Linking the unique molecular complexity of dissolved organic matter to flood period in the Yangtze River mainstream

Spatial variation of anthropogenic disturbances within watersheds determines dissolved organic matter composition exported to oceans

Global land-use changes alter the delivery of fluvial dissolved organic matter (DOM) along land-to-sea continuum. To study how spatial variations in watershed anthropogenic disturbances control chemodiversity and reactivity of DOM exported to oceans, we used fluorescent and ultra-high-resolution mass spectrometry to investigate spatial and seasonal variations of DOM properties along two subtropical coastal rivers with contrasting anthropogenic land-use distributions (North and West tributaries of Jiulong River, southeast China). Dissolved organic carbon (DOC) concentration and humic- and protein-like fluorescent DOM (FDOM) intensities were high in the mixed urban-agricultural impacted upper North River and lower West River. DOM molecular signatures suggested that the urban-sourced DOM is dominated by bio-labile, S-rich compounds, whereas the agricultural-sourced DOM is characterized by a mixture of bio-labile CHONS and bio-refractory CHON. This anthropogenic-induced spatial variation in DOM signatures was especially prominent during the dry season. Molecular analysis indicated that heteroatomic-containing (phosphorus-sulfur-nitrogen) DOM compounds are more biologically degradable, whereas most of the heteroatom-depleted and highly unsaturated CHO was stable during transport. Due to a longer transit distance and reservoir impoundment in North River, the urban-sourced aliphatic compounds were largely microbially removed or transformed into bio-refractory components, resulting in lower DOC fluxes and an increase of recalcitrance in the DOM exported to the ocean. Conversely, shorter transit times for anthropogenic inputs from the middle/lower West River increased watershed yield and export fluxes of DOC with higher bio-lability. Our study documents that transit history plays a crucial role in assessing the fate of anthropogenic DOM along the land-to-ocean continuum.

Characteristics of DOM and Their Relationships with Potentially Toxic Elements in the Inner Mongolia Section of the Yellow River, China

The characterization of dissolved organic matter (DOM) is important for better understanding of the migration and transformation mechanisms of DOM in water bodies and its interaction with other contaminants. In this work, fluorescence characteristics and molecular compositions of the DOM samples collected from the mainstream, tributary, and sewage outfall of the Inner Mongolia section of the Yellow River (IMYR) were determined by using fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). In addition, concentrations of potentially toxic elements (PTEs) in the relevant surface water and their potential relationships with DOM were investigated. The results showed that the abundance of tyrosine-like components increased significantly in downstream waters impacted by outfall effluents and was negatively correlated with the humification index (HIX). Compared to the mainstream, outfall and tributaries have a high number of molecular formulas and a higher proportion of CHOS molecular formulas. In particular, the O5S class has a relative intensity of 41.6% and the O5-7S class has more than 70%. Thirty-eight PTEs were measured in the surface water samples, and 12 found above their detective levels at all sampling sites. Protein-like components are positively correlated with Cu, which is likely indicating the source of Cu in the aquatic environment of the IMYR. Our results demonstrated that urban wastewater discharges significantly alter characteristics and compositions of DOM in the mainstream of IMYR with strongly anthropogenic features. These results and conclusions are important for understanding the role and sources of DOM in the Yellow River aquatic environment.

Estuarine hydrodynamic processes driving the molecular changes of terrestrial dissolved organic nitrogen: From mixing to biological modification

Estuaries receive substantial amounts of terrestrial dissolved organic nitrogen (tDON), which will be transported from the freshwater to the oceanic terminus through vigorous exchange processes. However, the intricate migration and transformation dynamics of tDON during this transportation, particularly at a molecular level, remain constrained. To address this knowledge gap, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was used for the analysis of DON molecular composition in the Pearl River Estuary (PRE), a river-dominated estuarine system influenced by intensified anthropogenic activities in southern China. The results showed a pronounced spatial-temporal variation in DON concentration in the study area. At the molecular level, tDON exhibited reduced unsaturation and aromaticity, coupled with an elevated abundance of DON compounds containing one‑nitrogen atom (1 N-DON, 53.17 %) and compounds containing carbon, hydrogen, oxygen, nitrogen, and sulfur (CHONS) (27.46 %). It was evident that lignin was depleted while more oxygenated tannin compounds were generated in the freshwater-seawater mixing zone. This transformation is attributed to heightened biological activities, likely influenced by the priming effect of terrestrial nutrient inputs. In summer, the prevailing plume combined with biological activities in the strong mixing area and outer estuary increased the abundance of 3 N-DON molecules and a concurrent rise in the abundance of DON compounds containing only carbon, hydrogen, oxygen, and nitrogen (CHON), DON compounds containing carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus (CHONSP), and CHONS. This trend also underscores the expanding role of marine plankton and microbes in the utilization of DON compounds containing carbon, hydrogen, oxygen, nitrogen, and phosphorus (CHONP). These findings provide details of tDON transformation processes at the molecular level in a river-dominated estuary and underline the estuarine hydrodynamics involved in transporting and altering DON within the estuary.

Dissolved organic matter composition and characteristics during extreme flood events in the Yangtze River Estuary

Understanding the molecular composition and fate of dissolved organic matter (DOM) during transport in estuaries is essential for gaining a comprehensive understanding of its role within the global biogeochemical cycle. In 2020, a catastrophic flood occurred in the Yangtze River basin. It is currently unknown whether differences in hydrologic conditions due to extreme flooding will significantly impact the estuarine to oceanic DOM cycle. We determined the DOM composition in the Yangtze River estuary (YRE) to the East China Sea by using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) during the high discharge and the flood period (monthly average discharge was 1.2 times higher) on the same trajectory. Our study found that the composition of DOM is more diverse, and more DOM molecules were introduced to the YRE during the flood, especially in the freshwater end member. The result revealed that the DOM was significantly labile and unstable during the flood period. A total of 1840 unique molecular formulas were identified during the flood period, most of which were CHON, CHONS, and CHOS compounds, most likely resulting from anthropogenic inputs from upstream. Only 194 of these molecules were detected in the seawater end member after transporting to the sea, suggesting that the YRE served as a 'filter' of DOM. However, the flood enhances the transport of a group of terrigenous DOM, that is resistant to photodegradation and biodegradation. As a result, YRE experienced ~1.6 times higher terrigenous DOC flux than high discharge period. Considering the increased frequency of future floods, our study provides a preliminary basis for further research on how floods affect the composition and characteristics of estuarine DOM. With the help of the FT-ICR MS technique, we can now better understand the dynamic of DOM composition and characteristics in large river estuaries.

Exploring the optical properties and molecular characteristics of dissolved organic matter in a large river-connected lake (Poyang Lake, China) using optical spectroscopy and FT-ICR MS analysis

River-connected lakes are complicated and dynamic ecosystems due to their distinctive hydrological pattern, which could significantly impact the generation, degradation, and transformation processes of dissolved organic matter (DOM) and further regulate DOM chemistry in lakes. However, the molecular compositions and characteristics of DOM in river-connected lakes are still poorly understood. Thus, here the spatial variations of optical properties and molecular characteristics of DOM in a large river-connected lake (Poyang Lake) were explored via spectroscopic techniques and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results showed high degree of spatial heterogeneity of DOM chemistry (variations in DOC concentrations, optical parameters, and molecular compounds) in Poyang Lake, and the diversity at the molecular level was primarily caused by the heteroatom compounds (N- and S- containing). Compared with classic lakes and rivers, DOM compositions of the river-connected lake had distinctive characteristics (differences in the AI_mod and DBE values, and CHOS proportions). And the composition characteristics of DOM between the southern and northern parts of Poyang Lake were different (such as the lability and molecular compounds), suggesting the changes of hydrologic conditions may affect the DOM chemistry. In addition, various sources of DOM (autochthonous, allochthonous, and anthropogenic inputs) were identified agreeably based on optical properties and molecular compounds. Overall, this study first characterizes the DOM chemistry and reveals its spatial variations in Poyang Lake at the molecular level, which could improve our understanding of DOM in large river-connected lake systems. Further studies are encouraged to investigate the seasonal variations of DOM chemistry under different hydrologic conditions in Poyang Lake to enrich the knowledge of carbon cycling in river-connected lake systems.

Spatiotemporal response of dissolved organic matter diversity to natural and anthropogenic forces along the whole mainstream of the Yangtze River

The Yangtze River, the largest river in Asia, plays a crucial role in linking continental and oceanic ecosystems. However, the impact of natural and anthropogenic disturbances on composition and transformation of dissolved organic matter (DOM) during long-distance transport and seasonal cycle is not fully understood. By using a combination of elemental, isotopic and optical techniques, as well as Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS), we investigated DOM abundance and composition along the whole mainstream at highly spatial resolution in the dry and early wet seasons. Our findings showed that the concentration and flux of dissolved organic carbon (DOC) in the Yangtze River was much lower compared with other worldwide larger rivers. The distribution of δ¹³C_DOC and higher abundance of humic-like fluorescent component and highly unsaturated and phenolics (HUPs) compound reflected a prominent contribution of allochthonous DOM. Further optical and molecular analysis revealed humic-like fluorescent components were coupled with CHO molecules and HUPs compound with higher aromatic, unsaturated, molecular weight and stable characteristics between upstream and midstream reaches. With increasing agricultural and urban land downstream, there were more heteroatomic formulae and labile aliphatic and protein-like compounds which were derived from human activities and in situ primary production. Meanwhile, DOM gradually accumulates with slow water flow and additional autochthonous organics. Weaker solar radiation and water dilution during the dry/cold season favours highly aromatic, unsaturated and oxygenated DOM compositions. Conversely, higher discharge during the wet/warm season diluted the terrestrial DOM, but warm temperatures could promote phytoplankton growth that releases labile aliphatic and protein-like DOM. Besides, chemical sulfurization, hydrogenation and oxygenation were found during molecular cycling processes. Our research emphasizes the active response of riverine DOM to natural and anthropogenic controls, and provides a valuable preliminary background to better understand the biogeochemical cycling of DOM in a larger river.

Innovative approach to reveal source contribution of dissolved organic matter in a complex river watershed using end-member mixing analysis based on spectroscopic proxies and multi-isotopes

Dissolved organic matter (DOM) in river watersheds dynamically changes based on its source during a monsoon period with storm event. However, the variations in DOM in urban and rural river watersheds that are dominated by point and non-point sources have not been adequately explored to date. We developed an innovative approach to reveal DOM sources in complex river watershed systems during pre-monsoon, monsoon, and post-monsoon periods using end-member mixing analysis (EMMA) by combining multi-isotope values (δ¹³C-DOC, δ¹⁵N-NO₃ and δ¹⁸O-NO₃) and spectroscopic indices (fluorescence index [FI], biological index [BIX], humification index [HIX], and specific UV absorbance [SUVA]). Several potential end-members of DOM sources were collected from watersheds, including top-soils, groundwater, plant group (fallen leaves, riparian plants, suspended algae), and different effluents (cattle and pig livestock, agricultural land, urban, industry facility, swine treatment facility and wastewater treatment facility). Concentrations of dissolved organic carbon, dissolved organic nitrogen, NO₃-N, and NH₄-N increased during the monsoon period with an increase in the input of anthropogenic DOM, which have higher HIX values owing to the flushing effect. The results of EMMA indicate that soil and agricultural effluents accounted for a substantial contribution of anthropogenic DOM at varying rates based on seasons. We also found that results of EMMA based on combining spectroscopic indices and δ¹³C-DOC isotope values were more accurate in tracing DOM sources with respect to land-use characteristics compared to applying only spectroscopic indices. The positive relationship between FI, BIX and δ¹⁵N-NO₃ were revealed that nitrate would be decomposed from DOM affected by intensive agricultural activities. In addition, consistent with the EMMA results, the molecular composition of the DOM was clearly evidenced by a large number of CHON formulas, accounting for over 50% of the total characterized compounds, including pesticides and pharmaceuticals used in agriculture farmland and livestock. Our results clearly demonstrated that EMMA based on combing multi-stable isotopes and spectroscopic indices could be trace the DOM source, which is important for understanding changes in the DOM quality, and application of nitrate isotopes and molecular analysis supports in-depth interpretation. This study provides easy and intuitive techniques for the estimation of the relative impacts of DOM sources in complex river watersheds, which can be verified in various ways rather than relying on a single technique approach.

Optical and molecular indices of dissolved organic matter for estimating biodegradability and resulting carbon dioxide production in inland waters: A review

Biodegradable dissolved organic carbon (BDOC) constitutes the most labile fraction of dissolved organic matter (DOM), which also functions as a source of CO₂ emissions from inland waters. However, no systematic review is available on DOM indicators of BDOC and CO₂ production potential. Optical and molecular indices can be used to track small changes in DOM composition during biodegradation. In this review, we identified four different methods for measuring BDOC together with their strengths and limitations. In addition, we discuss the potential of using documented optical indices based on absorption and fluorescence spectroscopy and molecular indices based on Fourier transform ion cyclotron mass spectrometry as proxies for estimating BDOC and biodegradation-induced CO₂ production based on previously reported relationships in the literature. Many absorbance- and fluorescence-based indices showed inconsistent relationships with BDOC depending on watershed characteristics, hydrology, and anthropogenic impacts. Nevertheless, several indices, including specific UV absorbance at 254 nm (SUVA₂₅₄), humification index (HIX), and terrestrial humic-like fluorescent DOM (FDOM) components, tended to have negative relationships with BDOC in tropical and temperate watersheds under baseflow or drought periods. Protein-like FDOM exhibited the strongest correlation with BDOC in different systems, except during storms and flood events. Despite the limited number of studies, DOM molecular indices exhibited consistent relationships with BDOC, suggesting that the relative abundance of aliphatic formulas and the molecular lability index could act as reliable proxies. The DOM optical indices explain up to 96% and 78% variability in BDOC and CO₂, respectively; nonetheless, there were limited studies on molecular indices, which explain up to 74% variability in BDOC. Based on literature survey, we recommend several sensitive indices such as SUVA₂₅₄, HIX, and terrestrial humic- and protein-like FDOM, which could be useful indicators of BDOC and dissolved CO₂ in inland water. Future research should incorporate a wider range of geographic regions with various land use, hydrology, and anthropogenic disturbances to develop system- or condition-specific DOM optical or molecular proxies for better prediction of BDOC and CO₂ emissions.

Hydrological isolation affected the chemo-diversity of dissolved organic matter in a large river-connected lake (Poyang Lake, China)

The transportation processes during aquatic systems regulate the ultimate chemistry of dissolved organic matter (DOM), and in recent years, climate changes and human activities have altered the hydrological patterns of many rivers and lakes, which generated some severe issues, such as hydrological isolation. However, how hydrological isolation affects variations of DOM chemistry in large lake systems is still poorly understood. Here, optical properties and molecular compositions of DOM samples derived from a large river-connected lake (Poyang Lake, China) and its nearby seasonal sub-lakes (formed by hydrological isolation) were characterized using Fourier transform ion cyclotron resonance mass spectrometry (FT ICR MS) and ultraviolet-visible (UV-Vis) spectroscopy. The results revealed more abundance of organic matter in sub-lakes than that in the main lake according to high dissolved organic carbon (DOC) concentrations and absorption coefficients (a₂₅₄ and a₂₈₀). Large proportions of CHOS formulas were identified by FT ICR MS in sub-lakes DOM, which were produced through Kraft reactions (sulfide/bisulfide + lignin CHO → CHOS) in the interface of sediment/water, and greatly contributed to aliphatic compounds. In addition, obvious variations of compounds (such as polyphenols, highly unsaturated and aliphatic compounds) and lability of DOM were observed between sub-lakes and main lakes, which were mainly caused by the different degradation pathways of DOM (photodegradation in sub-lakes while biodegradation in the main lake). Our results demonstrated that hydrological isolation has significant impacts on DOM chemistry, and provides an improved understanding of the DOM biogeochemistry process in Poyang Lake and supports the management of the large lake systems.

Metabolomics analysis of unresolved molecular variability in stoichiometry dynamics of a stream dissolved organic matter

Broad molecular classification based on stoichiometric ratio relationships has been used extensively to characterize the chemical diversity of aquatic dissolved organic matter (DOM). However, variability in the molecular composition within this classification has remained elusive, thus limiting the interpretation of DOM dynamics, especially with respect to transport versus transformation patterns in response to hydrologic or landscape changes. Here, leveraging high-frequency spatiotemporal sampling during rainfall events at a Critical Zone Observatory project site in Clear Creek, Iowa, we apply a metabolomics-based analysis validated with fragmentation using tandem mass spectrometry to uncover patterns in the molecular features of the DOM composition that were not resolved by classification based on stoichiometric ratios in the chemical formulae. From upstream to downstream sites, beyond the increased aromaticity implied by changes in the stoichiometric ratios, we identified an increased abundance of flavonoids and other phenylpropanoids, two important subgroups of aromatic compounds. The stoichiometric analysis also proposed a localized decline in the abundance of lipid-like compounds, which we attributed specifically to medium-chain and short-chain fatty acids; other lipids such as long-chain fatty acids and sterol lipids remained unchanged. We further determined in-stream molecular transitions and specific compound degradation by capturing changes in the molecular masses of terpenoids, phenylpropanoids, fatty acids, and amino acids. In sum, the metabolomics analysis of the chemical formulae resolved molecular variability imprinted on the stoichiometric DOM composition to implicate key molecular subgroups underlying carbon transport and cycling dynamics in the stream.

Fluorescence and molecular signatures of dissolved organic matter to monitor and assess its multiple sources from a polluted river in the farming-pastoral ecotone of northern China

The sources and composition of dissolved organic matter (DOM) in rivers are critical to water quality and aquatic ecosystems. Studies on detailed composition of organic matter in rivers in the farming-pastoral ecotone are relatively limited in the research community. To better understand the characteristics and dynamics of DOM, Yang River in North China was selected as the study area because of its profound influences on the farming-pastoral ecotone nearby. A combination of fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) techniques revealed that the DOM composition of Yang River is driven by land use. DOM in Yang River is predominantly imported from allochthonous inputs, together with agricultural runoff, pastureland, and urban sewage, causing a comprehensive impact on DOM. In detail, DOM associated with cropland inputs was dominated by lignin-like species, with higher nitrogen content. In comparison, DOM related to grassland is more diverse and susceptible to degradation. An increase in urban areas led to an increase in sulfur-containing compounds, while their oxygen, nitrogen, and aromaticity contents were significantly lower than those in cropland. Interestingly, urban-influenced lignin-like compounds may be associated with the effluents from the pulp and paper mill. Additionally, synthetic surfactants from the lower section of the river were also structurally identified by tandem mass spectrometry. Overall, this study could provide valuable insights into the DOM sources and their transformation dynamics at a molecular level, which could be an indicator for riverine water quality management and be applied to other farming-pastoral ecotones straightforward.

Effects of flood discharge on the water quality of a drinking water reservoir in China - Characteristics and management strategies

Stormflow runoff is an important non-point source of pollution in drinking water reservoirs. Rationally managed flood discharge processes at estuaries can reduce the high concentration of pollutants carried by runoff, and thus their impacts on water plant operations. In this study, the physical and chemical water quality parameters upstream from a dam were measured for a flood discharge process in the Jinpen Reservoir of Northwest China. The results showed that the time needed for the flood to reach the reservoir was ∼9 h after flooding began, and this lag effect meant that water in the metalimnion and hypolimnion were vented in advance. Consequently, the undercurrent intruded into the hypolimnion. The water temperature increased by 1.83 °C, and the Schmidt stability index decreased from 3291.37 J m^-2 to 2496.32 J m^-2. Flood discharge can effectively reduce the volume of pollutants; however, it cannot completely prevent the deterioration of water quality in the main reservoir. The turbidity of the reservoir still exceeded 300 NTU after the flood discharge. When the outflow discharge decreased, the critical height of aspiration also decreased, and dissolved pollutants could neither be vented nor precipitated quickly, not allowing their concentration within the reservoir to decrease further. A three-dimensional hydrodynamic model was successfully used to simulate flood processes and determine an optimal flood discharge plan. We found that the elevations of the outlet and undercurrent layer are essential to determine the efficiency of flood discharge, while appropriate timing and outflow discharge volumes may further enhance the results. Therefore, using a hydrodynamic model to predict the position of an undercurrent layer, and opening spillways near the elevation of the undercurrent layer can improve the efficacy of flood discharge. Our findings provide novel insights that may be used to improve the operation and management of source water reservoirs.

Sampling, storage and laboratory approaches for dissolved organic matter characterisation in freshwaters: Moving from nutrient fraction to molecular-scale characterisation

Recent research has highlighted the importance of dissolved organic matter (DOM) for ecosystem function and because of this paradigm shift, it has become crucial to not only quantify its contribution to river nutrient loads but also to characterise its composition. There has been a significant research effort utilising optical methods, such as fluorescence and UV-Vis spectrophotometry, in order to start exploring DOM character. However, these methods still lack the granularity to understand the chemical composition at the molecular level, which is vital to properly understanding its functional role in freshwater ecosystems. As a direct result, there has been a shift towards including molecular-scale analyses to investigate the in-stream processing of the material. Alongside this, recent methodological advancements, particularly in mass spectrometry are opening new opportunities for probing one of the most complex environmental mixtures. However, in order to fully exploit these opportunities, it is key that the way that samples are collected, processed and stored is considered carefully such that sample integrity is maintained. There are additional challenges when collecting water samples for analysis at molecular scale, for example the ultra-low concentrations of individual compounds within DOM means that the samples are sensitive to contamination. This paper discusses current sample collection, processing and storage protocols for this C, N and P quantification and characterisation in freshwaters, and proposes a new standardised protocol suitable for both nutrient fraction quantification and molecular scale analyses, based on method development and testing undertaken in our UK Natural Environment Research Council large grant programme, characterising the nature, origins and ecological significance of Dissolved Organic Matter IN freshwater Ecosystems (DOMAINE).

Eutrophication and watershed characteristics shape changes in dissolved organic matter chemistry along two river-estuarine transects

Dissolved organic matter (DOM) plays a crucial role in the coastal carbon cycle. However, eutrophication-induced algal blooms and lateral transport from connected tidal marshes may significantly affect DOM cycling, which remains poorly understood. By combining a suite of bulk and optical techniques, and the Fourier transform ion cyclotron resonance mass spectrometry and ion mobility quadrupole time-of-flight mass spectrometry, we determined DOM concentration and composition along two typical river-estuary transects (namely Liao and Daliao rivers), Northeast China, with contrasting eutrophic state and distribution of tidal marshes. The Daliao River is characterized by a higher eutrophication degree and is surrounded with lower reed coverage than the Liao River. Compared to the Liao River, significantly higher dissolved organic carbon concentrations were observed in the Daliao River, where higher stable carbon isotope (δ13C) values and protein-like fluorescent components, characterized relatively higher autochthonous DOM. Further molecular analysis revealed higher peptide and sugar-like compounds but lower isomeric percentages of several molecular formulas in the Daliao River, suggesting higher molecular lability but lower isomeric complexity than the Liao River. Associations between optical and molecular signatures among all DOM samples revealed that a red-shifted humic-like C3 component was significantly correlated with molecular formulas with lower molecular weight and aromaticity, and higher H/C, indicating that C3 was likely a result of phytoplankton production coupled with further heterotrophic processing. Moreover, we found that reed marshes could introduce to both rivers a series of carboxylic-rich alicyclic compounds, highly unsaturated compounds, and polyphenols with high molecular weight and low H/C. This study suggests that eutrophication and reed marsh affect the DOM quality and can be a potential source of recalcitrant DOM compounds to coastal rivers and estuaries, which warrants further investigations considering the increasing worldwide eutrophication and sea-level rise in coastal delta environments.

Source tracking of dissolved organic nitrogen at the molecular level during storm events in an agricultural watershed

Accelerated export of nitrogen-containing dissolved organic matter (DOM) or dissolved organic nitrogen (DON) to streams and rivers from agricultural watersheds has been reported worldwide. However, few studies have examined the dynamics of DOM molecular composition with the attention paid to the relative contributions of DON from various sources altered with flow conditions. In this study, end-member mixing analysis (EMMA) was conducted with the optical properties of DOM to quantify the relative contributions of several major organic matter sources (litter, reed, field soil, and manure) in two rivers of a small agricultural watershed. DOC and DON concentration increased during the storm events with an input of allochthonous DOM as indicated by an increase in specific ultraviolet absorbance at 254 nm (SUVA₂₅₄) and a decrease in biological index (BIX), fluorescence index (FI), and protein-like component (%C3) at high discharge. EMMA results based on a Bayesian mixing model using stable isotope analysis in R (SIAR) were more accurate in source tracking than those using the traditional IsoSource program. Manure (>30%) and field soil (also termed as "manure-impacted field soil") (>23%) end-members revealed their predominant contributions to the riverine DOM in SIAR model, which was enhanced during the storm event (up to 56% and 38%, respectively). The molecular composition of the riverine DOM exhibited a distinct footprint from the manure and manure-impacted field soil, with a larger number of CHON formulas and abundant polyphenols and condensed aromatics in peak flow samples in the studied rivers. The riverine DOM during peak flow contained many unique molecular formulas in both rivers (4980 and 2082) of which >60% originated from manure and manure-impacted field soil. Combining the EMMA with DOM molecular composition clearly demonstrated the effect of manure fertilizer on the riverine DOM of the watershed with intensive agriculture. This study provides insights into the source tracking and regulation of DON leaching from anthropogenically altered river systems worldwide.

Enhanced chemodiversity, distinctive molecular signature and diurnal dynamics of dissolved organic matter in streams of two headwater catchments, Southeastern China

Dissolved organic matter (DOM) is a complicated assembly of organic molecules, including thousands of molecules with various structures and properties. However, how the stream DOM sources respond to carbon compositions and the transformation processes remains unclear. In this study, the chemical characteristics and spectral and mass spectrometry (FT-ICR MS) of DOM were analyzed. Six sampling points of headwater stream (HWSs) were sampled, and an effluent polluted stream (WSR) and a main stream of the Changjiang River (DT) were also sampled for comparison. In situ degradation experiments and FT-ICR MS analysis were also performed to observe the dynamic processes of DOM in HWS. The results showed that the anthropogenic markers of sewage (i.e. sulfur (S) compounds and marker from antibiotics and estrogen) in HWS were higher than those in DT. The molecular weight decreased while the degradation products (S-containing compounds and unsaturated compounds (HU)) increased after in situ degradation due to the influence of both the photodegradation and biodegradation process. In addition, the KMD plots showed that the DOM homologue intensities in range 400-600 Da changed significantly after demethylation by biodegradation. The components of highly refractory substances and the degradation degree of DOM in DT was higher than that in HWS. We extracted the refractory DOM pool in HWS, which was mainly small molecular with molecular weights < 600 Da. These molecular will be difficult to remove in traditional drinking water treatment processes and easily produced disinfection byproducts (DBPs). This study emphasized the necessity of identifying the sources and transformation processes of DOM in HWS and clarified the types and characteristics of DOM that should be considered in future drinking water treatment.

Variability of dissolved organic matter in two coastal wetlands along the Changjiang River Estuary: Responses to tidal cycles, seasons, and degradation processes

Dissolved organic matter (DOM) in the coastal tidal marsh-estuary systems are complex mixtures with different source materials that vary with hydrological regimes, seasons, and environmental conditions and are modified by removal processes including photochemical and microbial degradations. Here, monthly surveys of DOM and its optical properties (i.e., absorbance and fluorescence of DOM) covering a complete semi-diurnal tidal cycle were conducted in two coastal marshes with distinct hydrological regimes (i.e., one freshwater and one brackish marsh) in the Changjiang River Estuary (CRE). Four fluorescent components were identified by excitation-emission matrix fluoresces combined with parallel factor analysis (EEMs-PARAFAC) as two terrestrial humic-like components and two autochthonous protein-like components. Results indicated that ebbing waters draining the marshes were consistently enriched with highly absorbing, more humic and highly aromatic DOM compared to the flood tidewaters. On a seasonal basis, DOM dynamics were largely modulated by the marsh productivity and the seasonal Changjiang runoff. Protein-like fluorophores, however, demonstrated a constant, less variable pattern on both the tidal and seasonal timescales. Onsite water incubation experiments with photochemical and microbial alterations revealed that photochemistry was primarily responsible for the removal of optically active components in the marsh DOM pools whereas the impact of microbes was minor. Principal component analysis (PCA) illustrated the processes regulating the DOM dynamics at the marsh-estuarine interface and allowed a clear distinction of samples between the two marshes, in addition to the samples under the influence of episodic weather events (i.e., Super Typhoon Lekima in summer 2009 and the basin-wide severe flood event occurred in summer 2020). This study underscores the importance of world large-rivers such as Changjiang on estuarine marsh DOM dynamics and also highlights the substantial heterogeneity of the marsh DOM across a river-dominated estuary.

Density currents affect the vertical evolution of dissolved organic matter chemistry in a large tributary of the Three Gorges Reservoir during the water-level rising period

Reservoirs have boomed for clean energy in recent decades and interrupted the natural river ecosystem severely. Riverine dissolved organic matter (DOM), which regulates aquatic food web dynamics, water quality, and carbon storage, has been significantly impacted by reservoir construction. However, the vertical evolution of DOM properties and its controlling mechanisms in large reservoirs with hydrological management are not well investigated, limiting the understanding of carbon cycling (e.g., CO₂ emissions and carbon burial) in reservoirs. To fill this knowledge gap, multiple complementary techniques including optical spectroscopy and ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry were applied to track composition and property changes of DOM along the vertical profile in a large deep tributary of the world largest Three Gorges Reservoir (TGR) during the water-level rising period. The results indicated that middle and bottom water have relatively more terrestrial input and recalcitrant DOM, while surface water has relatively more autochthonous input and labile DOM. Integrated with the comprehensive analysis of DOM chemistry in a high-resolution vertical profile, the primary production and photodegradation in surface water, the density currents induced water intrusion from mainstream to tributaries, in middle water, and the biodegradation in bottom water are main factors controlling the vertical heterogeneity of reservoir DOM during the water-level rising period. This vertical increase of DOM recalcitrance likely contributes to the enhancement of organic carbon burial in TGR during the water-level rising period. All in all, this study provides new insight into the vertical variations of riverine DOM induced by reservoir construction, and emphasizes the important role of reservoir construction in carbon sequestration.

Hydrologic heterogeneity induced variability of dissolved organic matter chemistry among tributaries of the Three Gorges Reservoir

Dissolved organic matter (DOM) chemistry in rivers regulates aquatic food web dynamics, water quality, and carbon storage. The operation of reservoirs represents one of the major human modifications on the natural flow of rivers, which can affect DOM chemistry. Although hydrologic heterogeneity has been observed in different segments of the reservoir, whether it will structure DOM chemistry is poorly assessed, which is critical to better constrain the carbon cycle in reservoirs. By the combination of a series of techniques including stable carbon isotopes, optical spectroscopy, and ultrahigh-resolution mass spectrometry, here we showed that hydrologic heterogeneity induced changes in DOM molecular composition between two large tributaries, named Shennongxi (SR) and Xiangxi (XR) rivers, of the Three Gorges Reservoir (TGR). With water intrusion from mainstream, SR had relatively higher terrestrial and more recalcitrant DOM than XR, where no water intrusion from the mainstream was observed. In contrast, the averaged relative abundance of autochthonous input and biological lability of DOM along upstream to downstream transect were higher in XR than that in SR. The presence or absence of water intrusion from mainstream to tributaries induced by hydrologic management is likely the main factor controlling DOM chemistry in different tributaries of the TGR. By linking DOM chemistry in the water column with that in surface sediments, we suggest that hydrologic management of reservoir likely affects the preferential preservation of recalcitrant DOM in surface sediments, which further affects the organic carbon burial and the river carbon cycle. With reservoir construction increasing worldwide, further studies are encouraged to investigate the DOM chemistry under different hydrologic management of reservoirs to better constrain and predict the carbon cycling in fluvial ecosystems.