Variations Of Dissolved Organic Matter Research Articles

Spectroscopic fingerprints to track the fate of aquatic organic matter along an alpine headstream on the Tibetan Plateau

The fragile aquatic ecosystem on the Tibetan Plateau is severely threatened by human activities and climate change. Dissolved organic matter (DOM) is a vital indicator of surface water quality; however, its comprehensive molecular analysis is challenged due to its low concentration (total organic carbon less than 0.5 mg/L) in alpine areas. This study proposes the fluorescence excitation-emission matrix (FEEM) to fingerprint DOM in a typical headstream in the Namco basin, one of the largest lake regions in Tibet. We found that the FEEM can sensitively detect low-concentration pollution traces and the variation of DOM along the flow from the ice sheet, through the wetland, eventually to the estuary of the lake. The fluorescence intensity indices for biodegradable carbon (fT/C) and humification (HIXem) responded drastically along the flow. Fluorescence regional integrals (FRIs) clearly reflected the overall increase of protein-like substances and decrease of humus-like substances along the flow, whereas this tendency was reversed when passing through the wetland. The FRIs-derived secondary parameters (HPP, HMP, WLP and SSP) further sensed likely variations in hydrophobicity, humification degree, excited-state fluorophore energy and Stokes shift. Parallel factor analysis (PARAFAC) and two-dimensional correlation spectroscopy (2DCOS) of the FEEM signals witnessed the trade-off among tyrosine-like organics (C1 peak), tryptophan-like byproducts (C2 peak) and humus-like remains (C3 peak) along the flow. The C1 component can be traced back to the vicinity of the ice sheet exit, presumably due to human and animal activities. The wetland can absorb or convert part of the C1 component into C2 or C3 products, demonstrating the function of regulating water quality and buffering environmental impacts. The spectroscopic indicators evaluated in this study may provide tools for diagnosing early traces of water pollution and ecological instability in alpine areas.

Characterization of dissolved organic matter (DOM) in an urbanized watershed using spectroscopic analysis

Landscape urbanization broadly alter watersheds ecosystems, yet the impact of nonpoint source urban inputs on dissolved organic matter (DOM) amount, composition and source is poorly understood. To systematically examine how DOM optical index and composition varied with urbanization, a unique long term observation dataset (4 years) of fluorescence excitation emission matrices (EEMs) was collected from two types of waters: urban waters and non-urban waters. Two humic-like DOM fluorescent components (C1 and C2) and one protein-like component (C3) were identified by parallel factor analysis (PARAFAC), and the results indicated that urbanization had an important influence on DOM concentration and composition, with urban waters having a high degree of DOM variation due to different land use surrounding each body of water. Urban waters presented higher DOM content, CDOM absorption and DOM fluorescence intensity (FI), a greater proportion of protein-like (26% > 21.3%), and less proportion of humic-like (51.9% < 57.6%) than non-urban waters, were dominated by allochthonous inputs. Moreover, the long-term observation of the urbanized DOM’s dynamics was conducted on monthly, seasonal and yearly timescales. The results reflected the response of DOM to regional climate. Higher DOM amount and FI appeared in the summer due to autochthonous production comes from algae growth and allochthonous input comes from rainfall. It also revealed that continuous increase in impervious artificial surfaces caused by urban expansion, contributed to the increase in DOM quantity and drove DOM composition to be more protein-like. Consequently, these findings filled the knowledge gap of the mechanism of land-water interaction on DOM properties in freshwater ecosystems.

Resource aromaticity affects bacterial community successions in response to different sources of dissolved organic matter

Microbe-mediated transformation of dissolved organic matter (DOM) contributes substantially to the carbon dynamics and energy flow of aquatic ecosystems; yet, the temporal dynamics of bacterial communities in response to diverse DOM sources are scarcely known. Here, we supplied four distinct sources of DOM (algae-derived, macrophyte-derived, sewage-derived, and soil-derived) to the same bacterial community to track the effects of these DOM sources on the carbon processing and successional dynamics of bacterial communities. Although by the end of the incubation the proportion of bio-degraded DOM was significantly lower in the soil-derived DOM treatment than for the other sources, rapid initial metabolism of protein-like and aliphatic compounds and increasing aromaticity and humification degree of DOM during the incubation period were observed for all sources. The role of stochastic processes in governing the community assembly decreased substantially from 61.4% on the first day to 16.7% at the end of the incubation. Moreover, stronger deterministic selection and lower temporal turnover rate were observed for the soil-derived than the other DOM sources, indicating stronger environmental filtering by the more aromatic DOM. Significant correlations were also observed between the humification index (HIX) of DOM and bacterial community diversities, co-occurrence patterns, habitat niche breadths, and the contribution of deterministic ecological processes. In addition, we demonstrated that taxa with different abundance patterns all play crucial but different roles in the response to DOM variation. Our results indicate the importance of DOM aromaticity as a predictor of the outcome of different DOM sources on bacterial community dynamics.

Spatial changes in molecular composition of dissolved organic matter in the Yangtze River Estuary: Implications for the seaward transport of estuarine DOM

The complexity of dissolved organic matter (DOM) limits our understanding of the estuarine carbon cycle. This study adopted a combination of bulk carbon isotope, optical techniques and ultra-high resolution mass spectrometry to study the spatial heterogeneity and compositional variations of DOM across a latitudinal transect of the Yangtze River Estuary (YRE). Results show that the whole section of YRE received high abundance of protein-like C4 fluorescent component (0.66 ± 0.08 R.U.) and high relative abundance of aliphatic compounds and peptides (8.28 ± 1.46%) from phytoplankton, which would contribute to the bioavailable DOM pool of the Eastern China Sea (ECS). However, multivariate analysis indicated that polycyclic aromatics and polyphenols from the Yangtze River experienced a significant decrease of 5% within the turbidity zone, creating a significant decrease of 0.08 in aromaticity index and modulating DOM content and compositions within the YRE. 1837 molecular formulae were identified to track dynamic behaviors of terrestrial DOM in the YRE. Molecular imprints showed the removal of terrestrial molecules in the turbidity zone indicated by the decrease of 753 in molecular quantity, when water masses mixing diluted the abundance of aromatic compounds. Adsorption and flocculation could serve important mechanisms to remove terrestrial DOM, promoting the burial of terrestrial DOM within estuarine sediments. Besides, some terrestrial molecular formulae were also detected in the ECS, suggesting the potential contribution of terrestrial DOM to the carbon stock of open seas after experiencing physical and photochemical transformations. This research provides a comprehensive insight into spatial variations of estuarine DOM composition, underlining the important role of estuaries in sorting and transporting DOM.

Synergistic co-digestion of waste commercial yeast and chicken manure: Kinetic simulation, DOM variation and microbial community assessment

Abstract Co-digestion of waste commercial yeast (Saccharomyces cerevisiae) and chicken manure was investigated in batch tests. The objective was to explore potential synergistic effects of the combination substrates. The biogas production rate and biomethane yield, batch kinetic simulation, metabolic dynamic and microbial community comparison were evaluated, respectively. The maximum biomethane yield of 364.79 mL/gVS was obtained at the optimum co-digestion feedstock (S.cerevisiae of 0.69 g/gVS) with a significant reaction rate increased of hydrolysis, acidogenesis, acetogenesis and methanogenesis. Person’s correlation analysis resulted that the dissolved organic matter (DOM) variation had a close correlation with indexes of NADH, NH4+-N, COD, VFAs and F420. The high-throughput sequencing showed that higher diversity was obtained both bacteria and archaea in the optimized condition with more abundant of synergistic bacteria. Moreover, community dynamic reflected that the abundances of hydrogentrophic Methanobacterium and Methanolinea raised from 11.74%, 0.23%–14.49%, 60.78%, respectively. While the proportion of the acetoclastic Methanosaeta was decreased from 88.04% to 24.72% with the increasing of S.cerevisiae dosages from 0.48 g/gVS to1.07 g/gVS.

Molecular Composition of Dissolved Organic Matter in Sediment Porewater of the Arctic Deep-Sea Observatory HAUSGARTEN (Fram Strait)

Over the last decades, the Arctic Ocean has suffered a substantial decline in sea-ice cover due to global warming. The impacts of these variations on primary productivity, fluxes of dissolved and particulate organic matter (OM) and turnover at the seafloor are still poorly understood. Here we focus on the characteristics and dynamics of the pool of marine dissolved OM (DOM) in surface sediments of the Arctic Ocean. To investigate spatial and temporal variations of DOM in relation to particulate OM input and benthic microbial community parameters, sediment porewater and overlying bottom water were collected from the long-term observatory HAUSGARTEN in June 2013 and 2014. The study area in the Fram Strait, which is partially covered by sea ice, was sampled along a bathymetric transect (1050-5500 m water depth), from east to west (7°0.2' E to 5°17' W), and from south to north (78°37' to 79°43' N). Molecular data on solid phase extracted DOM obtained via Fourier Transform Ion Cyclotron Resonance Mass Spectrometric analysis and a suite of bulk chemical parameters were related to benthic biogeochemical data. Our results demonstrate a close coupling between the production and input of OM from the surface ocean to the seafloor, and the concentration and composition of DOC/DOM in the deep sea. Surface porewaters collected in 2013 from shallower stations (≤ 1500 m water depth) in the eastern Fram Strait, had a signal of a larger and more recent input of OM (higher concentrations of phytodetritus), associated with higher numbers of molecular formulas, higher abundances of unsaturated aliphatic formulas and N-containing compounds, in concert with higher enzymatic activity, phospholipids, total organic carbon and protein content. In contrast, porewaters collected in 2014 from deeper stations and from the West, were associated with lower OM input, and showed higher abundances of aromatic and oxygen-poor compounds. Higher OM input was also reflected in higher DOC concentrations and fluxes from the sediment into the water column. Our study demonstrates that regional and temporal variations in OM input can quickly translate into changes in the quantity and quality of surface porewater DOM, the latter substantially altered by deep-sea sediment bacteria.

Regional-scale investigation of dissolved organic matter and lead binding in a large impacted lake with a focus on environmental risk assessment

Environmental risk assessment (ERA) increasingly relies on speciation modeling of bioavailability. Heavy metals are the most prevalent pollutants globally, and dissolved organic matter (DOM) plays an important role in speciation and bioavailability of heavy metals. Due to the variation of DOM properties in natural aquatic systems, improvements to the standard one-size-fits-all approach to modeling metal-DOM interactions are needed for ERA. In this study, we investigate variations in DOM and lead (Pb)-DOM binding in Lake Tai (Taihu), a large, impacted lake in eastern China that is characterized by a complex drainage network and is an important water resource at a regional level, and we assess implications of our findings within the context of ERA needs. In our study, DOM in water samples collected from across the 2,400 km2 area of Taihu was characterized using three-dimensional excitation-emission matrix and synchronous fluorescence spectroscopy spectra, the latter being used to calculate conditional stability constants for metal binding. Parallel factor analysis and peak picking were used to assess contributions of protein- and humic-like components of DOM, and fluorescence indices indicative of diagenetic processes were calculated. These quantities calculated from spectroscopic studies, in addition to water quality parameters, were analyzed by bivariate and multivariate analysis. Results show that different DOM components are highly variable across different regions of Taihu, and bivariate and multivariate analyses confirm that water quality and DOM characterization parameters are strongly interrelated. This reflects the different inputs, diagenetic and transport processes across the large expanse of Taihu. We find that the conditional stability constant of Pb-DOM binding is strongly affected by the water chemical properties and composition of DOM, though the conditional stability constant is not itself a parameter that differentiates lake water properties in different regions of the lake. The variability of DOM composition and Pb-DOM binding strength across Taihu is consistent with prior findings that a one-size-fits-all approach to metal-DOM binding may lead to inaccuracies in commonly used speciation models, and therefore such generalized approaches need improvement for regional-level ERA in complex watersheds. The approach taken here to obtain site-specific metal-DOM conditional stability constants for use in increasing the accuracy of speciation modeling is fit-for-purpose for ERA applications at regional levels because the approach is relatively simple, inexpensive, and amenable to high throughput analysis.

Spatiotemporal variations of spectral characteristics of dissolved organic matter in river flowing into a key drinking water source in China

The characteristics of dissolved organic matter (DOM) in inflowing river, flowing into drinking water, have profound influences on the quality and safety of water. Here, ultraviolet–visible (UV–vis) spectroscopy and three-dimensional fluorescence (EEM) spectroscopy were combined to investigate the spatiotemporal variations of DOM in Nanfei River flowing into Chaohu Lake in China. 24 water samples and 24 surface sediments samples (including dry-to-wet transition season and wet season in 2018) were collected from different types of outlets. Parallel factor analysis (PARAFAC) model and correlation analysis were conducted to identify the primary sources of DOM. Two humic-like components (C1 235–250 nm/385–430 nm and C3 255–270 nm/455–510 nm) and one tryptophan-like component (C2 270–290 nm/320–350 nm) were effectively identified by PARAFAC model. The results showed DOM concentration presented significant spatiotemporal variations. The concentration was much lower in water than in surface sediments in dry-to-wet transition season, but higher in the wet season. Fluorescence index (FI), biological index (BIX) and humification index (HIX) were used to judge characteristic of DOM origination. These indexes indicated that, DOM in Nanfei River had both the characteristics of humus and autogenous, but neogene autogenic feature was stronger, which was largely due to mixture of water, resuspension and desorption of DOM in sediments and photochemical degradation. In addition, the characteristic parameter of molecular mass of DOM (the values of M) had an obvious linear relationship with the fluorescence intensity ratio of fulvic acid-like to humic acid-like (C1/C3), indicating that macromolecular substances could be removed by adding or improving membrane treatment. These provided technical support for improving quality and comprehensive treatment of drinking water sources.

Revealing the correlation of biomethane generation, DOM fluorescence, and microbial community in the mesophilic co-digestion of chicken manure and sheep manure at different mixture ratio.

Batch co-digestion tests of chicken manure (CM) and sheep manure (SM) at different ratio (Rs/c) were conducted under mesophilic condition (35 °C). Batch kinetic analysis of bioCH4 production, excitation-emission matrix (EEM) fluorescence of dissolved organic matter (DOM), and microbial community were investigated. The well-fitted modified Gompertz model (R2, 0.98-0.99) resulted that the co-digestion markedly improved the methane production rate and shortened the lag phase time. The highest bioCH4 yield of 219.67 mL/gVSadd and maximum production rate of 0.378 mL/gVSadd/h were obtained at an optimum Rs/c of 0.4. Additionally, a significant variation of DOM was detected at the Rs/c of 0.4 with a consistent degradation of soluble microbial byproduct-like and protein-like organics. The positive synergy effects of co-digestion conspicuously enhanced the bioCH4 production efficiency. FI370 and NADH were significantly correlated to Rs/c (p < 0.05). Moreover, the correlations among process indicator, EEM-peaks and different environmental parameters were evaluated by Pearson correlation analysis. The high diversity of acetoclastic methanogens and hydrogenotrophic methanogens in the co-digestion improved the stability of process. Graphical Abstract.

Depth-dependent variations of dissolved organic matter composition and humification in a plateau lake using fluorescence spectroscopy

In aquatic systems, dissolved organic matter (DOM) plays an important role in regulating the reactivity and transport of environmental pollutants. Fluorescence excitation-emission matrix (EEM) analysis combined with fluorescence regional integration (FRI) and parallel factorial analysis (PARAFAC) were applied to investigate the composition, humification degree and depth-dependent variation of DOM in Lake Baihua, China. The results showed that humic-like materials with more than 60% of percentage fluorescence response (Pi,n) were dominant in DOM. The molecular complexity and structural condensation of PARAFAC components showed the order of C1 (humic-like components) > C3 (protein-like components) > C4 (fulvic-like components) > C2 (fulvic-like components). The lower maximum fluorescence intensities (Fmax) of components in surface layers were attributed to photo-oxidation of DOM by radiation. With depths less than 16 m, the Fmax increases and decreases were attributed to accumulation of recalcitrant humic substances and microbial/abiotic degradation of particulate DOM, respectively. A combination of biological and humification indices could be used as indicators for distinguishing different degrees of humification and sources of DOM. DOM from Lake Baihua mainly originated from an aquatic bacterial source that consisted of an important/intermediate autochthonous component with a weak humic character. The fluorescence indices of PI,n/PII,n, PI+II+IV,n/PIII+V,n, C2, C1/C2 and C2/(C3+C4) were more suitable to determine the humification degree. Stronger humic characters and higher humification degree for DOM were present at depths of 10–13 m. The fluorescence spectroscopy combined with fluorescence indices is convenient to investigate depth-dependent DOM characteristics and to assess water quality or pollution risk in lake systems.

Seasonal Variations of DOM Spectral Characteristics in the Surface Water of the Upstream Minjiang River

Alpine wetlands and valleys of northwestern Sichuan are the main catchment areas of Minjiang River, where dissolved organic matter (DOM) in surface waters comes mainly from the natural background environment. Sources and structure parameters of DOM are important for calibrating the flux and pattern of organic carbon exports from plateau wetlands and alpine rivers. In this study, surface water samples along the upstream Minjiang River were collected at the end of dry season (April) and rainy season (October). Excitation emission matrix (EEM) fluorescence spectroscopy coupled with parallel factor analysis (PARAFAC) was used to characterize seasonal variations of DOM along Minjiang River. Results showed fluorescence peaks (humic-like peaks A and C, protein-like peaks B and T) were different along the river. Peak A and peak C were more obvious at the end of dry season, while peak B and peak T were more obvious at the end of rainy season. PARAFAC produced a three-component model including two humic-like components[C1 (250-260/380-480 nm) and C2 (300-330/380-480 nm)] and one protein-like component[C3 (270-280/300-350 nm)], accounting relative intensity 48.68%-65.02% for C1, 23.17%-29.83% for C2, and 11.83%-21.53% for C3. Fluorescence components showed variations along the river more prominently in April than October, in which the most significant one was C1. Average fluorescence index (FI) values ranged from 1.4 to 1.9, indicating that DOM consisted of both autochthonous and allochthonous components. Moreover, higher degrees of humification, aromaticity and hydrophobicity were found in April than those in October, suggesting more terrigenous sources at the end of dry season and more biological sources at the end of rainy season. Additionally, chromophoric dissolved organic matter (CDOM)[a(355)] correlated significantly with humic-like substance[Fn(355)], which also indicated that DOM components originated from terrigenous input in the upstream Minjiang River. The results also showed significant positive correlation between C1 and C2 in April, with no significant correlation in October, which further proved that exogenous input and seasonal variations characterized DOM sources in the upstream Minjiang River.

Seasonal Variations of Dissolved Organic Matter in the East China Sea Using EEM-PARAFAC and Implications for Carbon and Nutrient Cycling

Dissolved organic matter (DOM) plays a very important role in the dynamics of different biogeochemical processes in the global marine carbon cycle. Seasonal variations of DOM were investigated using dissolved organic carbon (DOC), absorption spectroscopy, and fluorescence excitation emission matrix–parallel factor analysis (EEM-PARAFAC) in Niushan Island in the East China Sea, off southeast Zhejiang, in winter, spring, and summer 2017. A strong positive correlation between temperature and salinity was observed in winter, whereas negative correlations were obtained in spring and summer. The results suggest that the water in the study area originates from the Changjiang River water coinciding with the direction of winds in winter. On the other hand, the Taiwan Strait water flows northward on the eastern part of the study area in summer and spring. The DOC concentration showed significant seasonal variations and was much lower in summer than in winter, but largely independent of the temperature, degree of absorption and intensity of the fluorescence component. Three tryptophan-like components (C1, C2 and C4) and one humic-like component were identified by the PARAFAC model in winter, while two tryptophan-like components (C1 and C2) and two humic-like components were illustrated in summer. The seasonal variations of DOC concentrations and intensities of the fluorescence components in the study area from winter to summer suggest that the waters in this area were influenced mainly by mixing water of the Changjiang River and the Taiwan Strait. A strongly linear relationship was found between the nutrient and the fluorescence intensities of different components in winter, reflecting the profound impacts of the dynamics of DOM on nutrient cycling.

Influences of the alternation of wet-dry periods on the variability of chromophoric dissolved organic matter in the water level fluctuation zone of the Three Gorges Reservoir area, China

Dissolved organic matter (DOM) is a crucial driver of various biogeochemical processes in aquatic systems. Thus, many lakes and streams have been investigated in the past several decades. However, fewer studies have sought to understand the changes in DOM characteristics in the waters of the Three Gorges Reservoir (TGR) areas, which are the largest artificial reservoir areas in the world. Thus, a field investigation of dissolved organic carbon (DOC) concentrations and of chromophoric dissolved organic matter (CDOM) properties was conducted from 2013 to 2015 to track the spatial-temporal variability of DOM properties in the TGR areas. The results showed that the alternations of wet and dry periods due to hydrological management have a substantial effect on the quantity and quality of aquatic DOM in TGR areas. Increases in DOC concentrations in the wet period show an apparent “dilution effect” that decreases CDOM compounds with relatively lower aromaticity (i.e., SUVA254) and molecular weight (i.e., SR). In contrast to the obvious temporal variations of DOM, significant spatial variability was not observed in this study. Additionally, DOM showed more terrigenous characteristics in the dry period but weak terrigenous characteristics in the wet period. Furthermore, the positive correlation between SUVA254 and CDOM suggests that the aromatic component controls the CDOM dynamics in TGR areas. The first attempt to investigate the DOM dynamics in TGR areas since the Three Gorges Dam was conducted in 2012, and the unique patterns of spatial-temporal variations in DOM that are highlighted in this study might provide a new insight for understanding the role of DOM in the fates of contaminants and may help in the further management of flow loads and water quality in the TGR area.

Impacts of global changes on the biogeochemistry and environmental effects of dissolved organic matter at the land-ocean interface: a review.

Dissolved organic matter (DOM) is an important component in the biogeochemistry and ecosystem function of aquatic environments at the highly populated land-ocean interface. The mobilization and transformation of DOM at this critical interface are increasingly affected by a series of notable global changes such as the increasing storm events, intense human activities, and accelerating glacier loss. This review provides an overview of the changes in the quantity and quality of DOM under the influences of multiple global changes. The profound implications of changing DOM for aquatic ecosystem and human society are further discussed, and future research needs are suggested for filling current knowledge gaps. The fluvial export of DOM is strongly intensified during storm events, which is accompanied with notable changes in the chemical composition and reactivity of DOM. Land use not only changes the mobilization of natural DOM source pools within watersheds but also adds DOM of distinct chemical composition and reactivity from anthropogenic sources. Glacier loss brings highly biolabile DOM to downstream water bodies. The changing DOM leads to significant changes in heterotrophic activity, CO2 out gassing, nutrient and pollutant biogeochemistry, and disinfection by-product formation. Further studies on the source, transformations, and downstream effects of storm DOM, temporal variations of DOM and its interactions with other pollutants in human-modified watersheds, photo-degradability of glacier DOM, and potential priming effects, are essential for better understanding the responses and feedbacks of DOM at the land-ocean interface under the impacts of global changes.

Investigation of humic acids concentration in different seasons in a raw water canal, Bangkok, Thailand

Abstract This study presents an overview of the changes in humic acids, which are disinfection by-product precursors in a raw water canal, Bangkok, Thailand, during different seasons. Fluorescence excitation–emission matrix (EEM) spectroscopy was employed to characterize and quantify spatial and seasonal variations of dissolved organic matter (DOM) along a raw water canal of the Metropolitan Waterworks Authority (MWA) Bangkok, Thailand. A total of 276 raw water samples were collected from 23 stations during the winter, summer and rainy season. Results indicate that hydrophobic fractions made up the majority of DOM and this water source also had high specific UV absorbance (SUVA). Seasonal variation of DOM was found to be more pronounced than geographical variation along the canal. The presence of humic substances was the highest in the rainy season due to rainfall and surface runoff, while soluble microbial by-product-like substances were found only in summer. The results provide an insight into the prediction of humic acids in source water, which benefits the MWA in being aware of seasonal variation in water quality.

Assessing acid rain and climate effects on the temporal variation of dissolved organic matter in the unsaturated zone of a karstic system from southern China

Acid rain has the potential to significantly impact the quantity and quality of dissolved organic matter (DOM) leached from soil to groundwater. Yet, to date, the effects of acid rain have not been investigated in karstic systems, which are expected to strongly buffer the pH of atmospheric rainfall. This study presents a nine-year DOM fluorescence dataset from a karst unsaturated zone collected from two drip sites (HS4, HS6) in Heshang Cave, southern China between 2005 and 2014. Cross-correlograms show that fluorescence intensity of both dripwaters lagged behind rainfall by ∼1 year (∼11 months lag for HS4, and ∼13 months for HS6), whereas drip rates responded quite quickly to rainfall (0 months lag for HS4, and ∼3 months for HS6), based on optimal correlation coefficients. The rapid response of drip rates to rainfall is related to the change of reservoir head pressure in summer, associated with higher rainfall. In winter, low rainfall has a limited effect on head pressure, and drip rates gradually slow to a constant value associated with base flow from the overlying reservoir- this effect being most evident on inter-annual timescales (R2 = 0.80 for HS4 and R2 = 0.86 for HS6, n = 9, p < 0.01). We ascribed the ∼1 year lag of fluorescence intensity to the effect of the soil moisture deficit and the karst process on delaying water and solute transport. After eliminating the one year lag, the congruent seasonal pacing and amplitude between fluorescence intensity and rainfall observed suggests that the seasonality of fluorescence intensity was mainly controlled by the monsoonal rains which can govern the output of DOM from the soil, as well as the residence time of water in the unsaturated zone. On inter-annual timescales, a robust linear relationship between fluorescence intensity and annual (effective) precipitation amount (R2 = 0.86 for HS4 and R2 = 0.77 for HS6, n = 9, p < 0.01) was identified, implying that annual (effective) precipitation is the main determinant of DOM concentration in the aquifer. Conversely, the insensitivity of fluorescence intensity and fluorescence wavelength maxima to variations in the pH of local rainfall suggests that acid rain over the study period (∼pH 5.6 to ∼ 4.5) had no discernable effect on the quantity and quality of DOM in karst soil and soil solution, likely being strongly buffered by soil carbonates. Therefore, despite large increases in anthropogenic acid rain in recent Chinese history, hydrologic forcing is the predominant factor driving variations in DOM in karst aquifers.

Impacts of wheat straw addition on dissolved organic matter characteristics in cadmium-contaminated soils: Insights from fluorescence spectroscopy and environmental implications

Incorporating straw in soil can change the dissolved organic matter (DOM) content and also affect the mobility and solubility of soil pollutants. However, few studies have focused on the spectral features of DOM released by soil after amendment with crop residues. In this study, excitation-emission matrix (EEM) fluorescence and UV–visible absorbance spectroscopy were used to monitor the quality and quantity of the DOM derived from wheat straw incorporation in the soil. We found remarkable changes in the dissolved organic carbon (DOC) concentration (0.10–0.34 g kg−1). But the mean DOC concentrations were lower in the cadmium (Cd2+) added treatments compared with the control treatment due to additional Cd2+. The specific UV–visible absorbance at 254 nm was positively correlated with Cd2+ content, thereby suggesting a strong relationship between aromatic materials and heavy metals. Three fluorescent components were identified by EEM fluorescence combined with parallel factor analysis (PARAFAC) modeling, i.e., UVC humic-like, fulvic acid-like, and tryptophan-like. The relative abundances of these three components and dynamic variations in fluorescent indices predicted the changes in DOM composition during the wheat straws decomposition process. Therefore, the findings obtained in this study can be utilized to evaluate the variations in DOM for soil incorporated with straws in order to support agricultural production and overcome the heavy metal pollution.

Dynamics of dissolved organic matter (DOM) in a typical inland lake of the Three Gorges Reservoir area: Fluorescent properties and their implications for dissolved mercury species

Dissolved organic matter (DOM) plays an important environmental and ecological role in inland aquatic systems, including lakes. In this study, using fluorescence analysis, we investigated the seasonal dynamics of DOM characteristics in Changshou Lake, which is a typical inland lake in the Three Gorges Reservoir (TGR) area. We also discuss the environmental implications of DOM for mercury (Hg) dynamics. Based on the origins of two end-members, the variations in DOM observed in this study in Changshou Lake suggest that hydrological processes (e.g., terrestrial inputs resulting from runoff and humic-like component residences) and biological activities (e.g., microbial and algae growth) are the two main principal components controlling the seasonal dynamics of DOM characteristics. Furthermore, the dynamics of dissolved Hg co-varied with variations in DOM properties, rather than with dissolved organic carbon (DOC) concentrations. This indicates that the previously reported simple correlations between DOC and Hg were not comprehensive and may lead to misunderstanding the interactions between DOM and Hg. Therefore, we recommend that when using DOM-Hg correlations to evaluate the role of DOM in the environmental fate of Hg, especially in field investigations of the spatial and temporal distribution of Hg, the properties of DOM must be taken into account.

Evaluation of Spatial and Seasonal Variations of Dissolved Organic Matter in Maliau Basin, Sabah, Malaysia

The characteristics dissolved organic matter (DOM) and surface water quality in Maliau Basin, Sabah, Malaysia, were determined from December 2014 to September 2015. The objectives of this study were: (i). to distinguish the surface water quality of rivers which consisted of different types of land use: primary forest (PF), secondary forest (SF), main river (MR) and altered habitat (AH); (ii). to assess seasonal variations of the water quality. A total of 36 water samples were collected during the fieldwork campaign that extended over high and low rainfall periods. The water quality physico-chemical parameters such as water temperature, pH, dissolved oxygen (DO), conductivity, and salinity were measured in-situ with the YSI-multiparameter instrument, while dissolved organic carbon (DOC) concentration was evaluated using Shimadzu TOC-V-SCH analyzer with auto-sampler TOC-ASI-V equipped. Dissolved organic matter (DOM) was determined optically in the laboratory at 254 (a254), 340 (a340) and 410 (a410). Discriminant analysis (DA) was employed to organize independent variables into mutually-exclusive groups. The outcome of DA indicated that water temperature, total suspended solid (TSS), and conductivity were dominant at AH, whilst a340 was dominant at MR in March 2015. Our results also showed seasonal variations for water quality parameters. The highest mean concentrations of pH and a340 were found during the dry period (March 2015) and dissolved oxygen (DO) during the wet season (December 2014), suggesting spatio-seasonal variations of DOM and water quality parameters were determined by environmental factors such as precipitation, water velocity and discharge amount. UV-vis absorptions a254 and a340 show significant positive correlation with dissolved organic carbon (DOC), suggesting that it could be a suitable proxy for DOC concentration for tropical flowing water.

Diurnal variations of dissolved organic matter in the hydrothermal system of Green Island, Taiwan

Abstract Hydrothermal systems are increasingly recognized as important fields for the biogeochemical cycling of dissolved organic matter (DOM) in the ocean. While the quantity and quality of DOM have been characterized for various hydrothermal systems, little is known about the temporal variations of DOM and the underlying processes in these systems. Diurnal variations of DOM were investigated for the hot water area and the warm water area of the hydrothermal system in Green Island off southeast Taiwan at 3 h intervals, using measurement of dissolved organic carbon (DOC), absorption spectroscopy, and fluorescence excitation emission matrix-parallel factor analysis (EEM-PARAFAC). The results of temperature, salinity, pH and DOC indicated that the warm water area had a third freshwater source, besides the seawater and the hot fluids from the hot water area, that was probably groundwater. The DOC concentration showed significant diurnal changes within 84–130 μM at the warm water area. A negative correlation between the DOC concentration and temperature was observed at the warm water area, while the DOC concentration at the hot water area was largely independent of temperature and tide. Four fluorescent components were identified by PARAFAC, including tryptophan-like C1 and C4, and humic-like C2 and C3. The fluorescence intensities of C1 and C4 were higher at the hot and the warm water areas than those in the ambient seawater, suggesting that the former two areas were sources of fluorescent DOM for seawater. The fluorescence intensities of C1 and C2 at the hot water area were higher at night than those in the daytime, implying the effects of potential photobleaching. Overall, the low values of the humification index (HIX) and the high values of the fluorescence index (FI) and the autochthonous index (BIX) at the hot and the warm water areas revealed that the DOM might have a mainly autochthonous biological or bacterial origin.