Assessing the priming effect of dissolved organic matter from typical sources using fluorescence EEMs-PARAFAC

Seasonal effects of fish, seaweed and abalone cultures on dissolved organic matter and carbon sequestration potential in Sansha Bay, China

Coastal bays serve as undeniable dissolved organic matter (DOM) reactors and the role of prevalent mariculture in DOM cycling deserves investigation. This study, based on four seasonal field samplings and a laboratory incubation experiment, examined the source and seasonal dynamics of DOM and fluorescent dissolved organic matter (FDOM) in the seawater of fish (Larimichthys crocea, LC), seaweed (Gracilaria lemaneiformis, GL) and abalone (Haliotis sp., HA) culturing zones in Sansha Bay, China. Using three-dimensional fluorescence spectroscopy coupled with parallel factor analysis (EEMs-PARAFAC), three fluorescent components were identified, i.e. protein-like C1, protein-like C2, and humic-like C3. Our results showed that mariculture activities dominated the DOM pool by seasonal generating abundant DOM with lower aromaticity and humification degrees. Accounting for 40-95 % of total fluorescent components, C1 (Ex/Em = 300/340 nm) was regarded the same as D1 (Ex/Em = 300/335 nm) identified in a 180-day degradation experiments of G. lemaneiformis detritus, indicating that the cultured seaweed modulated DOM through the seasonal production of C1. In addition, the incubation experiment revealed that 0.7 % of the total carbon content of seaweed detritus could be preserved as recalcitrant dissolved organic carbon (RDOC). However, fish culture appeared to contribute to liable DOC and protein-like C2, exerting a substantial impact on DOM during winter but making a negligible contribution to carbon sequestration, while abalone culture might promote the potential export and sequestration of seaweed-derived carbon to the ocean. Our results highlight the influences of mariculture activities, especially seaweed culture, in shaping DOM pool in coastal bays. These findings can provide reference for future studies on the carbon accounting of mariculture.

Effects of polypropylene films and leached dissolved organic matters on bacterial community structure in mangrove sediments

Over the past decades, the accumulation of plastics in mangrove ecosystems has emerged as a significant environmental concern, primarily due to anthropogenic activities. Polypropylene (PP) films, one of the plastic types with the highest detection rate, tend to undergo intricate aging processes in mangrove ecosystems, leading to the release of dissolved organic matter (DOM) that may further influence the local bacterial communities. Yet, the specific effects of new and weathered (aged) plastic films and the associated leached DOM on bacterial consortia in mangrove sediments remain poorly understood. In this study, an incubation experiment was conducted to elucidate the immediate effects and mechanisms of the new and relatively short-term (45 or 90 days) aged PP films, as well as their leached DOM (PDOM), on characteristics of DOM and the bacterial community structure in mangrove sediments under different tidal conditions. Surface morphology and functional group analyses showed that both new and aged PP films exhibited comparable degradation profiles under different tidal conditions over the incubation period. As compared to the new PP film treatments, the introduction of the short-term aged PP films significantly affected the content of humic-like compounds in sediments, and such effects were partially ascribed to the release of PDOM during the incubation. Although the addition of PP films and PDOM showed minor effects on the overall diversity and composition of bacterial communities in the sediments, the abundance of some dominant phyla exhibited a growth or reduction tendency, possibly changing their ecological functions. This study was an effective attempt to investigate the relationship among plastic surface characteristics, sedimentary physicochemical properties, and bacterial communities in mangrove sediments. It revealed the ecological ramifications of new and short-term plastic pollution and its leachates in mangrove seedtimes, enhancing our understating of their potential impacts on the health of mangrove ecosystems.

Regulation of seawater dissolved carbon pools by environmental changes in Ulva prolifera originating sites: A new perspective on the contribution of U. prolifera to the seawater carbon sink function

The Ulva prolifera bloom is considered one of the most serious ecological disasters in the Yellow Sea in the past decade, forming a carbon sink in its source area within a short period but becoming a carbon source at its destination. To explore the effects of different environmental changes on seawater dissolved carbon pools faced by living U. prolifera in its originating area, U. prolifera were cultured in three sets with different light intensity (54, 108, and 162 μmol m-2 s-1), temperature (12, 20, and 28 °C) and nitrate concentration gradients (25, 50, and 100 μmol L-1). The results showed that moderate light (108 μmol m-2 s-1), temperature (20 °C), and continuous addition of exogenous nitrate significantly enhanced the absorption of dissolved inorganic carbon (DIC) in seawater by U. prolifera and most promoted its growth. Under the most suitable environment, the changes in the seawater carbonate system were mainly dominated by biological production and denitrification, with less influence from aerobic respiration. Facing different environmental changes, U. prolifera continuously changed its carbon fixation mode according to tissue δ13C results, with the changes in the concentrations of various components of DIC in seawater, especially the fluctuation of HCO3- and CO2 concentrations. Enhanced light intensity of 108 μmol m-2 s-1 could shift the carbon fixation pathway of U. prolifera towards the C4 pathway compared to temperature and nitrate stimulation. Environmental conditions at the origin determined the amount of dissolved carbon fixed by U. prolifera. Therefore, more attention should be paid to the changes in marine environmental conditions at the origin of U. prolifera, providing a basis for scientific management of U. prolifera.

Fluvial carbon dioxide emissions peak at the permafrost thawing front in the Western Siberia Lowland

In order to foresee the impact of permafrost thaw on CO2 emissions by high-latitude rivers, in-situ measurements across a permafrost and climate/vegetation gradient, coupled with assessment of possible physico-chemical and landscape controlling factors are necessary. Here we chose 34 catchments of variable stream order (1 to 9) and watershed size (1 to >105 km2) located across a permafrost and biome gradient in the Western Siberian Lowland (WSL), from the permafrost-free southern taiga to the continuous permafrost zone of tundra. Across the south-north transect, maximal CO2 emissions (2.2 ± 1.1 g C-CO2 m-2 d-1) occurred from rivers of the discontinuous/sporadic permafrost zone, i.e., geographical permafrost thawing boundary. In this transitional zone, fluvial C emission to downstream export ratio was as high as 8.0, which greatly (x 10) exceeded the ratio in the permafrost free and continuous permafrost zones. Such a high evasion at the permafrost thawing front can stem from an optimal combination of multiple environmental factors: maximal active layer thickness, sizable C stock in soils, and mobilization of labile organic nutrients from dispersed peat ice that enhanced DOC and POC processing in the water column, likely due to priming effect. Via a substituting space for time approach, we foresee an increase in CO2 and CH4 fluvial evasion in the continuous and discontinuous permafrost zone, which is notably linked to the greening of tundra increases in biomass of the riparian vegetation, river water warming and thermokarst lake formation on the watershed.

Biodegradation of mixed litter-derived dissolved organic matter with varying evenness in a temperate freshwater wetland

Litter-derived dissolved organic matter (DOM) plays an essential role in biogeochemical cycles. In wetlands, species relative abundance and its change have great influences on input features of litter-derived DOM, including chemical characteristics per se and functional diversity of chemical characteristics. Functional diversity is an important factor controlling organic matter biodegradation, but little is known in terms of the DOM. We mixed litter leachates of four macrophytes with a constant concentration (20 mg DOC L-1) but varying dominant species and volume ratios, i.e. 15:1:1:1 (low-evenness), 5:1:1:1 (mid-evenness), and 2:1:1:1 (high-evenness), generating a gradient of chemical characteristics and functional diversity (represented by functional dispersion index FDis). Based on a 42-d incubation, we measured degradation dynamics of these DOM mixtures, and analyzed potential determinants. After 42 days of incubation, the high-evenness treatments, along with mid-evenness treatments sometimes, had most degradation, while the low-evenness treatments always had least degradation. The degradation of mixtures related significantly to not only the volume-weighted mean chemical characteristics but also FDis. Furthermore, the FDis even explained more variation of degradation. The non-additive mixing effects, synergistic effects (faster degradation than predicted) in particular, on degradation of DOM mixtures were rather common, especially in the high- and mid-evenness treatments. Remarkably, the mixing effects increased linearly with the FDis values (r2adj. = 0.426). This study highlights the critical role of functional diversity in regulating degradation of mixed litter-derived DOM. Resulting changes in chemistry and composition of litter leachates due to plant community succession may exert substantial influences on biogeochemical cycling.

A field study of a novel permeable-reactive-biobarrier to remediate chlorinated hydrocarbons contaminated groundwater

Chlorinated hydrocarbons (CHs) pose significant health risks due to their suspected carcinogenicity, necessitating urgent remediation efforts. While the combination of zero-valent iron (Fe0) and microbial action shows promise in mitigating CH contamination, field studies on this approach are scarce. We devised a novel three-layer permeable reactive barrier (PRB) material incorporating Fe0 and coconut shell biochar, effectively implemented at a typical CH-contaminated site. Field monitoring data revealed conducive conditions for reductive dechlorination of CHs, characterized by low oxygen levels and a relatively neutral pH in the groundwater. The engineered PRB material consistently released organic carbon and iron, fostering the proliferation of CH-dechlorinating bacteria. Over a 250-day operational period, the pilot-scale PRB demonstrated remarkable efficacy in CH removal, achieving removal efficiencies ranging from 21.9% to 99.6% for various CH compounds. Initially, CHs were predominantly eliminated through adsorption and iron-mediated reductive dechlorination. However, microbial reductive dechlorination emerged as the predominant mechanism for sustained and long-term CHs removal. These findings underscore the economic viability and effectiveness of our approach in treating CH-contaminated groundwater, offering promising prospects for broader application in environmental remediation efforts.

Optical measurements of dissolved organic matter as proxies for CODMn and BOD5 in plateau lakes

The presence of organic matter in lakes profoundly impacts drinking water supplies, yet treatment processes involving coagulants and disinfectants can yield carcinogenic disinfection by-products. Traditional assessments of organic matter, such as chemical oxygen demand (CODMn) and biochemical oxygen demand (BOD5), are often time-consuming. Alternatively, optical measurements of dissolved organic matter (DOM) offer a rapid and reliable means of obtaining organic matter composition data. Here we employed DOM optical measurements in conjunction with parallel factor analysis to scrutinize CODMn and BOD5 variability. Validation was performed using an independent dataset encompassing six lakes on the Yungui Plateau from 2014 to 2016 (n = 256). Leveraging multiple linear regressions (MLRs) applied to DOM absorbance at 254 nm (a254) and fluorescence components C1-C5, we successfully traced CODMn and BOD5 variations across the entire plateau (68 lakes, n = 271, R2 > 0.8, P < 0.0001). Notably, DOM optical indices yielded superior estimates (higher R2) of CODMn and BOD5 during the rainy season compared to the dry season and demonstrated increased accuracy (R2 > 0.9) in mesotrophic lakes compared to oligotrophic and eutrophic lakes. This study underscores the utility of MLR-based DOM indices for inferring CODMn and BOD5 variability in plateau lakes and highlights the potential of integrating in situ and remote sensing platforms for water pollution early warning.

Co-inoculation of fungi and desert cyanobacteria facilitates biological soil crust formation and soil fertility

The inoculation of cyanobacteria for enriching soil nutrients and forming biological soil crusts (BSCs) is considered an effective means to restore degraded soil. However, there are limited studies on the application of co-inoculation of fungi and cyanobacteria for degraded soil remediation. In this study, a high exopolysaccharide-secreting fungi Zh2 was isolated from lichen BSCs in Hobq Desert, and co-inoculated with a cyanobacterial strain identified as Phormidium tenue in different proportions to form BSCs on sand during a 35 days incubation period. Results revealed significant differences in crust biomass and soil properties among crusts with different cyanobacterial/fungal inoculation ratios. Microbial biomass, soil nutrient content and enzyme activities in crusts co-inoculated with cyanobacteria and fungi were higher than those inoculated with cyanobacteria and fungi alone. The inoculation of cyanobacteria contributed to the fulvic-like accumulation, and the inoculated fungi significantly increased the humic-like content and soil humification. Redundancy analysis showed that the inoculation of cyanobacteria was positively correlated with the activities of urease and phosphatase, and the content of fulvic-like. Meanwhile, the inoculation of fungi was positively correlated with the contents of total carbon, total nitrogen and humic-like, the activities of catalase and sucrase. Cyanobacteria and fungi play distinct roles in improving soil fertility and accumulating dissolved organic matter. This study provides new insights into the effects of cyanobacteria and fungi inoculations on the formation and development of cyanobacterial-fungus complex crusts, offering a novel method for accelerating induced crust formation on the surface of sand.

Insights into priming effects of dissolved organic matter degradation in urban lakes with different trophic states

Priming effect (PE) is recognized as an important potential mechanism for dissolved organic matter (DOM) degradation in aquatic ecosystems. However, the priming effects (PEs) of various priming substances on the degradation of DOM pools in urban lakes along diverse trophic states remain unknown. To address this knowledge gap, the PEs and drivers of glucose and plant leachate of lake water with three trophic states were investigated. We reveal differences in the bioavailability of DOM in lake water, glucose, and plant leachate. The PE of the same priming substance was significantly higher in highly-eutrophic lake water than in mesotrophic lake. The priming intensity induced by glucose was significantly higher when compared to plant leachate. Regarding the addition of glucose, humic-like components (C1 and C3) showed slight PE, while the tyrosine-like component C2 showed negative PE. However, the positive PEs were observed on three components after adding plant leachate. The driver of PE by glucose shifted from nutrients to DOM components with increasing trophic levels. The PEs induced by plant leachate were affected by nutrients, chlorophyll-a (Chl-a), water chemistry, and DOM components in lightly/moderately-eutrophic lake water. This study revealed the intensities, directions, and drivers of PEs, providing essential insights into uncovering the DOM biogeochemical process in urban lakes.

Compensatory growth of Microcystis aeruginosa after copper stress and the characteristics of algal extracellular organic matter (EOM)

Cyanobacterial blooms can impair drinking water quality due to the concomitant extracellular organic matter (EOM). As copper is often applied as an algicide, cyanobacteria may experience copper stress. However, it remains uncertain whether algal growth compensation occurs and how EOM characteristics change in response to copper stress. This study investigated the changes in growth conditions, photosynthetic capacity, and EOM characteristics of M. aeruginosa under copper stress. In all copper treatments, M. aeruginosa experienced a growth inhibition stage followed by a growth compensation stage. Notably, although chlorophyll-a fluorescence parameters dropped to zero immediately following high-intensity copper stress (0.2 and 0.5 mg/L), they later recovered to levels exceeding those of the control, indicating that photosystem II was not destroyed by copper stress. Copper stress influenced the dissolved organic carbon (DOC) content, polysaccharides, proteins, excitation-emission matrix spectra, hydrophobicity, and molecular weight (MW) distribution of EOM, with the effects varying based on stress intensity and growth stage. Principal component analysis revealed a correlation between the chlorophyll-a fluorescence parameters and EOM characteristics. These results imply that copper may not be an ideal algicide. Further research is needed to explore the dynamic response of EOM characteristics to environmental stress.

Driving mechanism of water replenishment on DOM composition and eutrophic status changes of lake in arid and semi-arid regions of loess area

Water replenishment can be a key factor in driving lake eutrophication status. In arid and semi-arid regions of China, water replenishment for a lake has been widely carried out for not only improving water environmental quality, but also maintaining ecological system function. However, it is still unclear in terms of mechanism by which water replenishment drives lake eutrophication status. In this study, fluorescence excitation-emission matrix spectroscopy (EEMs) combined with multiple statistical analysis models (including parallel factor analysis, correlation analysis, redundancy analysis, and partial least squares structural equation modeling) was utilized to reveal potential driving mechanism and causality between water replenishment, dissolved organic matter (DOM) fractions and eutrophic status of Lake Shahu in China. Based on variations of DOM fractions, fulvic-like substances could be accumulated during the replenishment period, while nutrients carried along the replenishment might conduce to increase microbial activities during the non-replenishment period. This should be contributed to an alteration of prominent component from fulvic-like substances to tyrosine-like substances during the replenishment period to non-replenishment period. According to partial least squares structural equation modeling, two potential indirect paths were finally revealed, i.e., water replenishment derived the eutrophic status of Lake Shahu: water replenishment → microbial activity → algae → eutrophication, and water replenishment → microbial activity → eutrophication. This supposed that the water replenishment should indirectly drive the algae and eutrophication of the lake by promoting the transformation of DOM fractions. In addition, natural conditions could indirectly contribute to the eutrophication of the lake through impacting the algae growth. These findings should be conducive to trace the alteration of DOM fractions in lakes by water replenishment and in recognizing potential driving mechanisms of water replenishment on eutrophication of lakes by changing DOM fractions. This could provide basic theoretical support for policymakers to regulate and treat the eutrophication of lakes.

Effective abatement of ammonium and nitrate release from sediments by biochar coverage

Inorganic forms of N from sediments and runoff water, among others, remain some of the key sources of pollution of water bodies. However, the release of NH4+-N from sediment to water can be effectively reduced by biochar coverage due to high adsorption capacity, unlike NO3-N, where biochar has a low affinity. The feasibility of biochar coverage to abate NO3--N release needs to be evaluated. This study collected four sediments from Lake Taihu (China). Three types of biochar pyrolyzed from ordinary wastes, coconut shell (coBC), algal and excess sludge, were prepared to cover them and were incubated for 90 days. Results showed that the terminal total nitrogen (TN) and NO3--N concentrations decreased from 5.35 to 2.31-3.04 mg/L, 3.05 to 0.34-1.11 mg/L, respectively. CoBC coverage showed the best performance for reducing NO3--N release flux from 26.99 ± 0.19 to 9.30 ± 0.02 mg/m2·d (63.6 %). Potential denitrifiers, such as Flavobacterium and Exiguobacterium, were enriched in the biochar-coverage layer, and the absolute abundance of N-related functional genes (narG, nirS, nosZ and anammox) was increased by 1.76-4.21 times (p < 0.05). Jar tests by 15N isotope labeling further indicated that biochar addition increased the denitrification and anammox rates by 53.5-83.4 %. Experiments combining exogenous organic‑carbon addition and 15N labeling demonstrated that biochar's key role was regulating organic matter's bioavailability. Analysis with partial least square path modeling (PLS-PM) implied biochar with higher adsorption enhanced the denitrification and anammox processes in sediments via modifying the niche with suitable DOC, TN, and pH. This study suggested that biochar coverage could effectively abate NO3--N release from sediments by affecting the denitrification and anammox processes.

Characteristics of optical properties of DOM and nutrients in rainwater of different ecological areas of a large reservoir in China

Numerous studies have focused on the spectral characteristics and seasonal variations of dissolved organic matter (DOM) in rainwater. However, the relationship between the optical indices of DOM and nutrients in rainwater from different ecological areas of large reservoirs is poorly understood. A one-year monitoring study was conducted between March 2019 and February 2020 in the Danjiangkou Reservoir in Henan Province, China, to compare the composition, spectral characteristic parameters, and relationship between the optical indices of DOM and nutrients in rainwater under different ecological environments. The study showed that the average value of a300 in all samples was 5.29 ± 2.16 m-1 and showed a seasonal trend of higher in spring and winter and lower in summer and autumn as well as a regional difference of agricultural area > urban area > reservoir area. A three-dimensional fluorescence with parallel factor analysis (EEM-PARAFAC) revealed four components of the rainwater: C1 and C2 as UV humic-like substances, and C3 and C4 as protein-like substances. The protein-like components of rainwater from agricultural areas had a high fluorescence intensity, whereas the UV humic-like components of rainfall from urban and reservoir areas had a high fluorescence intensity. Analysis of the fluorescence indices showed that rainwater DOM humification was low and had a strong endogenous character in the Danjiangkou Reservoir. The redundancy analysis revealed that NO3--N, DTN, and SO42- mainly influenced the DOM optical indices of rainwater in urban areas, EC, DTN, and DOC had the highest interpretation of the DOM optical indices of rainwater in agricultural areas, and SO42-, DOC, and DTN had the highest interpretation of the DOM optical indices of rainwater in the reservoir. Overall, understanding the characteristics of rainfall DOM fluorescence and the relationships with nutrients in different ecological regions provides important information for comprehending biogeochemical processes in reservoirs.

Diurnal and seasonal variations of pCO2 and fluorescent dissolved organic matter (FDOM) in different polluted lakes

This study aimed to assess pollution and daily-to-seasonal dynamics of the partial pressure of CO2 (pCO2) and CO2 degassing flux concerning the fluorescent dissolved organic matter (FDOM) from tropical lakes. A membrane-enclosed pCO2 sensor and water quality multimeter analyzer was deployed to continuously record daily and seasonal variations in pCO2 and CO2 degassing flux in three lakes in Savar, Dhaka. During both wet and dry seasons, all lake water was supersaturated with CO2 in contrast to the atmospheric equilibrium (~400 μatm). The pCO2 values in the lake water during the dry season were relatively low in comparison, and the pCO2 levels in the wet season were much higher due to external inputs of organic matter from watersheds and direct inputs of CO2 from soils or wetlands. The estimated water-to-air CO2 degassing flux in the different levels of polluted lakes varies with the pollution context. Study areas calculated the carbon flux and three lakes released respectively 86.75×107g CO2 year-1, 13.8×107g CO2 year-1, and 9.17×107g CO2 year-1. Three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy combined with parallel factor (PARAFAC) analysis was used to investigate the distributions of fluorescent components in DOM. EEM-PARAFAC analysis identified humic-like, fulvic-like, protein-like, and more tyrosine-like FDOM components and their environmental dynamics. Terrestrial DOM may provide inputs to the terrestrial humic-like component in the lake water. In contrast, the biological activity of plankton-derived FDOM is the most likely source for the autochthonous humic-like component. FDOM and DO concentrations have negative correlations with pCO2, indicating that when the FDOM and DO level is decreased, the amount of pCO2 values increases.

Relevance of tributary inflows for driving molecular composition of dissolved organic matter (DOM) in a regulated river system

River regulation by dams can alter flow regimes and organic matter dynamics, but less is known about how unregulated tributaries regulate organic matter composition and processing in the regulated river below the confluence. This study reports on water chemistry, especially dissolved organic matter (DOM) concentration and composition (dissolved organic carbon (DOC), organic nitrogen (DON), organic phosphorus (DOP) and combined amino acids (DCAA)) along the regulated Tumut and unregulated Goobarragandra (tributary) rivers under different flow conditions (base flow vs storm event) in south-east Australia. The tributary was significantly different from regulated and downstream sites during base flow conditions with higher temperature, pH, buffering capacity, DOC and nutrient concentrations (DON, DOP, DCAA). DOM characterisation by spectrometry and size exclusion chromatography revealed that the tributary contained a higher proportion of terrestrially derived humic-like and fulvic-like DOM. In contrast, regulated and downstream sites contained higher proportion of microbially derived DOM such as low molecular weight neutrals and protein-like components. Storm pulses of tributary flows into the regulated system, influenced both concentration and composition of DOM at the downstream site, which more strongly resembled the tributary site than the regulated site during the storm event. Additionally, we found that the tributary supplied fresh DOM, including small organic molecules to the regulated system during storm events. The presence of these different types of labile DOM can increase primary productivity and ecological functioning within regulated river reaches downstream of tributary junctions. This has important implications for the protection of unregulated tributary inflows within regulated river basins.

The priming effects of plant leachates on dissolved organic matter degradation in water depend on leachate type and water stability

The modification of dissolved organic matter (DOM) degradation by plant carbon inputs represents a critical biogeochemical process that controls carbon dynamics. However, the priming effects (PEs) different plant tissues induce on the degradation of DOM pools with different stabilities remain unknown. In this study, PEs, induced by different tissue leachates of Phragmites australis, were evaluated via changes in DOM components and properties of both fresh and tidal water (with different stabilities). The results showed that DOM derived from different plant tissue leachates differed in composition and bioavailability. Inputs of tissue leachates induced PEs with different intensities and directions (negative or positive) on DOM degradation of fresh and tidal water. In fresh water, the PEs of leaf and root leachates were significantly higher than those of stem and rhizome leachates. The PE direction changed for DOM degradation between fresh and tidal water. The addition of leaf and root leachates tended to induce positive PEs on DOM degradation of fresh water, while resulting in negative PEs on DOM degradation of tidal water. Negative PEs for tidal water DOM may be due to preferential utilization of microbes, high salinity, and/or the promotion of exogenous DOM production from plant tissues. The results indicate that intensity and direction of PEs induced by plant leachates depend on both leachate type and water stability. The findings highlight the necessity to examine the nature of exogenous and native DOM when interpreting the interactive processes that regulate DOM degradation.

Composition, distribution, and source of organic carbon in surface sediments of Erhai Lake, China

Lake sediment is an important organic carbon (OC) sink. Nevertheless, few studies have been conducted on sediment organic carbon (SOC) in lakes, and the effects of environmental variables on SOC pools remain poorly understood. We combined physicochemical and spectroscopic analyses to investigate the composition, distribution, and source of OC in surface sediments of Erhai Lake, southwest China, and explored the relationships between environmental variables and its SOC pool. The SOC pool consists of relatively high proportions of labile organic carbon fractions, mainly from algal production, which are rapidly decomposed and exhibit high turnover rates. The relative content of humus carbon ranges from 13.5 % to 20.5 %, with fulvic acid carbon predominating (average 52.95 %), indicating weak humification and a relatively active humus carbon pool. The dissolved organic matter in water column and sediments of Erhai Lake is largely influenced by endogenous production, with a great contribution from phytoplankton. Surface sediments contained more protein-like components than overlying waters (80.0 % vs. 63.0 %), attributed mainly to abundant algal deposition and intense bacterial metabolism. Among environmental variables, sediment chlorophyll a showed the strongest relationship with the SOC pool, and was associated with rapid decomposition and promotion of the humification process, which supported the conclusion that algae had an important influence on the SOC pool. The SOC pool in the southern region of the lake is mainly contributed by algae, other microorganisms, and sewage, exhibiting a greater potential to release organic matters into the water column. The center and northern SOC pools show relatively stable characteristics and stronger OC sink capacity, mainly because of the input of terrestrial refractory organic matters from runoff. Our data shed light on the OC storage mechanisms in the surface sediments of Erhai Lake and provide theoretical bases for enhancing the OC sink of sediments in the lake.

Sources, Migration, Transformation, and Environmental Effects of Organic Carbon in Eutrophic Lakes: A Critical Review

Organic carbon (OC) plays a leading role in the carbon cycle of lakes and is crucial to carbon balances at regional and even global scales. In eutrophic lakes, in addition to external river inputs, the decomposition of endogenous grass and algae is a major source of organic carbon. Outbreaks of algal blooms (algal eutrophication) and the rapid growth of aquatic grasses (grass eutrophication) can lead to the accumulation and decay of large amounts of algae and aquatic grass debris, which increases the intensity of the carbon cycle of lakes and greatly impacts aquatic environments and ecosystems. The structures, decomposition processes, and distribution characteristics of algae and higher aquatic plant debris in eutrophic lakes are different from mesotrophic and oligotrophic lakes. Studying their accumulation dynamics and driving mechanisms is key to further understanding lake carbon cycles and their many interdependent pathways. This paper focuses on the carbon sources, tracing technologies, migration and transformation processes, and environmental effects of OC in eutrophic lakes. Based on the existing knowledge, we further combed the literature to identify the most important knowledge gaps preventing an in-depth understanding of the processes and driving mechanisms of the organic carbon cycle in eutrophic lakes.

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.

Characterising the interactions between Cu(II) and fulvic acid subcomponents using differential fluorescence spectroscopy combined with parallel factor analysis

Fulvic acid (FA) consists of various organic compounds that interact with metals in the aquatic environment. Interactions between subcomponents (FA3, FA5, FA7, FA9 and FA13) of FA and Cu(II) were investigated using fluorescence quenching titration by coupling differential fluorescence spectroscopy (DFS) and parallel factor analysis (PARAFAC). The material fluorescence intensities derived from FA subcomponents decreased with Cu(II) concentration increase, and stronger quenching was experienced at low Cu(II) concentrations. Humic-like materials of FA3 and protein-like materials of FA9 have relatively higher presence of carboxylic groups with greater molecular polarity, and preferential interaction of Cu(II) occurs. Fluorescence DFSs can be successfully broken down into five components as follows: fulvic-like components of a major compound containing carboxylic-like and phenolic-like groups, fulvic-like components of a major compound containing groups of other components, humic-like components of a major compound containing carboxylic-like and phenolic-like groups, humic-like components of a major compound containing carboxylic-like groups, and protein-like components. The fulvic-like components of a major compound containing carboxylic-like and phenolic-like groups exhibited stronger quenching with Cu(II) in five FA subcomponents. Static quenching is the dominant mechanism for the binding of Cu(II) by the FA subcomponent. The log K of quenching constants fitted using the modified Ryan-Weber equation (R2 = 0.9368-0.9985) ranged from 4.32 to 6.14 for five components derived from FA subcomponents. Subcomponents that were eluted earlier exhibited stronger binding affinity with Cu(II).

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.

Effects of Photodegradation on the Optical Indices of Chromophoric Dissolved Organic Matter from Typical Sources

Chromophoric dissolved organic matter (CDOM) plays important roles in aquatic environments, and its optical properties provide a series of indices for evaluating the source and composition of dissolved organic matter (DOM). However, little is known about the varying photodegradation of CDOM from different sources and the effects on the optical indices of DOM composition. This was studied for typical natural and anthropogenic sources (plant and leaf litter leachates, the influent and effluent of a wastewater treatment plant, and a river). The CDOM absorption (a₂₈₀) showed a lower degradability for the plant leachate than other sources, mainly due to its low molecular weight and aromaticity. Four fluorescent components were identified with excitation-emission matrices-parallel factor analysis (EEMs-PARAFAC), namely benzoic acid/monolignol-like C1, humic-like C2 and C3, and tryptophan-like C4. The plant leachate contained mainly C1, which was photodegraded moderately, while other sources had more C2 and C3 with higher photodegradability. C4 was photodegraded in most sources but was photoproduced in the leaf litter leachate. The absorption slope (S_275-295) and slope ratio (S_R) increased while the humification index (HIX) decreased, suggesting a decreasing molecular weight and humic content by photodegradation. This was consistent with the decreasing %C2 and %C3 but increasing %C4, which indicated preferential removal of humic-like components. The %C1, %C2, biological index (BIX), and fluorescence index (FI) were less affected by photodegradation than other indices for most sources. These results have implications for a better understanding of the photochemistry of CDOM and the applications of optical indices.

Effects of coastal aquaculture on sediment organic matter: Assessed with multiple spectral and isotopic indices

Sediment organic matter (SOM) is important in the biogeochemical cycling of carbon, nutrients, and pollutants in the coastal environment, which is increasingly disturbed by aquaculture that is particularly intense in China. However, the identification of aquaculture signals in SOM is rather challenging in the complex coastal environment that receives materials from a variety of sources. This was studied in a typical culture area of shellfish and algae in SE China from July 2019 to October 2020, using a combination of elemental (OC, TN, N/C), isotopic (δ¹³C and δ¹⁵N), spectral (absorption spectroscopy and fluorescence EEMs-PARAFAC), and statistical analysis (principal component analysis, PCA). All indices of SOM quantity and several spectral indices for the SOM composition correlated significantly with grain size, with lower SOM quantity and higher autochthonous contribution in coarse sediments. The strong correlations between elemental and spectral indices suggested that optical analysis could provide valuable indices for assessing the quantity of bulk organic matter. The comparison of SOM indices between different zones and between different months showed an overall limited influence of shellfish and laver culture. This indicated the sustainability of these types of aquaculture that require no manual addition of feeds and thus are generally clean. The further applications of end-member mixing analysis using the IsoSource program and PCA were more sensitive, which identified the removal of SOM by shellfish in the growing season and the contribution from shellfish residuals after the harvest and the cultured laver at some locations. Overall, our results have implications for a better understanding of the biogeochemical processes and ecosystem sustainability in the coastal environment under intense aquaculture activities.

Comparing the Pb(II) binding with different fluorescent components of dissolved organic matter from typical sources

Dissolved organic matter (DOM) is important for determining the speciation, environmental behavior, and effects of metal pollutants in aquatic environments. However, little is known about the difference between DOM from natural and anthropogenic sources for binding Pb(II). This study examined the Pb(II) binding with DOM from four typical sources including river, leaf litter leachate, and the influent and effluent of a wastewater treatment plant, using fluorescence quenching titration and excitation-emission matrices-parallel factor analysis (EEMs-PARAFAC). Four humic-like and one protein-like fluorescent components were identified, with much higher protein-like fraction and lower humification degree for the influent than for other sources. In the river water and leaf litter leachate, the abundant humic-like components were quenched by 6-17% while the protein-like component kept stable (2-4%) by the addition of Pb(II). In contrast, the influent DOM showed stronger fluorescence quenching of the protein-like component (46%) with higher conditional stability constant and binding fraction of fluorophore than the humic-like components (15-21%). The effluent DOM displayed weak quenching for all fluorescent components (4-6%) and thus weak complexation with Pb(II), indicating notable changes in the chemical composition and metal-binding affinity of DOM by wastewater treatments. These results demonstrated significant impacts of DOM source and chemical composition on its Pb(II) complexation properties, which have implications for understanding the interactions between DOM and heavy metals.

Photodegradation, bacterial metabolism, and photosynthesis drive the dissolved organic matter cycle in the Heilongjiang River

Dissolved organic matter (DOM) plays a vital role in the biogeochemistry of aquatic ecosystems. However, the mechanisms of DOM cycling in the water column during different seasons have not been fully elucidated to date. The differences in DOM degradation in summer, autumn, and winter water columns were evaluated in this study. The results showed that bacteria played an essential role in the degradation of DOM in the summer water column. Photochemical degradation was the primary degradation pathway of DOM in the autumn and winter water columns. Notably, while DOM is degraded, photosynthetic bacteria produce organic matter through photosynthesis to replenish the water column. EEM-PARAFAC analysis indicated more tryptophan component C1 in summer, but the contents of humic substance component C2 and terrestrial substance C3 were higher in autumn and winter. In summer, more tryptophan-like components were consumed by bacteria, and Cyanobacteria produced more organic matter through photosynthesis to replenish the water column. Moreover, a similar bacterial community structure and a more active tryptophan biosynthesis pathway were found in autumn and winter. Random forest models identified representative bacteria involved in the DOM transformation process in different seasons. The above findings may be helpful to explore the degradation characteristics of DOM in different seasons and predict the fate of DOM in the water column in the future.