Chromophoric Dissolved Organic Matter Fluorescence Research Articles

Optical closure in highly absorptive coastal waters: significance of inelastic scattering processes.

In hydrological optics, "optical closure" means consistency between the apparent optical properties (AOPs) determined from radiometric measurements and those derived from radiative transfer modelling based on concurrently measured inherent optical properties (IOPs) and boundary conditions (sea and sky states). Good optical closure not only provides confidence in the data quality but also informs on the adequacy of the radiative transfer parameterization. Achieving optical closure in highly absorptive coastal waters is challenging due to the low signal-to-noise ratio of radiometric measurements and uncertainties in the measurements of IOPs, namely the spectral absorption and backscattering coefficients. Here, we present an optical closure assessment using a comprehensive set of in situ IOPs acquired in highly absorptive coastal waters optically dominated by chromophoric dissolved organic matter (CDOM). The spectral remote sensing reflectance, Rrs(λ), was modeled using the software HydroLight (HL) with measured IOPs and observed boundary conditions. Corresponding in-water in situRrs(λ) was derived from radiometric measurements made with a Compact Optical Profiling System (C-OPS; Biospherical). The assessment revealed that the inclusion of inelastic scattering processes in the model, specifically sun-induced CDOM fluorescence (fDOM) and sun-induced chlorophyll fluorescence (SICF) from Chlorophyll-a ([chl]), significantly improved the optical closure and led to good agreement between measured and modeled Rrs (i.e., for 440 ≤ λ ≤ 710 nm with no inelastic processes: R2=0.90, slope=0.64; with inelastic processes: R2=0.96, slope=0.90). The analysis also indicated that fDOM and SICF contributed a substantial fraction of the green-red wavelength Rrs in these waters. Specifically, fDOM contributed ∼18% of the modeled Rrs in the green region and SICF accounted for ∼20% of the modeled Rrs in the red region. Overall, this study points out the importance of accounting for fDOM in remote sensing applications in CDOM-dominated waters.

Characteristics, sources and driving factors of riverine CDOM in a severe erosion basin on the Loess Plateau, China

Chromophoric dissolved organic matter (CDOM) plays a crucial role in aquatic ecosystems and biogeochemical processes. The characteristics, sources and influencing factors of riverine CDOM were determined with 56 samples collected in the Wuding River (WDR) Basin in spring and summer. The CDOM optical characteristics varied greatly during different seasons. The mean S275-295 was higher in spring than in summer, while SUVA254 exhibited the opposite result. The mean biological source index (BIX) was lower in summer than in spring, while the mean humification index (HIX) was higher in summer than in spring (p < 0.01). C1 and C3 (humic-like components) and C2 (tryptophan-like component) were extracted by parallel factor analysis (PARAFAC). C1 and C3 were prominent in summer, while C2 was prominent in spring. The effects of environmental factors on CDOM showed great differences. Chlorophyll a (Chla), dissolved organic carbon (DOC), electrical conductivity (EC) and dissolved oxygen (DO) had significant effects on the CDOM concentration (p < 0.05) in spring, while only DOC exerted significant effects on the CDOM concentration in summer. However, water quality parameters played important roles in evaluating the CDOM fluorescence components in summer. Total suspended matter (TSM) and turbidity (Tur) exerted significant effects on the humic-like fluorescence peaks (p < 0.05), while pH, Chla, DO, and EC had significant influences on tryptophan-like substances during this period. Based on redundancy analysis, precipitation, water erosion area and human activity intensity greatly affected the CDOM concentration, C1 and C3, while human activity intensity significantly affected C2 in the sub-basins in summer. CDOM was not significantly influenced by the surrounding environmental factors in spring except for the human activity intensity, which greatly influenced C1 and C3.

Radiative Transfer Modeling With Biogeochemical-Argo Float Data in the Mediterranean Sea.

A radiative transfer model was parameterized and validated using Biogeochemical Argo float data acquired between 2012 and 2017 across the Mediterranean Sea. Fluorescence‐derived chlorophyll a concentration, particulate optical backscattering at 700 nm, and fluorescence of chromophoric dissolved organic matter (CDOM) were used to parametrize the light absorption and scattering coefficients of the optically significant water constituents (such as pure water, non‐algal particles, CDOM, and phytoplankton). The model was validated with in situ downwelling irradiance profiles and apparent optical properties derived both from irradiance profiles and satellite data, such as the diffuse attenuation coefficients and remote sensing reflectance. Results showed that by using regional parameterizations that are not only related to chlorophyll concentration and vertical distribution, the model was able to capture a more accurate spectral response in the examined wavelength range compared to chlorophyll‐related (or Case 1) optical models. When using alternative models that incorporated also measurements of CDOM fluorescence or particulate optical backscattering, the model skill increased at all examined wavelengths. Finally, using a multi‐spectral optical configuration also enabled the estimation of the relative contribution of separate water constituents in the examined spectral range. Simulations including non‐algal particles and CDOM performed up to 61% and 79% better than when considering the optical properties of pure seawater alone. Moreover, a simulation including phytoplankton light absorption resulted in an error reduction of up to 42%, especially at 412 nm and with a more uniform response at the wavelengths considered.

Assessing the potential to use CDOM as an indicator of water quality for the sediment-laden Yellow river, China

Chromophoric dissolved organic matter (CDOM) in rivers is mainly affected by natural conditions and human activities and can reflect the watershed pollution status to a certain extent. The Yellow River is one of the largest contributors to the global riverine sediment flux from the land to ocean, and there is a paucity of information on how the optical properties of CDOM have the potential to serve as an indicator of water quality for the sediment-laden Yellow River. In this study, a three-dimensional fluorescence parallel factor (PARAFAC) analysis method was applied to investigate the seasonal and spatial variations in CDOM fluorescence components and spectral characteristics from the source region to the estuary in the mainstream of Yellow River. The relationships of CDOM with water quality indicators and trophic state were also analyzed. Six PARAFAC components (C1–C6) were identified and grouped into two categories: humic-like components (C1–C4), which accounted for 85.8 %, and protein-like components (C5 and C6), which accounted for only 14.2 %. The CDOM components, spectral parameters, and their clear correlations with the main ions (Na+ and Cl−) all indicated that the humic-like components may be primarily derived from nonpoint source erosion, and the protein-like components were mainly derived from point source discharges in the watershed. The combination of the CDOM absorption coefficient at 254 nm (a(254)), spectral slope ratio (SR), specific UV absorbance SUVA254, and fluorescence index (FI) had a good predictive ability for the key water quality indicators (total nitrogen (TN), dissolved total nitrogen (DTN), total phosphorus (TP), dissolved total phosphorus (DTP), and chlorophyll a (Chl a)) and trophic state index (TSI). Therefore, some fluorophores and UV spectral parameters of CDOM in the Yellow River can be used for rapid water quality monitoring and pollution source indication, especially pollutants related to nitrogen and phosphorus nutrients in the basin.

Binding of Calcium and Magnesium Ions to Terrestrial Chromophoric Dissolved Organic Matter (CDOM): A Combination of Steady-State and Time-Resolved Fluorescence Study

Revealing the binding properties of calcium ion (Ca2+) and magnesium ion (Mg2+) to terrestrial chromophoric dissolved organic matter (CDOM) facilities understanding the effect of natural water components on the photophysics of dissolved organic matter. Steady-state and time-resolved fluorescence spectrometry, and dynamic light scattering were applied to investigate the fluorescence quenching process of CDOM by Ca2+ and Mg2+. Due to a remarkable decrease of the steady-state fluorescence intensity and a slight decrease of fluorescence lifetime, the fluorescence quenching of CDOM by cations mainly occurred through a static process. The fluorescence quenching was profound under longer excitation and emission wavelengths. The binding constant (K, L/mol) for Ca2+ to CDOM ranged from 4.29 to 5.09 (lgK), which was approximately one order of magnitude higher than that of Mg2+ to CDOM (3.86 to 4.56). Nevertheless, the efficiency of CDOM fluorescence quenching by Ca2+, Mg2+ was much lower than that by Cu2+. Fluorescence decay became faster in the presence of a high concentration of Ca2+ (&gt;20 mg/L) and Mg2+ (&gt;50 mg/L). In the presence of these two metal ions, particularly for Ca2+, the lifetime of CDOM excited states shifted to the relatively small value side, indicating fluorescence quenching of CDOM mainly occurred through the interaction of Ca2+/Mg2+ with relatively long-lived fluorophores.

CDOM in the source regions of the Yangtze and Yellow Rivers, China: optical properties, possible sources, and their relationships with environmental variables.

The source regions of the Yangtze and Yellow Rivers on the Qinghai-Tibet Plateau are extremely important water resources and ecological functional areas in China, and the ecological environment is fragile and sensitive to climate change. Chromophoric dissolved organic matter (CDOM) is an important component that plays a crucial role in the biogeochemical cycle in aquatic ecosystems. However, knowledge of the distribution characteristics of CDOM in this area is limited. In this study, the optical properties, possible sources of CDOM, and their relationships with environmental variables were investigated in the two regions. The results indicated that the CDOM absorption spectra of these two source regions had a high degree of consistency, and the absorption coefficient aCDOM(355) was small, with a mean of 2.07 ± 1.10m-1. Two fluorescence components (C1 and C2) were identified and grouped into the humic-like component with parallel factor analysis (PARAFAC) of fluorescence excitation-emission matrices (EEMs), which exhibited highly similar (excitations/emission)max positions between each pair of components in the two regions. Comprehensive CDOM spectral absorption and fluorescence parameters suggested that CDOM was mainly derived from externally input humus, and the source region of the Yellow River showed stronger allochthonous sources. The dissolved organic carbon (DOC) gradients in the water affected the fluorescence intensity and indicated that the humic-like component was an important component of DOC. Water temperature (WT) and turbidity (Turb) positively affected the concentration of CDOM and the ability to absorb light in the aquatic ecosystems. Due to global warming, the rising temperature may lead to an increase in meltwater inflow in the source area and will also bring more external inputs through the runoff.

An ultraviolet to visible scheme to estimate chromophoric dissolved organic matter absorption in a Case-2 water from remote sensing reflectance

In a typical Case-2 coastal water environment (here, the Pearl River Estuary (PRE), China), chromophoric dissolved organic matter (CDOM) and suspended particulates dominate the water optical properties, and CDOM fluorescence contributes considerably to surface water reflectance. In this paper, an ultraviolet (UV) to visible scheme to retrieve CDOM absorption (ag)is developed based on a set of in situ observations. First, the CDOM UV absorption and spectral slope (Sg) are derived directly from the visible remote sensing reflectance; then the Sg is extrapolated to obtain the spectrum from UV to visible spectral range. This algorithm performs well, with an overall mean absolute percent difference (MAPD) of ∼30%, ∼5% and ∼6% for the estimation of ag in 250–450 nm, Sg over 250–400 nm, and 250–700 nm, respectively. The effectiveness and stability of the algorithm is further demonstrated in capturing the distribution pattern of CDOM absorption in the PRE from satellite ocean color imagery with multiple spatial and spectral resolution, namely: the Visible Infrared Imaging Radiometer Suite (VIIRS) (750 m/Multispectral), the Ocean and Land Color Instrument (OLCI) (300 m/Multispectral), the Hyperspectral Imager for the Coastal Ocean (HICO) (100 m/Hyperspectral), and the Landsat 8 Operational Land Imager (OLI) (30 m/Multispectral). The UV to visible scheme can benefit the CDOM absorption estimation in two aspects: 1) it is free from the disturbance of suspended matter; 2) it avoids uncertainties caused by the low signal-to-noise ratio (SNR) of ag measurements in the visible range. The algorithm is effective in revealing multiple scales of variation of CDOM absorption from ocean color observations.

Sources, Transport, and Transformation of Dissolved Organic Matter in a Large River System: Illustrated by the Changjiang River, China

AbstractThe sources, transport, and transformation of terrestrial dissolved organic matter (DOM) in large river systems are of great interest due to their importance in the regional and global carbon cycles. However, knowledge of these factors is limited, especially for different DOM compositions coupling with hydrodynamics at the basin scale. To investigate the dynamics of different DOM compositions in a large river system, we characterized the dissolved organic carbon (DOC), chromophoric DOM (CDOM), and dissolved lignin at both moderate (September–October 2009) and high (August 2012) discharges in the Changjiang River basin. We found that extensive soil erosion resulted in the increased DOC concentration in the middle to lower reaches in 2012. In 2009, the transformation of DOM was dominated by flocculation and photo degradation due to the low flow velocity and depressed particle suspension. In 2012, the degradation of CDOM and dissolved lignin was coupled and dominated by microbial degradation and photo degradation. In 2012, CDOM (absorbance and fluorescence) and dissolved lignin concentrations were positively correlated, demonstrating the potential of using CDOM to quantitatively estimate dissolved lignin. Mass balance estimations of the DOM fluxes further confirmed that 17.0% (6.1%) of DOC, 24.2% (32.3%) of dissolved lignin, and 14.5% (0.8%) of CDOM were degraded at high (moderate) discharge. This study reveals contrasting transformation processes of DOM compositions with different sources and reactivities along the Changjiang River under different hydrological conditions, and it provides new insights into the linkage between the chemical and optical properties of DOM in large river systems.

Characterization of CDOM in reservoirs and its linkage to trophic status assessment across China using spectroscopic analysis

Chromophoric dissolved organic matter (CDOM) represents the optically active component of the DOM pool, and originates from both allochthonous and autochthonous sources. The fluorescent characteristics of dissolved organic matter (FDOM) has been widely used to trace CDOM sources and infer its composition. However, little is known about the optical and fluorescent properties of CDOM in drinking water reservoirs, and the variability of CDOM properties along trophic gradients in these aquatic systems. A total of 536 water samples were collected between 2015 and 2017 from 131 reservoirs across China to characterize CDOM and FDOM properties using both light absorption and fluorescence spectroscopies, and examine relationships with water-quality condition as expressed by the modified trophic state index (TSIM) of the reservoirs (range: 12 < TSIM < 78). With increased reservoir trophic status, CDOM absorption coefficients at 254 nm (aCDOM(254)) and total fluorescence of FRI-EEMs (excitation-emission matrix coupled with fluorescence regional integration) increased significantly (p < 0.01). Our results indicated that the nutrients in aquatic systems and the social economy factors (including wastewater, sewage, cultivated land, GDP, population) and altitude affect the CDOM absorption and fluorescence significantly (p < 0.05). Most importantly, we proposed a new classification standard associated with eutrophic status via CDOM humification index (HIX) due to the significant correlation between each other (p < 0.01). HIX less than 4 was corresponding to TSIM less than 30 as oligotrophic states; 4 < HIX ≤ 12 was corresponding to TSIM less than 50 as mesotrophic states; 12 < HIX ≤ 18 was corresponding to TSIM less than 70 as eutrophic states; HIX more than 18 is defined as hypereutrophic status with TSIM > 70. This method highlights the importance of CDOM fluorescence for aquatic DOM input and strengthens the linkage between the trophic status index and CDOM characteristics, it also contributes to establish a new rapid assessing model with quick experimental measurements to monitor the trophic status of water reservoirs.

Terrestrial humic-like fluorescence peak of chromophoric dissolved organic matter as a new potential indicator tracing the antibiotics in typical polluted watershed

Natural surface waters are threatened globally by antibiotics pollution. In this study, we analyzed antibiotics and CDOM (Chromophoric dissolved organic matter) fluorescence in different water bodies using HPLC method and Excitation Emission Matrix- Parallel factor analysis, respectively. A combination of field studies in the Yinma River Watershed were conducted in rivers, reservoirs and urban rivers, and 65 CDOM and antibiotic samples were taken in April, May, July, and August 2016. EEM-PARAFAC analysis identified two components; a humic-like (C1) component and a tryptophan-like (C2) component. The redundancy analysis (RDA) demonstrated that CDOM could explain 38.2% (two axes) of the five antibiotics in reservoirs (N = 31), and 26.0% (two axes) of those in rivers and urban water (N = 30). Furthermore, the Pearson correlation coefficient between Sulfamethoxazole and C1 in reservoir water was 0.91 (t-test, 2-tailed, p < 0.01), and that between Sulfamethoxazole and C2 was 0.68 (t-test, 2-tailed, p < 0.01). This indicated that the humic-like component of CDOM PARAFAC fluorescence could detect Sulfamethoxazole contamination levels in the homogenized reservoir waters. Our results identified Sulfamethoxazole and Quinolones (Norfloxacin, 16.5 ng L−1; Enrofloxacin, 0.3 ng L−1; Ciprofloxacin, 30.9 ng L−1) at mean concentrations of 369.5 ng L−1 and 15.9 ng L−1, respectively, which were the higher levels in natural surface waters. The FTIR spectroscopy of the mixture of humic acid and sulfamethoxazole showed that the absorbance at 3415 cm−1 linked to OH stretching of OH groups and at 1386 cm−1 because of OH bending and vibration of COOH groups became weaker, indicating that COOH groups of humic acid can adsorb and react with –NH2 of sulfamethoxazole. The CDOM PARAFAC components can be adapted for online or in situ fluorescence measurements as an early warning of Sulfamethoxazole distribution and contamination in similar aquatic environments.

Seasonal Export of Dissolved Organic Matter from a New England Salt Marsh

ABSTRACT Schiebel, H.N.; Gardner, G.B.; Wang, X.; Peri, F., and Chen, R.F., 2018. Seasonal export of dissolved organic matter from a New England salt marsh. The seasonality of dissolved organic matter (DOM) export from a New England salt marsh was investigated using multiple estuary transects along with laboratory plant leaching experiments. From 2001 through 2013, 35 transects of the Neponset Estuary in Boston, Massachusetts, were conducted to measure high-resolution in situ chromophoric dissolved organic matter (CDOM) fluorescence, along with other standard oceanographic parameters, via sensors incorporated into an undulating depressor wing known as the mini-shuttle. Water samples were collected during each cruise for discrete dissolved organic carbon (DOC) and CDOM fluorescence analyses. Seasonal salt marsh DOC export rates ranged from 1.1 × 107 to 15 × 107 mol C season−1 (10–140 mol C m−2 season−1). The total export was found to be 23 ± 17 × 107 mol C y−1 (210 mol C m−2 y−1) with river discharge, temp...

Excitation-Dependent Fluorescence Quantum Yield for Freshwater Chromophoric Dissolved Organic Matter from Northern Russian Lakes

Advanced fluorescence analysis within the wide range of excitation wavelengths from 230 to 510 nm accompanied with chromatography was used to study natural chromophoric dissolved organic matter (CDOM) from three freshwater Karelian lakes. The influence of excitation wavelength (λex) on fluorescence quantum yield and emission maximum position was determined. The CDOM fluorescence quantum yield has reached a minimum at λex∼270–280 nm and a maximum at λex∼340–360 nm. It was monotonously decreasing after 370 nm towards longer excitation wavelengths. Analytical reversed-phase high-performance liquid chromatography with multiwavelength fluorescence detector characterized distribution of fluorophores between hydrophilic/hydrophobic CDOM parts. This technique revealed “hidden” protein-like fluorophores for some CDOM fractions, in spite of the absence of protein-like fluorescence in the initial CDOM samples. The humic-like fluorescence was documented for all hydrophilic and hydrophobic CDOM chromatographic peaks, and its intensity was decreasing along with peaks’ hydrophobicity. On contrary, the protein-like fluorescence was found only in the hydrophobic peaks, and its intensity was increasing along with peaks’ hydrophobicity. This work provides new data on the CDOM optical properties consistent with the formation of supramolecular assemblies controlled by association of low-molecular size components. In addition, these data are very useful for understanding the CDOM function in the environment.

The Human Health Assessment to Phthalate Acid Esters (PAEs) and Potential Probability Prediction by Chromophoric Dissolved Organic Matter EEM-FRI Fluorescence in Erlong Lake.

Phthalate acid esters (PAEs) are suspected to cause wide environmental pollution and have adverse effects on human health. Three priority control phthalates, namely dimethyl phthalate (DMP), diethyl phthalate (DEP), and dibutyl phthalate (DBP), were determined in 45 water samples from the largest drinking water source in Jilin Province. Chromophoric-dissolved organic matter (CDOM), which are composed of complex compounds and are a proxy for water quality, can be monitored using a fluorometer. This study attempted to understand the correlations of the CDOM fluorescence regional integration (FRI) components with PAEs and CDOM characteristics under seasonal and spatial variations in the Erlong Lake. The characteristics of the CDOM absorption parameters in different water samples showed a higher aromatic content and molecular weight in October because of increased terrestrial inputs. The Σ3PAEs concentrations ranged from 0.231 mg L−1 to 0.435 mg L−1 in water, and DEP contributed to more than 90% of the Σ3PAEs. The FRI method identified five fluorescence components: one tyrosine-like (R1), one tryptophan-like (R2), one fulvic-like (R3), one microbial protein-like (R4), and one humic-like (R5) component. However, significant relationships exist between DEP and R3 (R2 = 0.78, p < 0.001), R4 (R2 = 0.77, p < 0.001), and R5 (R2 = 0.58, p < 0.001). Quantifying the relationship between CDOM and PAEs was highly significant, because the results will simplify the componential analysis of pollutants from a spatiotemporal perspective as compared to traditional chemical measurements. The human health risk assessment results revealed no human health risk (HQ < 1) in the Erlong Lake basin.

Optical Characteristics and Distribution of Chromophoric Dissolved Organic Matter in Onega Bay (White Sea) during the Summer Season (Findings from an Expedition from June 22 to 26, 2015)

Onega Bay waters are characterized by a high content of chromophoric dissolved organic matter (CDOM). The absorbance spectra and fluorescence intensity (excitation wavelength 455 nm, emission wavelength >680 nm) were used to assess the distribution of CDOM content in water filtered through a GF/F filter. The CDOM content at different points in Onega Bay showed more than a fourfold difference, as inferred from the measured values. The CDOM content in surface waters was, as a rule, higher than in the deeper horizons. A higher CDOM content was measured near the Onega River, near the middle part of the Onega shore, and near the Pomor shore opposite the town of Belomorsk. River runoff is the major source of CDOM in Onega Bay water. The CDOM chemical composition in Onega Bay waters was heterogeneous. The ratio of the fluorescence intensity to the absorbance value was higher near the mouths of rivers and in intensive mixing zones than in water characterized by high salinity. A highly significant linear correlation (R2 = 0.7825) between water salinity and CDOM fluorescence intensity was demonstrated. The contribution of fluorescent compounds to river runoff CDOM is substantially higher than the contribution to the composition marine CDOM.

Formation of Chromophoric Dissolved Organic Matter by Bacterial Degradation of Phytoplankton-Derived Aggregates

Organic matter produced and released by phytoplankton during growth is processed by heterotrophic bacterial communities that transform dissolved organic matter into biomass and recycle inorganic nutrients, fueling microbial food web interactions. Bacterial transformation of phytoplankton-derived organic matter also plays a poorly known role in the formation of chromophoric dissolved organic matter (CDOM) which is ubiquitous in the ocean. Despite the importance of organic matter cycling, growth of phytoplankton and activities of heterotrophic bacterial communities are rarely measured in concert. To investigate CDOM formation mediated by microbial processing of phytoplankton-derived aggregates, we conducted growth experiments with non-axenic monocultures of three diatoms (Skeletonema grethae, Leptocylindrus hargravesii, Coscinodiscus sp.) and one haptophyte (Phaeocystis globosa). Phytoplankton biomass, carbon concentrations, CDOM and base-extracted particulate organic matter (BEPOM) fluorescence, along with bacterial abundance and hydrolytic enzyme activities (α-glucosidase, β-glucosidase, leucine-aminopeptidase) were measured during exponential growth and stationary phase (~3-6 weeks) and following six weeks of POM degradation. Incubations were performed in rotating glass bottles to keep cells suspended, promoting cell coagulation and, thus, formation of macroscopic aggregates (marine snow), more similar to surface ocean processes. Maximum carbon concentrations, enzyme activities, and BEPOM fluorescence occurred during stationary phase. Net DOC concentrations (0.19-0.46 mg C L-1) increased on the same order as open ocean concentrations. CDOM fluorescence was dominated by protein-like signals that increased throughout growth and degradation becoming increasingly humic-like, implying the production of more complex molecules from planktonic-precursors mediated by microbial processing. Our experimental results suggest that at least a portion of open-ocean CDOM is produced by autochthonous processes and aggregation likely facilitates microbial reprocessing of organic matter into refractory DOM.

Impacts of cyanobacterial blooms outbreak and decline on chromophoric dissolved organic matter in Lake Taihu

利用平行因子分析（PARAFAC）技术对藻华暴发季节太湖梅梁湾和开敞区水样中有色可溶性有机物（CDOM）的三维荧光光谱进行分析，获得代表类酪氨酸、类色氨酸和类腐殖酸等3种荧光组分，这3个组分的荧光得分值均与叶绿素a浓度呈极显著正相关，其中类腐殖酸物质荧光得分值占总分值的比例也与叶绿素a浓度极显著正相关，由此可以推测，蓝藻水华可能是太湖CDOM的一个重要来源，并极大地影响了湖泊CDOM的组成结构.为进一步探索蓝藻水华对湖泊CDOM的影响，于太湖梅梁湾湖岸进行藻华暴发和衰亡模拟试验.PARAFAC结果显示，模拟试验水样CDOM的三维荧光图谱同样含有3个组分，其中类酪氨酸和类色氨酸荧光峰位置与湖泊水样基本相同，而类腐殖酸物质荧光峰则有一定程度的蓝移，表明培养试验中有新的内源类腐殖酸物质产生.利用PARAFAC模型中的得分值计算后发现，初始叶绿素a浓度越高，类腐殖酸物质所占比例越大，而实验结束时添加了高浓度和中等浓度藻的2个处理中类腐殖酸物质对荧光的贡献率也有显著提高.因此，藻华的长期暴发可以显著改变湖泊CDOM的组成，导致类腐殖酸物质比例提高.;Parallel Factor Analysis (PARAFAC) was applied to assess the composition of chromophoric dissolved organic matter (CDOM) from the excitation-emission matrix (EEM) fluorescence spectra of the water samples collected from Meiliang Bay and the open area in Lake Taihu during a cyanobacterial bloom event, and three fluorescent components were identified:a tyrosine-like component, a tryptophan-like component and a humic-like component. Correlation analysis showed that the concentrations of chlorophyll-a(Chl.a) were significantly correlated with the fluorescence intensities of the three components, respectively, as well as the proportion of humic-like to total fluorescence intensities, suggesting that cyanobacteria might be an important source of CDOM and can greatly change the composition of CDOM in Lake Taihu. For further investigation, a simulation experiment was performed on the shore of Meiliang Bay. PARAFAC model showed that the number of components and the spectral characteristics of CDOM EEM data from the simulation experiment are similar in terms of the number of fluorescence peaks and their position to previously identified components in the water samples from Lake Taihu, except that the fluorescence peaks of humic acids showed a certain degree of blue shift, which suggested that endogenous humic substances were produced during the simulation experiment. It was found that the higher initial concentrations of Chl.a, the greater contribution of humic acids to total CDOM fluorescence intensity derived by the PARAFAC model. Moreover, at the end of the experiment, the ratios of humic substances to total fluorescent substances increased significantly in the two treatments with relatively high concentrations of cyanobacteria. Thus, the long-term outbreak of cyanobacterial blooms can significantly alter the composition of the CDOM in lakes, resulting in an increase of the proportion of humic-like substances.

Using submersible fluorescence sensors to track the removal of organic matter in decentralized wastewater treatment systems (DEWATS) in real time.

Decentralized wastewater treatment systems (DEWATS) using anaerobic treatment are increasingly being considered for wastewater treatment with options for non-potable water reuse at the community scale. One challenge for ensuring performance and reliability of DEWATS is the lack of suitable on-site sensors to monitor failure or contamination events. In this study, the aim was to use in situ fluorescence sensors to track the performance of a DEWATS, consisting of an anaerobic baffled reactor (ABR) coupled to anaerobic filter (AF) and constructed wetland (CW) treatment processes. A submersible in situ fluorometer equipped with tryptophan (TRP) and chromophoric dissolved organic matter (CDOM) sensors was deployed in each chamber of the ABR-AF-CW system, and results showed that TRP fluorescence was preferentially removed over CDOM fluorescence throughout the system. Significant relationships between TRP fluorescence and chemical oxygen demand (COD) also suggested that TRP fluorescence could be used as a surrogate for COD and soluble COD concentrations. Strong agreement between results obtained from the 1D in situ fluorometer and those obtained from a 3D benchtop fluorometer lends further support to the use of in situ fluorescence sensors to track DEWATS performance.

The contribution of component variation and phytoplankton growth to the distribution variation of chromophoric dissolved organic matter content in a mid-latitude subtropical drinking water source reservoir for two different seasons.

The distribution variation in chromophoric dissolved organic matter (CDOM) content in mid-latitude subtropical drinking water source reservoirs (MDWSRs) has great significance in the security of aquatic environments and human health. CDOM distribution is heavily influenced by biogeochemical processes and anthropogenic activity. However, little is known regarding the impact of component variation and phytoplankton growth on CDOM distribution variation in MDWSR. Therefore, samples were collected from a representative MDWSR (the Shanzai Reservoir) for analysis. CDOM absorption and fluorescence coupling with parallel factor analysis were measured and calculated. The results indicated that only two CDOM components were found in the surface water of Shanzai Reservoir, fulvic acid, and high-excitation tryptophan, originating from terrestrial and autochthonous sources, respectively. The types of components did not change with the season. The average molecular weight of CDOM increased in proportion to its fulvic acid content. The distribution variation in CDOM content mainly resulted from the variation in two CDOM components in summer and from high-excitation tryptophan in winter. Phytoplankton growth strongly influenced the distribution variation of CDOM content in summer; the metabolic processes of Cyanobacteria and Bacillariophyta consumed fulvic acid, while that of Cryptophyta produced high-excitation tryptophan.

Effects of titanium dioxide (TiO2) nanoparticles on the photodissolution of particulate organic matter: Insights from fluorescence spectroscopy and environmental implications

Widely used titanium dioxide (TiO2) nanoparticles are likely to accumulate ultimately in sediments and potentially pose a risk to water ecosystems. This study evaluated the effect of TiO2 nanoparticles on the photodissolution of particulate organic matter (POM) through fluorescence spectroscopy. Excitation-emission matrices and parallel factor analyses revealed that the fluorescent characteristics of produced dissolved organic matter (DOM) during photodissolution of suspended sediment and synthetic particulate organic matter (SPOM) were primarily humic-like. SPOM particles appeared to simulate well the photodissolution of suspended sediment. Quasi-complete increases in fluorescence intensity and chromophoric DOM (CDOM) abundance were reached after 90, 60, and 50 min irradiation for TiO2 concentrations of 0, 2, and 5 mg L−1, respectively. The faster increment of fluorescence intensity and CDOM abundance indicated the photocatalytic dissolution of SPOM, as opposite charges between TiO2 and SPOM at pH = 4 favored the adsorption of TiO2 onto SPOM. For sediments, the CDOM abundance and fluorescence intensity decreased with increasing TiO2 concentration, resulting from the photocatalytic degradation of photoproduced DOM from sediments. These results demonstrated that pH plays an important role in the photocatalytic dissolution of POM by TiO2. Therefore, appropriate pH controls should be implemented when TiO2 are used to treat sediments contaminated with organic pollutants. Finally, with increasing use of TiO2, its accumulation in sediments may affect the fate of carbon, nutrients, and heavy metals in shallow-water ecosystems.

Effect of a dam on the optical properties of different-sized fractions of dissolved organic matter in a mid-subtropical drinking water source reservoir

The presence of a dam on a river is believed to have a key role in affecting changes in the components of the chromophoric dissolved organic matter (CDOM) in reservoirs. However, questions remain about the mechanisms that control these changes. In this study, we used tangential ultrafiltration, fluorescence spectrum and phytoplankton cell density detection to explore the impacts of a dam on the CDOM components in the Shanzai Reservoir, a source of drinking water. The results demonstrated each CDOM size fraction comprised two main components, namely C1 (protein-like substance) and C2 (humic-like substance). The C1 content had a higher value in areas with slow flow than in the normal river channel, while the C2 contents were generally stable in the flow direction. The topography of the reservoir site affected the structure of the CDOM components based on changes in the hydraulic conditions caused by the dam. The variations in the CDOM components, hydraulic parameters and fluorescence indices in the river flow direction indicated that the contribution of the phytoplankton to the CDOM content increased as the distance to the dam decreased, phytoplankton metabolism enhanced C1 content of the 1–10kDa molecular weights range fraction. Further, the contributions of different phytoplankton biomass to C1 proved that the dam changed the hydraulic conditions, had secondary effects on the metabolism of the phytoplankton, and resulted in changes in the structure of the CDOM components.