Characteristics of the chromophoric dissolved organic matter of urban black-odor rivers using fluorescence and UV-visible spectroscopy

Urban black-odor water (BOW) is a typical phenomenon seen in the urban water environment; it is caused by excessive pollution by organic matter and other pollutants, such as nitrogen and phosphorous. Chromophoric dissolved organic matter (CDOM) is a major optical fraction of dissolved organic matter. In this study, optical properties and components of CDOM were obtained from 178 river samples collected from five cities in China, the sample were investigated using absorption and fluorescence spectroscopy. The collected included 89 ordinary water (OW) samples, 63 mild BOW (MBOW), and 26 heavy BOW (HBOW) samples. Significant differences were found in the absorption spectra of the HBOW, MBOW, and OW samples, particularly in their optical parameters (the slope of the spectrum (S_275-295), and the ratio of two absorption coefficients of CDOM (E₂:E₃)). Additionally, the fluorescence intensity of the humic acid-like component (F₅) and soluble microbial by product-like component (F₄) obtained via the fluorescence regional integration (FRI) method were 3 and 4.2 times higher in HBOW than in OW, respectively; this could be used as an indicator to distinguish OW from BOW in urban rivers. The results obtained using the redundancy method and the strong negative correlation between F₄ and dissolved oxygen (DO) (r = - 0.56) suggested that the composition of CDOM could change significantly under different urban water environments (p < 0.01). Different correlations were also found between F₅, and a355, E₂:E₃, S_275-295 in different BOW levels, suggesting that the optical parameters of CDOM were mainly determined by the polluted organic matter originating from terrestrial sources with large molecular humic acid-like compounds; optical parameter a355 could distinguish BOW from OW. These findings are conducive in understanding the dynamics of organic matter pollution and to discover the composition and optical properties of the CDOM in urban BOW and OW, thereby providing an effective method for tracking the spatial characteristics of BOW in urban rivers using remote sensing technologies in areas with multiple sources of pollution.

Empirical relationships for remote sensing reflectance and Noctiluca scintillans cell density in the northeastern Arabian Sea

A dinoflagellate under the ambit of Harmful Algal Blooms (HAB), the bioluminescent Noctiluca scintillans (NS), has been infesting the northern Arabian Sea increasingly over the last few decades during late winter. Their occurrence is found to be due to seasonal oscillations in the coastal currents. The physical and biogeochemical parameters associated with the seasonal blooms are reasonably well known. But accurate quantitative estimation capability using remote sensing sensors over the extensive oceanic regime is still lacking. This is especially due to a lack of information on bio-optical properties associated with cell density measurements. We attempted to show that remote sensing reflectance and chl-a show significant relationship e.g., R_rs(531)/R_rs(510) = 0.8261 + 6.06 × 10^-6NS + 0.02323chl-a (N = 19, R²_adj = 0.99, p = 2.5 × 10^-17, RMSE = 0.1083) which is applicable over diverse areas of the northeastern Arabian Sea e.g., coastal, shelf and offshore regions. The model is supported by a second dataset with an RMSE of 0.022893 (N = 8) for the R_rs(531)/R_rs(510) ratio. The NS cell densities were derived from the Rrs(510)/Rrs(531) band ratio within reasonable error and accuracy limits. Including sensor capability at 510 nm is suggested in future satellite launches.

Tracking flood debris using satellite-derived ocean color and particle-tracking modeling

Flood debris associated with Typhoon Lionrock from the Tumen River at the border between Russia and North Korea was traced using ocean color and a Lagrangian particle-tracking model. As debris is transported along with discharged water during floods, a means of tracing floodwater should also allow any associated debris to be tracked. By analyzing the anomalous distribution of colored dissolved organic matter (CDOM) and total suspended sediments (TSS) from the Geostationary Ocean Color Imager (GOCI), the southward movement of the floodwater was tracked along the eastern coast of the Korean Peninsula. This movement was driven by the North Korean Cold Current and was consistent with model results. The similarity between the satellite-derived and modeled datasets shows that CDOM and TSS can be used to track flood-derived debris for several hundreds of kilometers and locate hotspots of debris accumulation.

Microfluidic approach for controlled ultraviolet treatment of colored and fluorescent dissolved organic matter

Using microfluidic systems to address the optical properties of Colored and Fluorescent Dissolved Organic Matter (CDOM/FDOM) offers new ways for researching its interactions with the environment, and its response to rapid, as well as extreme, changes of abiotic conditions. Here we present a microfluidic device with an Ultraviolet (UV) component. The manufactured microfluidic device consists of passing a dissolved organic matter sample through a microchannel applying a combination of treatments using different UV wavelengths and exposure times. Here we test the workability of the microdevice by analyzing the effect of UV light on CDOM and FDOM, using as irradiations UVA and UVB to incite photodegradation, over different times. We then compare the absorbance and fluorescence, measured from both treated and non-treated samples. The analysis of the measurements is done by the calculation of the slope ratio, as indicative of molecular weight and dissolved organic carbon, besides the fluorescence humification index (HIX) as an overview of the difference between treated and non-treated of the excitation-emission matrices (EEMs). Our results show the efficiency of the microdevice by demonstrating a direct relation of degradation degree with exposure time. FDOM exposure to UVB shows a possible relation to humic-like fluorophores intensity, shown in HIX and the overview difference. Furthermore, the changes showed in the slope ratio demonstrate photodegradation in all treatments, with UVB exhibiting an increased influence. The combination of microfluidic sample treatment within in situ applications of optical sensors will enhance our capacities in addressing biogeochemical processes in the marine environment, which were not accessible with conventional bulk methods.

Strong linkages between dissolved organic matter and the aquatic bacterial community in an urban river

Aquatic bacterial communities play an important role in biogeochemical cycling in river ecosystems; however, knowledge of the linkages between bacterial communities and dissolved organic matter (DOM) in urban rivers is limited. Here, 16S rRNA amplicon sequencing and parallel factor (PARAFAC) modeling of excitation-emission fluorescence spectroscopy were used to analyze the compositions, co-occurrence patterns, and interactions with chromophoric DOM (CDOM) of bacterial communities in urban river water samples influenced by different human activities. The results revealed that two protein-like components accounted for 65.2 ± 9.56% of the total variability in all three fluorescence components, which suggests that CDOM in urban rivers is mainly a microbial source. In addition to pH and DO, CDOM is also an important factor affecting bacterial community structure, and the main classes (Gammaproteobacteria and Clostridia) and genera (Limnohabitans and Alpinimonas) showed strong positive correlations with terrestrial humic-like C1 and tryptophan-like C2, respectively. When autotrophic and heterotrophic bacteria coexist in urban rivers, the production and degradation of CDOM will occur simultaneously. Furthermore, the riverine bacterial co-occurrence network had a nonrandom modular structure, which was mainly driven by classification correlation and bacterial function. The high abundance of genes related to xenobiotic metabolism, carbon metabolism and nitrogen metabolism in the urban river indicated that anthropogenic activity may be the dominant selective force altering the bacterial communities. Overall, our results provide a novel view for the assembly of bacterial communities in urban river ecosystems under the influence of different human activities.

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

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

Linking bacterial community shifts with changes in the dissolved organic matter pool in a eutrophic lake

Aquatic bacterial communities play crucial roles in the circulation of nutrients in watershed ecosystems. However, the interaction between bacterial communities and chromophoric dissolved organic matter (CDOM) in freshwater ecosystems has not been studied in depth. In our study, we examined the constitution and interactions of CDOM with the bacterial community in Lake Chaohu and its inflow rivers under the influence of different exogenous pollutants. The results revealed that the bacterial community diversity in the inflow rivers was significantly lower than that in the lake sites. Clustering of different types of polluted inflow rivers integrated with the most abundant genera observed in specific areas indicated that environmentally guided species selection had a large impact on the composition of aquatic bacterial communities. Moreover, our study suggests that communities in lake environments may be more susceptible to interference through a variety of physiologies or via functional redundancy, allowing them to preserve their community structure. Through linear discriminant analysis effect size (Lefse) methods, we revealed that some taxa (from phylum to genus) were consistently enriched in the lake sites. Based on correlation network analysis results, the supersession niches of bacterial community members related to different CDOM in the biogeochemical process was determined. This study provides an ecological basis for the control of external pollution and the protection of the water environment in watershed ecosystems.

Optical properties of chromophoric dissolved organic matter (CDOM) and dissolved organic carbon (DOC) levels in constructed water treatment wetland systems in southern California, USA

Many removal mechanisms in treatment wetlands involve absorption to organic matter. Optical properties and DOC levels of waters entering and exiting 4 treatment wetland systems in Orange County, Southern California, were measured to characterize the dissolved organic matter pool. Average DOC levels decreased between the inlets and outlets, except for Forge Street (FS), which increased. For 3 wetlands, absorption coefficients decreased between inlet and outlet; the exception was FS, which increased. Average spectral slopes for the inlets and outlets were similar. Average intensities of terrestrial humic peaks A and C from 3D EEM fluorescence spectra decreased between the inlets and outlets for most wetlands. No EEM protein peaks were observed. Average flu/abs ratios for inlets and outlets were similar (high point for FS inlet excluded), suggesting chromophoric dissolved organic matter (CDOM) of a similar composition was present. The average FI value for the inlets and outlets was ∼1.5, consistent with terrestrial sources of CDOM. Average BIX values for the inlets and outlets were ∼0.8, suggesting limited contributions from autochthonous production of CDOM. Dominant plant species in the wetlands were cattail and bulrush. Humic peaks A and C, along with protein peaks, were observed in plant leachates. Protein peaks rapidly degraded with solar simulator irradiation. Results indicate that most of the wetlands are a net sink for CDOM, possibly due to absorption to sediments. The FS wetland appears to have a source of non-CDOM optically active organic carbon, possibly from a pollutant.

Dissolved organic matter in a tropical saline-alkaline lake of the East African Rift Valley

Saline-alkaline lakes of the East African Rift are known to have an extremely high primary production supporting a potent carbon cycle. To date, a full description of carbon pools in these lakes is still missing. More specifically, there is not detailed information on the quality of dissolved organic matter (DOM), the main carbon energy source for heterotrophs prokaryotes. We report the first exhaustive description of DOM molecular properties in the water column of a meromictic saline-alkaline lake of the East African Rift. DOM availability, fate and origin were studied either quantitatively, in terms of dissolved organic carbon (DOC) and nitrogen (DON) or qualitatively, in terms of optical properties (absorbance) and molecular characterization of solid-phase extracted DOM (SPE-DOM) through negative electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). DOM availability was high (DOC ∼ 8.1 mM in surface waters) and meromixis imprinted a severe quantitative and qualitative change on DOM pool. At the surface, DOM was rich in aliphatic and moderately in aromatic molecules and thus mirroring autochthonous microbial production together with photodegradation. At the bottom changes were extreme: DOC increased up to 5 times (up to 50 mM) and, molecular signature drifted to saturated, reduced and non-aromatic DOM suggesting intense microbial activity within organic sediments. At the chemocline, DOC was retained indicating that this interface is a highly reactive layer in terms of DOM processing. These findings underline that saline-alkaline lakes of the East African Rift are carbon processing hot spots and their investigation may broaden our understanding of carbon cycling in inland waters at large.

Absorption characteristics of CDOM in treated and non-treated urban lakes in Changchun, China

In urban settings, one may find (i) lakes that are non-treated (NT) and impacted by recurrent discharges of pollutants and nutrients, and (ii) lakes that, through restoration measures and active management, are treated (T) from external inputs. The optical properties of chromophoric dissolved organic matter (CDOM) have been used to assess the anthropogenic impact on lakes ecology, but their application in comparative assessments of urban lakes has not been attempted. For 2 years, we measured nutrients and CDOM properties in water samples collected from NT and T lakes in the city of Changchun, China. Significant differences in CDOM properties were found between the two types of lakes, and these results were supported by redundancy analysis. The NT lakes were eutrophic while the T lakes were mesotrophic, with mean trophic status index (TSI) of 74.2 and 50.3, respectively. The CDOM absorption coefficient at 350 nm, a(350), was 2-fold higher in NT than in T lakes (6.59 vs 3.21 m^-1). In the NT lakes, CDOM components predominantly comprised large molecular weight (MW > 1000-Da) humus-like substances of allochthonous origin, whereas in the T lakes CDOM was dominated by low MW (<1000-Da) substances from autochthonous production. Seasonal fluctuation has a great influence on the CDOM concentration, but a little influence on its molecular composition. The CDOM concentration were higher in summer than in other seasons. Weather conditions (rainfall, temperature) and biophysical processes (biodegradation, photo-bleaching) likely contributed to these variations. We found the water quality of the treated lakes was getting better from 2016 to 2018. In summary, the study results, not only revealed seasonal effects, but most importantly documented the impact of human activities on the characteristics of CDOM in urban lakes. Most specifically, the sharp difference between the lakes in regard to a(350) (2-fold lower in T than in NT lakes) demonstrated the suitability CDOM absorption coefficient as an early indicator of the impact of treatment measures on the hydrochemistry of DOM in urban lakes.

Remote sensing estimation of colored dissolved organic matter (CDOM) from GOCI measurements in the Bohai Sea and Yellow Sea

Colored dissolved organic matter (CDOM) is the main constituent of dissolved organic matter (DOM), also a key indicator of water quality conditions. Accurate estimation of CDOM is essential for understanding biogeochemical processes and ecosystems in marine waters. The use of remote sensing to derive the changes in CDOM is vital technology that can be used to dynamically monitor the marine environment and to document the spatiotemporal variations in CDOM over a large scale. In the present study, we develop a simple approach to estimate the CDOM concentrations based on the in situ datasets from four cruise surveys over the Bohai Sea (BS) and Yellow Sea (YS). Eight band combination forms (using Xi as a delegate, where i denotes the numerical order of band combination forms), including single bands, band ratios, and other band combinations by remote sensing reflectance, R_rs(λ), were trained to test the correlations with the CDOM concentrations. The obtained results indicated that X7, i.e., [R_rs(443) + R_rs(555)]/[R_rs(443)/R_rs(555)], was the optimal form, with correlation coefficient (R) values of 0.904 (p < 0.001). The X7-based fitting model was determined as the optimal model by the leave-one-out cross-validation method with relatively low estimation errors (mean relative error, MRE, 20%), and satellite match-up validation with in situ measurements indicated good performance MRE = 20.3%). Moreover, two spatial distribution patterns of CDOM in Jan. 2017 and Apr. 2018 (independent data) retrieved from Geostationary Ocean Color Imager (GOCI) data agreed well with those in situ observations. These results indicate that our proposed algorithm is feasible and robust for retrieving CDOM concentrations in this study region. In addition, we applied this method to GOCI data for the whole 2016 year in the BS and YS and produced the spatial distribution patterns from different temporal scales including monthly, seasonal, and annual scales. Overall, the findings of this study motivate the development and application of a simple but effective method of the CDOM estimation for those optically complex turbid coastal waters, like this study water areas.

Mapping the Photochemistry of European Mid-Latitudes Rivers: An Assessment of Their Ability to Photodegrade Contaminants

The abiotic photochemical reactions that take place naturally in sunlit surface waters can degrade many contaminants that pose concern to water bodies for their potentially toxic and long-term effects. This works aims at assessing the ability of European rivers to photoproduce reactive transient intermediates, such as HO^• radicals and the excited triplet states of chromophoric dissolved organic matter (³CDOM*), involved in pollutant degradation. A photochemical mapping of the steady-state concentrations of these transients was carried out by means of a suitable modeling tool, in the latitude belt between 40 and 50°N. Such a map allowed for the prediction of the photochemical lifetimes of the phenylurea herbicide isoproturon (mostly undergoing photodegradation upon reaction with HO^• and especially ³CDOM*) across different European countries. For some rivers, a more extensive dataset was available spanning the years 1990–2002, which allowed for the computation of the steady-state concentration of the carbonate radicals (CO₃^•−). With these data, it was possible to assess the time trends of the photochemical half-lives of further contaminants (atrazine, ibuprofen, carbamazepine, and clofibric acid). The calculated lifetimes were in the range of days to weeks, which might or might not allow for efficient depollution depending on the river-water flow velocity.

Environmental photodegradation of emerging contaminants: A re-examination of the importance of triplet-sensitised processes, based on the use of 4-carboxybenzophenone as proxy for the chromophoric dissolved organic matter

The photoreactions sensitised by the excited triplet states of chromophoric dissolved organic matter (³CDOM*) are very important in the photochemical attenuation of emerging contaminants in natural waters. Until quite recently, anthraquinone-2-sulphonate (AQ2S) was the only available CDOM proxy molecule to estimate the contaminant reaction kinetics with ³CDOM*, under steady-state irradiation conditions. Unfortunately, the AQ2S triplet state (³AQ2S*) is considerably more reactive than average ³CDOM*. We have recently developed an alternative protocol based on 4-carboxybenzophenone (CBBP), the triplet state of which (³CBBP*) is less reactive compared to ³AQ2S*. Here we show that in the case of ibuprofen (IBP), paracetamol (APAP) and clofibric acid (CLO), the reaction rate constants with ³CBBP* are more reasonable as ³CDOM* reactivity estimates than those obtained by using AQ2S. In contrast, similar rate constants are measured for the reaction of atrazine (ATZ) with either ³AQ2S* or ³CBBP*. Moreover, the reactivity of ATZ with both ³AQ2S* and ³CBBP* is very similar to that with ³CDOM*, available through a literature estimate. The possibility to validate the ATZ-³CBBP* reactivity estimate against the ³CDOM* data, and to accurately predict the reported IBP and CLO field lifetime, support the suitability of CBBP as CDOM proxy. The replacement of AQ2S with CBBP as proxy molecule does not reverse the qualitative prediction, according to which ³CDOM* would be the main process involved in the photodegradation of the studied contaminants in waters with high dissolved organic carbon (DOC). However, the CBBP-based data prompt for an important reconsideration of the estimated lifetimes at high DOC.

Centennial decline in North Sea water clarity causes strong delay in phytoplankton bloom timing

With climate warming, a widespread expectation is that events in spring, such as flowering, bird migrations, and insect bursts, will occur earlier because of increasing temperature. At high latitudes, increased ocean temperature is suggested to advance the spring phytoplankton bloom due to earlier stabilization of the water column. However, climate warming is also expected to cause browning in lakes and rivers due to increases in terrestrial greening, ultimately reducing water clarity in coastal areas where freshwater drain. In shallow areas, decreased retention of sediments on the seabed will add to this effect. Both browning and resuspension of sediments imply a reduction of the euphotic zone and Sverdrup's critical depth leading to a delay in the spring bloom, counteracting the effect of increasing temperature. Here, we provide evidence that such a transparency reduction has already taken place in both the deep and shallow areas of the North Sea during the 20th century. A sensitivity analysis using a water column model suggests that the reduced transparency might have caused up to 3 weeks delay in the spring bloom over the last century. This delay stands in contrast to the earlier bloom onset expected from global warming, thus highlighting the importance of including changing water transparency in analyses of phytoplankton phenology and primary production. This appears to be of particular relevance for coastal waters, where increased concentrations of absorbing and scattering substances (sediments, dissolved organic matter) have been suggested to lead to coastal darkening.

Reactivity of chromophoric dissolved organic matter (CDOM) to sulfate radicals: Reaction kinetics and structural transformation

Sulfate radical (SO₄^•-) has been extensively studied as a promising alternative in advanced oxidation processes (AOPs) for water treatment. However, little is known about its reactivity to the ubiquitous dissolved organic matter (DOM) in water bodies. SO₄^•- would selectively react with electron rich moieties in DOM, known as chromophoric DOM (CDOM), due to its light absorbing property. In this study, the reactivity and typical structural transformation of CDOM with SO₄^•- was investigated. Four well characterized hydrophobic DOM fractions extracted from different surface water sources were selected as model CDOM. SO₄^•- was produced through the activation of peroxymonosulfate (PMS) by Co(II) ions at pH 8 in borate buffer. The reactivity of CDOM was studied based on the decrease in its ultraviolet absorbance at 254 nm (UVA₂₅₄) as a function of time. The reactivity of CDOM changed with time where fast and slow reacting CDOMs (i.e., CDOM_fast and CDOM_slow) were clearly distinguished. A second-order rate constant of CDOM_fast with SO₄^•- was calculated by plotting UVA₂₅₄ decrease versus PMS exposure; where a R_ct value (i.e., ratio of sulfate radical exposure to PMS exposure) was calculated using pCBA as a probe compound. The transformation of CDOM was studied through the analysis of the changes in UVA_254, electron donating capacity, fluorescence intensity, and total organic carbon. A transformation pathway leading to a significant carbon removal was proposed. This new knowledge on the kinetics and transformation of CDOM would ultimately assist in the development and operation of SO₄^•--based water treatment processes.

Linking shifts in bacterial community with changes in dissolved organic matter pool in a tropical lake

Bacterioplankton communities have a pivotal role in the global carbon cycle. Still the interaction between microbial community and dissolved organic matter (DOM) in freshwater ecosystems remains poorly understood. Here, we report results from a 12-day mesocosm study performed in the epilimnion of a tropical lake, in which inorganic nutrients and allochthonous DOM were supplemented under full light and shading. Although the production of autochthonous DOM triggered by nutrient addition was the dominant driver of changes in bacterial community structure, temporal covariations between DOM optical proxies and bacterial community structure revealed a strong influence of community shifts on DOM fate. Community shifts were coupled to a successional stepwise alteration of the DOM pool, with different fractions being selectively consumed by specific taxa. Typical freshwater clades as Limnohabitans and Sporichthyaceae were associated with consumption of low molecular weight carbon, whereas Gammaproteobacteria and Flavobacteria utilized higher molecular weight carbon, indicating differences in DOM preference among clades. Importantly, Verrucomicrobiaceae were important in the turnover of freshly produced autochthonous DOM, ultimately affecting light availability and dissolved organic carbon concentrations. Our findings suggest that taxonomically defined bacterial assemblages play definite roles when influencing DOM fate, either by changing specific fractions of the DOM pool or by regulating light availability and DOC levels.

Chromophoric dissolved organic matter influence correction of algal concentration measurements using three-dimensional fluorescence spectra

In view of the adverse effects of CDOM (chromophoric or colored dissolved organic matter) on in vivo algal pigment concentration measurements in natural water bodies, a CDOM influence correction method for algal concentration measurements based on three-dimensional fluorescence spectra is investigated. The three-dimensional fluorescence spectra of five common species of algae belonging to five categories, HA (humic acid), and natural water sampled from the Dongpu reservoir, Hefei were analyzed, and the spectral similarity of endogenous/exogenous CDOM in the algal fluorescence spectra region was compared. HA was selected to represent the CDOM spectrum group. The CDOM modified algal pigment concentration measurement method was developed using three-dimensional fluorescence spectra coupled with non-negative weighted least squares linear regression analysis. The results show that under the presence of CDOM interference factors, the recognition accuracy rate of Pyrrophyta, Bacillariophyta, Cyanophyta, and Chlorophyta increased 100%, 100%, 40%, and 40%, respectively. The average recovery rate of Cryptomonas, Pyrrophyta, Bacillariophyta, and Chlorophyta increased 162.7%, 50.3%, 106.4%, and 19.1%, respectively. In addition, the classification accuracy of Pyrrophyta, Bacillariophyta, Cyanophyta, Chlorophyta increased 83.9%, 100%, 38.2%, and 48%, respectively. This was concluded by comparing these results with the results of the algal pigment concentration measurement method without the CDOM modification. This study provides an experimental basis for the development of accurate phytoplankton fluorescence classification monitoring technology.

Characterization of CDOM in saline and freshwater lakes across China using spectroscopic analysis

Colored dissolved organic matter (CDOM) is a major component of DOM in waters, and plays a vital role in carbon cycling in inland waters. In this study, the light absorption and three-dimensional excitation-emission matrix spectra (EEMs) of CDOM of 936 water samples collected in 2014-2017 from 234 lakes in five regions across China were examined to determine relationships between lake water sources (fresh versus saline) and their fluorescence/absorption characteristics. Results indicated significant differences regarding DOC concentration and a_CDOM(254) between freshwater (6.68 mg C L^-1, 19.55 m^-1) and saline lakes (27.4 mg C L^-1, 41.17 m^-1). While humic-like (F₅) and fulvic-like (F₃) compounds contributed to CDOM fluorescence in all lake waters significantly, their contribution to total fluorescence intensity (F_T) differed between saline and freshwater lakes. Significant negative relationships were also observed between lake altitude with either F₅ (R² = 0.63, N = 306) or F_T (R² = 0.64, N = 306), suggesting that the abundance of humic-like materials in CDOM tends to decrease with increased in lakes altitude. In high-altitude lakes, strong solar irradiance and UV exposure may have induced photo-oxidation reactions resulting in decreased abundance of humic-like substances and the formation of low molecular weight compounds. These findings have important implications regarding our understanding of C dynamics in lacustrine systems and the contribution of these ecosystems to the global C cycle.

Photochemical degradation of oil products in seawater monitored by 3D excitation emission matrix (EEM) fluorescence spectroscopy: implications for coloured dissolved organic matter (CDOM) studies

Fluorescence 3D excitation emission matrix (EEM) spectra of oil products in artificial seawater were monitored as a function of irradiation time in a solar simulator. EEMs were obtained for an excitation range of 240-400 nm and an emission range of 248-830 nm; this is the wavelength range typically used in chromophoric dissolved organic matter (CDOM) EEM studies in natural waters. This allows for comparison to prior work on CDOM in an oil-contaminated salt marsh that attributed a fluorescent component in the tryptophan/tyrosine protein-region to oil. For comparison, EEMs were also measured for a broader excitation range of 220-400 nm typically used in oil related studies to capture the primary oil peak at lower excitation wavelengths. Fluorescence intensities in both excitation wavelength ranges decayed exponentially with irradiation time consistent with first-order kinetics. There was little change in wavelength for primary oil peaks. However, in the CDOM, wavelength range peaks typically shifted to longer excitation and shorter emission wavelengths, moving into the protein peak region of the CDOM EEM spectrum. This is consistent with a decrease in the complexity of the structure of the organic material. Half-lives for photodegradation ranged from 0.36 to 7.2 days for the oil wavelength range and 0.14 to 28 days for the CDOM wavelength range. Higher density oils typically had higher degradation rate constants. Peak locations and peak behaviour are consistent with the primary fluorophore in the oil products being PAH-related.

[Analysis of Absorption Characteristics of Urban Black-odor Water]

The urban black-odor water body has become a serious problem of urban water environment and identifying their optical characteristics in urban areas is the prerequisite and basis for their detection by remote sensing. A total of 85 samples of urban black-odor water, from Changsha, Nanjing, and Wuxi, were collected from 2016 to 2017, and a total of 80 samples were collected from non-black-odor (smelly) water samples. The water quality parameters such as suspended matter and the absorption coefficient were measured. The results showed that:①The total black-odor water body particulate matter absorption coefficients and non-pigmented particulate matter absorption coefficients were generally higher than those for non-black-odor water bodies and they had a certain degree of discrimination at 440 nm, but their effect was not significant. The absorption of relatively high levels of particulate matter in black-odor water was mainly due to high absorption of non-pigmented particles. In the data collected, non-pigmented particles in black-odor water accounted for more than 50% of the total particles. ② The Colored Dissolved Organic Matter (CDOM) absorption coefficients for black-odor water bodies and non-black-odor water bodies differed. The average absorption coefficient of CDOM at 440 nm for black-odor water bodies was 1.7 times higher than that for non-black-odor water bodies. Thus, the black-odor water body can be distinguished by using the slope of the absorption coefficient curve fitted in the characteristic wavelength band of 440 nm, as well as in different wavelength bands; the overall effect is good. However, as the wavelength increases, the discrimination effect decreases. Analysis of the absorption characteristics of urban black-odor water bodies will provide effective technical support for their detection by remote sensing and supervision, and the density of CDOM in black-odor water was generally high, which could be used as an important reference for identification.

Implications of irradiance exposure and non-photochemical quenching for multi-wavelength (bbe FluoroProbe) fluorometry

Multi-wavelength fluorometers, such as the bbe FluoroProbe (FP), measure excitation spectra of chlorophyll a (Chl-a) fluorescence to infer the abundance and composition of phytoplankton communities as well as the concentration of chromophoric dissolved organic matter (CDOM). Experiments were conducted on laboratory cultures and on natural communities of freshwater phytoplankton to determine how the response of phytoplankton to high irradiance might affect fluorometric estimates of community composition and concentrations of Chl-a and CDOM. Cultures of a representative cyanobacterium, bacillariophyte, synurophyte, cryptophyte, and chlorophyte revealed changes in Chl-a excitation spectra as irradiance was increased to saturating levels and non-photochemical quenching (NPQ) increased. The degree of change and resulting classification error varied among taxa, being strong for the synurophyte and cryptophyte but minimal for the cyanobacterium. Acute-exposure experiments on phytoplankton communities of varying taxonomic composition from five lakes yielded variable results on apparent community composition. There was a consistent decrease in CDOM estimates, whereas Chl-a estimates were generally increased. Subsequent exposure to low PAR relaxed NPQ and tended to reverse the effects of high irradiance on composition, total Chl-a, and CDOM estimates. Relaxation experiments on near-surface communities in a sixth, large lake, Georgian Bay, showed that total Chl-a estimates increased by 44% on average when dark treatments were used to relax NPQ, though, in contrast to the findings from the small lakes, there was little effect on CDOM estimates. We observed a statistically-significant, negative linear relationship between the photon flux density of in situ irradiance and the accuracy of taxonomic assignment by FP in Georgian Bay. Not discounting the correlations between light intensity and the accuracy of the FP that were observed in this study, we conclude that the applicability of the reference spectra to the system under investigation is a more important consideration than variability in natural irradiance conditions.

[Comparative Characteristics of Optical Absorption in Waters from Yiluo River and Huntai River in Spring]

The absorption characteristics, composition, spatial variability, and relative contribution of optically active constituents over the range of photosynthetically active radiation (PAR, 400-700 nm) were analyzed from samples collected in the Yiluo and Huntai Rivers in May 2017 and 2013, respectively. Results demonstrated that the absorption curves of total suspended particulates[a_p(λ)] were similar to those of non-algae particles[a_d(λ)]. Significant correlations between a_p(λ) and a_d(λ) were obtained, especially at 440 nm, with r=0.968 in the Yiluo River and r=0.899 in the Huntai River. Meanwhile, positive correlations between a_p(λ) and the absorption of phytoplankton[a_ph(λ)] were observed at 675 nm. a_p(λ) in the two basins was dominated by a_d(λ). Moreover, the composition of auxiliary pigments and chloropyhll a concentration[Chla] showed more significant spatial variations based on a_ph(440)/a_ph(675) in the Yiluo River than in the Huntai River. In addition, CDOM absorption slopes (S_CDOM) indicated that CDOM composition in the Yiluo River was dominated by exogenous substances; in comparison, the Huntai River had more endogenous substances. S_CDOM in the Yiluo River was lower than in the Huntai River, indicating that the CDOM composition of the Yiluo River was inclined towards high molecular weights. Furthermore, M_r[a_CDOM(250)/a_CDOM(365)] showed greater ranges and lower mean values in the Yiluo River than in the Huntai River, confirming that CDOM molecular weight varied greatly in the former. CDOM molecular weight in the Huntai River was also lower than in the Yiluo River, in accordance with S_CDOM results.

Relationship changes between CDOM and DOC in the Songhua River affected by highly polluted tributary, Northeast China

In this study, the dissolved organic carbon (DOC) concentrations, chromophoric dissolved organic matter (CDOM) absorption coefficient a(254), and excitation-emission matrix fluorescence (EEM) were examined in the Songhua River (SHR) and its highly polluted tributary of Northeast China. Fluorescence regional integration (FRI) was used to identify five fluorescent regions: one tyrosine-like (R1), one tryptophan-like (R2), one fulvic-like (R3), one microbial by-product-like (R4), and one humic-like (R5) regions. The five EEM-FRI regions for all water samples have site-specific properties. Principle component analysis (PCA) was conducted to assess variations in the five FRI regions and the humification index (HIX) for all water samples. For the water samples from the mainstream of SHR, CDOM absorption coefficient a(254) was correlated with either DOC or FRI fluorescent regions (R3 and R5), respectively. FRI R3 region was also correlated with R5 region for the water samples in the mainstream of SHR. However, the determination coefficients (R²) and slopes of these relationships among CDOM absorption, fluorescent regions, and DOC all decreased when the SHR waters were influenced by the highly polluted tributary of Yinma River (YMR) and Yitong River (YTR), which has a negative effect on the estimation of DOC flux transported by the SHR to oceans.

Characterization of CDOM absorption of reservoirs with its linkage of regions and ages across China

The absorption of chromophoric dissolved organic matter (CDOM) is an important part of light absorptions in aquatic systems. The increasing eutrophication of reservoirs and regional characteristics would affect the CDOM properties sensitively which would be important for the application of remote sensing monitoring. The highest (4.07 ± 2.31 m^-1) and lowest (0.79 ± 0.67 m^-1) CDOM concentrations of reservoirs were observed in the northeastern lake region (NER) and Tibetan Plateau lake region (TPR), respectively. The differences between S_275-295 among the five lake regions were significant (p < 0.05) in which the steepest S_275-295 (0.0173 ± 0.0026 nm^-1) was observed in TPR and the shallowest (0.0326 ± 0.0152 nm^-1) in Yungui Plateau lake region (YGR). The strong relationships between a_CDOM(355) and DOC appeared in the NER (R² = 0.43), eastern lake region (EAR) (R² = 0.69), Mengxin lake region (MXR) (R² = 0.61), and YGR (R² = 0.79) which would be a good proxy for DOC in regional reservoirs. Most of all, the correlation between reservoir's establishing time and CDOM absorption under oligotrophic states was relatively strong in the EAR and MXR regions. It indicated that the establishing time of reservoirs affected the CDOM absorption to some extent under the oligotrophic states without much human disturbance. Our results indicate CDOM absorption varies with regions, and the relationships between CDOM and DOC are variable for different regions. Therefore, DOC estimation in reservoirs through CDOM absorption needs to be considered according to lake regions and trophic states.

Characterization and sources of colored dissolved organic matter in a coral reef ecosystem subject to ultramafic erosion pressure (New Caledonia, Southwest Pacific)

The eastern lagoon of New Caledonia (NC, Southwest Pacific), listed as a UNESCO World Heritage site, hosts the world's second longest double-barrier coral reef. This lagoon receives river inputs, oceanic water arrivals, and erosion pressure from ultramafic rocks, enriched in nickel (Ni) and cobalt (Co). The aim of this study was to characterize colored dissolved organic matter (CDOM), as well as to determine its main sources and its possible relationships (through the use of Pearson correlation coefficients, r) with biogeochemical parameters, plankton communities and trace metals in the NC eastern lagoon. Water samples were collected in March 2016 along a series of river/lagoon/open-ocean transects. The absorption coefficient at 350nm (a₃₅₀) revealed the influence of river inputs on the CDOM distribution. The high values of spectral slope (S_275-295, >0.03m^-1) and the low values of specific ultraviolet absorbance (SUVA₂₅₄, <4Lmg-C^-1m^-1) highlighted the photodegradation of CDOM in surface waters. The application of parallel factor analysis (PARAFAC) on excitation-emission matrices (EEMs) allowed the identification of four CDOM components: (1) one humic- and one tyrosine-like fluorophores. They had terrestrial origin, exported through rivers and undergoing photo- and bio-degradation in the lagoon. These two fluorophores were linked to manganese (Mn) in southern rivers (r=0.46-0.50, n=21, p<0.05). (2) A tryptophan-like fluorophore, which exhibited higher levels offshore. It would be potentially released from the coral reef. (3) A second tyrosine-like ("tyrosine 2-like") fluorophore. Linked to Prochlorococcus cyanobacteria (r=0.39, n=47, p<0.05), this fluorophore would have an oceanic origin and enter in the lagoon through its southern and northern extremities. It also displayed relationships with Ni and Co content (r=0.53-0.54, n=21, p<0.05). This work underlines the diversity of CDOM sources in the NC eastern lagoon.