Dynamic Process And Control Factors Of Dissolved Organic Matter In Xiangxi Bay Of Three Gorges Reservoir

River damming is one of the most significant human activities that interfere with the terrestrial-ocean material cycle.There are more than 70,000 large dams in the world,and the reservoir formed has more than seven times the storage capacity of natural rivers.The tributary bay is one of the important components of the reservoir.It not only has the typical water environment formed after the river damming,but also the "sensitive area" affected by the reservoir.Dissolved organic matter（DOM）is the largest bioavailable carbon pool in the natural water environment,the variation of which has a profound impact on the balance of ecosystems.In this context,the study of the source and dynamic of the tributary bay under the influence of reservoir regulation is not only the key to evaluating the impact of the reservoir on the regional carbon cycle,but also important for reservoir regulation management and global carbon cycle evolution prediction.Therefore,Xiangxi Bay,a typical tributary bay of the world’s largest reservoir,was selected and the complex sources of dissolved organic matter（DOM）,particulate organic matter（POM）,and sedimentary organic matter（SOM）were identified.On the basis of the DOM sources identification,the impact of reservoir regulation on the evolution of DOM in Xiangxi Bay was evaluated,and the main controlling factor under reservoir regulation was clarified.Then the degradation behavior of DOM under the influence of reservoir regulation was further analyzed.The main knowledge gained is as follows:1.Through the analysis of the bulk properties characteristics,optical properties and molecular composition of 145 DOM samples in all seasons of the Xiangxi Bay,the mixed sources including autochthonous,anthropogenic,and allochthonous inputs were identified.Protein like components（28-67%）and humic like components（33-72%）of CDOM were identified quantitatively。High molecular weight,aromaticity,and degree of unsaturation of terrestrial CDOM and low molecular weight aromaticity of autochthonous CDOM were presented through the application of correlation of optical and molecular techniques.2.Through multi-scale analyzing of properties at different stages（drainage period,transitional period,and storage period）of Xiangxi Bay,significant variation of DOM properties were revealed.Autochthonous input: drainage period > transitional period > storage period,anthropogenic input: drainage period > transitional period > storage period,allochthonous input: drainage period < transitional period < storage period.3.Through the comprehensive analysis of the various influencing factors including alga bloom,water level variation,runoffs variation,and hydrologic modes variation under the regulation of reservoir,the hydrologic modes（medium-upper layer reverse gravity flow pattern,bottom layer inverted gravity flow pattern,normal slow flow pattern）under different regulation stages are proposed to be major factor controlled the DOM seasonal evolution in Xiangxi Bay.The hydrologic modes combined with algal blooms,water level variations,and runoff changes has formed a complex DOM evolution mechanism in Xiangxi Bay。Evolution mode were also presented.Then,the composition of organic carbon pools in the DOM at various regulation stages of the Xiangxi Bay was quantified.There were 0.6×10³t terrestrial DOC and 1.1×10³t autochthonous and anthropogenic DOC in drainage period,0.7×10³t terrestrial DOC and 0.6×10³t autochthonous and anthropogenic DOC in transitional period,1.3×10³t terrestrial DOC and 0.4×10³t autochthonous and anthropogenic DOC in storage period.Besides that,significant variations of spatial characteristics of DOM were also revealed,which were turned out to be resulted from hydrologic modes.4.The degradation abilities（photodegradation,biodegradation）of DOM at various regulation stages were revealed,which turned out that the reservoir regulation exerted a significant control on degradation abilities.Biodegradation ability: drainage period > transitional period > storage period,photodegradation ability: drainage period < transitional period < storage period.The reservoir regulation not only influenced the composition and evolution dynamics of DOM,but also effected the degradation ability and might also devoted to the variation of CO₂ emission of reservoir.