Characteristics Of Phosphorus Transport And Transformation And Its Influence Mechanism In The Three Gorges Reservoir

The anti-seasonal water level control in the Three Gorges Reservoir?TGR?has changed the hydrodynamic characteristics of the Yangtze River,and then influenced the nutrient transport circulations with water and sand as transport carriers.This thesis mainly focused on the characteristics of phosphorus transport and transformation and its influence mechanism in the TGR by means of field investigation and laboratory simulation.The major source of phosphorus in the TGR was from phosphorus inputs of upstream rivers,whose total phosphorus?TP?fluxes accounted for 79.73%of the total amount of TP inputs of the TGR.Based on the field investigation results in the TGR from 2014 to 2016,total dissolved phosphorus?TDP?was the predominant phosphorus fraction in water which accounted for 45%⁹6%of TP.The concentrations of TDP and TP in water in the TGR mainstream presented a complex spatial-temporal heterogeneity,and total particulate phosphorus?TPP?in water showed decreasing trends from the upstream of the Yangtze River to the TGR.On the contrary,concentrations of particulate phosphorus in a unit mass of suspended particles?PP?and extractable organic phosphorus?Exo-P?gradually increased along the direction of water flow,but detrital phosphorus?Det-P?gradually decreased.PP and Exo-P concentrations in suspended particles were negatively correlated with particle size,but were positively correlated with total organic matter and manganese contents in suspended particles.Det-P concentrations in suspended particles had a significantly positive correlation with particle size.The seasonal variation of drainage basin hydrology and the anti-seasonal reservoir scheduling are two important factors affecting phosphorus transport and transformation process in water and suspended particles of the TGR.Under the combined influence of the two factors,the particle size was finer and PP,Exo-P concentrations in suspended particles during the impounding period and high water level period were higher than that during the sluicing period and low water level period.Furthermore,the TGR impoundment slowed down the hydrodynamic condition and enhanced the selective deposition of suspended particles in the TGR.These caused that Det-P tended to settle down along the path with the coarse particles and Exo-P moved farther away together with fine particles or discharged into the middle and lower reaches of the Yangtze River.On the whole,10%⁵6%of PP that imported from upstream rivers were deposited in the TGR during the four reservoir scheduling periods.Nevertheless,the simulation results of phosphorus adsorption by suspended particles in the TGR showed that phosphate adsorption-desorption equilibrium mass concentrations?EPC₀?of suspended particles were higher than phosphate concentrations(PO₄^3-)in water,meaning that suspended particles tended to desorb exchangeable phosphorus?Exc-P?into the water in the transport process in the TGR mainstream.Particulate phosphorus imported from upstream rivers were mainly deposited in the sediment zone and water-level-fluctuating zone?WLF?of the TGR.The simulation results showed that EPC₀ concentrations of WLF soils were also higher than PO₄^3-concentrations in water.Therefore,during the impounding period and high water level period,Exc-P was easy to be desorbed from the submerged WLF soils to water,and the reduction of dissolved oxygen?DO?in the submerged WLF soils may also lead to the release of iron bound phosphorus?Fe-P?by anaerobic reduction reaction.The measured results by the diffusive gradients in thin-films technique?DGT?experiment showed that the diffusion fluxes of labile phosphorus across the sediment-water interface in the TGR mainstream and tributary?the Daning River?were in the range of 0.110⁰.581mg/?m²·d?,indicating that sediment tended to be release sources of phosphorus for overlying water.Sediment phosphorus release in the TGR mainstream was controlled by iron reductive dissolution under anoxic sediment conditions,while sediment algal decomposition was the main cause for phosphorus release in surface sediments of the algal bloom occurring area of the Daning River.The backwater from the TGR mainstream was the major supply source of phosphorus in the Daning River.During the impounding period and high water level period,the phosphorus supply from the backwater of the TGR mainstream increased water bloom risk in the Daning River in spring and summer.