Study Of Spatio-temporal Varitation Charateristics Of Nitrogen And Phosphorus Nutrients In The Three Gorges Reservoir After Its Impoundment

Nitrogen cycle and phosphorus cycle are indispensable part of biogeochemical circulation in basin.The health and safety of aquatic ecosystem,as well as the stability and balance of water environment,is determined by the nitrogen and phosphorous nutrients in rivers.The impoundment of dams has disturbed the continuity of nitrogen and phosphorous nutrients in rivers,exerting significant influences on the transport of nitrogen and phosphorous nutrients.At present,studies on nitrogen and phosphorus nutrients in water have always been the hot spots around the world.Regarded as the largest hydraulic works of the world,the characteristics of nitrogen and phosphorous nutrients in Three Gorges Dam?TGD?have a profound impact on the whole Yangtze river.The changes of nitrogen and phosphorus nutrients in import water of TGD is one of the reflection of water qualities in the upper reaches of Yangtze river basin,while the variations of nitrogen and phosphorus nutrients in export water of TGD affect the health of water body environment in the middle and lower reaches of the Yangtze river.Thus,it is necessary for us to clarify the characteristics of nitrogen and phosphorus nutrients in the inflow and outflow of TGD.This paper collects the monthly monitored water quality data,such as NO₃^—-N,NO₂^—-N,NH₄⁺-N,TN and TP at the cross-sections of Zhutuo,Cuntan,Qingxichang,and Huanglingmiao,combined with runoff and sediment data after the impoundment of the TGD from 2003 to 2015,and studies the spatio-temporal variation of the concentration and fluxes of nitrogen and phosphorous nutrients.Variations of nitrogen and phosphorus nutrients fluxes in the Three Gorges Reservoir is also discussed in order to deepen the understanding of nitrogen and phosphorus nutrients in Three Gorges Reservoir area,and it will promote the managements of water qualities and risks in reservoir and downstream areas.The major results of this study are as follows:?1?Annual average dissolved inorganic nitrogen?DIN?concentration in Zhutuo,Cuntan,Qingxichang and Huanglingmiao station are all greater than1.0 mg/L,annual average total nitrogen?TN?concentration is above 1.5 mg/L.Both annual average DIN concentration and annual average TN concentration increase year by year.From 2003 to 2015,flux of TN and DIN in Zhutuo,Cuntan,and Qingxichang station have risen along the main stream,which shows the cumulative effect in space.It also has significant periodicity in time,the flux of nitrogen is large in flood season,when it is small in non-flood season.The shortage of runoff in 2006 and 2011 is cause of low nitrogen fluxes.?2?The average proportion of NO₃^—-N NH₄⁺-N NO₂^—-N in DIN is respectively 91%,8%,1%at the studied stations.NO₃^—-N Nitrogen nitrate?NO₃^—-N?is dominated in the dissolved inorganic nitrogen?DIN?in water body.From 2003 to 2015,DIN concentration is positively related to the TN concentration in Zhutuo,Cuntan and Qingxichang.station,except for Huanglingmiao station.However,flow of the four studied stations has no significant correlation to the TN concentration.There is a significant positive correlation between the concentration of NO₃^—-N,NO₂^—-N and NH₄⁺-N in Zhutuo,Cuntan and Qingxichang.station.There is also a significant positive correlation between NH₄⁺-N concentration and flow in Cuntan and Qingxichang station.In Huanglingmiao station,a hysteresis exists between the NH₄⁺-N concentration peak and flow peak.?3?In the inflow and outflow water body of the Three Gorges Reservoir,particulate total phosphorus?TPP?accounts for the majority of total phosphorus?TP?.The fluxes of TP,TDP and TPP also show the characteristics of accumulation along Zhutuo,Cuntan and Qingxichang station.In addition,there is a significant periodicity of phosphorus flux in time,it is small in dry season and large in flood season.The TP and TPP concentration in Zhutuo,Cuntan,Qingxichang and Huanglingmiao station respectively have a good positive correlation with the sediment concentration and flow,moreover,the positive correlation with sediment is better.TPP concentration is more affected by sediment than TP.However,there is a relative bad correlation between TDP concentration and sediment concentration.On the contrary,TDP concentration has a negative correlation with flow.In general,phosphorus sink is greater in Three Gorges Reservoir.Take 2006 as an example,total phosphorus?TP?interception in Three Gorges Reservoir is 5.5×10⁴ t,the total phosphorus interception rate is above 70%.The biggest inflow part of total phosphorus into Three Gorges Reservoir is total phosphorus from the upstream.?4?The nitrogen-phosphorus ratio?TN/TP?of Zhutuo,Cuntan,Qingxichang is greater than Huanglingmiao.The nitrogen-phosphorus ratio?TN/TP?of Zhutuo,Cuntan,Qingxichang has no consipicuous variation along the river.The nitrogen-phosphorus ratio?TN/TP?of Zhutuo,Cuntan,Qingxichang fluctuates greatly and the seasonal variation is significant.While The nitrogen-phosphorus ratio?TN/TP?of Huanglingmiao relatively changes smoothly during the year.In addition,Three Gorges Reservoir evidently has changed the nitrogen-phosphorus ratio?TN/TP?in the studied control stations,and the change of nitrogen-phosphorus ratio of discharged water in reservoir area is affected by sediment trapping in reservoir,which have an important impact on the aquatic ecosystem in the downstream of Three Gorges Reservoir area.