Colored dissolved organic matter dynamics and anthropogenic influences in a major transboundary river and its coastal wetland.

Effects of land use on the characteristics and composition of fluvial chromophoric dissolved organic matter (CDOM) in the Yiluo River watershed, China

A multiband Quasi-Analytical Algorithm (QAA) was proposed by Lee et al. in 2002. It can be used to quickly retrieve the total absorption coefficients, particle back scattering coefficients, and absorption coefficients of phytoplankton pigment and colored dissolved organic matter (CDOM) etc for optically deep water. Effects of inelastic scattering, e.g., Raman scattering and CDOM fluorescence, were not taken into account. Contribution of Raman scattering for clear ocean waters, and effect of CDOM fluorescence for high CDOM concentration waters, however, were not neglectable, so their effects were simulated using the Hydrolight numerical simulation technique. It is found that contributions of Raman scattering to remote sensing reflectance are mainly between 400 nm and 500 nm, and are ~9% to 25%; effects of CDOM fluorescence are mainly between 400 nm and 550 nm, and about 1% to 15%; however, effects of Raman scattering and CDOM fluorescence to the inversion QAA algorithm are not significant (less than 5%), and may not to be corrected during inversion

The role of estuaries in sourcing and transforming dissolved organic matter - the largest reservoir of organic carbon in the ocean - still presents many unknowns for coastal biogeochemical cycles, and is further complicated by increasing human pressures and a changing climate. Here, we examined the major drivers of colored dissolved organic matter (CDOM) dynamics in Long Island Sound (LIS), a heavily urbanized estuary of National Significance with a storied water quality past. A comprehensive new optical dataset, including measurements of CDOM absorption and fluorescence signatures, was integrated with biological and hydrological measurements to capture the spatiotemporal heterogeneities of LIS, including its urban-to-rural gradient, dynamic river mouths, and blue carbon ecosystems across seasons, following episodic storm events, and over five years. Results reveal longitudinal gradients in both DOM amount and quality. While carbon-rich and humic terrigenous DOM was dominant in the heavily riverine-influenced Central to Eastern LIS, an uncoupling between CDOM absorption (aCDOM) and dissolved organic carbon (DOC) concentration in Western LIS, and a stronger correlation with Chlorophyll-a, indicated increased autochthonous CDOM production. Closer to the New York City urban core, aCDOM was highly correlated to turbidity, consistent with increased wastewater influences. Fluorescence PARAFAC analysis provided strong evidence for seasonal processing of CDOM in LIS, related to increased summertime photochemical degradation of humic-like components and shoulder-season microbial processing. Riverine CDOM export was influenced by discharge amount, residence time, and coastal wetlands acting as additional sources of strongly humic and aromatic organic matter. These measurements allowed us to assess how hydrologic, biological, and anthropogenic processes impact DOM dynamics and, subsequently, biogeochemical variability and trophic status in this complex urbanized estuary, with implications for water quality management and policy. Results discussed here are applicable beyond LIS, as urbanized estuaries globally face similar hydrological and anthropogenic forcings.

Increasing levels of dissolved organic matter (DOM) in watercourses in the northern hemisphere are mainly due to reduced acid rain, climate change, and changes in agricultural practices. However, their impacts vary in time and space. To predict how DOM responds to changes in environmental pressures, we need to differentiate between allochthonous and autochthonous sources as well as identify anthropogenic DOM. In this study we distinguish between allochthonous, autochthonous, and anthropogenic sources of DOM in a diverse watercourse network by assessing effects of land cover on water quality and using DOM characterization tools. The main sources of DOM at the studied site are forests discharging allochthonous humic DOM, autochthonous fulvic DOM, and runoff from urban sites and fish farms with high levels of anthropogenic DOM rich in protein-like material. Specific UV absorbency (sUVa) distinguishes allochthonous DOM from autochthonous and anthropogenic DOM. Anthropogenic DOM differs from autochthonous fulvic DOM by containing elevated levels of protein-like material. DOM from fishponds is distinguished from autochthonous and sewage DOM by having high sUVa. DOM characteristics are thus valuable tools for deconvoluting the various sources of DOM, enabling water resource managers to identify anthropogenic sources of DOM and predict future trends in DOM.

Measurements by volunteer scientists using participatory science methods in combination with high resolution remote sensing can improve our ability to monitor water quality changes in highly vulnerable and economically valuable nearshore and estuarine habitats. In the Chesapeake Bay (USA), tidal tributaries are a focus of watershed and shoreline management efforts to improve water quality. The Chesapeake Water Watch program seeks to enhance the monitoring of tributaries by developing and testing methods for volunteer scientists to easily measure chlorophyll, turbidity, and colored dissolved organic matter (CDOM) to inform Bay stakeholders and improve algorithms for analogous remote sensing (RS) products. In the program, trained volunteers have measured surface turbidity using a smartphone app, HydroColor, calibrated with a photographer's gray card. In vivo chlorophyll and CDOM fluorescence were assessed in surface samples with hand-held fluorometers (Aquafluor) located at sample processing "hubs" where volunteers drop off samples for same day processing. In validation samples, HydroColor turbidity and Aquafluor in vivo chlorophyll and CDOM fluorescence were linear estimators of standard analytical measures of turbidity, chlorophyll and CDOM, respectively, with R2 values ranging from 0.65 to 0.85. Updates implemented in a new version (v2) of HydroColor improved the precision of estimates. These methods are being used for both repeat sampling at fixed sites of interest and ad-hoc "blitzes" to synoptically sample tributaries all around the Bay in coordination with satellite overpasses. All data is accessible on a public database (serc.fieldscope.org) and can be a resource to monitor long-term trends in the tidal tributaries as well as detect and diagnose causes of events of concern such as algal blooms and storm-induced reductions in water clarity.

A comprehensive spectral-biogeochemical database was developed for the Wabash River and the Tippecanoe River in Indiana, United States. This database includes spectral measurements of river water, coincident in situ measurements of water quality parameters (chlorophyll (chl), non-algal particles (NAP), and colored dissolved organic matter (CDOM)), nutrients (total nitrogen (TN), total phosphorus (TP), and dissolved organic carbon (DOC)), water-column inherent optical properties (IOPs), water depths, substrate types, and bottom reflectance spectra collected in summer 2014. With this dataset, the temporal variability of water quality observations was first analyzed and studied. Second, radiative transfer models were inverted to retrieve water quality parameters using a look-up table (LUT) based spectrum matching methodology. Results found that the temporal variability of water quality parameters and nutrients in the Wabash River was closely associated with hydrologic conditions. Meanwhile, there were no significant correlations found between these parameters and streamflow for the Tippecanoe River, due to the two upstream reservoirs, which increase the settling of sediment and uptake of nutrients. The poor relationship between CDOM and DOC indicates that most DOC in the rivers was from human sources such as wastewater. It was also found that the source of water (surface runoff or combined sewer overflow (CSO)), water temperature, and nutrients were important factors controlling instream concentrations of phytoplankton. The LUT retrieved NAP concentrations were in good agreement with field measurements with slope close to 1.0 and the average estimation error was 4.1% of independently obtained lab measurements. The error for chl estimation was larger (37.7%), which is attributed to the fact that the specific absorption spectrum of chl was not well represented in this study. The LUT retrievals for CDOM experienced large variability, probably due to the small data range collected in this study and the insensitivity of Rrs to CDOM change. It is concluded that the success of the LUT method requires accurate spectral measurements and enough a priori information of the environment to construct a representative database for water quality retrieval. Therefore, future work will focus on continuing data collection in other seasons of the year and better characterization of the study area.

化学需氧量（COD）、五日生化需氧量（BOD₅）及溶解性有机碳（DOC）是指示湖泊水质的重要指标，然而上述指标测定通常耗费大量时间、试剂及人力物力且排放大量有害废液.有色可溶性有机物（CDOM）是溶解性有机物（DOM）中可以强烈吸收光谱中的紫外光和可见光的部分，数据测定耗时短、方便快捷，且样品处理过程环境友好，能在很大程度上反映湖泊水质.本研究基于2016年2、5和8月在太湖均匀布设的32个采样点进行样品采集，运用光谱吸收与三维荧光-平行因子分析（EEMs-PARAFAC）探究太湖CDOM的光谱吸收和荧光组分，探讨CDOM光谱指标对湖泊BOD₅、COD及DOC浓度等湖泊环境质量指标的可替代性.结果表明：（1）运用EEMs-PARAFAC方法解析出3种荧光组分：类腐殖酸C1、类酪氨酸C2和类色氨酸C3.（2）COD和BOD₅和DOC在空间上呈现出相似的分布趋势，不同水期的最高值均出现在竺山湾和梅梁湾，由西北湖区至中部敞水区、东南湖湾递减.（3）在不同水期，COD、BOD₅、DOC浓度和C1组分均表现为丰水期极显著大于枯水期和平水期，a₂₅₄在丰、平、枯水期间无显著性差异，最大值出现在丰水期；C2与C3组分均在枯水期和平水期极显著大于丰水期.（4）在不同水文时期，COD、BOD₅和DOC浓度均与a₂₅₄、类腐殖酸C1呈显著正相关，丰水期太湖COD、BOD₅和DOC浓度与CDOM光谱指标的线性相关性要优于枯水期和平水期.（5）CDOM光谱指标在不同水文时期均能很好地替代COD、BOD₅和DOC等作为反映太湖水体中有机物污染程度及湖泊水质的指标.;Chemical oxygen demand (COD), biochemical oxygen demand (BOD₅), and dissolved organic carbon (DOC) are important indicators tracing lake water quality, but their measurement processes are time-consuming and require a large amount of reagent, manpower and material resources, and discharges harmful waste during measurements. Chromophoric dissolved organic matter (CDOM) is the fraction of dissolved organic matter (DOM) that can strongly absorb light at the ultraviolet region. The CDOM measurement is time-saving and convenient, and environmentally friendly, and the optical indices of CDOM can be used to trace lake water quality. We collected samples from 32 sampling sites in Lake Taihu for three occasions in February, May and August of 2016, using spectral absorption and the combination of excitation-emission matrices and parallel factor analysis (EEMs-PARAFAC) to explore the optical variability of CDOM in Lake Taihu and the applicability of using CDOM indices to trace the variability of BOD₅, COD, and DOC concentrations. Three fluorescent components were identified using EEMs-PARAFAC, including a humic-like C1, a tyrosine-like C2, and a tryptophan-like C3. COD, BOD₅, DOC and the optical indices of CDOM all showed similar spatial distribution trends with the highest values of these indices in different hydrological scenarios being found in Zhushan Bay and Meiliang Bay in the inflowing regions of the lake. The means of COD, BOD₅, DOC, and humic-like C1 in the rainy season were all significantly higher than those in the dry season and the wet-to-dry transition season. We found no significant difference between the mean of a₂₅₄ during the rainy, dry season, and the wet-to-dry transition season. The means of protein-like C2 and C3 were significantly higher in the dry season and wet-to-dry transition season than those in the rainy season. In different hydrological scenarios, COD, BOD₅, and DOC are all significantly positively correlated with a₂₅₄ and humic-like C1, and the linear correlation between COD, BOD₅, DOC and CDOM indices in Lake Taihu in the rainy season is better than that in the dry season and wet-to-dry transition season. CDOM optical indices including a₂₅₄, C1, C2, C3 are potential indices tracing the variability of COD, BOD₅ and DOC and are useful indicators tracing water quality in Lake Taihu and potentially a wider aquatic environment in different hydrological scenarios.

The development of techniques for real-time monitoring of water quality is of great importance for effectively managing inland water resources. In this study, we first analyzed the absorption and fluorescence properties in a large subtropical reservoir and then used a chromophoric dissolved organic matter (CDOM) fluorescence monitoring sensor to predict several water quality parameters including the total nitrogen (TN), total phosphorus (TP), chemical oxygen demand (COD), dissolved organic carbon (DOC), and CDOM fluorescence parallel factor analysis (PARAFAC) components in the reservoir. The CDOM absorption coefficient at 254 nm (a(254)), the humic-like component (C1), and the tryptophan-like component (C3) decreased significantly along a gradient from the northwest to the lake center, northeast, southwest, and southeast region in the reservoir. However, no significant spatial difference was found for the tyrosine-like component (C2), which contributed only four marked peaks. A highly significant linear correlation was found between the a(254) and CDOM concentration measured using the CDOM fluorescence sensor (r(2) = 0.865, n = 76, p < 0.001), indicating that CDOM concentrations could act as a proxy for the CDOM absorption coefficient measured in the laboratory. Significant correlations were also found between the CDOM concentration and TN, TP, COD, DOC, and the maximum fluorescence intensity of C1, suggesting that the real-time monitoring of CDOM concentrations could be used to predict these water quality parameters and trace the humic-like fluorescence substance in clear aquatic ecosystems with DOC <2 mg/L and total suspended matter (TSM) concentrations <15 mg/L. These results demonstrate that the CDOM fluorescence sensor is a useful tool for on-line water quality monitoring if the empirical relationship between the CDOM concentration measured using the CDOM fluorescence sensor and the water quality parameters is calibrated and validated.

We studied the temporal variation in ultraviolet (UV) radiation transparency in the water column of a low dissolved organic carbon (DOC) lake (range: 0.12–0.65 mg L−1) located above the treeline. After ice-break, the water column was spatially segregated into an upper zone (0–4 m depth) with relatively constant UV transparency and a lower zone (4–9 m depth) where transparency decreased over time, for example at 380 nm, by up to 70%. This temporal decrease in UV transparency was significantly correlated with the increase in phytoplankton chlorophyll a. For the entire study period, there was no clear relationship between changes in UV transparency and colored dissolved organic matter (CDOM) absorptivity. Furthermore, changes in DOC concentration were not reflected in CDOM absorptivity or fluorescence, indicating a discrepancy between estimates of concentration and optical properties of this material. Use of spectral slope to track signals of CDOM photobleaching resulted in no consistent pattern. An indication of the low proportion of UV-absorbing chromophores in the dissolved organic matter (DOM) pool was obtained by applying size-exclusion chromatography combined with simultaneous measurements of DOC and UV absorption. These results indicated that more than the 50% of the DOM did not have significant UV absorption. Electrospray mass spectrometry analysis of DOM also showed a low diversity of organic compounds which exhibited relatively low molecular weight (< 250 D). These results, in combination with measurements of DOC-specific absorptivity and fluorescence, indicated that DOM in this lake is mainly autochthonous. Overall, our results show that factors which have been established to explain temporal changes in UV transparency of high DOC lakes may have little application in this and similar alpine lakes. Finally, reconstruction of the UV-exposure history in alpine lakes may have to take into account the discrepancy between DOC and CDOM.

Characterization of dissolved organic matter fluorescence in the South Atlantic Bight with use of PARAFAC model: Interannual variability

New insight into and characterization of DOC, DON and CDOM for urban waters in the lower reaches of the Yellow River, China

Chromophoric dissolved organic matter and dissolved organic carbon in Chesapeake Bay

Seasonal dynamics of chromophoric dissolved organic matter in Poyang Lake, the largest freshwater lake in China

Abstract. We investigated how absorption of sunlight by chromophoric dissolved organic matter (CDOM) controls the degradation and export of DOM from Imnavait Creek, a beaded stream in the Alaskan Arctic. We measured concentrations of dissolved organic carbon (DOC), as well as concentrations and characteristics of CDOM and fluorescent dissolved organic matter (FDOM), during ice-free periods of 2011–2012 in the pools of Imnavait Creek and in soil waters draining to the creek. Spatial and temporal patterns in CDOM and FDOM in Imnavait Creek were analyzed in conjunction with measures of DOM degradation by sunlight and bacteria and assessments of hydrologic residence times and in situ UV exposure. CDOM was the dominant light attenuating constituent in the UV and visible portion of the solar spectrum, with high attenuation coefficients ranging from 86 ± 12 m−1 at 305 nm to 3 ± 1 m−1 in the photosynthetically active region (PAR). High rates of light absorption and thus light attenuation by CDOM contributed to thermal stratification in the majority of pools in Imnavait Creek under low-flow conditions. In turn, thermal stratification increased the residence time of water and DOM, and resulted in a separation of water masses distinguished by contrasting UV exposure (i.e., UV attenuation by CDOM with depth resulted in bottom waters receiving less UV than surface waters). When the pools in Imnavait Creek were stratified, DOM in the pool bottom water closely resembled soil water DOM in character, while the concentration and character of DOM in surface water was reproduced by experimental photo-degradation of bottom water. These results, in combination with water column rates of DOM degradation by sunlight and bacteria, suggest that photo-degradation is the dominant process controlling DOM fate and export in Imnavait Creek. A conceptual model is presented showing how CDOM amount and lability interact with incident UV light and water residence time to determine whether photo-degradation is "light-limited" or "substrate-limited". We suggest that degradation of DOM in CDOM-rich streams or ponds similar to Imnavait is typically light-limited under most flow conditions. Thus, export of DOM from this stream will be less under conditions that increase the light available for DOM photo-degradation (i.e., low flows, sunny days).

2004年3月对太湖梅梁湾有色可溶性有机物(CDOM))的吸收和荧光等光学行为进行研究,并由此探讨了CDOM的空间分布.结果表明,溶解性有机碳(DOC)的浓度在10.48-19.72 mg/L间变化,其均值为13.20±2.79 mg/L)CDOM在280 nm,355 nm和440 nm的吸收系数分别为18.73-31.91 m^-1(平均值23.19±4.36 m^-1)、4.63-7.14 m^-1(平均值5.76±0.91 m^-1)、1.45-2.99 m^-1(平均值1.92±0.40 m^-1))355 nm波长处CDOM的比吸收系数为0.34-0.57 L/(mg·m),平均值0.44±0.06 L/(mg·m))表征CDOM分子大小的比值a(250)/a(365)变化范围为5.05-7.55)355 nm的激发波长、450 nm的发射波长处的荧光值的变化范围0.79-3 04 nm^-1(平均值1.69±0.77 nm^-1).CDOM吸收系数、DOC浓度、荧光强度的分析显示CDOM浓度呈现从河口往湾内、湾口递减的趋势.CDOM吸收与DOC浓度的相关性随波长的降低而增加,在短波部分存在明显的正相关.355 nm处的荧光值、DOC浓度与CDOM吸收系分别存在如下显著性正相关关系:F_n(355)=0.692(±0.135)a(355)-2.297(±0.786),a(355)=0.233(±0.061)DOC+2.690(±0.816).280-500 nm、280-360 nm、360-440 nm指数函数斜率S值分别为13.86±0.91、18.54±1.11、12.93±0.92μm^-1,S值与比吸收系数之间存在显著的负线性相关关系,而与a(25)/a(365)值则存在显著的正线性关系.比吸收系数越大,a(250)/a(365)值和S值就越小,对应的CDOM分子量就越大,腐质酸的比例就越高.;Chromophoric dissolved organic matter (CDOM) is the light absorption fraction of dissolved organic carbon (DOC) that absorbs light in both the ultraviolet and visible ranges. The spatial and temporal distribution of CDOM in aquatic ecosystems can have important effects on ecosystem productivity, negatively impacting primary productivity due to CDOM absorbs light at both ultraviolet and visible wavelengths while positively impacting secondary productivity by fueling of microbial respiration by photo-degraded CDOM. In water color remote sensing, overlaps of pigment absorption spectra with CDOM absorption at blue wavelength generally complicate the use of chlorophyll a retrieval algorithms that are based on remotely sensed ocean color and lead to overestimated chlorophyll a concentration. In addition, high concentrations of CDOM can also act as a photoprotectant against UV damage for aquatic organisms. However, the protection against UV radiation provided by dissolved humic material for aquatic biota may be diminished if photodegradation of CDOM by UV radiation and acidification increase the UV transparency in lakes. CDOM absorption measurements and their relationship with DOC, and fluorescence are presented in Meiliang Bay of Lake Taihu based on a field investigation and lab analysis to show the spatial distribution of CDOM. Absorption spectral of CDOM was measured from 240 to 800 nm. Concentrations of DOC ranged from 10.48 to 19.72 mg/L with an average of 13.20 ±2.78 mg/L. CDOM absorption coefficients at 280 nm and 355 nm were in the range 18. 73-31. 91 m^-1 ( average 23.19±4.36mm^-1) and 4. 63-7. 14 mm^-1 (average 5. 76±0. 90 mm^-1),respectively. The values of the DOC-specific absorption coefficient at 355 nm ranged from 0. 34 to 0. 57 L/( mg · m). Fluorescence emission at 450 nm, excited at 355 nm, had a mean value of 1. 69 ±0. 77 nmm^-1. A significant lake zone difference is found in DOC concentration, CDOM absorption coefficient and fluorescence. This regional distribution pattern was in agreement with the location of sources of yellow substance : highest concentrations close to river mouth under the influence of river inflow, lower values in outlet of Meiliang bay. CDOM fluorescence and absorption coefficient were significantly and positively correlated. The results show a good correlation between CDOM absorption and DOC coefficients during 280-500 nm short wavelength intervals. The coefficient of variation between CDOM absorption and DOC concentration decreased with the increase of wavelength from 280 to 500 nm. The linear regression equations between fluorescence, DOC concentration and absorption coefficients at 355 nm are: F_n ( 355 ) = 0.692( ±0. 135)a(355)-2.291 (±0.786),a(355) =0.233(±0.061)DOC +2.690(±0.816),respectively. The exponential slope coefficient ranged from 11.0 to 14.9 um^-1 with a mean value 13. 86 ± 0. 91um^-1, 15. 8-20. 7 um^-1 with a mean value 18. 54 ± 1. 11 um^-1 and 9.9-13. 9 um^-1 with a mean value 12. 93 ± 0. 92um^-1 over the 280-500 nm, 280-360 nm and 360-440 nm intervals. A significant negative linear correlation was found between spectral slope coefficient and DOC specific absorption coefficient, but a significant positive linear correlation for spectral slope coefficient and the ratio of a( 250)/a( 365 ). Larger specific absorption coefficient corresponds to smaller a (250)/a (365) and spectral slope coefficient, which shows higher ratio humic in CDOM.