Variation of Concentration,Form and Flux of Phosphorus in the Three Gorges Reservoir from 1998 to 2019
The changes in total phosphorus(TP)concentration and form in the Three Gorges Reservoir(TGR)from 1998 to 2019 were studied,and P fluxes were calculated.The entire research period was divided into three stages:1998-2002(stage I,before the operation of the TGR),2003-2012(stage II,as a transitional period),and 2013-2019(stage III,after the TGR achieved the normal water level of 175 m and Xiangjiaba Reservoir began to operate).Comparative analysis was mainly on Stages I and III.The results show that the concentrations of suspended sediment(SS)in the TGR decreased to a very low level(average 42 mg/L during 2013-2019 period and 96 mg/L in wet season).From 1998 to 2019,the inflow and outflow of sediment amount showed a downward trend.A huge amount of sediment was retained at the bottom of the reservoir during 2003-2012(average 154.6 million t/a),while in 2013-2019,the amount of sediment deposition significantly decreased and the sedimentation amount was only 37%of that in 2003-2012.Year 2013 marked a turning point in the trend of phosphorus pollution in TGR from increasing to decreasing.Afterwards,concentrations of TP and DP(dissolved phosphorus)showed a significant downward trend.The form of phosphorus in TGR changed from mainly granular to dissolved,and the percentage of DP in TP in the perennial backwater area increased from 39%before TGR formation to 69%during 2017-2019.2004-2012 was a period with high deposition for TP when the amount of TP entering the reservoir was large and fluctuated vastly between years,and approximately 60%of phosphorus was trapped(82220 t/a).From 2013 to 2019,there occurred significantly decrease in the TP amount entering the reservoir in accompany with diminishing interannual fluctuation,tending to a balance between input and output of P.Fluxes of TP,DP,and PP(particulate phosphorus)all show a pattern of wet season>even season>dry season.The main driving forces for changes in TP concentration,flux,and form are the reduction in SS concentration and phosphorus pollution level.
Yangtze Riverthe Three Gorges Reservoirphosphorusfluxform
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