Sources,Dynamics And Transformation Of Organic Matter In Wujiang River Basin

Dissolved organic matter（DOM）is of allochthonous and autochthonous origin which is,on average,composed of molecular weights ranging from less than 100 to over 300,000 Da in natural waters.The DOM components are generally involved to take part into key biogeochemical processes,such as global carbon cycle,nutrient dynamics,photosynthesis,biological activity and finally as energy sources in the aquatic environments.Among the DOM components,only a limited fraction of organic compounds shows fluorescence properties.These compounds are termed as the fluorescent DOM（FDOM）.The fluorescent properties of terrestrial humic-like substance are used as tracers of soil organic matter in freshwater and seawater environments.In addition,the fluorescent properties of humic-like substance can also be used to distinguish between river water and seawater and to trace their source.Organic matter is an important component in water and sediments.The organic matter in water can be decomposed and mineralized,released into the surface water,and re-entered into the nutrient cycle.At the same time,the source of organic matter is used to assess the local environmental quality and the source of pollutants.Therefore,the research on the source,kinetics and transformation of organic matter in the water environment will be one of the main research directions in the field of environment research in surface water.Solar radiation is a common phenomenon in the biosphere and it is vital to all life on Earth.It involves the physical,chemical,light-induced and biological processes of most organic matter in natural waters.The light-induced or photolyzed process can maintain pH,water transparency;promote thermal decomposition,redox reaction and so on.The organism can use the substrate produced by photolysis to produce nutrients,dissolve inorganic carbon（DIC）,dissolve organic carbon（DOC）,and form chlorophyll on the surface to increase photosynthesis.Finally,solar radiation is a major source of energy that is important in natural water ecosystems.The photoinduced degradation of DOM and its effect on natural water depends on the spectral range of sunlight,i.e.UV-A（315-400 nm）,UV-B（280-315 nm）or visible light（400-700 nm）.Depending on the wavelength,there is a significant change in the penetration of sunlight in the water.DOM is generally able to absorb ultraviolet radiation from seawater and lakes,thereby controlling the penetration of ultraviolet light into deeper water.The penetration depth of ultraviolet radiation in natural waters varies greatly.The typical penetration depth in UV-B of seawater is 20 m and the ultraviolet radiation is 50 m.In malnourished seawater,the penetration of UV-B radiation is 5-10 meters,while in fresh water it is0.5-3 meters.Therefore,changes in the wavelength of radiation or increasing in global temperature may have a large impact on the various biogeochemical processes mentioned above.However,researchers pay less attention to light-induced processes to assess biogeochemical processes in natural waters.DOM is one of the most important organic components in the carbon cycle,and researchers’understanding of the cycle process is not thorough.As early as the beginning of the 20th century,investigations have shown that sunlight is the main cause fading of organic matter on the surface water.In recent years,through spectroscopy and photochemical research,scientists have begun to realize that organic matter undergoes a variety of photochemical processes under the action of solar radiation.FDOM is the most important light absorbing component in water environments and is easily converted by photochemical processes.For decades,researchers have become increasingly aware of the importance of the photochemical conversion process of DOM to the organic carbon cycle in water environments.For example,DOM can be photolyzed into inorganic components to DIC and carbon monoxide（CO）,or to degrade some of the large molecular weight components into bioavailable low molecular weight carbonyl compounds,indirectly stimulating microbial activity and speed up the DOM turnaround process.Some studies have shown that photochemical process can transform bioavailable DOM into anti-biodegradable matters.Therefore,the research on the mechanism and properties of DOM photolysis has important significance for the migration and transformation of organic pollutants in water and the causes of global warming.In this study,the DOM of the Hongjiadu Reservoir,the Silin Reservoir and the Yinpan Reservoir in the upper,middle and lower reaches of the Wujiang River Basin were studied.The parallel factor analysis of three-dimensional fluorescence spectra（EEM-PARAFAC）was used to explore the DOM components with the source,dynamics,migration and transformation of the drainage and revealing the influence mechanism and spatial distribution characteristics of the organic matter in different seasons of the Wujiang River Basin which included inflowing water,the surface and deeper water in the reservoir area and outflowing water.At the same time,the surface water of the Dongfeng Reservoir in the Wujiang River Basin was collected for photodegradation experiments.The DOC concentration,EEM-PARAFAC,NO₃^-concentration and other measurements were used to investigate the organic matter and NO₃^-concentration in the water during 30 days’irradiation.Simultaneously,it can understand the process by which they are produced from phytoplankton while degrading without adding any precursors.The main findings are as follows:（1）There is a significant time-space difference of DOM in the water of upper,middle and lower reaches of the Wujiang River Basin.In this study,the source difference of samples at each sampling site was considered.EEM-PARAFAC analysis was analyzed separately for the upper,middle and lower reaches,with inflowing water,the surface and deeper water of the reservoir area and outflowing water.For upper reaches,the surface and deeper water in the reservoir area have three fluorescent components during cold time including terrestrial humic-like substance（peak C at Ex/Em=300-360/449-458 nm and peak A at Ex/Em=260-270/449-452 nm）,microbial humic-like substance（peak M=300-305/411-420 nm and peak A=230-235/411-420nm）and protein-like substance(peak T=270-275/335-344 nm and peak T_UV=230/335-344 nm).Only terrestrial humic-like substance were detected in the outflowing water（peak C=305/407 nm and peak A=265/407 nm）.In contrast,in the warm season samples,only three fluorescent components were detected in the inflowing water.Under the conditions of high solar intensity in warm season,the microbial humic-like substance in the surface and deeper water of the reservoir area was degraded.The same as cold season,in the warm time also has only one fluorescent component,which is caused by physical process caused by dam barrier effect.A similar conclusion can be drawn for the Silin Reservoir in the middle reaches.Three fluorescent components were detected in all cold season samples,but these three fluorescent components were only detected in the surface water of the warm season reservoir area.Under the action of photodegradation,microbial degradation and physical process,the components of the surface and deeper in the reservoir area and the outflowing water had followed similar degradation processes of upper reaches waters.Similar results were obtained in the lower reaches compared with upper reaches and middle reaches.In the cold season,all three fluorescent components were detected,while in warm season,these three fluorescent components were only detected in the surface and deeper water of the reservoir area.While in the inflowing and outflowing water,there are only two components.These studies show that the river-reservoir system has a range of physical,chemical and biological processes.Photodegradation,microbial degradation,and physical process caused by dam barrier effects can significantly alter the composition of DOM in the surface and deep water of the reservoir.（2）Fluorescence characteristics of DOM and its controlling factors in river-reservoir water.From the data of fluorescence intensity,terrestrial humic-like substance（C type）was gradually decreased in surface and deeper waters and subsequently in outflowing waters,suggesting that it may originate from terrestrial source and could always alter during its transportation from incoming to reservoir-surface and deeper waters and then in outflowing waters by photochemical,microbial and dam barrier-affected physical processes.The fluorescence intensity of microbial humic-like substances（M type）and protein-like or tryptophan-like or tyrosine-like substances were increased either in the surface or deeper waters of the reservoir and then decreased in outflowing waters.This indicates that these fluorescent substances are autochthonously produced from phytoplankton that simultaneously reproduced and degraded in surface and deeper waters by photochemical,microbial and dam barrier-affected physical processes,thereby depicting a transformation dynamics of overall DOM in reservoir waters.（3）Increasing DOC concentration from photo-and microbial degradation suggests that photochemical and microbial respiration of phytoplankton is responsible for increasing DOC concentration in the euphotic zone than in deeper water of lakes and oceans.（4）The EEM-PARAFAC results of photodegradation experiment from the original unfiltered samples showed four fluorescence components.Component 1 was recognized as terrestrial humic-like substances with fluorescence peaks at Ex/Em=350/431 nm（peak C）peak Ex/Em=260/431 nm（peak A）.Component 2 was recognized as fulvic-like substances with fluorescence peaks at Ex/Em=305/398 nm（peak M）peak Ex/Em=230/398 nm（peak A）.The third component was donated as modified protein-like substance with two fluorescence peaks at Ex/Em=230/322 nm（peak Tuv）and peak T at Ex/Em=260/322 nm.The last component was donated as protein-like substances with single fluorescence peaks at Ex/Em=274/342 nm（peak T）.For original filtered samples,it was the same results for three components.Six fluorescent components were identified in sunlight irradiated unfiltered samples.Three components of them were the same as the samples which were collected before irradiation.The forth component was protein-like substances which have two main peaks at Ex/Em=265/346 nm and Ex/Em=290/346.The fifth component was tyrosine-like substance with two peaks at Ex/Em=230/321 nm（peak Tuv）and peak at Ex/Em=270/321 nm（peak T）.The last component was phenylalanine-like substance with two peaks at Ex/Em=225/308 nm（peak Tuv）and peak at Ex/Em=255/308 nm（peak T）.The filtered irradiated samples were the same five components as the filtered samples except the last component which was donated as newly produced protein-like substance.There were four components in dark condition unfiltered samples.The first is fulvic-like substance with peaks at Ex/Em=265/436 nm（peak A）and Ex/Em=340/436 nm（peak C）.The second is fulvic-like substance with peaks at Ex/Em=240/391 nm（peak A）and Ex/Em=305/391 nm（peak M）.The third component was tryptophan-like substance which has the peak of Ex/Em=225/325 nm(peak T_uv)and Ex/Em=265/325 nm（peak T）.The forth component was the protein-like substances which has single peak at Ex/Em=275/341 nm（peak T）.The dark filtered samples have three components.The component of fulvic-like substance was the same as the unfiltered samples.Another component was protein-like substance with the peak Ex/Em=230/331 nm(peak T_uv),Ex/Em=275/331 nm（peak T）.The last component was fulvic-like substance which was different from the unfiltered samples.It had the peak of Ex/Em=230/398 nm（peak A）and Ex/Em=305/398 nm（peak C）.（5）DOC concentrations and fluorescent intensities in irradiated samples has significantly increased than in dark incubated samples over 30 days solar irradiation period.Our field and experimental evidence shows that phytoplankton degradation plays an important role in contributing FDOM in water,and thus affects nutrient cycling and the optical properties of water.Although laboratory experiments do not duplicate the actual environmental conditions in nature,results from our degradation experiment suggest that the production of FDOM and nutrients from phytoplankton degradation can be a very rapid process that could also occur under field conditions.Photoinduced respiration on DOM production from lake organic matter（e.g.phytoplankton）are significantly higher and effective than in microbial processes.（6）A conceptual model of phytoplankton production and degradation is built to explain the simultaneous production and degradation of phytoplankton in the irradiated and dark condition.Such conceptual model can provide some useful evidences for increasing DOC concentrations in upper surface waters than in deeper water along with occurrence of the harmful algal blooms in the stagnant surface waters（lakes,estuaries and oceans）in the world.（7）EEM-PARAFAC could give a preliminary useful infrmation to further understand the mechanisms underlying FDOM changes in inflowing,surface,deeper and outflowing waters.At the same time,in the photodegradation experiment,parallel factor analysis can separate each component of FDOM,which is crucial for the exploration of phytoplankton production and degradation mechanism.