| The changes of aquatic environment and the transportation of nitrogen and phosphorus caused by hydropower development in the Lancang River Basin are important issues for the downstream countries and the international community.At present,there are few studies on the effects of cascade dam on nitrogen and phosphorus Nutrients in the rivers.Therefore,it is very important to study the temporal and spatial characteristics of nitrogen and phosphorus nutrients and the influence of watershed aquatic ecosystem in the middle and upper reaches of Lancang River.This paper focused on the middle and upper reaches of the Lancang River,through the monitoring the nutrients within dry and flood seasons in middle and upper reaches of the Lancang River in 2015-2016,to study the seasonal and annual variability of nitrogen and phosphorus concentrations in the cross-section of each station and its spatial distribution,as well as the analysis of nitrogen and phosphorus Nutrients and water environmental factors and the eutrophication status of reservoirs.The main findings are as follows:(1)There was a certain seasonal,annual and spatial differentiation of total nitrogen concentration in different sections.The TN concentration in the front section of the two dams in 2015 was significantly higher than that in the flood season.The concentration of TN was gradually increased from 2015 in the area of Xiaowan hydrological station.The dry season of TP concentration in two flow sections was significantly lower than that in flood season.The seasonal variation of TP concentration in the front/dams section of Gongguoqiao and Miaowei was significant,and the flood season was significantly higher than that in dry season.The TN concentration in the four dams/hydrological station sections,which was closely related to the dams in 2016,is significantly higher than that in the flood season.TP concentration decreased significantly from the cross section of Gongguoqiao hydrological station to Xiaowan hydrological station.(2)The total nitrogen concentration in the flood season was significantly higher than that in the flood season of Gongguoqiao cross section.The total phosphorus concentration in the flood season in 2016 was significantly lower than that in 2015,and the flood season was significantly larger than that in the dry season.The total nitrogen concentration of the cross section of Gongguoqiao hydrological station is higher in the two years of dry season,and the values are higher than the Primary standard water quality of surface water.The total nitrogen concentration of Xiaowan dam front section in the dry season of 2015 is lower than that in the dry season of 2016,and the concentration of dry season in the two years is larger than that in the flood season.The dry season of total phosphorus concentration in the section of the Xiaowan dams/hydrological station in 2016 was significantly larger than that in 2015.The retention effect of nitrogen and phosphorus nutrients concentration in Xiaowan reservoir area was obvious,the retention efficiency of total phosphorus in flood season was higher than that of total nitrogen,and the retention efficiency of total phosphorus in flood season was 94%.(3)The relationship between the main water quality index and its relationship with nutrient showed that the water temperature was slightly increasing from upstream to downstream,and the water temperature in the front section of the dam was higher than that of the dam hydrological station.The turbidity from the upstream to the downstream And the dissolved oxygen in each section is generally higher than that in the main flood season,and the dissolved oxygen in the front section of the dam is lower than that of the dam hydrological station.The conductivity gradually declines from the upstream to the downstream.The main water environmental factors that play an important role in the study of nitrogen and phosphorus nutrients in the study area are turbidity,dissolved oxygen and temperature,which affect the TN and TP contents and temporal and spatial distribution characteristics of the study area.In contrast,the effect of conductivity is small.(4)Eutrophication analysis showed that:The nutrients structure of dams in the study area is dominated by phosphorus.and the tributaries are mainly nitrogen limitation.The results of linear fitting show that the ratio of nitrogen to phosphorus in the middle and upper reaches of the Lancang River is mainly affected by the fluctuation of phosphorus. |
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