Spatial Variation Characteristics And Control Factors Of Dissolved Organic Matter In The Yangtze River

Dissolved organic matter(DOM)is a crucial component of ecosystem,its composition significantly influences the species composition,material cycling,energy flow,and information transmission within river ecosystems.As continuous aquatic ecosystems,rivers are subject to profound influences from biogeochemical processes occurring upstream,which subsequently affect downstream ecosystem functioning.However,there is relatively limited research on the spatial patterns of DOM composition and the controlling factors at both longitudinal and lateral scales of rivers.This study focuses on the large-scale Strahler 6th order main stem of the Yangtze River and its upstream small-scale tributaries in the Longcangou watershed,specifically investigating 27 Strahler 1st order headwater streams.By employing fluorescence spectroscopy techniques to analyze the dissolved organic matter components in the water,and integrating results from geographical,climatic,water chemistry,dam construction,microbial community composition,and diversity analyses,this study aims to explore the spatial variation and controlling factors of dissolved organic matter distribution in the Yangtze River.Furthermore,the study aims to elucidate the role of microbial community structure in the formation of dissolved organic matter.The main research findings are as follows:(1)The dissolved organic carbon(DOC)concentration in the main stem of the Yangtze River ranges from 0.28 to 3.40 mg/L,with an average of 1.52 mg/L.The tryptophan-to-tyrosine ratio(Trypto/Tyro)is higher than 0.5,indicating poor bioavailability of proteins in the Yangtze River.In the main stem,there is spatial variation of DOM components across three geographical tiers,where fluorescence index(FI)and freshness index(β/α)significantly decrease from upstream to downstream in the natural segments,while humic substance content and humification index(HIX)significantly increase.Different calculation methods for humic substance content and HIX show variability among different geographical ladders.Linear relationships between FI,β/α,humic substance content,HIX,and spatial distance indicate a gradual decrease in DOM quality from upstream to downstream in the Yangtze River.Redundancy analysis(RDA)explains 49.68% of the spatial variation in DOM components.Among the environmental factors,NO3--N and altitude are the most important factors which affect DOM variation.Variation partitioning analysis(VPA)also suggests that aquatic environmental factors(25.30%)play a greater role in DOM spatial variation than geographical factors(9.40%).There is no significant difference in DOM components between river segments before and after dams,but the quality of DOM in the overall second-ladder segment is improved.The spatial longitudinal variation of DOM components in the Yangtze River follows an internal transformation model rather than a river continuum concept.The main sources of DOM in the Yangtze River are refractory terrestrial materials,and the quality of DOM in the water is mainly regulated by internal transformation and dam construction.(2)The dissolved organic carbon concentration in the 27 Strahler 1st-order tributaries of the Longcangou watershed ranges from 0.35 to 1.50 mg/L,with an average of 0.85 mg/L.The Trypto/Tyro ratio has an average value of 0.76,indicating poor bioavailability of proteins.β/α has an average value of 0.61,indicating low microbial activity.FI,Trypto/Tyro,humic substance content,and β/α are significantly negatively correlated with altitude,suggesting that the bioavailability of DOM decreases with increasing altitude.Besides altitude,DOM components are not significantly affected by topography,watershed area,and climatic factors.Stepwise regression analysis shows that altitude,slope,temperature,p H,oxidation reduction potential,DOC,Ca/Mg,and NO3-concentration all have an effect on DOM components.Structural equation modeling results indicate that in the Strahler 1st-order headwater rivers,DOM component characteristics are mainly controlled by variations in rock weathering and input of allochthonous materials influenced by human activities.Changes in Ca/Mg resulting from rock weathering and variations in NO3-concentration influenced by human activities are the primary controlling factors of DOM component variation.In small-scale headwater rivers,the enhancement of allochthonous dissolved organic matter and the decrease in microbial activity occur with increasing altitude.From the perspective of source river regulation,the characteristics of water DOM components are mainly regulated by rock weathering influenced by geographical factors,particularly altitude,and changes in nutrient levels influenced by human activities.(3)The dominant microbial species in the water of the main stem of the Yangtze River at the genus level are Limnohabitans,Cyanobacteria,Methylopumilus,and Thermus.Dominant species in the free-living microbial community include Paracoccus and Pseudarcicella,while Cyanobacteria is dominant in the particle-attached microbial community.Particle-attached microbial communities have higher OTU(operational taxonomic unit)numbers and community diversity than free-living communities.Parameters related to microbial community composition that correlate with DOM components are humic substance content,FI,and β/α,which may be related to the utilization of humic substances by Pseudarcicella in the free-living microbial community and primary production of Cyanobacteria in the particle-attached microbial community.Variance analysis of microbial diversity parameters and nonmetric multidimensional scaling(NMDS)analysis of OTU clustering annotation results show significant differences in microbial community structure between the first-ladder and third-ladder,indicating spatial variation in microbial community structure and diversity in the main stem of the Yangtze River.Among the water environmental parameters,nitrogen(TN,NO3--N,NH4+-N),carbon(TC,TOC,DOC),and environmental background(temperature,electrical conductivity,p H)are the main factors influencing microbial community structure.The microbial community is not the dominant controlling factor for the characteristics of DOM components in the main stem of the Yangtze River,as the variation of DOM components is more influenced by other environmental factors.The integrated analysis indicates that DOM in the Yangtze River basin exhibit longitudinal spatial variation at both large and small spatial scales and are influenced by the water’s chemical environment.Although DOM is composed of allochthonous substances,the main controlling process for DOM in higher-order rivers is internal transformation,which is different from the input of catchment areas in headwater rivers.Therefore,the dominant hydrochemical factors influencing DOM component variation differ across rivers of different scales(NO3-in the main stem of the Yangtze River and Ca/Mg in Longcangou).This study investigates the spatial variation and controlling factors of dissolved organic matter(DOM)components in the Yangtze River,and provides a preliminary discussion on the relationship between DOM components and microbial community structure.The results of this research can help clarify the spatial variation and controlling factors of dissolved organic matter components in rivers of different spatial scales.They offer a potential explanatory direction for understanding the variation in microbial community structure within river systems and provide insights for assessing the impact of human activities on river ecosystems.