Response Of Spatio-temporal Variation Of Nitrogen And Phosphorus To Reservoir Construction In Lower Reaches Of The Lancang River

The changes of downstream water ecological environment caused by hydropowers development on the mainstream of the Lancang river are the focus of riverine countries in the Mekong basin even the international community.In this paper,the lower reaches of the Lancang River were selected as the research area including the six sections(Jiajiu hydrologic station?downstream of Dachaoshan dam?Jinglin bridge?downstream of Nuozhadu?Jinghong bridge and Guanlei Port)with a one-year(from January to December of 2016)monitoring data.Based on the monitoring data,temporal and spatial differences of nutrients were analyzed to revealed the response of nutrients to program of reservoir construction,quantify the degree of influence of cascade hydropower development on river nutrient transport,the water quality and eutrophication condition were assessed by a set of methodologies.Finally,conclusions are drawn through the analysis results as following.(1)Temporal and spatial variation of nutrient concentration in lower reaches of the Lancang River:On the time scale,nutrients changed evidently within the past seasons,TN with high concentration in dry season and low concentration in flood season;the regularity of TP is on the contrary.At the spatial scale,there was no spatial significant difference of TN in dry season but decreased along the river and then increased in flood season;the spatial distribution of TP in the dry season and flood season is the similar,from upstream to downstream,the trend of TP is increase first,then decrease and then increase,and the upstream concentration is higher than the lower reaches.(2)Response of nutrients to reservoir construction:On the time scale,Manwan reservoir,the first reservoir on the mainstream,was taken into consider the temporal effects of dam blocking on the flowing nutients.Our results indicated that the concentration of nitrogen and phosphorus increased significantly after the dam construction,the cumulative effect is significant,may be due to the interception of the upstream dam,the nutrient concentration decreased after the upstream reservoirs stored water.At the spatial scale,data from the Nuozhadu reservoir show that the nutrient concentration of the outflow reservoir was much lower than that of the inflow reservoir,dam blocking effect is obvious,and the dry season is higher than the flood water period,the TP is higher than TN.(3)Evaluation of nutrient status and water quality:water quality is good as a whole in lower reaches of the Lancang River,but the concentration of TP is high.TP as the evaluation factor,the sampling sites were identified as class four of Chinese water quality standard.In addition,evaluate the eutrophication of the lower reaches of Lancang River,there is a potential eutrophication risk,but because of the high concentration of nutrients in the flood season,the rapid flow reduces the probability of eutrophication.