| Dissolved organic matter（DOM）is one of the most active carbon pools in the ocean,the compositions,sources,and the properties of DOM have great significance for the study of marine carbon cycle in estuarine and coastal waters.Estuarine and coastal regions are not only the most strongly affected by the intensification of human activities and economic and social development,but also the most vulnerable regions in the ecosystem.The biogeochemical process of DOM in estuarine region is complicated.Different chemical components in DOM have different size fractionation ranges,and DOM components with different size fractionation have different biogeochemical behavior.At present,there has been some progress in the study of DOM with different size fractionation in estuarine region,but there are few reports on the systematic study of the spatial and temporal distribution characteristics and controlled factors of its size spectrum.In this dissertation,water samples were collected from the freshwater end-member in the Changjiang River,its Estuary and the adjacent East China Sea.Dissolved organic carbon（DOC）and chromorphoric dissolved organic matter（CDOM）were analyzed.The temporal and spatial variation characteristics and controlled factors of CDOM in the Changjiang River estuary were investigated.Combining fluorescence excitation-emission matrix and parallel factor analysis,the temporal and spatial variations of CDOM with different size fractionation in the Changjiang Estuary were systematically investigated by the ultrafiltration technique.Through comparison and analysis with other rivers,it is helpful to understand the differences of CDOM distribution,sources and degradation and transformation process among different freshwater in the world.The main research results of this dissertation are as follows:（1）The concentrations and properties of particulate organic carbon（POC）and DOC had significant non-conservative behaviors in the Changjiang River estuary.In the study area,the concentrations of DOC and POC gradually decreased with the increase of salinity from land to sea.The local increase of POC and DOC concentrations in the salinity front and maximum turbidity zone confirmed the non-conservative behavior of POC and DOC during transport.The changes in the distribution of DOC and POC within the study area were controlled by different factors,with the former being mainly affected by the mixing of freshwater and seawater and the latter by changes of suspended particulate matter（SPM）concentration.Under the background of increased discharge in the flood season,the diluted water of the Changjiang River would transport more DOC to the adjacent sea area.The"non-conservative"increase of DOC concentration in the region was often accompanied by a high POC（%）value.Based on the above phenomena,it is speculated that there may be interaction between DOC and POC in estuarine area,and even mutual transformation.（2）The concentration,property and size fractionation distributions of DOM from freshwater end-member of Changjiang River showed obvious seasonal variation.The DOC concentration of end-member in freshwater of Changjiang River was high in the flood season and low in the dry season,while CDOM concentration was high in both flood season and dry season.In addition,the responses of DOC and CDOM concentrations to discharge were different.The former showed a positive correlation with discharge,while the latter showed a negative correlation in dry season and a positive correlation with hysteretic phenomena in flood season.In terms of the internal distribution of CDOM size fractionation,the proportion of low molecular weight of CDOM（<1 k Da）was 66.7%,and the remaining 33.3%was mainly 1 k Da-0.7μm.Compared with the dry season,the molecular weight of CDOM was higher and the degree of aromatization was higher.Compared with other rivers in the world,the DOC concentration,the degree of DOM aromatization and the percentage of colloidal in the Changjiang River,which is significantly affected by human activities,are significantly lower.Based on the correlation between S275-295（SR）and colloidal percentage,it is found that CDOM size fractionation can be used as an important index to identify the composition and properties of end-member DOM in freshwater in global rivers.（3）The concentration,size fractionation and sources about fluorescence components of fluorescent dissolved organic matter（FDOM）from lower Changjiang River showed obvious seasonal variation trend.There were mainly four types of fluorescent components exported from the lower Changjiang River,including two types of proteins-like（tryptophan-like C1 and tyrosin-like C4）and two types of humus-like（C2 and C3）.The ratio of C2/C3,which represented the degree of photodegradation,showing that the flood season with extreme discharge was lower than the flood season with high discharge,and the extreme discharge was accompanied by high molecular weight DOM,indicating that the degree of photodegradation may be related to the discharge.The variations of fluorescence index（HIX,BIX and FI）indicated that the freshwater end-member CDOM source is autochthonous and terrestrial,and the high discharge in flood season will carry more terrestrial humus.The relative contribution rates of different fluorescent components of CDOM indicated that lower Changjiang River were affected by terrestrial sources,field biological production and human activities in the flood season,and were more significantly affected by the latter two in the dry season.The size fractionation of CDOM and its different fluorescent components are concentrated in<1 k Da.In the colloidal size fractionation range,the size distribution of CDOM and humic-like mainly concentrated in the lower size fractionation range（1-10 k Da）,and the particle size distribution of protein-like mainly distributed in the large size fractionation range（10 k Da-0.7μm）.In addition,the lower colloidal percentage of CDOM and four fluorescent components in the dry season further indicated that the microbial degradation is stronger in the dry season during the study period.These results indicate that size fractionation of FDOM and chemical composition are important parameters for analyzing the source and seasonal variation of DOM in natural water.（4）The results of different cruises（dry season,ordinary flood season and extreme flood season）in the Changjiang River Estuary and its adjacent waters showed that the size spectrum characteristics of CDOM in the dissolved state had obvious spatial and temporal distribution trend.The average CDOM size fractionation decreased significantly from land to sea with the increase of salinity.The colloidal percentage（1 k Da-0.7μm）in freshwater end-member was about 20%-40%,while the colloidal percentage in offshore areas was often less than 1%.Under the background of extreme flood,the increase of discharge transported higher concentration and higher molecular weight of CDOM to the Changjiang River Estuary and its adjacent East China Sea.The average CDOM size fractionation at the mouth of the Changjiang River increased locally,which reflected the influence of flocculation and other processes on the average CDOM size fractionation,and further confirmed that the non-conservative behavior of CDOM in the mixing process of salty and light water at the mouth of the Changjiang River would be reflected in the migration and transformation law within the dissolved state of CDOM.（5）The concentration and size fractionation distribution about fluorescent components of FDOM had different mixing behavior in the Changjiang River estuary.The two types of humic-like components from the Changjiang River estuary were controlled by terrigenous input and physical mixing process,and showed conservative mixing behavior with salinity.The two types of protein-like exhibited non-conservative behavior with salinity due to their instability and microbial degradation processes.The concentrations of fluorescence components at the stations located in the Turbidity Maximum Zone of the three cruises were usually higher than those at the other stations,mainly due to the sediment re-suspension process in the region.The results of fluorescence,absorption spectrum and salinity indicated that DOM photodegradation was stronger in the high salinity region.In addition,the two colloidal humic-like in the study area were mainly controlled by discharge,while the two colloidal protein-like were affected and had different responses to flocculation,microbial degradation processes and discharge.These results indicated that the concentration,properties and size fractionation distributions of FDOM fluorescence components could provide effective information for the biogeochemical processes of DOM in aquatic continuum.In this dissertation,the freshwater end-member in the Changjiang River,the Changjiang River Estuary and its adjacent East China Sea were taken as the research objects.Combined with the relevant studies of rivers in other regions,the distribution characteristics,removal and transformation rules of CDOM concentration,optical parameters and size spectrum characteristics of the Changjiang River estuary were explored.In the process of transmission from land to sea,CDOM imported from rivers will be affected by many factors,such as discharge,the mixing of fresh and brackish water,“filter”effect caused by Turbidity Maximum Zone,photodegradation and microbial degradation.The concentration,properties and size spectrum characteristics of CDOM would change,and the average size fractionation and proportion of larger size components of CDOM would decrease significantly.It is helpful to enrich our understanding of the biogeochemical processes of DOM in estuarine and coastal regions to distinguish the components of CDOM with different size fractionation by using ultrafiltration technique. |
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