Source Of Chromophoric Dissolved Organic Matter Research Articles

Dynamics and source characterization of chromophoric dissolved organic matter (CDOM) in a tropical monsoon driven lagoon

While there is a significant body of research on the dynamics of chromophoric dissolved organic matter (CDOM) in coastal and offshore waters, our understanding of CDOM dynamics in tropical inland water bodies remains limited. To bridge this knowledge gap, a monthly in-situ investigation was carried out at 33 stations along a monsoon driven lagoon, Chilika, on the southeast coast of India for one year i.e., from July 2018 to June 2019. CDOM absorption at 440 nm [aCDOM(440)] data were analyzed as a proxy for CDOM concentration which varied between a range of 0.04–65.14 m−1 with an average value of 2.77 ± 5.23 m−1. A gradient in aCDOM(440) was observed from the river discharge dominated shallower northern sector (4.91 ± 8.32) m−1 to the more isolated and less fresh water influenced deeper southern sector (1.21 ± 1.55) m−1. Spectral slope (S), spectral slope ratio (SR), and molecular weight proxy (M) were computed to understand the possible source and fate of CDOM in the lagoon. The average spectral slope of S280–500 and S350–500 vary between 0.002 and 0.096 nm−1 and 0.001–0.095 nm−1, respectively. The (SR) and (M) values ranged between of 0.01–6.81 and 0.31–52.28. It was observed that large-sized, high molecular weight CDOM from terrestrial origin was prevalent during monsoon with lower (S280–500), (SR), and (M)values. In contrast, lower molecular weight CDOM fractions were prevalent during pre- and post-monsoon which were mainly of autochthonous origin with higher (S350–500), (SR), and (M) values. Our findings indicate significant spatio-temporal variations of CDOM in Chilika Lagoon, influenced by the monsoon-driven influx of freshwater and the mixing of fresh and marine water. In-situ changes in CDOM are likely shaped by microbial breakdown and photodegradation.

Increased dominance of terrestrial component in dissolved organic matter in Chinese lakes

The source composition of chromophoric dissolved organic matter (CDOM) in lakes is closely related to regional environmental changes, human activities, and the carbon cycle. The spectral slope ratio (SR) is an important parameter of CDOM optical components, and combined with remote sensing technology, the source composition of CDOM can be tracked comprehensively and efficiently in large regions. Here, we proposed a CDOM source tracking remote sensing model (CDOM-SR) based on the hue angle (α) to assess the spatial pattern and long-term trend of the CDOM source composition in Chinese lakes (surface area ≥ 1 km2) from 1986 to 2021. Validation results show that the CDOM-SR model has a good SR estimation performance with a median absolute percentage difference, root mean square deviation, median ratio, and median deviation of 17.91 %, 0.23, 1.02, and 0.03, respectively. We found that the average SR of Chinese lakes presents an obvious spatial pattern of high in the west and low in the east due to the difference in human activity intensity and the natural geographical environment. Additionally, we found that the average SR of Chinese lakes from 1986 to 2021 decreased at a rate of - 0.06/10 years, of which 64.37 % of lakes decreased significantly, 15.42 % of lakes had no significant change, and only 20.20 % of lakes increased. The widespread decrease in the average SR indicates that the increasing human activity discharge of terrestrial organic matter has had an important impact on the source composition of the CDOM in Chinese lakes. Our results provide a new resource for remote sensing monitoring of CDOM sources and important insights into lake carbon cycling under the influence of ongoing human activities.

Dynamics of chromophoric dissolved organic matter in a highly productive Amundsen Sea polynya

Dissolved organic carbon (DOC) is the largest organic carbon pool in the ocean, and is the most active component in respect to the ocean carbon cycling. However, its study in Antarctica has been limited due to challenges associated with sample collection. In this study, we conduct an investigation on the chromophoric dissolved organic matter (CDOM) in a highly productive Amundsen Sea Polynya (ASP), where phytoplankton blooms occur annually during the austral summer, serving as the primary source of DOC and CDOM. The relative abundances of CDOM, as indicated by the absorption coefficient at 254 nm (a254), exhibit significant variability, reaching up to 6.34 m−1. Four fluorescent components, including two humic-like components (C1 and C4) and two protein-like components (C2 and C3), are identified by excitation emission matrix coupled with parallel factor analysis (EEM-PARAFAC). Our findings suggest that heterotrophic metabolism primarily contributes to the formation of humic-like fluorescent components and DOC removal. Water mass, solar radiation, primary productivity as well as microbial degradation are identified as the main factors influencing CDOM dynamics in ASP. This study bears significant implications for advancing our understandings of the CDOM and DOC dynamics in the coastal polynyas of Antarctica, thus facilitating improved evaluation of carbon cycle in the Antarctica.

Regulated vs. unregulated rivers: Impacts on CDOM dynamics in the eastern James Bay

The eastern James Bay (EJB) coast harbors numerous rivers, but there is a dearth of knowledge concerning dissolved organic matter (DOM) in the downstream coastal water. Here we report a four-year (2018–2021) and multi-seasons field study on the mixing behavior and characteristics of the chromophoric DOM (CDOM) in the nearshore EJB. Freshwater discharged from the extensively regulated La Grande River (LGR) was constantly depleted in DOM compared with the unregulated rivers (URRs), being on average 3.35 times lower in CDOM absorption coefficient at 440 nm (aCDOM(440)) and 2.50 times lower in dissolved organic carbon (DOC). In contrast, the absorption spectral slope between 275 and 295 nm (S275–295, a proxy of molecular weight) and the specific absorption coefficient at 254 nm (a*CDOM(254), an indicator of aromaticity) of the LGR CDOM were only 10.6% lower and 11.7% higher than those of the URRs CDOM, respectively. Riverine input was found to be the dominant source of CDOM in the study area, with little influence from sea ice formation or melting. CDOM distribution fell into two distinct regimes: the LGR-influenced low-CDOM area in the north and the URRs-influenced high-CDOM area in the south. The two areas showed strong conservative but separate aCDOM(440)–salinity relationships converging at a common marine endmember (salinity ∼25) with little seasonality. The composite data combing both areas and all seasons and years exhibited non-linear relationships between S275–295, a*CDOM(254) and aCDOM(440) and a robust simple linear correlation of DOC to aCDOM(440). This study suggests a strong impact of river regulation on CDOM input into the EJB, reveals low seasonal variability of CDOM mixing dynamics and characteristics, and demonstrates the feasibility of using remote sensing from space for real-time and synoptical assessment of DOM dynamics and the associated biogeochemical cycles in the EJB.

Contrasting effects of different light regimes on the photoreactivities of allochthonous and autochthonous chromophoric dissolved organic matter

Chromophoric dissolved organic matter (CDOM) plays an important role in ultraviolet (UV) light absorption in the ocean. CDOM is known to originate from either an allochthonous or autochthonous source and has varying compositions and levels of reactivity; however, the effects of individual radiation treatments and the combined effects of UVA and UVB on allochthonous and autochthonous CDOM remain poorly understood. Thus, here, we measured changes in the common optical properties of CDOM collected from China's marginal seas and the Northwest Pacific, using full-spectrum, UVA (315–400 nm), and UVB (280–315 nm) irradiation to induce photodegradation over the same time period (60 h). Excitation–emission matrices (EEMs) combined with parallel factor analysis (PARAFAC) identified four components: marine humic-like C1, terrestrial humic-like C2, soil fulvic-like C3, and tryptophan-like C4. Although the behaviours of these components during full-spectrum irradiation exhibited similar decreasing tendencies, three components (C1, C3, and C4) underwent direct photodegradation under UVB exposure, whereas C2 was more susceptible to UVA degradation. The diverse photoreactivities of the source-dependent components to different light treatments led to differing photochemical behaviours of other optical indices [aCDOM(355), aCDOM(254), SR, HIX, and BIX]. The results indicate that irradiation preferentially reduced the high humification degree or humic substance content of allochthonous DOM, and promoted the transformation from the allochthonous humic DOM components to recently produced components. Although values for the samples from different sources overlapped frequently, principal component analysis (PCA) indicated that the overall optical signatures could be linked to the original CDOM source features. The degradation of CDOM humification, aromaticity, molecular weight, and autochthonous fractions under exposure can drive the CDOM biogeochemical cycle in marine environments. These findings can aid in a better understanding of the effects of different combinations of light treatments and CDOM characteristics on CDOM photochemical processes.

Examining superoxide dynamics in irradiated natural waters

AbstractSuperoxide () is a reactive oxygen species (ROS) that is primarily produced by the one‐electron transfer of photooxidized chromophoric dissolved organic matter (CDOM) to O2 in sunlit natural waters. Here we examine the environmental and chemical parameters (pH, ionic strength, buffer, and halides) that may influence photochemical production rates and decay pathways in natural water. Using the enzyme superoxide dismutase and H2O2 measurements, we present results from an irradiated freshwater CDOM source indicating that reductive decay pathways (P/PSOD) dominate with increased pH and NaCl additions and maximal photoproduction rates () increase with carbonate compared to borate buffer. Over 2 h of irradiation, a significant decline in was seen for all samples along with a minor increase in oxidative pathways. These results imply shifts in decay pathways and production rates that seem to vary across natural waters and as a function of irradiation history.

Seasonal Variations of Dissolved Organic Matter in Urban Rivers of Northern China

Dissolved organic matter (DOM) is ubiquitously present in aquatic environments, playing an important role in the global carbon cycle and water quality. It is necessary to reveal the potential sources and explore spatiotemporal variation of DOM in rivers, especially in urban zones impacted by human activities. It was designed to aim to explore spatiotemporal variations of DOM in urban rivers and ascertain the influencing factors. In this study, dissolved organic carbon (DOC) concentrations, UV-Vis absorption spectroscopy, and 3D fluorescence spectroscopy combined with parallel factor analysis were utilized to characterize DOM composition in urban rivers (the Jiyun, Chaobai, and Yongding rivers) in Tianjin city, northern China. The results showed that DOC (1.28 to 25.85 mg·L−1), generally, was at its highest level in spring, followed by summer, and lowest in autumn and winter, and that the absorption parameters E250:365 (condensation degree/molecular weight, 7.88), SUVA254 (aromaticity, 3.88 L mg C−1 m−1), a355 (content of chromophores, 4.34 m−1), a260 (hydrophobicity, 22.02 m−1), and SR (molecular weight, 1.08) of CDOM (chromophoric DOM) suggested that DOM is mainly composed of low-molecular-weight fulvic acid and protein-like moieties, and had the capability of participating in pollutant migrations and transformations. The results demonstrated significant seasonal differences. Generally, high DOC content was detected in rivers in urban suburbs, due to anthropogenic inputs. Three fluorescence components were identified, and the fluorescence intensity of the protein class reached the highest value, 294.47 QSU, in summer. Different types of land use have different effects on the compositions of riverine DOM; more protein-like DOM was found in sections of urban rivers. The correlation between DOC concentration and the CDOM absorption coefficient was found to be unstable due to deleterious input from industrial and agricultural wastewater and from domestic sewage from human activities. HIX and BIX elucidated that the source of CDOM in three river watersheds was influenced by both terrestrial and autochthonous sources, and the latter prevailed over the former. Geospatial data analysis indicated that CDOM in autumn was sourced from plant detritus degradation from forest land or from the urban green belt; construction land had a great influence on DOC and CDOM in riparian buffer areas. It was revealed that DOM in the watershed is highly impacted by nature and human activities through land use, soil erosion, and surface runoff/underground percolation transport; domestic sewage discharge constituted the primary source and was the greatest determiner among the impacts.

Chromophoric Dissolved Organic Matter and Dissolved Organic Carbon in Lakes Across an Elevational Gradient From the Mountains to the Sea

AbstractDissolved organic matter (DOM) in lakes across elevation gradients is a complex function of topography, climate, vegetation coverage, land use, and lake properties. To examine sources and processing of DOM from sea level to mountain lakes (3–1,574 m), we measured dissolved organic carbon (DOC) concentrations and chromophoric dissolved organic matter (CDOM) optical properties, lake characteristics, and water quality parameters in 62 freshwater lakes in the Pacific Northwest, USA. Higher elevation lakes had lower DOC concentrations and absorbance. These lakes had higher forest cover and minimal wetlands in their watershed, in addition to low nutrients, water temperatures, and chlorophyll a in the lake itself. Two humic‐like and one protein‐like fluorescent component were identified from excitation‐emission matrix spectroscopy. The index of recent autochthonous contribution (BIX), fluorescence index (FIX), and SR optical indices showed that most lakes were dominated by terrestrially derived material. The humification index (HIX) and specific ultra‐violet absorbance (SUVA254) were consistent with more aromatic humic CDOM at lower elevations. The lower fluorescence of humic‐like components at higher elevation was attributed to lower inputs from vegetation. The relative contribution of the protein‐like component increased at higher elevation. This may be due to reduced allochthonous terrestrial inputs relative to in situ production of autochthonous material or increased photochemical/biological degradation of allochthonous material. Differences in optical characteristics associated with the amount and source of CDOM were observed across the elevational gradient. These differences were driven by characteristics at both within‐lake and watershed scales.

Assessing the sources and dynamics of organic matter in a high human impact bay in the northern Beibu Gulf: Insights from stable isotopes and optical properties

Severe human activities in coastal areas have greatly impacted the sources and biogeochemical behaviors of organic matter (OM), including particulate OM (POM) and dissolved OM (DOM). However, few studies have incorporated the indices of POM and DOM to address this issue. Here, a dataset of the combination of stable isotopes of carbon and nitrogen in POM and the optical properties of DOM was presented in Xi Bay, a semi-enclosed bay with a highly developing industrial port in Beibu Gulf, South China, to reveal the origin, distribution, and fate of OM during the rainy season. In the upper bay, depleted δ13C suggested that particulate organic carbon (POC) mainly originated from terrestrial sources. However, the negative relationship between chromophoric DOM (CDOM) and particulate nitrogen (PN) suggested that bacterial-mediated decomposition of POM may be the primary source of CDOM. The negative correlation between humic-like fluorescent components (C1 and C2) and salinity suggested that those two components were mainly affected by terrestrial input. The significant correlation between the protein-like component (C3) and Chl a suggested that C3 was mainly derived from phytoplankton production in the upper bay. In the lower bay, the increase of δ13C values indicated an increased contribution of marine POC. The high levels of CDOM may be due to the decomposition of marine (fresh) POM. However, the low levels of C1 and C2 might be affected by dilution with seawater, and the increased levels of the protein-like C3 were due to enhanced primary production. In addition, the enhancement of δ15N values in both the upper and lower bays indicated serious nitrogen pollution in the bay. This study highlights that biological production fueled by excess nutrients is the dominant OM dynamic process in the bay with high human impact in Beibu Gulf.

Estimation of chromophoric dissolved organic matter and its controlling factors in Beaufort Sea using mixture density network and Sentinel-3 data

With the warming of the high-latitude regional climate, melting of permafrost, and acceleration of hydrological cycles, the Arctic Ocean (AO) has undergone a series of rapid changes in the past decades. As a dominant optical component of the AO, the variations in chromophoric dissolved organic matter (CDOM) concentration affect the physiological state marine organisms. In this study, machine learning retrieval model based on in situ data and mixture density network (MDN) was developed. Compared to other models, MDN model performed better on test data (R2 = 0.83, and root mean squared error = 0.22 m−1) and was applied to Sentinel-3 OLCI data. Afterward, the spatiotemporal distribution of CDOM during the ice-free (June–September) from 2016 to 2020 in the Beaufort Sea was obtained. CDOM concentration generally exhibited an upward trend. The maximum monthly average CDOM concentration appeared in June and gradually decreased thereafter, reaching its lowest value in September of each year. The maximum value appeared in June 2020 (0.91 m−1), and the minimum value was observed in September 2017 (0.81 m−1). The CDOM concentration nearshore was higher than that in other areas; and gradually decreased from offshore to the open sea. CDOM was highly correlated with salinity (R2 = 0.49) and discharge (R2 = 0.53), and the tight correlation between salinity and CDOM further suggested that terrestrial inputs were the main source of CDOM in the Beaufort Sea. However, sea level pressure contributed to the spatial variations in CDOM. When southerly wind prevailed and wind direction was aligned with the CDOM diffusion direction, the wind accelerated the diffusion of CDOM into the open sea. Meanwhile, seawater was diluted by the sea ice melting, resulting in decrease in CDOM concentration. Herein, this paper proposed a robust and near real-time method for CDOM monitoring and influence factor analysis, which would promote the understanding of AO CDOM budgets.

Dynamics of absorption properties of CDOM and its composition in Likas estuary, North Borneo, Malaysia

Chromophoric Dissolved Organic Matter (CDOM) is a vital water constituent in aquatic ecosystems that contributes to water colour, affects light penetration, and impacts primary production. This study aims to determine the spatial and monsoonal variability of CDOM absorption properties in the Likas estuary, characterise the source of CDOM, and investigate the correlations between CDOM absorption properties and salinity. Likas estuary is a small estuary located in Kota Kinabalu city on the west coast of Sabah, facing the South China Sea. A mangrove ecosystem surrounds it with manufactured structures such as residential areas and public facilities. Surface water samples were collected at 19 stations: upstream of rivers to the river mouth and coastal area during spring tides every month, from June 2018 to July 2019, for 14-months. The distribution of aCDOM(440) in the study area is predictable as a signature in a coastal area with a decreasing gradient from the upstream towards coastal water (0.29 ± 0.19 m−1 to 1.05 ± 0.39 m−1). There are increasing spatial patterns of spectral slopes S275-295 and SR. However, S350-400 and S300-600 declined spatial gradients from the upstream to coastal water. Thus, S300-600 indicates a linear relationship between aCDOM(440), which unconventional results in coastal water. We suspect this is due to a small coverage of the study site with a distance of 0.5 m intervals of each station. This could be why the S300-600 had constant values throughout the study area (with no statistical difference between stations). In addition, S300-600 was merely varied in the stations located at the river mouth and coastal water. Based on the spectral slope ratio (SR), most of the stations located in the Darau, Inanam, and Bangka-Bangka rivers had SR values less than 1. Hence, CDOM in these stations is a terrestrial-dominated source. Therefore, from our observations during the study period, monsoonal variation could alter the source of CDOM in the study area.

Chromophoric dissolved organic matter (CDOM) release by Dictyocha fibula in the central Bohai Sea

Phytoplankton blooms can be an important source of autochthonous chromophoric dissolved organic matter (CDOM) in surface oceans. Here we report the first detection and optical characterization of CDOM produced during a Dictyocha fibula bloom occurring in the western central Bohai Sea in mid-summer 2019. The mean CDOM absorption coefficient at 330 nm (aCDOM(330)) in the surface water of the bloom area (3.16 ± 0.48 m−1) was 2.5 times higher than that of the adjacent bloom-absent area (1.26 ± 0.34 m−1). The aCDOM(330) increased exponentially to a maximum with increasing chlorophyll a and cell abundance of D. fibula, indicating a steady state was reached between production and consumption of CDOM. The biological index (BIX) of the fluorescent DOM (FDOM) showed little variation between bloom and background areas. The combination of these observations points to an extensive production of autochthonous CDOM by D. fibula in the bloom area. Mass budgeting indicates that the CDOM source from D. fibula (aCDOM(330): 1.98 ± 0.48 m−1) accounted for 63% of the CDOM stock in the bloom area. The spectral slope coefficients over 275–295 nm (S275–295) in the bloom area (range: 0.0228–0.0241 nm−1; mean: 0.0235 nm−1) were substantially lower than those in the bloom-absent area (range: 0.0240–0.0311 nm−1; mean: 0.0271 nm−1), suggesting a high-molecular-weight nature of the D. fibula-derived CDOM. The freshly produced CDOM contained two humic-like FDOM components and one protein-like FDOM component, and the three FDOM components were depleted in fresh CDOM relative to their counterparts in CDOM within the background area. Owing to the episodic nature of algal bloom-driven CDOM production, future attention should be paid to accelerated biogeochemical cycles (e.g., spiked oceanic trace gas emission) associated with CDOM photochemistry during algal blooms which can be easily missed for pre-scheduled field surveys.

Source and Optical Dynamics of Chromophoric Dissolved Organic Matter in the Watershed of Lake Qinghai

Lake Qinghai is the largest lake in China and is of great significance to maintain the ecological security of the Qinghai-Tibet Plateau. Few studies have been carried out to investigate the optical composition and source of chromophoric dissolved organic matter (CDOM) in large lakes on the Qinghai-Tibet Plateau. It is of great significance to study the source and optical dynamics of CDOM in Lake Qinghai watershed for water quality protection and filling in the gaps in the knowledge of CDOM variability in a remote area. Two sampling campaigns in the Lake Qinghai watershed were carried out, and excitation-emission matrices coupled with parallel factor analysis (EEMs-PARAFAC) were used to unravel the optical composition and the sources of CDOM. Our results indicated that the mean dissolved organic carbon (DOC) concentration, a250:a365, and the spectral slope of CDOM absorption S275-295 in the lake were significantly higher than that in the inflow river (P<0.0001, t-test), whereas the mean absorption coefficient of CDOM a350, humification index (HIX), fluorescence peak integration ratio IC:IT, and specific ultraviolet absorbance at 254 nm SUVA254 of CDOM were shown to be lower in the lake than in the inflow river (P<0.0001), indicating that compared with the lake itself, CDOM in the inflow was humic-rich and highly aromatic. Four fluorescent components were obtained using PARAFAC, including a terrestrial human-like component C1, a microbial human-like component C2, a tyrosine-like C3, and a tryptophan-like C4. The mean DOC concentration, S275-295, and a250:a365in the headwater streams of the Lake Qinghai watershed were lower than those in the downstream estuary, indicating that the CDOM abundance increased, and the molecular weight decreased, from the headwaters to the downstream river mouths. The mean of SUVA254, C1, and the first axis of principal component analysis were positively related to terrestrial input (i.e., the PC1 values were significantly higher in rivers than in lakes (P<0.001)), indicating that the aromaticity of CDOM in rivers was higher than that in lakes. Particularly, the contribution of terrestrial humic-like C1 was higher in the Quanji River, Shaliu River, and Khargai River compared with that in other tributaries due to an intensified cultivated land use at the downstream estuary of these rivers.

Photochemical and microbial alteration of chromophoric dissolved organic matter (CDOM) in aquatic ecosystems associated with different sources

城市非点源污染向水生生态系统中输入大量的溶解有机物(DOM)，对生态系统健康产生重要影响. 有色可溶性有机物(CDOM)是广泛分布于自然水体中的一类成分和结构复杂、含有多种高活性化学官能团的大分子聚合物，是DOM的重要组分，对水生生态系统健康、能量流动及生物地球化学循环有重要影响. 光化学反应和微生物代谢过程被认为是控制水体CDOM转化、降解和循环的主要影响因素. 然而，对城市化如何影响CDOM组成以及光化学和微生物如何相互作用影响城市水体CDOM动态的理解是不足的. 因此，为评估光化学过程和微生物代谢对不同城市水体CDOM降解与转化的贡献，解析不同城市水体CDOM光化学/微生物降解作用机理，本研究在英国伯明翰选择3类具有典型DOM来源的水体样本，通过实验室9 d受控培养实验，对比分析光化学以及微生物影响下CDOM来源和组成的变化. 结果表明：(1)城市河流由于接受上游污水排放及较短的水力滞留时间，含有丰富的芳香性碳，其CDOM光化学活性明显高于湖泊，光化学降解率为16.60%;(2)城市湖泊CDOM受人类活动影响，自生源类荧光成分富集，生物活性高，在微生物培养过程中CDOM增加了62.16%，而相较于城市湖泊，非城市湖泊由于接受周围景观土壤输入的大量腐殖质类CDOM，光照对其降解转化作用较为明显; (3)光化学过程促进了陆源CDOM中大分子类腐殖质物质降解成为生物活性高的小分子化合物，刺激微生物代谢生成类蛋白质类有机物; 以类蛋白质组分为主导的CDOM在光照过程中被转化为难降解状态，生物活性降低，CDOM微生物代谢过程被抑制. 研究成果为城市水体不同CDOM来源及活性差异特征研究提供了新的思路，有助于城市河流的可持续开放与管理.;Urban non-point source pollution contributes large amounts of dissolved organic matter (DOM) to aquatic ecosystems and has a significant impact on ecosystem health. Chromophoric dissolved organic matter (CDOM) is a group of macromolecular polymers with complex composition and structure, containing a variety of highly reactive chemical functional groups, which are widely distributed in natural waters. It has an important impact on aquatic ecosystem health, energy flow and biogeochemical cycling. Photochemical reactions and microbial metabolic processes are the main controlling factors on the transformation, degradation and cycling of CDOM in water bodies. However, understanding how urbanization source input impacts CDOM quantity and quality and how photochemical and microbial interact to affect CDOM dynamic are deficient. To assess the contribution of photochemical processes and microbial metabolism to the degradation and transformation of different sources of CDOM in urban waters, and to elucidate the mechanisms of photochemical/microbial degradation of different sources of CDOM in aquatic systems (non-urban lake with humic, mostly terrestrial CDOM; urban lake with autochthonous biological production CDOM; urban river with mixed humic and protein-like CDOM), this study compares and analyses the changes in the sources and composition of CDOM under the influence of photochemistry and microorganisms in a controlled 9-day laboratory incubation experiment in three water bodies with typical urban characteristics, selected in Birmingham, UK. Changes in CDOM concentration, optical properties and chlorophyll concentration were measured at the initial and end of incubations. The results show that: (1) due to the acceptance of upstream sewage discharge and shorter hydraulic retention times, the CDOM of urban rivers is dominated by aromatic substances, with the highest photochemical degradation rate of 16.60%; (2) the CDOM of urban lakes, which is affected by human activities, is dominated by autochthonous substances, and could be increased by 62.16% via microbial metabolism, while compared to urban lakes, non-urban lakes have a more pronounced effect of light on their degradation and transformation due to receiving a large input of humus-like CDOM from the surrounding landscape soils; (3) the photochemical process could degrade terrestrial humic-like substances into small molecular compounds with high biological activity, which could stimulate microbial metabolism to produce protein-like CDOM, however, photochemical degradation could inhibit microbial metabolism by transform the autochthonous protein-like CDOM into a refractory state. The study provides a novel perspective for the differences in the sources and activities of CDOM in different urban aquatic systems and the changes in CDOM characteristics during photochemical and microbial processes.

Characterizing Sources and Composition of Chromophoric Dissolved Organic Matter in a Key Drinking Water Reservoir Lake Tianmu

Lake Tianmu is an important source of drinking water, and its water quality can influence ecosystem service functions. Unraveling the sources and composition of chromophoric dissolved organic matter (CDOM) that can affect water treatment processes is necessary to maintain water supply safety and ecosystem service functioning of Lake Tianmu. Samples were collected monthly in 2017 and analyzed for CDOM absorbance and fluorescent spectra using parallel factor analysis (PARAFAC) to investigate the spatial and temporal variations of CDOM sources and composition in Lake Tianmu. PARAFAC results showed that CDOM in Lake Tianmu was mainly composed of a microbial humic-like component C1 (44.2%±9.8%), followed by a tryptophan-like component C2 (29.2%±4.3%), tyrosine-like component C3 (17.2%±13.1%), and terrestrial humic-like component, C4 was the lowest (9.4%±2.4%). The CDOM abundance a(254) and fluorescence intensities of C1 and C2 were significantly higher in the river mouths than in the downstream lake regions, whereas the spectral slope S275-295 was significantly lower in the river mouths (t-test, P<0.05), indicating that allochthonous inputs cause an elevated degree of humification and relative increase in the molecular weight of CDOM in the inflowing river mouths. Seasonal differences in CDOM composition were mainly ascribed to the a(254) and fluorescence intensities of C1, C2, and C4 being significantly higher in the summer and autumn than in the winter and spring (t-test, P<0.05). Our results showed that the influences of different seasons on CDOM composition comprise differences in rainfall and runoff input, as well as water temperature, thermal stratification, phytoplankton biomass, and mineralization of CDOM by light and microbes.

Long‐termwater color and flow trends in the Mississippi River Headwaters, 1944–2010

AbstractIntensification of brown color in surface waters has been observed over several decades in many areas. We examined a 64‐yr daily record (1947–2010) of visual water color, a measure of chromophoric dissolved organic matter (CDOM), in the Mississippi River at Minneapolis, Minnesota. Although no monotonic trends in daily or mean annual color were evident, our analyses revealed trends in seasonal metrics, for example, mean winter color, on decadal scales related to changes in flow (hence climatic conditions). A pattern of high color (CDOM) in late spring and summer, corresponding with higher flow, was found across the period. Daily flow accounted for ~ 50% of the variance in color, and a lag of four days was found between peak responses of flow and color, supporting a CDOM source from wetlands in northern parts of the basin. The slope of the color‐flow relationship increased over the 64 yr, driven by increased CDOM flushing in late summer‐early fall. Based on trends in seasonally aggregated color and discharge, minimum and mean color and flow increased during winter over the 64 yr, potentially due to higher temperatures. Summer months did not show increases, but color became less variable. As a result, the color difference between summer and winter became smaller over the study period. During high flow events (ice‐out or high precipitation), some hysteretic color patterns were observed consistent with observations on other large rivers. Our results indicate that long‐term color (CDOM) trends in the Mississippi Headwaters reach are related to seasonally dominant changes in climatic conditions.

Sources and compositional characterization of chromophoric dissolved organic matter in a Hainan tropical mangrove-estuary

Chromophoric dissolved organic matter (CDOM) is a major optically active component of DOM in estuaries that plays a significant part in maintaining the ecosystem sustainability of mangrove forests via affecting nutrient and micronutrient ((micro)nutrient) biogeochemical processes and pathways. This study was conducted in Hainan Dongzhai Harbor to better understand the sources, composition, and behaviors of CDOM in a tropical mangrove estuary. Water samples were collected from the upper fluvial mangrove zones to the lower marine endmember along the estuarine salinity gradient and analyzed by absorption and fluorescence spectroscopy combined with parallel factor analysis (PARAFAC). DOC, TDN, TDSi, Mn, Ba, and optical characteristics, i.e., a254, SUVA, HIX, and the abundance of humic-like CDOM, are negatively correlated with salinity. Terrestrial/anthropogenic inputs, riverine runoff, and mangrove porewater exchange are sources of (micro)nutrients and aromatic CDOM into the estuary. In contrast, pH, DO, TDP, SR, BIX, and tryptophan-like CDOM are positively correlated with salinity. This indicates saltwater dilution, tidal mixing, photo-oxidation, and microbial degradation processes promote the enrichment of low MW autochthonous CDOM and TDP, together with the removal of humic-like substances and other (micro)nutrients in the lower estuary. Overall, these findings have important implications for biogeochemical cycling processes driven by CDOM composition and sources in mangrove estuaries.

CDOM Spatiotemporal Variability in the Mediterranean Sea: A Modelling Study

This study investigates the spatial and temporal variability of chromophoric-dissolved organic matter (CDOM) in the Mediterranean Sea. The analysis is carried out using a state-of-the-art 3D biogeochemical model. The model describes the plankton dynamics, the cycles of the most important limiting nutrients, and the particulate and dissolved pools of carbon. The source of CDOM is directly correlated to the dynamics of dissolved organic carbon (DOC) by a fixed production quota. Then CDOM degrades by photobleaching and remineralization. The main innovation of the system is the inclusion of a bio-optical radiative transfer model that computes surface upwelling irradiance, and therefore simulates remotely sensed reflectance (Rrs). Simulation results of three model configurations are evaluated using satellite Rrs, particularly at 412 nm, 443 nm, and 490 nm. All simulations show a winter minimum in Rrs for the considered bands. However, different parameterizations of DOC-release induce a different accumulation of CDOM, especially in the eastern Mediterranean, and a different Rrs signature: a more active microbial loop during summer implies a decrease of Rrs at 412 nm. We demonstrate how the usage of a bio-optical model allows us to corroborate hypotheses on CDOM-cycling based on blue–violet Rrs data, supporting the importance of this complementary data stream with respect to satellite-derived chlorophyll.

Submarine mud volcanoes as a source of chromophoric dissolved organic matter to the deep waters of the Gulf of C\xe1diz

Seafloor structures related to the emission of different fluids, such as submarine mud volcanoes (MVs), have been recently reported to largely contribute with dissolved organic matter (DOM) into the oceans. Submarine MVs are common structures in the Gulf of Cádiz. However, little is known about the biogeochemical processes that occur in these peculiar environments, especially those involving DOM. Here, we report DOM characterization in the sediment pore water of three MVs of the Gulf of Cádiz. Estimated benthic fluxes of dissolved organic carbon (DOC) and chromophoric DOM (CDOM) were higher than in other marine sediments with an average of 0.11 ± 0.04 mmol m−2 d−1 for DOC and ranging between 0.11 and 2.86 m−1 L m−2 d−1, for CDOM. Protein-like components represented ~ 70% of the total fluorescent DOM (FDOM). We found that deep fluids migration from MVs (cold seeps) and anaerobic production via sulfate-reducing bacteria represent a source of DOC and FDOM to the overlying water column. Our results also indicate that fluorescent components can have many diverse sources not captured by common classifications. Overall, MVs act as a source of DOC, CDOM, and FDOM to the deep waters of the Gulf of Cádiz, providing energy to the microbial communities living there.

Optical Properties and Photochemical Transformation of the Dissolved Organic Matter Released by Sargassum

Chromophoric dissolved organic matter (CDOM) is the dominant absorber of ultraviolet radiation in the ocean, but its sources within the ocean, as well as its chemical composition, remain uncertain. One source of marine CDOM is Sargassum, an epipelagic marine macro brown alga common to the Gulf of Mexico, Caribbean, and Western North Atlantic. Furthermore, Sargassum contains phlorotannins, a class of polyphenols that may have similar optical properties to terrestrial polyphenols. Here, we analyze Sargassum CDOM optical properties, acquired from absorption and fluorescence spectra of filtered samples collected during Sargassum exudation experiments in seawater tanks. To further evaluate the structural basis of Sargassum CDOM optical properties, Sargassum CDOM was collected by solid phase extraction (SPE) and its chemical composition was tested by pH titration and sodium borohydride reduction. These chemical tests revealed that Sargassum CDOM absorption spectra respond similarly to pH titration and borohydride reduction when compared to terrestrially-derived materials, but Sargassum CDOM has unique absorbance peaks in difference spectra that have not been observed in terrestrially-derived CDOM. These absorbance features are consistent with the deprotonation of modified Sargassum phlorotannins, which are likely highly related phenolic acids and polyphenols. Sargassum CDOM was also more rapidly photodegraded when compared to terrestrial CDOM such as Suwannee River Natural Organic Matter. Similar to terrestrial DOM, ultrahigh resolution mass spectrometry revealed that sunlight decreases relative abundances of m/z ions and molecular formulas with an average O/C ratio of 0.6 and an average H/C ratio of 0.9, suggesting preferential photodegradation and/or phototransformation of hydrogen-deficient and oxygenated compounds, such as Sargassum phlorotannins. Assuming a large fraction of Sargassum CDOM is quickly mineralized to CO2 during its rapid photodegradation, Sargassum could play a major role in marine photochemical carbon mineralization during its annual growth cycle.