Spectral Slope Ratio Research Articles

Variation in the content and fluorescent composition of dissolved organic matter in soil water during rainfall-induced wetting and extract of dried soil

Dissolved organic matter (DOM) is present in all soils, providing a readily available carbon source for microorganisms, which influences microbially mediated biogeochemical processes. Rainfall-induced wetting can alter the content and composition of soil DOM. However, conventional methods commonly used to extract DOM from soils involve air- or oven-drying followed by extraction with water, and the results vary considerably in terms of indexes used. Therefore, this study aimed to determine the variation in DOM content and composition of soil gravity water and capillary water during wetting, and establish a better method to obtain real soil DOM information. Following simulated rainfall (50 mm h−1, 0–24 h), gravity water and capillary water in fresh soil samples were separated using a high-speed refrigerated centrifuge. Additionally, DOM in dried soil samples was extracted using various soil/water ratios after drying by different methods. The DOM data obtained by conventional methods were compared with capillary water data. The results showed that DOM degradation occurred mainly in capillary water close to the surface of soil particles. Among the six fluorescent components of DOM identified, a tryptophan-like component (Ex/Em = 295/335 nm and 230/335 nm) was possibly derived from terrestrial plants, and a tyrosine-like component (Ex/Em = 265/305 nm) was likely derived from microbial secretion. Except for little variation in the fluorescence index, dissolved organic carbon concentrations in capillary water were double those in dried soil extracted by conventional methods. The humification index and spectral slope ratio of DOM extracted by conventional methods also markedly varied, and no clear patterns were observed for the variation in specific UV absorbance at 254 nm. These findings allow real information to be obtained regarding soil DOM during wetting, and better selection of the extraction method and indexes when studying soil DOM.

River network travel time is correlated with dissolved organic matter composition in rivers of the contiguous United States

AbstractMost terrestrial allochthonous organic matter enters river networks through headwater streams during high flow events. In headwaters, allochthonous inputs are substantial and variable, but become less important in streams and rivers with larger watersheds. As allochthonous dissolved organic matter (DOM) moves downstream, the proportion of less aromatic organic matter with autochthonous characteristics increases. How environmental factors converge to control this transformation of DOM at a continental scale is less certain. We hypothesized that the amount of time water has spent travelling through surface waters of inland systems (streams, rivers, lakes, and reservoirs) is correlated to DOM composition. To test this hypothesis, we used established river network scaling relationships to predict relative river network flow‐weighted travel time (FWTT) of water for 60 stream and river sites across the contiguous United States (3090 discrete samples over 10 water years). We estimated lentic contribution to travel times with upstream in‐network lake and reservoir volume. DOM composition was quantified using ultraviolet and visible absorption and fluorescence spectroscopy. A combination of FWTT and lake and reservoir volume was the best overall predictor of DOM composition among models that also incorporated discharge, specific discharge, watershed area, and upstream channel length. DOM spectral slope ratio (R2 = 0.77) and Freshness Index (R2 = 0.78) increased and specific ultraviolet absorbance at 254 nm (R2 = 0.68) and Humification Index (R2 = 0.44) decreased across sites as a function of FWTT and upstream lake volume. This indicates autochthonous‐like DOM becomes continually more dominant in waters with greater FWTT. We assert that river FWTT can be used as a metric of the continuum of DOM composition from headwaters to rivers. The nature of the changes to DOM composition detected suggest this continuum is driven by a combination of photo‐oxidation, biological processes, hydrologically varying terrestrial subsidies, and aged groundwater inputs.

Photochemical and Microbial Degradation of Chromophoric Dissolved Organic Matter Exported From Tidal Marshes

AbstractWetlands export chromophoric dissolved organic matter (CDOM) to estuaries, where CDOM is removed and transformed through biotic and abiotic process, subsequently impacting nutrient cycling, light availability, ecosystem metabolism, and phytoplankton activity. We examined the bioavailability and photoreactivity of CDOM exported from four Chesapeake Bay tidal marshes across three seasons and along an estuarine salinity gradient using three incubation treatments: 14‐day microbial (MD), 7‐day combined photochemical/microbial (PB + MD), and 7‐day microbial incubation after photobleaching (MD after PB). CDOM absorption at 300 nm (aCDOM300) and dissolved organic carbon (DOC) concentrations showed strong seasonality, with minima in winter, but CDOM quality (absorption spectral slopes, fluorescence component ratios) was less variable seasonally. PB + MD over 7 days decreased aCDOM300 (−56.0%), humic‐like fluorescence (−67.6%), and DOC (−17.8%), but increased the spectral slope ratio SR (=S275–295/S300–350) (+94.8%), suggesting a decrease in CDOM molecular weight. Photochemistry dominated the PB + MD treatment. Photoreactivity was greater during the winter and in marsh/watershed versus down‐estuary sites, likely due to less previous light exposure. Prior photobleaching increased the bioavailability of marsh‐exported CDOM, resulting in a greater loss of aCDOM300 and DOC, and a greater increase in humic‐like fluorescence (−6.0%, −5.9%, and +18.4% change, respectively, over 7‐day MD after PB incubations, vs. −2.8%, −5.5%, and +2.6% change, respectively, over 14‐day MD incubations). CDOM exported from a marsh downstream of a major wastewater treatment plant showed the greatest photoreactivity and bioavailability. This highlights the significance of human activity on estuarine CDOM quality and biogeochemical cycles.

Characterizing the origins of dissolved organic carbon in coastal seawater using stable carbon isotope and light absorption characteristics

Abstract. In order to determine the origins of dissolved organic matter (DOM) occurring in the seawater of Sihwa Lake, we measured the stable carbon isotope ratios of dissolved organic carbon (DOC-δ13C) and the optical properties (absorbance and fluorescence) of DOM in two different seasons (March 2017 and September 2018). Sihwa Lake is enclosed by a dike along the western coast of South Korea, and the water is exchanged with the Yellow Sea twice a day through the sluice gates. The DOC concentrations were generally higher in lower-salinity waters in both periods, and excess of DOC was also observed in 2017 in high-salinity waters. Here, the excess DOC represents any DOC concentrations higher than those in the incoming open-ocean seawater. The excess DOC occurring in the lower-salinity waters originated mainly from marine sediments of tidal flats, based on the DOC-δ13C values (-20.7±1.2 ‰) and good correlations among the DOC, humic-like fluorescent DOM (FDOMH), and NH4+ concentrations. However, the origins of the excess DOC observed in 2017 appear to be from two different sources: one mainly from marine sources such as biological production based on the DOC-δ13C values (−19.1 ‰ to −20.5 ‰) and the other mainly from terrestrial sources by land–seawater interactions based on its depleted DOC-δ13C values (−21.5 ‰ to −27.8 ‰). This terrestrial DOM source observed in 2017 was likely associated with DOM on the reclaimed land, which experienced extended exposure to light and bacterial degradation as indicated by the higher spectral slope ratio (SR) of light absorbance and no concurrent increases in the FDOMH and NH4+ concentrations. Our study demonstrates that the combination of these biogeochemical tools can be a powerful tracer of DOM sources and characteristics in coastal environments.

Consequences of Increased Variation in Peatland Hydrology for Carbon Storage: Legacy Effects of Drought and Flood in a Boreal Fen Ecosystem

Globally important carbon (C) stores in boreal peatlands are vulnerable to altered hydrology through changes in precipitation and runoff patterns, groundwater inputs, and a changing cryosphere. These changes can affect the extent of boreal wetlands and their ability to sequester and transform C and other nutrients. Variation in precipitation patterns has also been increasing, with greater occurrences of both flooding and drought periods. Recent work has pointed to the increasing role of algal production in regulating C cycling during flooded periods in fen peatlands, but exactly how this affects the C sink-strength of these ecosystems is poorly understood. We evaluated temporal trends in algal biomass, ecosystem C uptake and respiration (using static and floating chamber techniques), and spectroscopic indicators of DOM quality (absorbance and fluorescence) in a boreal rich-fen peatland in which water table position had been experimentally manipulated for 13 years. Superimposed on the water table treatments were natural variations in hydrology, including periods of flooding, which allowed us to examine the legacy effects of flooding on C cycling dynamics. We had a particular focus on understanding the role of algae in regulating C cycling, as the relative contribution of algal production was observed to significantly increase with flooding. Ecosystem measures of gross primary production (GPP) increased with algal biomass, with higher algal biomass and GPP measured in the lowered water table treatment two years after persistent flooding. Prior to flooding the lowered treatment was the weakest C sink (as CO2), but this treatment became the strongest sink after flooding. The lower degree of humification (lower humification index, HIX) and yet lower bioavailability (higher spectral slope ratio, Sr) of DOM observed in the raised treatment prior to flooding persisted after two years of flooding. An index of free or bound proteins (tyrosine index, TI) increased with algal biomass across all plots during flooding, and was lowest in the raised treatment. As such, antecedent drainage conditions determined the sink-strength of this rich fen—which was also reflected in DOM characteristics. These findings indicate that monitoring flooding history and its effects on algal production could become important to estimates of C balance in northern wetlands.

Seasonal Variation of DOM Spectral Characteristics of Rivers with Different Urbanization Levels in the Three Gorges Reservoir Area

The urbanization process of the Three Gorges Reservoir area (TGRA) shows important impacts on the formation, migration, and transportation of dissolved organic matter (DOM) in rivers. In this study, a high-urbanized river (Taohuaxi) and low-urbanized river (Pulihe) in the TGRA were selected and water samples were collected in spring (April) and summer (August) to analyze the seasonal and spatial characteristics of DOM in the rivers using UV-Vis absorption spectroscopy and three-dimensional fluorescence spectroscopy. The DOC, S275-295 (the spectral slope of the short wavelength range), SR (spectral slope ratio), BIX (recent autochthonous contribution), and Fn(280) (relative importance of protein-like substances) of the Taohuaxi and Pulihe Rivers were higher in summer (August) than in spring (April), and CDOM concentration, CDOM/DOC, SUVA254 (aromatic structure), SUVA260 (hydrophobic component), E3/E4 (proportion of fulvic acid), S350-400 (the spectral slope of the long wavelength range), and HIX (the degree of humification) were higher in spring than in summer. The overall averages of the DOC, CDOM concentration, SR, Fn (280), BIX, and FI were higher in the high-urbanized Taohuaxi River than the low-urbanized Pulihe River, whilst the CDOM/DOC, SUVA254, and SUVA260 were lower in the Taohuaxi River. With the increase of urbanization along the water flow pathways, SUVA254 and SUVA260 gradually decreased, while Fn (280) gradually increased. The humus source (FI) of the two rivers in spring and summer is mixed and includes terrigenous and biological sources (plankton and algae) but is mostly endogenous. Furthermore, DOM is characterized by a moderately recent autochthonous contribution. Our study implies a large impact of urbanization on the composition and sources of DOM.

The relative importance of photodegradation and biodegradation of terrestrially derived dissolved organic carbon across four lakes of differing trophic status

Abstract. Outgassing of carbon dioxide (CO2) from freshwater ecosystems comprises 12 %–25 % of the total carbon flux from soils and bedrock. This CO2 is largely derived from both biodegradation and photodegradation of terrestrial dissolved organic carbon (DOC) entering lakes from wetlands and soils in the watersheds of lakes. In spite of the significance of these two processes in regulating rates of CO2 outgassing, their relative importance remains poorly understood in lake ecosystems. In this study, we used groundwater from the watersheds of one subtropical and three temperate lakes of differing trophic status to simulate the effects of increases in terrestrial DOC from storm events. We assessed the relative importance of biodegradation and photodegradation in oxidizing DOC to CO2. We measured changes in DOC concentration, colored dissolved organic carbon (specific ultraviolet absorbance – SUVA320; spectral slope ratio – Sr), dissolved oxygen, and dissolved inorganic carbon (DIC) in short-term experiments from May–August 2016. In all lakes, photodegradation led to larger changes in DOC and DIC concentrations and optical characteristics than biodegradation. A descriptive discriminant analysis showed that, in brown-water lakes, photodegradation led to the largest declines in DOC concentration. In these brown-water systems, ∼ 30 % of the DOC was processed by sunlight, and a minimum of 1 % was photomineralized. In addition to documenting the importance of photodegradation in lakes, these results also highlight how lakes in the future may respond to changes in DOC inputs.

Spectral characteristics of dissolved organic matter in sediment pore water from Pearl River Estuary

As key parts of land-sea transition zones, estuary ecosystems play a very important role in the ocean carbon cycle processes. The sources, degradation, and preservation of dissolved organic matter (DOM) in estuaries have long been the subject of intense study. To examine the aforementioned issues, this study examined three-dimensional fluorescence spectroscopy and ultraviolet-visible absorption spectroscopy to determine the spatial distribution and sources of DOM in the pore water of three sedimentary cores from the Pearl River Estuary (S1, S2 and S3, with increasing salinity). Using the parallel factor analysis (PARAFAC) method to analyze the three-dimensional fluorescence spectrum data, five fluorescent components were obtained—three humic-like components (C1, C3, and C4), and two protein-like components (C2 and C5). C2 exhibited a significant positive correlation with the sediment microbial deoxyribose nucleic acid (DNA) concentration (R2=0.69, P < 0.01), indicating that the protein-like component C2 might be derived from the catabolism of in situ microbes. C5 displayed a relatively weak correlation with DNA concentration (R2=0.40, P < 0.05), presumably due to the incorporation of phenolic compounds, which have a fluorescence peak very similar to that of protein-like components. The source of humic-like fluorescent components is extremely complex. The content at station S1 was relatively high (1.45–8.83 R.U.), which implies that terrestrial inputs had a significant influence. The three humic-like components showed similar distributions at S2 and S3, and the fluorescence intensity was rather low; this result indicates that the DOM at these two stations was more likely affected by the metabolism of algae and microorganisms. The humification index (HIX) and the fluorescence intensity of protein-like components increased and decreased, respectively, with depth. There was a significant positive correlation between the relative content of protein-like components and the spectral slope ratio (SR), which indicates that DOM transitioned from low-molecular-weight protein-like components in the surface sediment to high-molecular-weight humic-like components in the subsurface. This study provides valuable information for understanding the pore water size/reactivity (PWSR) model of DOM and its biochemical processes occurring in estuary sediments.

Rainstorm events shift the molecular composition and export of dissolved organic matter in a large drinking water reservoir in China: High frequency buoys and field observations

Rainstorm events can flush large amounts of terrestrial organic-rich material into lakes that are used for drinking water. To date, few studies have been carried out to investigate how rainstorm events change the molecular composition, bio-lability, and flux of upstream-imported dissolved organic matter (DOM), which can impact the odor and taste of drinking water as well as the efficiency of wastewater treatment. We undertook high-frequency buoy monitoring and point sample collection (n = 495), during high, moderate, and low inflow discharge, in Lake Qiandao, a key drinking water source for about 10 million people. Data from two online fluorescent DOM sensors deployed and field samples collected at the river site, Jiekou, and the lake site, Xiaojinshan, showed that rainstorm events increased the specific UV absorbance (SUVA254), humification index (HIX), humic-like components (C1-C2), and FT-ICR MS derived condensed aromatic and polyphenolic compounds (p < 0.001) and decreased the spectral slope of DOM (S275–295), spectral slope ratio (SR), biological index (BIX), and highly bio-degradable peptide-like and aliphatic substances (p < 0.001). Our results suggest that rainstorm events enhanced the export to the lake of colored, hydrophobic, and aromatic DOM. Upstream-derived dissolved organic carbon (DOC) concentrations decreased (p < 0.001), while DOC bio-availability (BDOC) increased only slightly (p < 0.05) during rainstorm events. The loss rate of DOC in Lake Qiandao is 0.82 × 104 t C yr−1, of which 0.30 × 104 t C yr−1 is highly bio-labile, and higher occurrences of both ≥ 25 mm d − 1 and ≥ 50 mm d − 1 rainfall events are anticipated by linear fittings for this region in the future. The application of in situ fluorescence sensors provides an early warning of DOC surge incidents caused by rainstorm events and may be useful in advising drinking water treatment plant managers of changes in raw water DOM quality and treatability.

Assessment of dissolved organic carbon and iron effects on water color between a forest and pasture-dominated fine-scale catchment in a Central Appalachian region, West Virginia.

Dissolved organic carbon (DOC) and iron (Fe) have been observed to be the important contributors to surface water brownification. Additionally, the DOC quality influences water color by forming Fe-DOC complexes that provide additive effects and is influenced by dominant land use type within watersheds. However, the influence of quantity and quality of DOC on Fe and water color is poorly understood in headwater streams. The aim of this study was to investigate the effects of DOC and Fe on water color in forest (FC) and pasture (GFC) fine-scale watersheds to remove the confounding effects of climate and soil parent material. Significant differences of DOC, Fe, and water absorbance at 420 nm (a420) between FC and GFC were found (p < 0.05). A dominant contribution to water color was from DOC (95.5 - 63.7%) with a decreasing trend when Fe increased from 0.011 to 0.258 mg L-1. There were no significant interactions between FC and GFC and Fe on either a420/DOC (p = 0.06) or specific ultraviolet absorbance at 254 nm (SUVA254) (p = 0.30). Increasing values of a420/DOC and SUVA254 were significantly associated with increasing Fe concentration (p < 0.01). Significant interactions were found between FC and GFC and Fe on spectral slope ratio (S ratio) (p < 0.01). The response rate of S ratio with increasing Fe per unit was 0.235 for GFC while it was - 11.043 for FC. These differences indicate that land use may change the quality of DOC, influence Fe-DOC interactions, and thus affect water color. Linking the effects of soil Fe and DOC and headwater Fe and DOC may help identify optimal management practice to mitigate surface water brownification.

Distribution Characteristics and Influencing Factors of Chromophoric Dissolved Organic Matter in a Northern-Side River of the Qinling Mountains in Summer

UV-visible absorption spectroscopy, fluorescence spectroscopy, and parallel factor analysis were used to analyze the composition of chromophoric dissolved organic matter (CDOM) in the waters of the Wangchuan River in summer, and the source of this CDOM was explored. The redundant analysis method and Pearson correlation were used to analyze the correlation between optical parameters and water quality parameters. The results showed that the CDOM of the Wangchuan River is composed of the tryptophan-like component C1 (245, 300/335 nm), the short-wave humus component C2 (240, 320-340/405 nm), and the long-wave humus component C3 (270, 350-370/470 nm), in which components C1 and C2 have some homology (r=0.859, P<0.001). CDOM absorption coefficient α(355) indicates that the CDOM concentration in the water body of the Wangchuan River is at a low level, and the correlation between α(355) and DOC concentration is significant (r=0.850, P<0.001), which is conducive to the establishment of a DOC inversion model. Water fluorescence index FI (2.36±0.20), HIX (3.66±2.47), BIX (1.56±0.82), and freshness index (β:α) (1.33±0.62), and the spectral slope ratio SR (0.76±0.25) indicate that the CDOM of the Wangchuan River has strong self-generated characteristics, weak humification characteristics, and more new CDOM. Redundancy analysis showed that the humic components (C2, C3) are affected by algae metabolism and microbial action, while tryptophan-like components (C1) are related to land-based input, and negatively correlated with dissolved total nitrogen. The humic components C2 and C3 are positively correlated with total phosphorus, dissolved total phosphorus, and dissolved organic carbon. This paper clarifies the characteristics and influencing factors of CDOM in the Qinling valley, and provides a theoretical basis for water body management in the Qinling valley.

Spatiotemporal variability, size and photoreactivity of chromophoric dissolved organic matter in the Bohai Sea and the northern Yellow Sea

The distribution, composition, and photoreactivity of chromophoric and fluorescent dissolved organic matter (CDOM and FDOM) in the Bohai Sea (BS) and the northern Yellow Sea (NYS) were investigated during three cruises (summer 2017, winter 2017, and spring 2018) by combining size fractionation through a series of micro- and ultra-filtrations and irradiation experiments. A total of 484 excitation-emission matrix spectra were modeled by parallel factor analysis to yield four humic-like (C1–3 and C5) and three protein-like (C4 and C6–7) fluorescent components. CDOM in the BS and the NYS was primarily affected by terrestrial inputs. Microbial degradation processes and autochthonous production were considered to be important factors driving the dynamics of fluorescence components. A total of 73 triple CDOM absorption and fluorescence spectra (n = 219) from the study regions were measured on 0.2, 0.45, and 0.7 μm filters in winter; water samples were also collected from the Yellow River Estuary (YRE) to the NYS, which was further size fractionated through ultrafiltration membranes (10 kDa and 1 kDa) to investigate variability between spectra of different filter pore sizes. The significant mean difference of absorption coefficient and fluorescence spectra, and no consistent trend between spectral slope (S275–295, S350–400), slope ratio (SR) from these filtrates suggested that the choice of different filter pore sizes considerably influenced the CDOM optical assessment. Higher values of CDOM and FDOM in the particulate phases (0.2–0.7 μm) fraction were more distributed in the NYS where terrestrial inputs were less and DOM dynamics were more linked with phytoplankton production. As pore size decreased, the percentage of humic-like fluorescence increased suggesting that humic-like components of CDOM were more dominated by low-molecular-weight (LMW) materials compared with the protein-like components. Consistent with its higher terrestrial CDOM content, photodegradation resulted in losses of CDOM and FDOM and increasing of the spectral slope ratios (SR). These finding suggested that CDOM molecules were photochemically degraded to LMW substances. Our results indicated that the proportion of the LMW CDOM decreased from estuary to open sea, which likely resulted from high-molecular-weight (HMW) organic material produced during phytoplankton production in off-shore regions, and LMW substances produced during CDOM photochemical processes in coastal waters.

Use of Geospatial, Hydrologic, and Geochemical Modeling to Determine the Influence of Wetland-Derived Organic Matter in Coastal Waters in Response to Extreme Weather Events

Flooding from extreme weather events (EWE), such as hurricanes, exports large amounts of dissolved organic matter (DOM) to both estuaries and coastal waters globally. Hydrologic connectivity of wetlands to adjacent river channels during flood events is potentially a major control on the DOM exported to coastal waters after EWEs. In this study, a geographic information system based flood model was used to: 1) determine the volume of flooded wetlands in a river corridor following Hurricane Matthew in 2016; 2) compute the resulting volume fluxes of DOM to the Neuse River Estuary-Pamlico Sound (NRE-PS), in eastern North Carolina and 3) use the flood model to quantify the wetland contribution to DOM export. The flood model-derived contributions were validated with a Bayesian Monte Carlo mixing model combining measurements of DOM quality: specific UV Absorbance at 254 nm (SUVA254), spectral slope ratio (SR), and stable isotope ratios of dissolved organic carbon (13C-DOC). Results indicated that 1) hydrologic connectivity of the freshwater riparian wetlands caused the wetlands to become the primary source of organic matter (OM) that was exported into the NRE-PS after Matthew and 2) this source lingered in these coastal waters in the months after the storm. Thus, in consideration of the pulse-shunt concept, EWE such as Hurricane Matthew cause pulses of DOM from wetlands, which were the primary source of the OM shunted from the terrestrial environment to the estuary and sound. Wetlands constituted ca. 48% of the annual loading of DOC into the NRE and 16% of DOC loading into the PS. Results were consistent with prior studies in this system, and other coastal ecosystems, that attributed a high reactivity of DOM as the underlying reason for large CO2 releases following EWE. Adapting pulse-shunt concept to estuaries requires the addition of a “processing” step to account for the DOM to CO2 dynamics, thus a new pulse-shunt process is proposed to incorporate coastal waters. Our results suggest that with increasing frequency and intensity of EWE, strengthening of the lateral transfer of DOM from land to ocean will occur and has the potential to greatly impact coastal carbon cycling.

Formation of assimilable organic carbon (AOC) during drinking water disinfection: A microbiological prospect of disinfection byproducts

Disinfection processes might alter the chemical structure of biological recalcitrant natural organic matter (NOM) in source water to form assimilable organic carbon (AOC), which can be readily utilized by microbes for growth. However, AOC has not been classified as disinfection byproducts (DBPs) before and little is known about the chemical and structural nature of AOC. This study, for the first time, considers the disinfection-induced AOC as DBPs from a microbiological perspective. The AOC formation by three types of disinfection processes, i.e., chlorination, UVC irradiation (254 nm) and photocatalysis represented by TiO2-UVA in drinking water containing two reference NOM materials of Suwannee River and Nordic Reservoir (SRNOM and NRNOM, respectively) were comparatively benchmarked using Pseudomonas aeruginosa as inoculum. Results showed that chlorination caused a substantial increase in AOC content, whereas TiO2-UVA led to a moderate increase in AOC content and UVC rendered the AOC content unchanged, independent of the types of NOM. Molecular weight indicated by spectral slope ratio and fluorescence fingerprint were found to not provide critical information about the AOC formation potential. FTIR and FT-ICR-MS results indicated that the AOC formation by chlorination was attributed to the oxidation and chlorine substitution on aromatic molecules to form molecules with carboxylic- and alcohol- functionalities, as well as chlorinated aromatics. These molecules could be metabolized and assimilated by Pseudomonas species by a catechol pathway. The results obtained in this study can provide valuable insight regarding the selection of proper technologies for disinfection to prevent microbial growth/regrowth in the distributing system and is intended to encourage more thinking and research on AOC as a new prospect of DBPs during disinfection of drinking water.

Structural variation of coloured dissolved organic matter during summer and winter seasons in a tropical estuary: A case study.

The diurnal variations in water quality and optical properties of organic matter were studied in the Gautami-Godavari estuary during two contrasting seasons. Dissolved inorganic nitrogen (DIN) and silicate showed similar patterns with the tide during summer, whereas in winter contrasting trends were noticed. Three-folds higher N to P ratio was recorded in winter than in summer. The spectral slope ratio (SR) and specific ultra violet absorption coefficient (SUVA) peaked during summer (1.28 ± 0.09 and 3.95 ± 0.2) followed by winter (1.10 ± 0.18 and 1.91 ± 0.35). The parallel factor (PARAFAC) analysis of excitation emission matrix (EEM) fluorescence spectra was extracted three humic (C1, C2, C3) and one protein-like (tryptophan (C4)) fluorophore components. Humic like fluorophores inversely correlated with the tide in both the seasons, due to influence of seawater. In summer, the Chlorophyll a (Chl a) and dissolved organic carbon (DOC) showed positive correlations with humic like and C4 fluorophores, suggesting the insitu organic matter production.

Predictive role of spectral slope ratio towards 17α-ethynylestradiol photodegradation sensitized by humic acids

Humic acids (HAs) have been shown to dominate the photodegradation of steroid estrogens in natural waters. Nevertheless, how the photosensitizing ability of HAs relates to their structural and optical characteristics remains largely unknown. In this study, 17α-ethynylestradiol (EE2) was selected as a model compound to study to what extent easily-measurable characteristics of HAs might be used to predict their photosensitization potency. HAs were extracted from sediments of two different sources, and then subjected to structural and optical properties characterization using elemental analyzer, UV–vis spectroscopy and fluorescence spectroscopy. Photochemical experiments show that the HAs from the two sources can effectively meditate EE2 photodegradation. Although with drastically different structural and optical properties, the photosensitizing ability of these HAs towards EE2 can be well described by simple linear regressions using a spectroscopic index, the spectral slope ratio (SR). This optical indicator is correlated with various physicochemical properties of HAs, including the molecular weight, lignin content, charge-transfer interaction potential, photobleaching extent and sources. No universal prediction model could be established for predicting EE2 photodegradation kinetics on the basis of SR, but in specific waters SR could be a powerful indictor for predicting the EE2 photodegradation sensitized by HAs.

Optical Characterization of Chromophoric Dissolved Organic Matter (CDOM) and Fe(II) Concentrations in Soil Porewaters Along a Channel-Bank Transect in a Salt Marsh

Chromophoric dissolved organic matter (CDOM) optical properties were measured in surface and porewaters as a function of depth and distance from the channel in a transect up the bank in a southern California salt marsh. Higher absorbance coefficients and fluorescence intensities in porewaters at depth vs. surface waters and shallower porewaters suggest soil porewater is a reservoir of CDOM in the marsh. Higher values were observed at the marsh sites compared to the channel site, suggesting increased production and storage in the marsh sites, and reduced leaching into overlying surface waters, is occurring. Spectral slope ratios decreased with depth, consistent with more aromatic, higher molecular weight material in the deeper porewaters, possibly due to different bacterial processing in the anaerobic vs. aerobic zones. Fe(II) concentrations, indicative of anaerobic bacterial processing, increased significantly at depth to values > 1000 μM, consistent with active anaerobic microbial processing occurring at depth. The transitions to higher reduced iron concentrations correlated with increased absorbance and fluorescence, suggesting processing by anaerobic iron-reducing bacteria in these deeper zones may not mineralize as much carbon as in the shallower aerobic zones. Alternatively, this may be due to reduction of solid iron oxides coated with organic matter releasing both DOM and Fe(II). The ratio of humic-like fluorescence to the absorption coefficient decreased with increasing iron concentration, possibly due to optical interference by iron species. Taken together, the data indicate that marsh sites in the salt marsh act as a reservoir for higher molecular weight, more aromatic organic matter.

Shining light on the storm: in-stream optics reveal hysteresis of dissolved organic matter character

The quantity and character of dissolved organic matter (DOM) can change rapidly during storm events, affecting key biogeochemical processes, carbon bioavailability, metal pollutant transport, and disinfection byproduct formation during drinking water treatment. We used in situ ultraviolet–visible spectrophotometers to concurrently measure dissolved organic carbon (DOC) concentration and spectral slope ratio, a proxy for DOM molecular weight. Measurements were made at 15-minute intervals over three years in three streams draining primarily agricultural, urban, and forested watersheds. We describe storm event dynamics by calculating hysteresis indices for DOC concentration and spectral slope ratio for 220 storms and present a novel analytical framework that can be used to interpret these metrics together. DOC concentration and spectral slope ratio differed significantly among sites, and individual storm DOM dynamics were remarkably variable at each site and among the three sites. Distinct patterns emerged for storm DOM dynamics depending on land use/land cover (LULC) of each watershed. In agricultural and forested streams, DOC concentration increased after the time of peak discharge, and spectral slope ratio dynamics indicate that this delayed flux was of relatively higher molecular weight material compared to the beginning of each storm. In contrast, DOM character during storms at the urban stream generally shifted to lower molecular weight while DOC concentration increased on the falling limb, indicating either the introduction of lower molecular weight DOM, the exhaustion of a higher molecular weight DOM sources, or a combination of these factors. We show that the combination of high-frequency DOM character and quantity metrics have the potential to provide new insight into short-timescale DOM dynamics and can reveal previously unknown effects of LULC on the chemical nature, source, and timing of DOM export during storms.

Concentration and Characterization of Colored Dissolved Organic Matter in the Surface Microlayer and Subsurface Water of the Yellow Sea and the East China Sea

The distribution and chemical properties of colored dissolved organic matter (CDOM) in the Yellow Sea and the East China Sea during December 2011-January 2012 were investigated. The input of freshwater and biological activities had an evident influence on the CDOM levels (characterized by the light absorption coefficient at the wavelength of 355 nm a355) in the study area. The spatial distribution of CDOM levels displayed a gradually decreasing trend from the coastal waters (0.37 m−1) to the open sea (0.18 m−1). The spectral slope ratio (the slope ratio SR defined as S275−295:S350−400) during the cruise was correlated with salinity, and exhibited a large variation from inshore (average of 2.515) to offshore sites (average of 5.327) compared with the distribution of a355. The values of SR were related to CDOM molecular weight (MW). The a355, SR, and chlorophyll a in 37 samples collected from the surface microlayer were significantly correlated with those in the corresponding subsurface water samples, implying a strong exchange action between the microlayer and bulk water. The a355 and SR of CDOM exhibited significant microlayer enrichment, with mean enrichment factors (EFs) of 1.72 and 1.62, respectively.

Study on Different Fractions of Organic Molecules in the Baltic Sea Surface Microlayer by Spectrophoto- and Spectrofluorimetric Methods

The sea surface microlayer (SML), created by surface active organic molecules (called: surfactants), is a highly active interface between the sea and the atmosphere. In this study we used the dissolved organic matter absorption and fluorescence properties for description of changes in molecular size and weight distribution of organic molecules as well as in their composition in the SML compared to subsurface water layer (SS), of 1 m depth. Data were collected during three research cruises in the Baltic Sea. Spectral values of the CDOM absorption coefficient were higher in the SML about 29 % (in the UV light) to 17 % (in a blue spectral range), while spectral slope coefficients at different spectral ranges, S_Δλ, were lower in SML by 2.3 and 10.5% compared to SS in 275-295 nm and 350-400 nm, respectively. The fluorescence intensities of main Excitation Emission Matrix spectra peaks belonging to the main fluorescing components of marine organic matter, called: A, C, M, T, were higher in SML by 41, 43, 41 and 14% compared to SS. The ratio of fluorescence intensities, ((M+T))⁄((A+C) ) and humification index, HIX, values in SML were, respectively higher by 17.9% and lower by 10.7 % compared to SS. These two parameters where coupled with inverse non-linear relationship. This relationship indicated faster removal of humic components of high MW in the SML compared to SS. We have observed decreasing trend of spectral slope ratio, S_R, with increasing salinity. The S_R decreased 33.5 and 23.6 % in the SML and SS, respectively in the salinity gradient from 4.5 to 7.94 (of practical salinity ). The fluorescence intensity of all FDOM peaks decreased by in the same salinity gradient. The decrease rate was higher in SML for all fluorescing peaks by 34, 36 and 26 % for A, C and M, respectively than in the SS. Decrease rate indicated the susceptibility to photochemical degradation of respective peaks. This effect was strongest for C while T peak was almost unbleached. The fluorescence intensity decrease rate was smaller in SS what indicated shielding effect of the SML.