The Flux Of Biogenic Elements Exported From Chinese Major Rivers And Their Sources And Transformation In Large River Estuaries

Carbon,nitrogen,phosphorus,silicon,and other biogenic elements serve as the foundation of material cycling and energy flow in natural water bodies,playing a crucial role in the ecological systems of rivers and oceans.As vital conduits connecting land and sea,rivers transport an amount of terrestrial material to estuaries and coastal areas.The flux and composition changes of biogenic elements in river transport,as well as the biogeochemical processes of these elements at the land-sea critical interface,have long been a hot topic of academic interest.Especially since the21^stcentury,the cumulative impacts of intensive human activities on river,estuarine,and nearshore ecosystems have spread worldwide,causing a series of pressing issues that need to be addressed,such as climate change,frequent droughts and floods,eutrophication of water,and reductions in arable land and forested areas.The study of biogeochemical cycling and the influencing factors of biogenic elements in the process of land-ocean transport has of great scientific significance for understanding regional material cycling and ecological environmental evolution.This paper focuses on eight typical rivers within China,including the Changjiang,Huanghe,Zhujiang,Liaohe,Haihe,Huaihe,Qiantangjiang,and Minjiang,as well as the Changjiang River Estuary and adjacent East China Sea shelf as the study region.Several important terrestrial substances,such as suspended particulate matter（SPM）,nutrients(NO₃^-,Si O₃^2-,PO₄^3-,NH₄⁺and NO₂^-),dissolved organic carbon（DOC）,chromorphoric dissolved organic matter（CDOM）,and dissolved organic phosphorus（DOP）,were selected as research subjects to systematically analyze their temporal and spatial variations and influencing factors during river transport processes,as well as their temporal and spatial distribution,dynamic sources,and transformation in the estuarine and nearshore regions.Based on the aforementioned research,this paper primarily concludes the following:（1）The concentrations of nutrients exported by the lower Changjiang River exhibited significant seasonal variations,and the increasing human activity in river basins and global climate change can significantly alter the concentration and flux of nutrients in different time scales.The seasonal variations of NO₃^-and PO₄^3-were mainly controlled by diluting effect caused by water discharge,while the daily variations during the flooding period were mainly controlled by flushing effect.Combining the latest data from this study with historical data from previous literature,the conclusion showed that the nutrient concentrations and fluxes in the lower Changjiang River have reached a turning point in the past decade.The concentrations of NO₃^-,PO₄^3-,and NH₄⁺began to decline in 2012 after a previously increasing trend,while the concentration of Si O₃^2-began to decline from 2014.The long-term trend changes in the concentrations of NO₃^-and PO₄^3-can be attributed to the application of nitrogen and phosphorus fertilizers in the Changjiang River Basin,while the decreasing trend in Si O₃^2-concentration may be related to the construction of numerous reservoirs and dams in the basin.（2）The sources,composition,and influencing factors of dissolved organic carbon（DOM）in the eight major rivers in China show significant differences.Overall,the CDOM fluorescent components in the eight rivers were mainly composed of humic-like substances,and some components in the protein-like and humic-like fluorescence may come from the same source.The fitting analysis of CDOM and DOC fluxes in the eight rivers shows an excellent linear result,indicating that CDOM fluxes could be used to estimate DOC fluxes in Chinese rivers.The areal yields of DOC,CDOM and nutrients of the eight rivers are negatively correlated with latitude,indicating that the latitude-related control factors,including climatic factors and human activities,have a significant effect on the DOM and nutrients fluxes from Chinese rivers.In addition,the higher the latitude of the river,the smaller the molecular weight of CDOM,and the less input of allochthonous macromolecular compounds.The composition of DOM in the Liaohe,Haihe,and Huaihe is more significantly affected by human activities,as evidenced by the smaller particle size of the CDOM output from these three major rivers.In contrast,CDOM output from the Zhujiang,Minjiang,and Qiantangjiang in southern China exhibited larger particle sizes and greater amounts of terrestrial plant debris components.The particle size and composition of DOM output from the Changjiang and Huanghe fall between the two aforementioned groups of rivers,indicating that both human activities and climate change can to a large extent determine the particle size and chemical composition of DOM in these rivers.（3）There are significant temporal and spatial distribution differences of dissolved phosphorus in the Changjiang River Estuary and the adjacent ECS shelf,while the severe flood in the Changjiang River will significantly alter the distribution patterns of dissolved phosphorus and exacerbate its non-conservative behavior.Overall,DIP exhibits a decreasing trend from land to sea in the study area during both winter and summer,while DOP shows a patchy high-value distribution.DIP dominates the research area within 123°E in the surface water during summer,mainly due to high DIP content in the Changjiang Diluted Water（CDW）,while DIP dominates the total dissolved phosphorus pool in the bottom water,possibly due to the combined effect of sediment resuspension caused by oceanic intrusion and CDW input disturbances,summer stratification,and DOP remineralization processes.During flood periods,the high SPM and DIP concentrations of the surface water from the Changjiang River will expand significantly beyond 123°E,while the DIP concentration in the bottom water is significantly higher than this during normal summer,which is believed to be mainly due to the supplementation of bottom particles and sedimentary organic phosphorus remineralization.At the same time,high turbidity floodwaters significantly reduce the seawater transmittance in the study area,thereby weakening the absorption and utilization of DIP by phytoplankton,resulting in a significant decrease in autochthonous biomass,and causing the DOP concentration in the entire study area to be significantly lower than that during summer.The non-conservative behavior of dissolved phosphorus in the study area mainly manifests as follows:During normal summers,both source and sink effects of DIP and DOP can be observed in the surface seawater of the study area,and there is complementarity between the source and sink effects of DIP and DOP,indicating mutual conversion between the two during transmission.During the flood periods,the study area shows a sink effect for all DIP and a corresponding source effect for DOP,with the sink effect of DIP usually stronger than that of DOP.We believe that the stronger sink effects may be related to the adsorption of both components of dissolved phosphorus by high SPM concentrations in the extended CDW.（4）SPM in the Changjiang River Estuary and the adjacent ECS shelf exhibits different dynamic sources and transformation pathways.By utilizing on-site observation instrument data,it is possible to distinguish three types of SPM sources,i.e.,land-sourced SPM,biological particles,and in-situ flocculation.In this study,some optical observation instruments such as the OBS and the LISST were used during the Changjiang River flood period in 2016 to determine the concentration,properties,and size distribution of SPM.We proposed to differentiate land-derived particles and autochthonous biological particles by using the ratio between the total volume concentration measured by LISST and the turbidity measured by OBS.Additionally,we also identified flocculation processes near the estuary mouth successfully.Land-derived particles were found to be the dominant source of SPM in the bottom water of both the inner and outer stations,while autochthonous biological particles were the primary source of SPM in the upper water of the outer station.Furthermore,the boundary between upper and lower water layers at each station was typically located near the pycnocline layers,where the higher density of the lower water facilitated the aggregation of biological particles.In the mouth of the Changjiang River Estuary,in-situ flocculation was observed at some stations,which resulted in SPM particles with larger diameters that even exceeded the measuring range of the LISST.This study is of great significance for understanding the distribution and sources of SPM in the Changjiang River Estuary and the adjacent ECS shelf,as well as its impacts on marine ecosystems and coastal environments.