Hydrological management affected dissolved organic matter chemistry and organic carbon burial in the Three Gorges Reservoir

Meta-Analysis of Optical Surrogates for the Characterization of Dissolved Organic Matter

Optical surrogates, derived from absorbance and fluorescence spectra, are widely used to infer dissolved organic matter (DOM) composition (molecular weight, aromaticity) and genesis (autochthonous vs allochthonous). Despite the broad adoption of optical surrogates, several limitations exist, such as context- and sample-specific factors. These limitations create uncertainty about how compositional interpretations based on optical surrogates are generalized across contexts, specifically if there is duplicative or contradictory information in those interpretations. To explore these limitations, we performed a meta-analysis of optical surrogates for DOM from diverse sources, both from natural systems and after water treatment processes (n = 762). Prior to analysis, data were screened using a newly developed, standardized methodology that applies systematic quality control criteria before reporting surrogates. There was substantial overlap in surrogate values from natural and treated samples, suggesting that the gradients governing the surrogate variability can be generated in both contexts. This overlap provides justification for using optical surrogates originally developed in the context of natural systems to describe DOM changes in engineered systems, although the interpretations may change. Absorbance-based surrogates that describe the amount of spectral tailing (e.g., E2:E3 and S275-295) had a high frequency of strong correlations with one another but not to specific absorbance (SUVA254) or absorbance slope ratio (SR). The fluorescence index (FI) and biological index (β/α) were strongly correlated with one another and to the peak emission wavelength but not to the humification index (HIX). Although SUVA254 and FI have both been correlated to DOM aromaticity in prior research, there was a lack of reciprocity between these optical surrogates across this data set. Additionally, there were patterns of deviations in the wastewater subset, suggesting that effluent organic matter may not follow conventional interpretations, urging caution in the use of optical surrogates to track DOM in water reuse applications. Finally, the meta-analysis highlights that three aspects should be captured when optical spectra are used for DOM interpretation: specific absorbance, absorbance tailing, and the extent of red-shifted fluorescence. We recommend that SUVA254, E2:E3, and FI or β/α be prioritized in future DOM studies to capture these aspects, respectively.

Hydrological alteration drives chemistry of dissolved organic matter in the largest freshwater lake of China (Poyang Lake)

As the largest reactive organic carbon pool, dissolved organic matter (DOM) plays an important role in various biogeochemical processes in lake ecosystems. Recently, climate change-induced extreme events (e.g., floods and droughts) have significantly modified the hydrological patterns of lakes worldwide, and regulated the quality and quantity of DOM. However, the responses of DOM chemistry to hydrological alteration in lakes remain poorly understood. Here we investigated the influences of hydrological alteration on sources, composition, and characteristics of DOM in Poyang Lake, the largest freshwater lake in China, using a combination of bulk chemical, optical and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) techniques. Results show various sources of DOM (autochthonous, allochthonous, and anthropogenic inputs) and significant variations in DOM chemistry across four hydrological periods (the retreating, dry, rising, and flooding periods) in Poyang Lake. During the retreating, rising, and flooding periods, DOM was characterized by higher aromaticity, humification degree, and recalcitrance, and exhibited pronounced allochthonous signatures. In contrast, DOM contained more S-containing molecules and aliphatic compounds during the dry period, displaying relatively stronger autochthonous features. Terrestrial inputs and the lignin-CHOS formation process are likely the primary underlying mechanisms shaping the differences in DOM chemistry in Poyang Lake. Our research demonstrates the significant impacts of hydrological alteration on DOM dynamics, and provides an improved understanding of DOM biogeochemical cycles and carbon cycling in large aquatic systems under global climate change.

In-reservoir transformation of dissolved organic matter as a function of hydrological flow

Reservoirs are vital to meet the ever-increasing demands for freshwater in a warming climate. Dissolved organic matter (DOM) represents an important pool of carbon and can be a major concern in drinking water sources. However, insights into DOM dynamics in temperate, semi-arid reservoirs remain limited. Therefore, we investigated the variations in DOM properties in Lake Diefenbaker, a large reservoir on the Canadian Prairies, by analyzing eight years of DOM concentrations and composition through linear mixed effect modeling. Contrary to expectations, reservoir dissolved organic carbon (DOC) concentration showed no correlation with inflow from the South Saskatchewan River (p = 0.12), while dissolved organic nitrogen (DON) increased with decreasing inflow (p = 0.002). DOM optical indices (SUVA254 and E4:E6 ratio) and DOC:DON ratio revealed a pronounced influence of inflow on reservoir DOM composition (p < 0.001), i.e., allochthonous characteristics increased with increasing flow, and autochthonous characteristics increased with declining flow. Travel time corrected comparison of approximately the same water parcel along the reservoir length revealed that increasing water residence time in downstream regions led to a significant transformation in DOM composition, favoring autochthonous characteristics (mean SUVA254 reduced by 0.52 L mg-C-1 m-1, and the E4:E6 and spectral slope ratio increased by 1.6 and 0.06, respectively). Autochthonous DOC inputs likely offset the allochthonous DOC losses, which resulted in a relatively stable DOC concentration throughout the reservoir (mean 3.7 mg L-1). Additionally, the effect of a large aquaculture operation on reservoir DOM properties was investigated, but no effect was detected. The results have significant implications for managing large river-reservoirs. Autochthonous DOM poses challenges to water processing, necessitating monitoring of DOM composition for reservoir drinking water quality. Insights on climate-induced changes in DOM properties will also assist with understanding changes to habitat conditions and contaminant transport.

Effects of reservoir construction on optical and molecular characteristics of dissolved organic matter in a typical P-contaminated river

The source and composition characteristics of dissolved organic matter (DOM) are crucial to identify and evaluate the sources of pollution in the watershed. The construction of reservoirs changes the hydrological condition and pollutant fate of the river. However, the effects of reservoirs' construction on DOM in the watershed and the underlying mechanisms are still unclear. This study aims to examine and compare the characteristics of DOM in reservoirs and streams in the Huangbai River, a typical reservoir-affected and P-contaminated river within the Yangtze River catchment. The results showed that DOM in reservoirs was characterized by more contribution from autochthonous source, under the influence of reservoirs' construction; while, DOM in rivers was mainly originated from terrestrial input. Reservoirs had more lipid-like and protein-like compounds, while rivers contained more oxy-aromatic-like compounds. The percentage of CHOP molecules in reservoirs was significantly higher than that in rivers. The underlying mechanism is that more suitable conditions were created for plankton to grow after constructing reservoirs, which converted inorganic orthophosphate into organic phosphorus, and over time, organic phosphorus was gradually enriched in reservoirs, which exacerbated the risk of eutrophication in the reservoir water body. This study can provide theoretical support for monitoring and evaluation of water quality in reservoir-affected rivers.

Niche differentiation of microbial community shapes vertical distribution of recalcitrant dissolved organic matter in deep-sea sediments

Sedimentary organic matter provides carbon substrates and energy sources for microorganisms, which drive benthic biogeochemical processes and in turn modify the quantity and quality of dissolved organic matter (DOM). However, the molecular composition and distribution of DOM and its interactions with microbes in deep-sea sediments remain poorly understood. Here, molecular composition of DOM and its relationship with microbes were analyzed in samples collected from two sediment cores (∼40 cm below the sea floor), at depths of 1157 and 2253 m from the South China Sea. Results show that niche differentiation was observed on a fine scale in different sediment layers, with Proteobacteria and Nitrososphaeria dominating the shallow sediments (0-6 cm) and Chloroflexi and Bathyarchaeia prevailing in deeper sediments (6-40 cm), indicating correspondence of microbial community composition with both geographical isolation and the availability of organic matter. An intimate link between the DOM composition and microbial community further indicates that, microbial mineralization of fresh organic matter in the shallow layer potentially resulted in the accumulation of recalcitrant DOM (RDOM), while relatively low abundance of RDOM was linked to anaerobic microbial utilization in deeper sediment layers. In addition, higher RDOM abundance in the overlying water, as compared to that in the surface sediment, suggests that sediment might be a source of deep-sea RDOM. These results emphasize the close relation between the distribution of sediment DOM and different microbial community, laying a foundation for understanding the complex dynamics of RDOM in deep-sea sediment and water column.

Occurrence profiling and environmental risk assessment of veterinary antibiotics in vegetable soils at Chongqing region, China

Veterinary antibiotics (VAs) are emerging contaminants in soils as they may pose high risks to the ecosystem and human health. Identifying VAs accumulation in soils is essential for assessing their potential risks. Therefore, we investigated the distribution of VAs in soils from vegetable fields and evaluated their potential ecological and antimicrobial resistance risks in the Chongqing region of the Three Gorges Reservoir area, China. Results indicated that twenty-six species of VAs, including nine sulfonamides (SAs), seven quinolones (QNs), four tetracyclines (TCs), four macrolides (MLs), and two other species of VAs were detected in soils, with their accumulative levels ranging from 1.4 to 3145.7 μg kg^-1. TCs and QNs were the dominant VAs species in soils with high detection frequencies (100% TCs and 80.6% for QNs) and accumulative concentration (up to 1195 μg kg^-1 for TCs and up to 485 μg kg^-1 for QNs). Risk assessment indices showed that VAs (specifically SAs, TCs, and QNs) in most vegetable soils would pose a medium to high risk to the ecosystem and antimicrobial resistance. Mixture of VAs posed a higher risk to soil organisms, antimicrobial resistance, and plants than to aquatic organisms. Modeling analysis indicated that socioeconomic conditions, farmers' education levels, agricultural practices, and soil properties were the main factors governing VAs accumulation and environmental risks. Farmers with a high educational level owned large-scale farms and were more willing to use organic fertilizers for vegetable production, which eventually led to high VAs accumulation in vegetable soil. These findings would provide a reference for sustainable agricultural and environmental production under the current scenario of chemical fertilizer substitution by organic products and green agricultural development.

Development of subsurface chlorophyll maximum layer and its contribution to the primary productivity of water column in a large subtropical reservoir

The phenomenon of subsurface chlorophyll maximum (SCM) layer emerging at a certain water depth is commonly found in stratified water bodies. Also, it is a crucial contributing region to the primary productivity of the water column. Currently, there is a lack of concern about the occurrence of SCM phenomena in studies targeting inland water bodies such as natural lakes and artificial reservoirs. This led to a significant underestimation of the level of primary productivity in these water bodies and their trophic state. In this study, a subtropical reservoir (the Xinanjiang Reservoir, XAJR) was investigated, to understand the characteristics of SCM layer in deep-large reservoir and its contribution to the primary productivity of the water column. Water sampling were conducted from September 2020 to August 2021, and in September 2022. Buoy station data for this reservoir between 2019 and 2021 were also collected. Based on the detailed observations of the water column profile in riverine area (X1), transitional area (X2), and central area (X3 and X4) of this reservoir, it was found that there was an obvious SCM phenomenon, which was closely related to the characteristics of seasonal thermal stratification. The SCM layer of XAJR appeared at depth around 3-5 m underwater from May to August, and as the thermal stratification strength increased, so did the depth and thickness of the SCM layer. It was estimated that gross primary productivity of euphotic layer of XAJR ranged from 347.9 to 4508.6 mgC·m^-2·d^-1. The average primary productivity level of the SCM layer reached 1411.7mgC·m^-2·d^-1, accounting for about 40-90% of the gross primary productivity of euphotic layer. This study contributes to a better understanding of the factors influencing changes in the development of the SCM layer in large reservoirs, as well as its critical role in the inland water carbon cycle.

Seasonal variation and provenance of organic matter in the surface sediments of the three gorges reservoir: Stable isotope analysis and implications for agricultural management

The construction of the Three Gorges Dam has altered the hydrology and increased the trapping of sediment in the reservoir. This has also changed the composition and export of particulate organic matter in the Yangtze River. To understand the seasonal variations and sources of organic matter in sediments, total organic carbon (TOC), total nitrogen (TN), δ¹³C_org and δ¹⁵N in surface sediment samples from the mainstream and tributaries of the Three Gorges Reservoir were measured in the summer (July) and winter (December) of 2017, respectively. The results showed that the concentrations of TOC and TN in the surface sediments of the Three Gorges Reservoir were 0.79 %-1.46 % and 0.07 %-0.13 %, respectively. The ranges of δ¹³C_org and δ¹⁵N were - 26.35 ‰ to-24.70 ‰ and 2.59 ‰ to 5.67 ‰, respectively. According to δ¹³C_org and the TOC/TN ratio, the source range of organic matter was determined, and the contributions of different organic matter sources were quantified using a Bayesian mixed model. The results showed that soil organic matter and river plankton were the main sources of surface sediment organic matter in summer, whereas soil organic matter and aquatic vascular plants were the main sources in winter. The source of organic matter is related to biological factors in summer, whereas it is mainly caused by hydrodynamic conditions in winter. The analysis of δ¹⁵N further reveals that there are obvious external pollutants in the Three Gorges Reservoir, mainly related to artificial nitrogen fertiliser and domestic sewage. This study highlights the influence that soil nitrogen loss may be an important reason for the impact of agricultural non-point source pollution in the reservoir area, showing seasonal differences which were mainly affected by rainfall in summer and controlled by impoundment in winter. Hence, fine nitrogen management is required to reduce pollution in the Three Gorges Reservoir.

Response of dissolved organic matter to thermal stratification and environmental indication: The case of Gangnan Reservoir

Dissolved Organic Matter (DOM), an important part of the carbon cycle in reservoir ecosystems, has a great impact on aquatic environment to recognize the conversion process of different media DOM. The distribution, spectral characteristics, and sources of DOM in Gangnan Reservoir during thermal stratification were analyzed using ultraviolet-visible absorption spectroscopy and excitation-emission matrix spectroscopy. Three humic-like components (C2, C3, and C4) and two protein-like components (C1 and C5) were identified. The proportions of the humic-like components increased with the progression of thermal stratification (C2 and C3 were dominant), whereas the protein-like components decreased in proportion, and the trend in the interstitial water was constant (C3 and C4 were dominant). The proportion of the humic-like components in the sediments was highest during the stationary period of thermal stratification (C2 and C3 were dominant). C2 and C3 were significantly correlated in the water body and interstitial water (P < 0.001), while C1 and C5 were correlated in the sediment (P < 0.05). In the water body, C2 and C3 were negatively correlated during the formative period of thermal stratification (slope = -1.85; R² = 0.52), strongly positively correlated during the stationary period (slope = 0.76; R² = 0.94), and positively correlated during the weakening period of thermal stratification (slope = 0.46; R² = 0.30). With the progression of thermal stratification, the relative contribution of endogenous substances decreased gradually, whereas the humification degree increased in the water body and interstitial water. The protein-like components and key physicochemical factors (Fe, Mn, TN, TP, and COD_Mn) were significantly correlated during the formative period (P < 0.05), and humic-like components and key physicochemical factors (NO₂^--N and TN) were significantly correlated during the stationary and weakening periods (P < 0.05). C1, C4, and C5 indicated NO₃^--N during the formative period; C2 and C3 indicated NO₃^--N during the stationary period; and C2 and C4 indicated NO₃^--N during the weakening period in the water body. These findings enhance the understanding the mutual transformation processes of DOM in reservoir ecosystems and could guide water quality management.

Optical properties of sedimentary dissolved organic matter in intertidal zones along the coast of China: Influence of anthropogenic activities

The intertidal zone, due to its location in the transition zone of terrestrial and marine ecosystems, is seriously disturbed by anthropogenic activities such as fuel combustion and industrial production, causing significant increase in dissolved organic matter (DOM). However, the distribution and properties of DOM in intertidal sediments at the large scale and their correlations with local socio-economic indicators remain unclear. In this study, we collected sediment samples from 13 intertidal zones across 11 coastal provinces in China and analyzed optical properties and compositions of sedimentary DOM. The results showed that the physico-chemical properties of sediment, such as pH and texture, affected the content of organic matter, thereby influencing the concentration of sedimentary DOM indirectly. The contents of fulvic acid- and protein-like components were relatively higher than humic acid-like component at all sampling sites. Moreover, urbanization could lead to the release of aromatic and humified organic matters into intertidal zones. Unlike coal, oil consumption exhibited positive correlation with SUVA₂₅₄, indicating that the combustion of oil released more aromatic compounds. These findings revealed the impact of anthropogenic activities on sedimentary DOM and provided theoretical basis for predicting and regulating intertidal carbon sink.

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.

The influence of the deep subtropical reservoir on the karstic riverine carbon cycle and its regulatory factors: Insights from the seasonal and hydrological changes

Reservoirs are widely established worldwide with considerable environmental impacts, especially on the riverine carbon cycle. However, the influence of reservoirs on the cycling of different forms of carbon and its regulation factors (e.g., seasonal variations versus hydrological management) have not been simultaneously studied. To fill this knowledge gap, seasonal water samples from the deep subtropical reservoir (Longtan reservoir) in the Pearl River were collected, and the concentrations and stable carbon isotopes of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC) were determined. The variations in stable carbon isotopes of DIC (-11.4‰ to -5.2‰), DOC (-32.2‰ to -26.2‰), and POC (-38.9‰ to -25.3‰) in the river-reservoir system indicated active production and degradation processes in different layers. We estimated that up to 23.0% of DIC, 20.5% of DOC, and most POC were intercepted or degraded within the reservoir. Our results further illustrated that hydrological management (water storage regulation) and seasonal variations from different perspectives controlled the cycling of different forms of carbon in the reservoir. In addition, with the gradual increase in the number of reservoirs, hydrological management can be considered as a potentially effective strategy to adjust the carbon biogeochemical cycling of reservoirs in the future.

Dynamics and internal links of dissolved carbon in a karst river system: Implications for composition, origin and fate

Dissolved carbon (DC) deciphers biotic and abiotic processes in aquatic ecosystems, representing a critical component of global carbon cycling. However, underlying drivers of riverine DC dynamics and internal links have yet to be studied. Here, we investigated fluvial physicochemical characteristics, dissolved inorganic carbon (DIC) species, carbon dioxide (CO₂) exchange, dissolved organic carbon (DOC) compositions and properties in a karst river system Qijiang, Southwest China. Carbonate dissolution combined with photosynthetic uptake could explain dynamics of DIC species. Carbon sequestration caused low-magnitude of partial pressure of aqueous CO₂ (pCO₂, 620.3 ± 1028.7 μatm) and water-air CO₂ flux (F, 154.3 ± 772.6 mmol/m²/d), yielding an annual CO₂ emission of 0.079 Tg CO₂/y. Relatively high biological index (BIX, 0.77-0.96 on average) but low humification index (HIX, 0.67-0.78 on average) indicated notable autochthonous processes. Humic-like component was the predominant DOC, accounting for 39.0%-75.2% with a mean of 57.2% ± 6.17%. Meanwhile, tryptophan-like component (5.84% ± 2.31%) was also identified as collective DOC by parallel factor analysis (PARAFAC) across samples. Biological metabolism established internal linkages between DIC and DOC in the karst river system. Our findings highlighted biological process as a determinant for DC cycling in karst aquatic ecosystems.

Lake reclamation alters molecular-level characteristics of lacustrine dissolved organic matter - A study of nine lakes in the Yangtze Plain, China

In recent decades, the reclamation of lakes has captured 42% of the total lake area of the Yangtze Plain in China and introduced additional pressure on lacustrine water quality. While lacustrine dissolved organic matter (DOM) is critical in regulating biogeochemical processing and aquatic biodiversity, the impact of reclamation on the molecular-level characteristics of lacustrine DOM remains unexplored. Here, the DOM characteristics altered by reclamation practices in the Yangtze Plain lakes were investigated using fluorescence spectroscopy, nuclear magnetic resonance spectroscopy, and Fourier transform ion cyclotron resonance mass spectrometry. Results demonstrated that reclamation not only elevated the quantity (on average +32%) but also altered the characteristics and composition of lacustrine DOM. Compared to the natural water sites close by, reclamation sites did not significantly alter the DOM aromaticity but significantly lowered the average molecular weight and increased the biolability of DOM. The chromophoric DOM and humic-like fluorescent components were remarkably elevated, but not the protein-like fluorescent components. More lipid-like and condensed aromatic-like components were detected in the lacustrine DOM as compared to the lignin-like, carbohydrate-like, and protein-like components, which may be driven by the increased microbial processing. Overall, the significant alteration in characteristics and composition of lacustrine DOM highlights the potential impact of reclamation on the DOM biogeochemical cycle and the environmental quality in aquatic ecosystems.

Natural versus anthropogenic controls on the dissolved organic matter chemistry in lakes across China: Insights from optical and molecular level analyses

Dissolved organic matter (DOM) plays an essential role in the global carbon biogeochemical cycle for aquatic ecosystems. The complexity of DOM compounds contributes to the accurate monitoring of its sources and compositions from large-scale patterns to microscopic molecular groups. Here, this study demonstrates the diverse sources and compositions for humic-rich lakes and protein-rich lakes for large-scale regions across China with the linkage to optical components and molecular high-resolution mass spectrometry properties. The total fluorescence intensity of colored DOM (CDOM) for humic-rich lake regions (0.176 Raman unit; R.U.) is significantly (p<0.05) higher than that of the protein-rich lake region (0.084 R.U.). The combined percentages of CDOM absorption variance explained by the anthropogenic and climatic variables across the five lake regions of Northeastern lake region (NLR), Yungui Plateau lake region (YGR), Inner Mongolia-Xinjiang lake region (MXR), Eastern lake region (ELR), and Tibetan-Qinghai Plateau lake region (TQR) were 86.25%, 82.57%, 80.23%, 88.55%, and 87.72% respectively. The averaged relative intensity percentages of CHOS and CHONS formulas from humic-rich lakes (90.831‰, 10.561‰) were significantly higher than that from the protein-like lakes (47.484‰, 5.638‰), respectively. The more complex molecular composition with higher aromaticity occurred in the humic-rich lakes than in the protein-rich lakes. The increasing anthropogenic effects would significantly enhance the sources, transformation, and biodegradation of terrestrial DOM and link to the greenhouse gas emission and the carbon cycle in inland waters.

Characterization of dissolved organic matter processing between surface sediment porewater and overlying bottom water in the Yangtze River Estuary

Dissolved organic matter (DOM) exchange in the sediment-water interface of estuaries is essential for the global elemental cycle. To clarify the interface DOM processing, this study applies optical techniques and ultrahigh-resolution mass spectrometry to assess DOM composition of surface sediment porewater and bottom (overlying) water across the Yangtze River Estuary (YRE). Results suggested that DOM exchange in the sediment-water interface mainly followed from sediment porewater to bottom water driven by a significant dissolved organic carbon concentration gradient and hydrodynamic force. We also characterized two porewater DOM sources, including microbial production and byproducts of processed sediments. High microbial activities resulted in the enrichment of protein-like fluorescent components and N-bearing compounds in porewater, potentially decreasing the oxygen concentration of bottom water due to the high lability. And the deamination of N-bearing compounds in the sediment-water interface could likely serve as a N-bearing nutrient source to bottom water. Moreover, due to sediment-specific features in different areas driven by hydrologic sorting and local phytoplankton supply, porewater DOM of muddy areas accumulated more aromatic substances from the degradation of terrestrial organic matter. The release and oxic transformation of oxygen-deficient aromatic compounds could contribute to the refractory carbon pool of estuarine water (carboxyl-rich alicyclic molecules, CRAM), modulating the quality of organic carbon mobilized from the land to the coastal ocean. Considering strong hydrodynamic force in numerous estuaries worldwide, DOM exchange and processing at the sediment-water interface has a meaningful influence on the biogeochemistry of estuarine water columns, which warrants further studies.

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.