Spatial-temporal Distributions And Influence Factors Of Organic Carbon With Different Size Fractions In Changjiang River-estuary-coastal Continuum

Organic carbon in the ocean is one of the important carbon reservoirs in the world and plays a crucial role in global carbon cycling and biogeochemical patterns.Rivers are a key pathway for terrestrial material to be transported to the ocean,and the content,properties,and biogeochemical behavior of terrestrial organic carbon undergo significant changes during transport from rivers to the ocean,which has important implications for estuarine and coastal environments.The degree of transformation of organic carbon in the interaction zone,the complexity of biogeochemical processes,and long-term variations in carbon flux have always been hotspots and challenges in oceanography research.This study considers the river-estuary and the adjacent ocean continuum as an integrated framework and employs a two-way organic linkage between"basin-to-ocean"and"ocean-to-basin"to analyze the migration and transformation patterns of organic carbon during transport from land to sea.It explores the response processes of the Changjiang river basin and the estuarine and coastal areas to global changes and human activities and provides important scientific support for the coordinated development of the basin and the ocean.In estuarine and coastal studies,the parameter of size was generally used in differentiating suspended particulate matter（SPM）materials with sizes larger than 1μm and dissolved matter below theμm level.Due to constraints such as technical means,colloidal（1 k Da–0.7μm）,and true dissolved（<1 k Da）matter have rarely been further distinguished.In this study,ultrafiltration was also conducted for selected samples to fractionate the bulk dissolved organic carbon（DOC）into different size-fractions to examine changes in the molecular weight distribution of DOC.The parameter of dissolved molecular size will be introduced into the marine biogeochemical studies in order to clarify the spatial distribution of organic carbon components with different molecular sizes during transport from the Changjiang river to the ocean,and to investigate the seasonal and temporal variations in the content and flux of organic carbon components with different molecular sizes in the freshwater end member of the Changjiang river and to elucidate the contributions of biogenic molecules with different sizes to the non-conservative behaviors of biogenic elements in the Changjiang Estuary and the adjacent area and finally to study the responses of molecular sizes of organic carbon to the in situ flocculation processes generally found near the Changjiang river mouth.Based on the key scientific questions above,this paper mainly drew the following conclusions:（1）The organic carbon concentration,properties,molecular size distributions,and degradation pathways in the Changjiang river basin showed significant spatial distribution differences.Dams and enhanced human activities within the basin could complicate the sources and biogeochemical behavior of organic carbon in the middle and lower reaches of the river.The particulate organic carbon（POC）and DOC concentrations in the middle and lower reaches of the Changjiang river were significantly higher than those in the upper reaches.The POC content in the reservoir area（as a percentage of suspended particulate matter,%）had significantly increased and was higher than that in the upstream reach,indicating an increase in the proportion of autochthonous organic carbon in the Changjiang river basin reservoirs.Based on the variations inδ¹⁵N values,it was found that besides terrestrial materials and in situ production,anthropogenic pollutant are also a major source of organic carbon in the middle and lower reaches of the Changjiang river.The molecular size distributions and fluorescence signals of DOC further indicated that DOC has also affected by photochemical degradation and flocculation during transportation in the Changjiang river,and these degradation pathways were mainly concentrated in the middle and lower reaches of the river after the Three Gorges Dam intercepts the water discharge.（2）The concentration,properties,and molecular size distributions of organic carbon in the lower Changjiang river at Xuliujing station（representing the end-member river water）showed significant seasonal variations,and under the combined effects of intensive anthropogenic activities and global climate change,these new decadal evolution trends of POC and DOC regimes in the lower Changjiang river would have been revealed.The elevated Changjiang River discharges in summer probably had a flushing effect on DOC,with increasing DOC concentrations during flood seasons.The seasonal variations of POC were characterized with higher proportions of autochthonous components in summer but lower in winter,as manifested in POC（%）,δ¹³C,δ¹⁵N,and POC/PN values.During flood seasons,the bulk DOC pool was more contributed by the<1 k Da LMW-DOC（low-molecular-weight DOC）and the 10 k Da–0.7μm HMW-DOC（high-molecular-weight DOC）.Together with the available literature data,DOC concentrations,POC（%）,δ¹³C,andδ¹⁵N all showed a significant increase over the past decades,likely resulting from the fact that the Changjiang River has changed from a“primitive”（with more allochthonous contribution from river basin）to a“civilized”river system（with more autochthonous contribution from in situ biological processes）.（3）Using the ratio of total volume to turbidity and size spectra,SPM from all the three cruises of different seasons could be successfully classified into three groups of terrestrial sediments,their flocculation products,and autochthonous-derived biological particles.Theδ¹³C values were utilized to study the response of biogeochemical parameters to the flocculation.Phytoplankton blooms occurring in May largely resulted in discontinuous variations in laser in situ scattering and transmissometry（LISST）parameters andδ¹³C from March to July.Although the pattern of SPM size spectra involved in flocculation differed among seasons,flocculation processes were always contributed by smaller particles with sizes of several tens ofμm aggregating into larger ones>300μm.Using LISST andδ¹³C measurements combined greatly improved our understanding of SPM dynamics in estuarine and coastal areas where estuarine flocculation was a critical component.（4）The molecular size of organic carbon during the land-to-sea transport in the Changjiang river estuary was very complex,including the transformation between materials from the particulate and dissolve phases,as well as the competition among different molecular size in the dissolved phase.In the different seasons,DOC molecule sizes always showed decreasing trends from land to sea,in which those with sizes>1k Da generally contributed 18 to 33%of the bulk DOC in the freshwaters but always disappeared at the offshore stations.During the summer cruises,there was often a significant increase in both the molecular size and large DOC size fractions near the salinity front and the maximum turbidity zone.These results reflected the variations in the molecular size of DOC under the control of flocculation and other processes,as well as the fact that higher primary productivity in the summer often leads to an increase in the proportion of 10 k Da–0.7μm fractions.By simultaneously measuring the proportions of POC,HMW-DOC,and LMW-DOC during the same period,it was found that the largest POC and the smallest LMW-DOC showed an obvious opposite trend in their variations,and their higher and lower values almost occurred simultaneously.This indicates that there were mainly two kinds of inter-conversion relationships between POC and DOC during the process of mixing freshwater and seawater:LMW-DOC with smaller molecular sizes aggregated into larger HMW-DOC,and colloids aggregated into POC mainly in low salinity areas;while in areas from the mouth of the river to the open sea,large particles disaggregated into smaller particles and large molecules disaggregated into small molecules,resulting in an increase in the content of LMW-DOC.（5）The degradation and transformation of organic carbon in the Changjiang river estuary were not only reflected in variations in concentrations but are also largely related to the molecular sizes of various components.The dissolved organic matter（DOM）of different chemical compositions and DOC contained different molecular sizes.Chromophoric dissolved organic matter（CDOM）and humic-like substances mainly existed in small particle size fractions（1–10 k Da）,while protein-like substances mainly existed in large particle size fractions（10 k Da–0.7μm）.In addition,the non-conservative behavior of each molecular size fractions during the mixing of freshwater and seawater was also reflected in the amplitude of removal or addition relative to the conservative mixing curve.For example,the removal rate of terrestrial humic-like substances and CDOM was highest in the open sea,while the"non-conservative"behavior of protein-like components at the maximum turbidity zone station has increased.These results indicated that flocculation and photochemical degradation processes were the main causes of non-conservative mixing of DOM,and different chemical components had different responses to the non-conservative process.Combining the results of potential source-sink rate measurements from in situ incubation experiments,it was also found that most of the chemical components of the DOM in the estuarine serves as a source,but POC and sea-derived humic-like substances serves as a source.