Relationship Of Water Environment And Internal Sediment Nutrient Release In Gaoyang Lake Of Pengxi River In The Three Gorges Reservoir

Since the Three Gorges Reservoir(TGR)has been impounded,the original river ecosystem has been transformed into a reservoir ecosystem,and the hydrological,hydrodynamic and water quality characteristics of the water body have changed dramatically.The change leads to frequent algal blooms in tributaries.Pengxi River's water eutrophication and blooming phenomenon was more serious than other tributaries in TGR,and has aroused the concern of the community.Studies have shown that the stratification of water and hydrological conditions are considered to be the key factors in the formation of water,and nitrogen and phosphorus nutrition are considered as limiting factors in the growth of algae.Tributary sediment and fluctuating zone experienced a drastic change in a year and played important functions of "Source" and "Sink" in the reservoir area.Therefore,based on researching the algal biomass,water quality and hydrology in Gaoyang Lake,the middle reaches of the Pengxi River which is the largest tributary in northern bank of TGR,this study focuses on the relationship among water quality,total phosphorous and phosphorus forms existing and being released from sediment in order to understand the relationship between biochemical process in sediment and water quality of tributaries in TGR.In this paper,sampling and analysis of the water and sediments of Gaoyang Lake were carried out from January to October in 2016.The results are as follows:(1)Water of Gaoyang Lake stratified into thermocline + hysteresis layer model,and the mixed layer was not obvious.Chl-a mainly existed in the 0~10m depth,and thermocline was the mainly area of algae to grow.Stratification is key condition for algal bloom to form;(2)The contents of TN and TP in Gaoyang Lake during the sampling duration were 1.793 mg·L-1 and 0.062 mg·L-1 respectively,which are much higher than eutrophication threshold of both TN and TN.The main form of N and P were dissolved nitrogen,phosphorus in winter,spring and autumn,ie,DTN and DTP were about 32.3% and 84.4% of TN and TP respectively,while in summer,they are accounted for 21.25% of TN and 46.12% of TP respectively.10.41% of the samples' TN/TP<22(mass ratio,the same below);56.25% were 22<TN/TP <32,and 33.34% of the samples TN/TP?32.Therefore,phosphorus is still the limiting factor for eutrophication and blooms.(3)The variation of TP content in the sediment of Gaoyang Lake was 0.49 ~ 0.66 g·kg-1,with an average at 0.59 g·kg-1,which was in the middle of sediment P among the middle and lower reach of Yangtze and its related lakes.Sediment TP content with the seasons shows: spring <winter <summer <autumn.Thus,it could be conclude that in spring sediment was the “Source” of the water phosphorus,while in summer,autumn and winter it is the “Sink” of water phosphorus.NH4Cl-P?BD-P?NaOH-P?HCl-P?Res-P accounted for 0.24%,2.08%,71.81%,9.01% and 16.85% of TP respectively.The inorganic phosphorus in the sediment was 83.15% and the organic phosphorus content was at a low level.There is a potential risk of phosphorus pollution regarding NaOH-P;(4)Chl-a showed a significant positive correlation with NH4Cl-P in sediments(p<0.05)and showed a significant negative correlation with HCl-P(p<0.05),but not not significant with TN.Since water TP was most significantly and negatively correlated with sediment BAP,NaOH-P and HCl-P(p<0.01),the release of BAP,NaOH-P and HCl-P in the sediment mainly contribute to water TP;(5)There was no significant correlation between water phosphorus forms with sediment pH,water content,bulk density and TOC.These environmental factors had slight effect on the distribution and release of phosphorus in the sediments of Gaoyang Lake.On the other hand,the redox potential(Eh)was most significantly and negatively correlated with TP(p <0.01),significantly and negatively correlated with NaOH-P(p <0.05),and most significantly and positively correlated with NH4Cl-P(p <0.01).The Eh value played an important role in the release of phosphorus from the sediments,which was an important factor affecting the form and distribution of phosphorus among all of the environmental factors;The change trend of phosphorus in the sediment was consistent with the change of microbial PLFAs(phospholipid fatty acids)and APA(alkaline phosphatase activity)sediment,which indicates that the microorganism plays an important role in the distribution,adsorption and release of phosphorus in the sediment.