the average retention rates of TN and TP in Jinsha River were 9%and 23%respectively.There was no significant difference between the two rivers.The average conversion rates of NH4+in Lancang River and Jinsha River were 17.7%and 3.5%,respectively.The average conversion rates of SRP were 6.7%and 2.4%,respectively,while the average conversion rates of NO3-were-8.8%and-2.8%,respectively.In the reservoirs of the two rivers,the conversion rates of NH4+and SRP were positively correlated with HRT,while the conversion rate of NO3-was negatively correlated with HRT.(3)After stripping the input along the cascade basin,the fluxes of nitrogen and phosphorus and their various forms decreased significantly.Among them,TP flux decreased most significantly(p<0.05),and TN flux decreased overall but showed an upward trend along the way.In terms of nitrogen and phosphorus forms,the proportion of SRP and NH4+in TP and TN showed an increasing trend along the way,and the proportion of outflow was higher than that of inflow.The proportion of NO3-decreased along the way,and the proportion of outbound was lower than that of inbound.Through the research,the main conclusions are as follows:(1)The Lancang River Basin and Jinsha River Basin are dominated by nitrogen source input along the way,mainly due to the non-point source pollution such as agricultural fertilizer residues and animal manure in the confluence area.The nitrogen source input of each reservoir in the confluence area is mainly contributed by the dissolved non-point source input generated by land use and livestock and poultry breeding.The phosphorus source input is mainly PP formed by the adsorption of residual phosphorus in soil and particulate matter.(2)TN mostly exists in the dissolved state,which is mainly affected by the transformation in the reservoir and continues to be transported downstream.TP is mainly in the granular state,and HRT is the key factor that makes PP settle.The retained nitrogen and phosphorus increased the relative content of NH4+and SRP,reduced the relative content of NO3-,and improved the availability of nitrogen and phosphorus transported downstream.According to the single factor linear regression analysis,there was a positive correlation between the conversion rate of NH4+and SRP and HRT.(3)Exogenous input along the way is an important factor affecting the fluxes of nitrogen,phosphorus and various forms of storage.There was no significant difference in the proportion and conversion rate of various forms of nitrogen and phosphorus before the input along the stripping route,indicating that the transformation process of nitrogen and phosphorus forms along the cascade was mainly contributed by the transformation in the reservoir.This study pointed out that although the cascade dam construction would intercept part of the nitrogen and phosphorus nutrients in the river,it changed the form of nitrogen and phosphorus nutrients,and the proportion of SRP to TP and NH4+to TN increased along the way.The transformation in the reservoir rather than the input along the river basin is the main contributor to the morphological transformation of nitrogen and phosphorus nutrients.The hydraulic retention time of the reservoir is the main controlling factor of the morphological transformation.The longer the hydraulic retention time,the more dissolved nitrogen and phosphorus nutrients that can be absorbed and utilized by aquatic animals and plants,which improves the availability of nitrogen and phosphorus in the water transported downstream.This finding indicates that cascade dam construction can improve the bioavailability of nitrogen and phosphorus nutrients in downstream water bodies,and provide an important reference for the biogeochemical cycle of nitrogen and phosphorus in dammed rivers.