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Rivers are crucial pathways for transporting organic carbon from land to ocean, playing a vital role in the global carbon cycle. Dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) are major components of dissolved organic matter and have significant impacts on maintaining the stability of river ecosystems and driving the global carbon cycle. In this study, the in situ samples of aCDOM(355) and DOC collected along the main stream of the Songhua River were matched with Sentinel-2 imagery. Multiple linear regression and five machine learning models were used to analyze the data. Among these models, XGBoost demonstrated a superior, highly stable performance on the validation set (R2 = 0.85, RMSE = 0.71 m−1). The multiple linear regression results revealed a strong correlation between CDOM and DOC (R2 = 0.73), indicating that CDOM can be used to indirectly estimate DOC concentrations. Significant seasonal variations in the CDOM distribution in the Songhua River were observed: aCDOM(355) in spring (6.23 m−1) was higher than that in summer (5.3 m−1) and autumn (4.74 m−1). The aCDOM(355) values in major urban areas along the Songhua River were generally higher than those in non-urban areas. Using the predicted DOC values and annual flow data at the sites, the annual DOC flux in Harbin was calculated to be approximately 0.2275 Tg C/Yr. Additionally, the spatial variation in annual CDOM was influenced by both natural changes in the watershed and human activities. These findings are pivotal for a deeper understanding of the role of river systems in the global carbon cycle. [ABSTRACT FROM AUTHOR]

The vegetation surrounding aquatic ecosystems contributes allochthonous sources of organic matter to the water bodies. In Andean-Patagonian catchments, several non-native species grow among native ones. Different sources of dissolved organic matter (DOM) may vary in their contribution, potentially affecting aquatic functioning. This study evaluates differences in DOM contributions from native and non-native plants, analyzing DOM reactivity through its optical properties. An outdoor experiment was conducted with DOM leaf leachates from two native (Maytenus boaria and Nothofagus dombeyi) and two invasive trees (Salix fragilis and Pinus contorta). We characterized DOM concentration and quality optically by UV–Visible and fluorescence spectroscopy and found differences in both quantity and quality between the different leaf leachates. The exotic S. fragilis presented up to 7 times higher dissolved organic carbon (DOC) values than the other species. We found that leachates response to light exposure depended on its DOM composition. Decreasing patterns of DOC absorbance and fluorescence were observed in all leachates over time. We detected a gradient in leaf leachate photoreactivity due to solar exposure: (S. fragilis > P. contorta > M. boaria = N. dombeyi). Overall, our work shows that the invasive plants studied produce larger amounts of degradable DOM than some local native species. [ABSTRACT FROM AUTHOR]

Abstract We studied chromophoric (CDOM) and fluorescent (FDOM) dissolved organic matter (DOM) and dissolved organic carbon in surface waters to determine the factors controlling the geographical distribution of DOM along two meridional transects in the Indian Ocean. For CDOM, we calculated the absorption coefficients, spectral slope, and absorption coefficient ratio from the observed absorption spectra. For FDOM, we calculated the biological (BIX) and humification (HIX) indices from the excitation emission matrices (EEMs); parallel factor analysis of the EEMs identified three fluorescent components, i.e., two humic-like and one protein-like. Using these DOM parameters, a factor analysis extracted fewer latent variables than the observed variables to account for the geographical distributions. We obtained three factors (F1, F2, and F3), which explained ~ 84% of the variance in the observed data. From the factor loadings, F1, F2, and F3 were interpreted as the effects of net primary production-derived DOM and its horizontal transport, photodegradation, and vertical transport by physical processes. We characterized seven marine biogeochemical provinces by factor scores. F1 scores gradually decreased from the northernmost to the Antarctic province, with a small maximum around the subtropical front. F2 scores were highest in the subtropical province and decreased in both the northward and southward directions. F3 scores were high in the Antarctic and northernmost provinces, and lowest in the subtropical province. Only BIX was insufficiently explained by these factors. BIX was highest in the northern part of the subtropical province, where photodegradation of DOM was the most intense. This suggests that the possible interaction between photodegradation, autochthonous production, and reworking by heterotrophic bacteria of DOM occurs in the subtropical province.

The Red Sea is a hotspot of biodiversity susceptible to oil pollution. Besides, it is one of the warmest seas on the Earth with highly transparent waters. In this study, we estimated the oil dissolution rates under natural sunlight spectra and temperature conditions using coastal oil slicks collected after the 2019 Sabiti oil spill in the Red Sea. Optical analyses revealed the significant interactive effect of sunlight and temperature in enhancing the dissolution of oil into dissolved organic matter (DOM). The highest oil dissolution rate (38.68 g C m−3 d−1) was observed in full-spectrum sunlight. Oil dissolution significantly enhanced total organic carbon (TOC) and polycyclic aromatic hydrocarbons (PAHs) in seawater. High nucleic acid (HNA) bacteria, likely the oil degraders, proliferated from 30 to 70 – 90% after 4 days. The heavier stable carbon isotopic composition of methane (δ13C-CH4) and lighter stable carbon isotopic composition of carbon dioxide (δ13C-CO2) indicate the putative role of bacterial processes in the natural degradation of crude oil. The results indicated that the combined effect of temperature and solar radiation enhanced the biological and photochemical dissolution of oil on the Red Sea surface. [ABSTRACT FROM AUTHOR]

The epipelagic macroalgae of Ulva prolifera and Sargassum are the primary contributors to widespread seaweed tides globally. Both ocean plants release large amounts of chromophoric dissolved organic matter (CDOM) into the surrounding seawater. The photochemical reactivity of this CDOM, however, has not been adequately addressed. In this study, we extracted CDOM from Ulva prolifera and Sargassum, examined their ultraviolet (UV)-visible absorption characteristics, and quantified their broadband apparent quantum yields (AQY) of absorbance photobleaching and photomineralization (in terms of CO2, CO, and CH4 photoproduction). On a per-unit-weight basis, Sargassum leached 3.5 times more CDOM than did Ulva prolifera in terms of the absorption coefficient averaged over 254-500 nm. Both Ulva prolifera and Sargassum CDOM were characterized by quasi-exponential decay absorption spectra, with Sargassum CDOM exhibiting a distinct shoulder over 310-350 nm suggestive of mycosporine amino acids. The Sargassum CDOM had a higher photobleaching AQY but lower photomineralization AQYs compared to Ulva prolifera CDOM. The photobleaching and photomineralization AQYs of both macroalgal CDOM are, however, orders of magnitude higher than those of CDOM in various natural waters. Potential photoproduction rates of CO2 and CO from the Ulva prolifera CDOM and Sargassum CDOM during the bloom periods are several times to orders of magnitude higher than the air-sea fluxes of these gases in the absence of the macroalgae. This study demonstrates that CDOM released by Ulva prolifera and Sargassum is extremely prone to photobleaching and photomineralization, rendering floating mats of these plants in oceans as potential "hotspots" of greenhouse gas emissions to the atmosphere. This photochemical feedback should be considered when assessing ocean afforestation as a CO2 removal approach to mitigate climate warming. [ABSTRACT FROM AUTHOR]

We studied chromophoric (CDOM) and fluorescent (FDOM) dissolved organic matter (DOM) and dissolved organic carbon in surface waters to determine the factors controlling the geographical distribution of DOM along two meridional transects in the Indian Ocean. For CDOM, we calculated the absorption coefficients, spectral slope, and absorption coefficient ratio from the observed absorption spectra. For FDOM, we calculated the biological (BIX) and humification (HIX) indices from the excitation emission matrices (EEMs); parallel factor analysis of the EEMs identified three fluorescent components, i.e., two humic-like and one protein-like. Using these DOM parameters, a factor analysis extracted fewer latent variables than the observed variables to account for the geographical distributions. We obtained three factors (F1, F2, and F3), which explained ~ 84% of the variance in the observed data. From the factor loadings, F1, F2, and F3 were interpreted as the effects of net primary production-derived DOM and its horizontal transport, photodegradation, and vertical transport by physical processes. We characterized seven marine biogeochemical provinces by factor scores. F1 scores gradually decreased from the northernmost to the Antarctic province, with a small maximum around the subtropical front. F2 scores were highest in the subtropical province and decreased in both the northward and southward directions. F3 scores were high in the Antarctic and northernmost provinces, and lowest in the subtropical province. Only BIX was insufficiently explained by these factors. BIX was highest in the northern part of the subtropical province, where photodegradation of DOM was the most intense. This suggests that the possible interaction between photodegradation, autochthonous production, and reworking by heterotrophic bacteria of DOM occurs in the subtropical province. [ABSTRACT FROM AUTHOR]

The Secchi disc depth (Dsd) measurement is widely used to monitor eutrophication and the quality of the aquatic environment. This study aimed to investigate the relationship between Dsd and various factors, including the coefficient of attenuation of photosynthetically active radiation [Kd (PAR)], the depth of the euphotic zone (Deu), PAR at the Secchi disk depth (Esd) and the absorption coefficient of PAR (F) in the Neva Estuary, one of largest estuaries of the Baltic Sea. Environmental variables impacting these indices were identified using data collected from midsummer 2012 to 2020. The Dsd values in the estuary ranged from 0.3 to 4.0 m, with an average value of 1.8 m, while the Deu/Dsd ratio ranged from 1.5 to 6.0 with an average value of 2.8. These values were significantly lower than those observed in the open waters of the Baltic Sea. The highest Deu/Dsd ratio was observed in turbid waters characterized by high Kd(PAR) and low Dsd. Contrary to expectations, Dsd did not exhibit a significant relationship with the concentration of chlorophyll a, raising doubts about the utility of historical Dsd data for reconstructing phytoplankton development in the estuary. Principal component analysis did not identify the primary environmental variables strongly affecting the optical characteristics of water. However, recursive partitioning of the dataset using analysis of variance (CART approach) revealed that the concentration of suspended mineral matter (SMM) was the primary predictor of Deu/Dsd, Kd(PAR), and F. This SMM was associated with the frequent resuspension of bottom sediments during windy weather and construction activities in the estuary. Concentrations of suspended organic matter and the depth of the water area were found to be less significant as environmental variables. Furthermore, the CART approach demonstrated that different combinations of environmental variables in estuarine waters could result in similar optical indicator values. To reliably interpret the data and determine the optical characteristics of water in estuaries from Dsd, more complex models incorporating machine learning and neural connections are required. Additionally, reference determinations of Esd in various regions with specific sets of environmental variables would be valuable for comparative analyses and better understanding of estuarine systems. [ABSTRACT FROM AUTHOR]

Rivers are crucial pathways for transporting organic carbon from land to ocean, playing a vital role in the global carbon cycle. Dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) are major components of dissolved organic matter and have significant impacts on maintaining the stability of river ecosystems and driving the global carbon cycle. In this study, the in situ samples of aCDOM(355) and DOC collected along the main stream of the Songhua River were matched with Sentinel-2 imagery. Multiple linear regression and five machine learning models were used to analyze the data. Among these models, XGBoost demonstrated a superior, highly stable performance on the validation set (R2 = 0.85, RMSE = 0.71 m−1). The multiple linear regression results revealed a strong correlation between CDOM and DOC (R2 = 0.73), indicating that CDOM can be used to indirectly estimate DOC concentrations. Significant seasonal variations in the CDOM distribution in the Songhua River were observed: aCDOM(355) in spring (6.23 m−1) was higher than that in summer (5.3 m−1) and autumn (4.74 m−1). The aCDOM(355) values in major urban areas along the Songhua River were generally higher than those in non-urban areas. Using the predicted DOC values and annual flow data at the sites, the annual DOC flux in Harbin was calculated to be approximately 0.2275 Tg C/Yr. Additionally, the spatial variation in annual CDOM was influenced by both natural changes in the watershed and human activities. These findings are pivotal for a deeper understanding of the role of river systems in the global carbon cycle.

Estuaries are characterized by steep biogeochemical gradients that are often overlooked by discrete water sampling which is especially critical for large ecosystems like the Río de la Plata estuary (RLP). To overcome this limitation, high-resolution in situ measurements of salinity, temperature, pH, turbidity, chlorophyll a (Chl-a), colored dissolved organic matter (CDOM) complemented by laboratory analysis of Chl-a and suspended particle (SPM) grain size distribution were employed to characterize the fluvio-marine transition along seven legs covering 1500 km in the RLP. In situ records performed each 400 m permitted to detect the anthropogenic discharge of CDOM in the upper freshwater Buenos Aires–La Plata metropolitan sector (1.7–3.6 mg C l−1) and the sharp turbidity increase at the beginning of the saline intrusion (S: 0.2–2) resulting in a consistent turbidity maximum zone formation (TMZ: 180–350 NTU). High-resolution data at the TMZ also revealed a significant positive covariation of Chl-a with turbidity indicating the retention of freshwater algae. The relative increase of phaeopigments in the South TMZ sector supported the detrital signature of the Paraná River load contrasting with the less turbid and fresher signal of the North corridor influenced by the Uruguay River flow. CDOM showed an opposite pattern and decreased with turbidity at the TMZ suggesting partial colloid precipitation from the surface's 50-cm layer sampled by the boat, followed by a rapid recovery and a linear decrease with salinity (2.9 ± 0.7 to 0.3 ± 0.4 mg C l−1). Turbidity and SPM grain size measurements also reflected the North–South asymmetry of the estuary with lower turbidities enriched in finer fractions along the Northern corridor. A consistent transport pathway of partially mixed, turbid waters enriched in Chl-a, CDOM, and clays concentrated in the TMZ with intermediate salinities was traced downstream along the South shore as a significant transport pathway of suspended solids to the sea. [ABSTRACT FROM AUTHOR]

Dissolved organic carbon (DOC) in lakes, as a regulatory agent and light-absorbing compound, is a key component of the global carbon cycling in lacustrine ecosystems. Hence, continuous monitoring of the DOC concentration in arid regions is extremely important. This study utilizes the QAA-CDOM semi-analytical model, which has good accuracy in retrieving the CDOM (colored dissolved organic matter) concentration of Lake Ebinur. We chose to invert the CDOM time-series data from May to October during the 2018–2022 period. A DOC estimation model was then established using the linear regression approach based on the CDOM inversion data and the field DOC measurements. In general, the DOC concentration in Lake Ebinur exhibited an increasing trend from 2018 to 2022, typically lower in May and higher in June. When comparing the average values of DOC in Lake Ebinur for the same months across different years, it can be observed that the month of September exhibits the greatest variability, whereas June shows the least variability. In sum, this study successfully retrieved CDOM concentrations for a saline lake within an arid region and developed a DOC estimation model, thereby providing a reference for investigating carbon cycling in typical lakes of arid areas. [ABSTRACT FROM AUTHOR]

River estuaries along western Hudson Bay, Canada, are important summer habitats for beluga whales (Delphinapterus leucus (Pallas, 1776)) and subject to increasing industrial development activities including vessel traffic. The feasibility of establishing a National Marine Conservation Area (NMCA) in western Hudson Bay is under consideration, requiring baseline studies and habitat monitoring. In this study, beluga whale locations were identified using aerial photographs collected during summer 2018 of the Seal, Knife, Churchill, and Nelson River estuaries. Sentinel 2 wavelength bands were used to outline river plume boundaries for the Seal, Knife, and Churchill Rivers. Multiple discriminant analysis was used to differentiate between beluga habitat areas according to their environmental characteristics including concentration of total suspended sediments (TSS), and coloured dissolved organic matter (CDOM). The Seal River estuary, Knife River estuary, Churchill River outer estuary, Churchill River estuary, and Nelson River estuary were identified as distinct habitat areas. Resource selection functions and model selection were used to determine that habitat variables related to prey availability were important for beluga habitat selection, including TSS, CDOM, and the distance to river mouth or river plume. Identification of preferred habitat and habitat areas in this study are imperative for future management decisions including establishment of an NMCA.

The Secchi disc depth (Dsd) measurement is widely used to monitor eutrophication and the quality of the aquatic environment. This study aimed to investigate the relationship between Dsd and various factors, including the coefficient of attenuation of photosynthetically active radiation [Kd (PAR)], the depth of the euphotic zone (Deu), PAR at the Secchi disk depth (Esd) and the absorption coefficient of PAR (F) in the Neva Estuary, one of largest estuaries of the Baltic Sea. Environmental variables impacting these indices were identified using data collected from midsummer 2012 to 2020. The Dsd values in the estuary ranged from 0.3 to 4.0 m, with an average value of 1.8 m, while the Deu/Dsd ratio ranged from 1.5 to 6.0 with an average value of 2.8. These values were significantly lower than those observed in the open waters of the Baltic Sea. The highest Deu/Dsd ratio was observed in turbid waters characterized by high Kd(PAR) and low Dsd. Contrary to expectations, Dsd did not exhibit a significant relationship with the concentration of chlorophyll a, raising doubts about the utility of historical Dsd data for reconstructing phytoplankton development in the estuary. Principal component analysis did not identify the primary environmental variables strongly affecting the optical characteristics of water. However, recursive partitioning of the dataset using analysis of variance (CART approach) revealed that the concentration of suspended mineral matter (SMM) was the primary predictor of Deu/Dsd, Kd(PAR), and F. This SMM was associated with the frequent resuspension of bottom sediments during windy weather and construction activities in the estuary. Concentrations of suspended organic matter and the depth of the water area were found to be less significant as environmental variables. Furthermore, the CART approach demonstrated that different combinations of environmental variables in estuarine waters could result in similar optical indicator values. To reliably interpret the data and determine the optical characteristics of water in estuaries from Dsd, more complex models incorporating machine learning and neural connections are required. Additionally, reference determinations of Esd in various regions with specific sets of environmental variables would be valuable for comparative analyses and better understanding of estuarine systems.

Colored dissolved organic matter (CDOM) is a significant constituent of aquatic systems and biogeochemical cycles. Satellite CDOM retrievals are challenging in inland waters, due to overlapped absorption properties of bio-optical parameters, like Total Suspended Matter (TSM). In this framework, we defined an accurate CDOM model using Sentinel2-MSI (S2-MSI) data in Pertusillo Lake (Southern Italy) adopting a classification scheme based on satellite TSM data. Empirical relationships were established between the CDOM absorption coefficient, aCDOM (440), and reflectance band ratios using ground-based measurements. The Green-to-Red (B3/B4 and B3/B5) and Red-to-Blue (B4/B2 and B5/B2) band ratios showed good relationships (R2 ≥ 0.75), which were further improved according to sub-region division (R2 up to 0.93). The best accuracy of B3/B4 in the match-ups between S2-MSI-derived and in situ band ratios proved the exportability on S2-MSI data of two B3/B4-based aCDOM (440) models, namely the fixed (for the whole PL) and the switching one (according to sub-region division). Although they both exhibited good agreements in aCDOM (440) retrievals (R2 ≥ 0.69), the switching model showed the highest accuracy (RMSE of 0.0155 m−1). Finally, the identification of areas exposed to different TSM patterns can assist with refining the calibration/validation procedures to achieve more accurate aCDOM (440) retrievals. [ABSTRACT FROM AUTHOR]

Sampling for dissolved organic carbon (DOC) in the Arctic is challenging given the limited access and because it is not yet possible to measure with instruments deployed in situ. Compared to DOC, colored dissolved organic matter (CDOM) absorption spectroscopy is an easy‐to‐measure, relatively quick and cost‐effective approach which is often closely related to DOC concentrations in water samples. Here we present an algorithm based on quantitative and qualitative metrics of CDOM to provide DOC estimates derived from a Pan‐Arctic data set (n = 3,302) spanning rivers to deep ocean, with DOC ranging between 31 and 1,958 μM. The algorithm provided robust DOC estimates (r2 = 0.94; p < 0.0001) and could reproduce DOC profiles and mixing plots across different locations in the Arctic Ocean. Besides its simplicity, this method is capable of capturing the extremely broad range of DOC within the strong gradients observed between Arctic riverine and marine systems. Plain Language Summary: Surface waters in the Arctic Ocean have high content of dissolved organic carbon (DOC), which plays an important role in CO2 fluxes and absorbing heat, thus warming the ocean. Measuring DOC requires resource‐intensive lab analysis. The optical properties of colored dissolved organic matter (CDOM) are easier, quick and cost‐effective to measure, and can therefore be sampled at a higher frequency or with autonomous sensors. Here we develop an algorithm that uses CDOM optical properties to estimate DOC concentrations across the entire Arctic Ocean, spanning from rivers (high DOC) to the deep waters (low DOC). The algorithm provides reliable estimates and can reproduce regional features, which shows that the method is effective and can be employed to increase sampling efforts for DOC in remote Arctic waters. Key Points: Spectral properties of colored dissolved organic matter can predict dissolved organic carbon concentrations across a range of Arctic aquatic systemsWe developed a pan‐Arctic algorithm from a comprehensive data set spanning from rivers to the deep Arctic watersThe algorithm provides robust estimates allowing for reproduction of regional features such as vertical profiles and mixing lines [ABSTRACT FROM AUTHOR]

Reservoirs are globally ubiquitous and critical for inland carbon (C) cycling, but the coupled influence of land use and hydro-climatic fluctuations on reservoir C source and composition is relatively unexplored. In this study, we used chromophoric dissolved organic matter (CDOM) absorbance and fluorescence excitation and emission matrices modeled by parallel factor analysis to characterize the source and composition of reservoir CDOM. Water samples were collected from reservoirs along a land use gradient in Missouri, USA during a wet and dry summer, respectively. Dissolved organic C (DOC) and CDOM quantity assessed by absorbance at 350 nm (a350) were significantly (r2 = 0.5; p < 0.001) correlated, with CDOM quantity per unit DOC increasing with percentage agriculture. Stronger correlations were observed in the wet summer compared to the dry summer. The influence of land use and hydro-climatic variability was further confirmed by redundancy analysis between CDOM compounds, reservoir limnological parameters, watershed characteristics, and precipitation anomaly (Palmer Drought Severity Index). Agricultural reservoirs were shallow and abundant in a350, aromatic, humic acid- and fulvic acid-like components (C2), chlorophyll-a, and particulate organic matter. These OM signatures in agricultural reservoirs were more pronounced during wet summer conditions, indicating the greater watershed influence. In comparison, CDOM composition in deeper reservoirs was less influenced by watershed influxes. Further, reservoirs with longer residence times experienced internal CDOM processing and an abundance of low-molecular-weight, microbial (C3), and protein-like (C4) CDOM, particularly during drier conditions. Understanding the sensitivity of reservoir CDOM dynamics to watershed land use and variable hydro-climatic conditions will be useful for effective monitoring and resource management. [ABSTRACT FROM AUTHOR]

Seasonal wetlands are complex ecosystems highly dependent on climatic variation due to their small size and shallowness. Wetlands are carbon (C) reservoirs and the major fraction of the C is in the dissolved phase as part of the dissolved organic matter (DOM). Due to limited information about the characteristics of the DOM regarding source, molecular size/weight and diagenetic state, we aimed to performed an optical DOM characterization of 12 temporary wetlands located in the Argentinean Patagonia. As a result, we found that wetlands located in the austral forest have variable amount of dissolved organic carbon (DOC), ranging from 26.04 to 0.9 mg L−1 in a strong longitudinal gradient from west to east, respectively. The proxy for terrestrial input (a350:DOC) showed high values suggesting an allochthonous origin of the DOM. The DOM molecular size (S275–275) showed that most of the wetlands were characterized by high molecular size/weight DOM. A multivariate analysis (RDA) ordinated the wetlands according to their longitudinal position, in which samples collected at west were characterized by high aromaticity and humification, and low recently produced DOM (low BIX). Meanwhile, at eastern side wetlands are characterized by low DOC, low humic DOM components but increasing values of recently produce DOM (high BIX). This study contributes to understand the interaction between terrestrial and aquatic systems in a pristine region which is increasingly impacted by global climate change. [ABSTRACT FROM AUTHOR]

Water quality monitoring in coastal areas and estuaries poses significant challenges due to the intricate interplay of hydrodynamic, chemical, and biological processes, regardless of the chosen monitoring methods. In this study, we analyzed the applicability of different monitoring sources using in situ data, uncrewed-aerial-vehicle (UAV)-mounted hyperspectral sensing, and Sentinel-2-based multispectral imagery. In the first part of the study, we evaluated the applicability of existing empirical algorithms for water quality (WQ) parameter retrieval using hyperspectral, simulated multispectral, and satellite multispectral datasets and in situ measurements. In particular, we focused on three optically active WQ parameters: chlorophyll a ( C h l , a ), turbidity (TUR), and colored dissolved organic matter (CDOM) in oligotrophic coastal waters. We observed that most existing algorithms performed poorly when applied to different reflectance datasets, similar to previous findings in small and optically complex water bodies. Hence, we proposed a novel set of locally based empirical algorithms tailored for determining water quality parameters, which constituted the second part of our study. The newly developed regression-based algorithms utilized all possible combinations of spectral bands derived from UAV-generated hyperspectral data and exhibited coefficients of determination exceeding 0.9 for the three considered WQ parameters. The presented two-part approach was demonstrated in the semi-enclosed area of Kaštela Bay and the Jadro River estuary in the Central Eastern Adriatic Sea. This study introduces a promising and efficient screening method for UAV-based water quality monitoring in coastal areas worldwide. Such an approach may support decision-making processes related to coastal management and ultimately contribute to the conservation of coastal water ecosystems. [ABSTRACT FROM AUTHOR]

Abstract Sampling for dissolved organic carbon (DOC) in the Arctic is challenging given the limited access and because it is not yet possible to measure with instruments deployed in situ. Compared to DOC, colored dissolved organic matter (CDOM) absorption spectroscopy is an easy‐to‐measure, relatively quick and cost‐effective approach which is often closely related to DOC concentrations in water samples. Here we present an algorithm based on quantitative and qualitative metrics of CDOM to provide DOC estimates derived from a Pan‐Arctic data set (n = 3,302) spanning rivers to deep ocean, with DOC ranging between 31 and 1,958 μM. The algorithm provided robust DOC estimates (r2 = 0.94; p

The study explores the spatio-temporal variation of water quality parameters in the Hooghly estuary, which is considered an ecologically-stressed shallow estuary and a major distributary for the Ganges River. The estimated parameters are chlorophyll-a, total suspended matter (TSM), and chromophoric dissolved organic matter (CDOM). The Sentinel-3 OLCI remote sensing imageries were analyzed for the duration of October 2018 to February 2019. We observed that the water quality of the Hooghly estuaries is comparatively low-oxygenated, mesotrophic, and phosphate-limited. Ongoing channel dredging for maintaining shipping channel depth keeps the TSM in the estuary at an elevated level, with the highest amount of TSM observed during March of 2019 (41.59g m−3) at station A, upstream point. Since the pre-monsoon season, TSM data shows a decreasing trend towards the mouth of the estuary. Chl-a concentration is higher during pre-monsoon than monsoon and post-monsoon periods, with the highest value observed in April at 1.09 mg m−3 in station D during the pre-monsoon period. The CDOM concentration was high in the middle section (January–February) and gradually decreased towards the estuary's head and mouth. The highest CDOM was found in February at locations C and D during the pre-monsoon period. Every station shows a significant correlation among CDOM, TSM, and Chl-a measured parameters. Based on our satellite data analysis, it is recommended that SNAP C2RCC be regionally used for TSM, Chl-a, and CDOM for water quality product retrieval and in various algorithms for the Hooghly estuary monitoring. [ABSTRACT FROM AUTHOR]

Inland waters are important components of the global carbon cycle as they regulate the flow of terrestrial carbon to the oceans. In this context, remote monitoring of Colored Dissolved Organic Matter (CDOM) allows for analyzing the carbon content in aquatic systems. In this study, we develop semi-empirical models for remote estimation of the CDOM absorption coefficient at 400 nm (aCDOM) in a tropical estuarine-lagunar productive system using spectral reflectance data. Two-band ratio models usually work well for this task, but studies have added more bands to the models to reduce interfering signals, so in addition to the two-band ratio models, we tested three- and four-band ratios. We used a genetic algorithm (GA) to search for the best combination of bands, and found that adding more bands did not provide performance gains, showing that the proper choice of bands is more important. NIR-Green models outperformed Red-Blue models. A two-band NIR-Green model showed the best results (R2 = 0.82, RMSE = 0.22 m−1, and MAPE = 5.85%) using field hyperspectral data. Furthermore, we evaluated the potential application for Sentinel-2 bands, especially using the B5/B3, Log(B5/B3) and Log(B6/B2) band ratios. However, it is still necessary to further explore the influence of atmospheric correction (AC) to estimate the aCDOM using satellite data. [ABSTRACT FROM AUTHOR]

Understanding the immediate impacts of oil spills is essential to recognizing their long-term consequences on the marine environment. In this study, we traced the early (within one week) signals of crude oil in seawater and plankton after a major oil spill in October 2019 in the Red Sea. At the time of sampling, the plume had moved eastward, but we detected significant signs of incorporation of oil carbon into the dissolved organic carbon pool, resulting in a 10–20% increase in the ultraviolet (UV) absorption coefficient (a254) of chromophoric dissolved organic matter (CDOM), elevated oil fluorescence emissions, and depletion of the carbon isotope composition (δ13C) of the seawater. The abundance of the picophytoplankton Synechococcus was not affected, but the proportion of low nucleic acid (LNA) bacteria was significantly higher. Moreover, specific bacterial genera (Alcanivorax, Salinisphaera, and Oleibacter) were enriched in the seawater microbiome. Metagenome-assembled genomes (MAGs) suggested that such bacteria presented pathways for growing on oil hydrocarbons. Traces of polycyclic aromatic hydrocarbons (PAHs) were also detected in zooplankton tissues, revealing the rapid entry of oil pollutants into the pelagic food web. Our study emphasizes the early signs of short-lived spills as an important aspect of the prediction of long-term impacts of marine oil spills. [ABSTRACT FROM AUTHOR]

The coefficient of attenuation of photosynthetically active radiation (Kd(PAR)) and the depth of the euphotic zone (Deu) are widely used to study biogeochemical processes and eutrophication in marine ecosystems. However, determination of Kd(PAR) and Deu in estuaries is hampered by the simultaneous influence of many environmental factors. In this study, we analyzed the relationship between water turbidity (Turb), Kd(PAR), Deu, and environmental variables in the Neva Estuary, the largest estuary in the Baltic Sea. The summer values of Kd(PAR) and Deu were significantly higher than in the open waters of the Baltic Sea and varied in the range of 0.5–9.1 m−1 and 0.5–8.5 m, respectively. Mixed-effects regression analysis showed that the concentration of suspended mineral matter primarily determined the Turb. This variable fluctuated widely due to frequent wind-induced resuspension of bottom sediments and the periodic construction of port infrastructure in the shallow upper reaches of the estuary. Deu was determined by the depth of the water area, concentration of chlorophyll a, and concentrations of suspended mineral and organic matter. The average efficiency of using PAR energy for gross primary production (PP) was about 2%. However, PP did not depend on the amount of radiation incident on the water surface, but was mostly determined by underwater light and nutrient conditions. The study showed that more research on the impact of environmental variables on underwater light conditions in different regions is needed to predict the impact of climate change and anthropogenic factors on phytoplankton productivity in coastal areas. [ABSTRACT FROM AUTHOR]

Dissolved organic carbon (DOC) in lakes, as a regulatory agent and light-absorbing compound, is a key component of the global carbon cycling in lacustrine ecosystems. Hence, continuous monitoring of the DOC concentration in arid regions is extremely important. This study utilizes the QAA-CDOM semi-analytical model, which has good accuracy in retrieving the CDOM (colored dissolved organic matter) concentration of Lake Ebinur. We chose to invert the CDOM time-series data from May to October during the 2018–2022 period. A DOC estimation model was then established using the linear regression approach based on the CDOM inversion data and the field DOC measurements. In general, the DOC concentration in Lake Ebinur exhibited an increasing trend from 2018 to 2022, typically lower in May and higher in June. When comparing the average values of DOC in Lake Ebinur for the same months across different years, it can be observed that the month of September exhibits the greatest variability, whereas June shows the least variability. In sum, this study successfully retrieved CDOM concentrations for a saline lake within an arid region and developed a DOC estimation model, thereby providing a reference for investigating carbon cycling in typical lakes of arid areas.

Colorful dissolved organic matter (CDOM) is an optically important active substance in marine habitats that feature significant biochemical conservatism. Additionally, CDOM is an important tracer and indicator of pollution status in marine water masses. In this study, we use sampling data collected in September 2020 and May 2021 from the marine areas around the Leizhou Peninsula. This information is combined with the Delft 3D numerical simulation analysis to carry out a study on the relationship between the ag (355) in the marine areas and to determine the regional marine pollution levels. The results show that (1) the nearshore areas of the Leizhou Peninsula in spring and autumn are mainly lightly polluted. Compared with the same period in history, the pollution level has significantly decreased, and the ag (355), DIN, and DIP concentrations show a trend of high near the shore and low far from shore. The peak concentrations are mainly in the Zhanjiang Bay, Qiongzhou Strait, Wailuo Port, Liusha Port and Guangxi Tieshan Port. (2) ag (355) can be used as an indicator to characterize the degree of water pollution in the nearshore waters of the Leizhou Peninsula. In spring, the ag (355) interval of [0.115, 0.200] m−1, [0.201, 0.245] m−1 and [0.246, 0.484] m−1 can be considered clean, lightly polluted, and moderately polluted water, respectively. In autumn, the ag (355) interval of [0.180, 0.275] m−1, [0.276, 0.478] m−1, [0.479, 0.642] m−1, [0.643, 0.921] m−1 can be considered clean, lightly polluted, moderately polluted and heavily polluted water, respectively. (3) The response models between the ag (355) and pollutant dispersion trends were constructed in the Zhanjiang Bay and Tieshan Port. The RMSEs of the ag (355) validation points in the models were lower than 0.12 mg/L, and the MAPEs were lower than 30 %, the pollution traceability of CDOM is verified.

Colored dissolved organic matter (CDOM) is highly spatiotemporally varied due to the effects of complex environmental factors within a catchment or system. The seasonal nutritional status and potential risks of heavy metals in the coastal rivers of the Liaohe River basin were evaluated based on 40 water samples in January, April, May, and September. Meanwhile, the effects of environmental factors on CDOM, especially human activities, were quantitatively analyzed. The trophic state index (TSI) and the potential ecological risk index (RI) of heavy metals in the Liaohe River basin exhibited significant differences. The rivers were mesotrophic in January, lightly eutrophic in May, and highly eutrophic in April and September. An extremely high RI was shown in April and May, while a high RI was exhibited in September. CDOM exhibited great seasonal characteristics and showed significant seasonal correlations with environmental factors. Based on multiple general linear model analysis, total phosphorus (TP) was the most influential factor and significantly explained 62.1% of aCDOM(440) (p < 0.01) among the water parameters, followed by total alkalinity (38.3%). The percentages of built-up area exerted significantly positive effects on aCDOM(440) (R2 = 0.44), while distance from oil extraction sites significantly negatively affected aCDOM(440) (r = − 0.328, p < 0.05). Polluting enterprises showed non-significant correlation with CDOM (r = 0.314, p = 0.178). [ABSTRACT FROM AUTHOR]

Polar ecosystems are vulnerable to climate warming and are characterized by a rapid decline in sea ice. In this study, we collected surface water samples from the Southern Ocean off Prydz Bay during and after sea ice melting season (SIM). After removing the particles using 0.7‐μm pore size filters, we measured the concentration of dissolved organic carbon (DOC) and the composition of dissolved organic matter (DOM) by using the total organic carbon analyzer, optical spectrometry, and high‐resolution mass spectrometry. The mean DOC concentration during the SIM is 1.3‐fold higher than that during the post‐SIM, whereas the a350 value, an indicator of chromophoric DOM, showed the opposite trend by a factor of 2.8. Five components, including three humic‐like and two protein‐like substances were identified by fluorescence excitation emission matrices‐parallel factor analysis, in which protein‐like biolabile components were enriched during the SIM and humic‐like refractory components were enriched during the post‐SIM. The DOM in the water samples from the SIM had higher proportions of polycyclic aromatics, polyphenols, and compounds within molecular lability boundary, whereas the DOM from the post‐SIM had a higher proportion of highly unsaturated compounds. Statistical analysis revealed significant correlations between DOM and sea‐ice melting/associated environmental factors (e.g., water temperature, salinity, and microbial activity). These collective results suggest that sea ice melting is an important factor driving a transition from more labile to more refractory DOM in the surface water off Prydz Bay, which may have a profound impact on the marine ecosystem and carbon cycle in the polar ocean. Plain Language Summary: The polar ocean is sensitive to climate warming and characterized by rapid sea ice decline. It is important to understand how sea‐ice melting influences the marine carbon cycle in the Southern Ocean. In this study, we collected 73 surface water samples from the Southern Ocean close to Prydz Bay, East Antarctica, during the two cruises that corresponded to the sea ice melting season and the post‐sea ice melting season. While analyzing the total dissolved organic carbon (DOC) concentration, optical properties, and molecular formulas, we found substantial differences in the concentration and composition of dissolved organic matter (DOM) between the two cruises. Water samples from the sea ice melting season had higher bulk DOC concentrations, more protein‐like components, and more compounds within molecular lability boundary. In contrast, water samples from the post‐sea ice melting season had lower DOC concentrations, more humic‐like components, and higher proportions of highly unsaturated compounds. There were significant correlations between the DOM parameters and sea ice melting/associated environmental changes (e.g., water temperature, salinity, and microbial activity). These results suggest that rapid sea‐ice changes in the Southern Ocean can cause substantial changes in the concentration and composition of DOM in surface water. Key Points: Dissolved organic matter (DOM) were examined for two batches of surface water samples off Prydz Bay from sea ice melting season (SIM) and post‐SIMDOM, chromophoric DOM, and fluorescent DOM differ significantly between SIM and post‐SIMSea ice melting is an important factor driving a transition from more labile to more refractory DOM [ABSTRACT FROM AUTHOR]

Dissolved organic nitrogen (DON) in inland water bodies plays an important role in the global nitrogen and carbon cycles. A total of 14 fresh water and 5 brackish water bodies (electrical conductivity (EC) threshold value = 1200 μS cm−1) were selected to assess the dynamics of DON for inland water bodies in the semiarid Songnen Plain of Northeast China. The spatial characteristics of DON concentrations, chromophoric dissolved organic matter (CDOM) absorption, CDOM fluorescent components (two humic-like C1 and C3, one tryptophan-like C2) and their correlations with water quality for these 19 water bodies were firstly evaluated using excitation-emission matrix (EEM) fluorescence spectroscopy coupled with parallel factor (PARAFAC) with the data collected in May 2021. Results showed that there were significant differences in DON concentrations, CDOM absorption a(254) and CDOM fluorescent components C1–C3 between fresh water and brackish water bodies (t test, p < 0.01), respectively. DON concentrations were moderately correlated with EC (R2 = 0.766, p < 0.01), trophic state index (TSI) (R2 = 0.757, p < 0.01) and chemical oxygen demand (COD) (R2 = 0.740, p < 0.01) for all 19 water bodies, respectively, suggesting that DON can be used as the potential predictors for TSI and COD. Further, there was a significant positive linear relationship between the CDOM fluorescent component C1 and TN (R2 = 0.654, p < 0.01), DON (R2 = 0.746, p < 0.01) or COD (R2 = 0.893, p < 0.01), respectively, indicating that the nitrogen and carbon in CDOM were originated from similar terrestrial humic-like substances. These results demonstrated that CDOM humic-like fluorescent component C1 can be used for the best optical predictors for nutrients providing a possible way to directly monitor DON and associated with water quality for inland water bodies in the semiarid regions of Northeast China, which has potential implication for inland water bodies with similar characteristics. [ABSTRACT FROM AUTHOR]

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Previous studies have shown that climate change makes Norwegian fjords prone to deoxygenation and water darkening (reduced light penetration) with ecological implications across the food web from phytoplankton to fish. While deoxygenation occurs in deep fjord basins due to reduced water renewal, water darkening has primarily been linked to increased loads of colored dissolved organic matter (CDOM) of terrestrial origin in rivers draining to the Baltic Sea, the North Sea and ultimately to the Norwegian Coastal Current and associated coastal waters. Here, we investigate the hypothesis that water darkening is also linked to deoxygenation of fjord basins. We measure the downwelling irradiance in a water column with hypoxic and anoxic water and compare it with a nearby, well-oxygenated water column. Our data show increased darkening in the hypoxic and anoxic layers, likely due to elevated concentrations of CDOM that is produced locally in these layers. We discuss the ecological implication of this result, which we believe is particularly relevant for the mesopelagic habitat. • Downwelling irradiance was measured in oxygenated, hypoxic and anoxic fjord water. • Light attenuation was higher in low-oxygen water than in well-oxygenated water. • Oxygen-related darkening was likely linked to CDOM from microbial degradation. • Climate change makes fjords prone to a two-sided darkening effect. • The two-sided darkening involves locally produced CDOM in addition to increased supplies of CDOM of terrestrial origin. [ABSTRACT FROM AUTHOR]

• Relationship between polarized light field and the Sun-induced chlorophyll fluorescence was evolved. • Vector radiative transfer accurately reproduced the in-water hyperspectral polarization observations. • Polarized retrival of chlorophyll fluorescence was developed and validated at different water types. • Distinction between the polarized and un-polarized retrievals was illustrated at varied geometries. The impact of the un-polarized nature of chlorophyll fluorescence, which causes a dip in the degree of polarization of the underwater light field matching the fluorescence spectrum, led to the development of a theoretical relationship indicating that the resulting fractional reduction in observed polarization is linearly proportional to the magnitude of the fluorescence causing it. To evaluate this relationship, we used a vector radiative transfer code (VRTE) for the coupled atmosphere-ocean system using measured inherent optical properties (IOPs) for a variety of oligotrophic and eutrophic waters as inputs. The VRTE was used to simulate elastic components of the underwater reflectance as well as the degree of linear polarization (DoLP) for these different conditions. These values were compared with the underwater reflectances and the DoLPs measured by a multi-angular hyperspectral polarimeter to determine the magnitude of the fluorescence component in the reflectance spectra at 685 nm, and the decrease of the DoLP due to the fluorescence impact at the same wavelength. Fluorescence magnitudes retrieved from the differences between simulated and measured reflectances were found to match well the magnitudes estimated through the relationship based on the drop of the DoLP. It is noted that retrieval accuracies increase for both larger fluorescence and larger underlying DoLP values. Furthermore, a new method is evolved from the measured polarization analysis, Polarization-Curve Fluorescence Height (PCFH). Measured polarization were used to approximate the elastic signal and derive the inelastic un-polarized signal (fluorescence) from the difference in the fluorescence vicinity. These results open possibilities for estimating the magnitude of natural fluorescence using polarization measurements below or above the water surface,.in-situ , or remotely from aircraft or future satellites. Results of ongoing work on potential sensitivities and retrieval accuracies for these applications will be reported. [ABSTRACT FROM AUTHOR]

Monthly water samples were collected from the lower Mississippi and Pearl Rivers between January 2009 and August 2011 to investigate the heterogeneity in the dynamic variations of dissolved organic carbon (DOC), colloidal organic carbon, chromophoric and fluorescence dissolved organic matter (CDOM and FDOM), PARAFAC-derived fluorescent components, and other optical properties including spectral slope, specific UV absorbance (SUVA), and fluorescence indices between the two contrasting river systems. The lower Mississippi River exhibits relatively lower concentrations of DOC (306 ± 41 μM C) and CDOM (27.9 ± 5.7 m−1 at 254 nm), featuring lower aromaticity (indicated by SUVA 254) and apparent molecular weight (or higher spectral slope) with weak seasonal variability. The Pearl River, in contrast, has elevated levels of DOC (537 ± 212 μM C) and CDOM (66.4 ± 31.4 m−1), characterized by higher aromaticity, higher molecular weight, and significant seasonality, primarily originating from soil-derived allochthonous sources. The abundance of the >1 kDa colloidal DOC was, on average, 58 ± 3 % of the bulk DOC in the lower Mississippi River and 68 ± 6 % in the lower Pearl River. The >1 kDa high-molecular weight DOM (HMW-DOM) consistently had lower spectral slope and biological index (BIX) values, but higher humification index (HIX) values compared to both bulk DOM and low-molecular-weight DOM (LMW-DOM) counterparts. These trends could be representative of other similar large and small rivers. Four PARAFAC-derived fluorescent components (three humic-like and one protein-like) were identified for both rivers. A positive correlation between discharge and terrestrial humic-like fluorescent components indicated their dominant allochthonous sources, while the protein-like component decreased with increasing discharge, consistent with its autochthonic source and a dilution effect during high flow seasons. The occurrence of large flood events during the sampling period contributed to large DOC pulses, with DOM of higher aromaticity and HMW-DOM. This has important implications for coastal ocean ecosystems, which are increasingly impacted by river flooding events under changing climate conditions. Our results also provide an improved understanding of DOM dynamics in two representative rivers and establish a baseline dataset for future studies to assess changes in sources and composition of DOM and their impacts on the coastal ocean in response to climate, hydrological, and anthropogenic influences. • LMR exhibits lower DOC, CDOM and SUVA254, but higher spectral slope and weak seasonality • LPR has elevated DOC and CDOM, characterized by higher aromaticity and MW and significant seasonality • Colloidal DOC was lower (58 ± 3 %) in LMR but higher (68 ± 6 %) in LPR, with lower S and BIX but higher HIX in both rivers • Four PARAFAC-derived fluorescent components (three humic-like and one protein-like) were identified • Flood events were associated with large changes in DOM quantity and quality [ABSTRACT FROM AUTHOR]

Colored dissolved organic matter (CDOM) is an indicator and optical proxy of terrestrial processes such as land use with allochthonous material fluxes, biogeochemical cycles, and water quality in coastal zones influenced by rivers. However, the role of land use changes on the spatial and temporal availability of CDOM has been poorly explored in Chile. Here, we studied two watersheds with similar climates and contrasting land use patterns in northern Patagonia considering the sampling of CDOM in their estuarine and adjacent coastal ocean. An empirical algorithm with the coefficients adjusted to our study areas to estimate CDOM was applied to Landsat 7 and 8 images to examine temporal variability of CDOM est from 2001 to 2011 and 2013–2020. Our results showed an increasing trend of CDOM est in both areas. Different trends in land use patterns between the two watersheds showed a significant correlation with CDOM est and contrasting associations with environmental variables. Higher humification was found in Yaldad in comparison with Colu. In both areas, allochthonous materials predominated, especially during austral spring according to the low values of the Fluorescence Index (FI). Our results highlight the potential of CDOM est to parameterize biogeochemical cycling models and to further understand the dynamics of CDOM in coastal ecosystems. [Display omitted] • CDOM can be used as an indicator of land use in coastal zone of Northern Patagonia. • Land use change had a high influence on the availability and quality of CDOM. • Inter annual time series of coastal CDOM showed an increasing trend in both basins. [ABSTRACT FROM AUTHOR]

Colored dissolved organic matter (CDOM) is a significant constituent of aquatic systems and biogeochemical cycles. Satellite CDOM retrievals are challenging in inland waters, due to overlapped absorption properties of bio-optical parameters, like Total Suspended Matter (TSM). In this framework, we defined an accurate CDOM model using Sentinel2-MSI (S2-MSI) data in Pertusillo Lake (Southern Italy) adopting a classification scheme based on satellite TSM data. Empirical relationships were established between the CDOM absorption coefficient, aCDOM (440), and reflectance band ratios using ground-based measurements. The Green-to-Red (B3/B4 and B3/B5) and Red-to-Blue (B4/B2 and B5/B2) band ratios showed good relationships (R2 ≥ 0.75), which were further improved according to sub-region division (R2 up to 0.93). The best accuracy of B3/B4 in the match-ups between S2-MSI-derived and in situ band ratios proved the exportability on S2-MSI data of two B3/B4-based aCDOM (440) models, namely the fixed (for the whole PL) and the switching one (according to sub-region division). Although they both exhibited good agreements in aCDOM (440) retrievals (R2 ≥ 0.69), the switching model showed the highest accuracy (RMSE of 0.0155 m−1). Finally, the identification of areas exposed to different TSM patterns can assist with refining the calibration/validation procedures to achieve more accurate aCDOM (440) retrievals.

Water quality monitoring in coastal areas and estuaries poses significant challenges due to the intricate interplay of hydrodynamic, chemical, and biological processes, regardless of the chosen monitoring methods. In this study, we analyzed the applicability of different monitoring sources using in situ data, uncrewed-aerial-vehicle (UAV)-mounted hyperspectral sensing, and Sentinel-2-based multispectral imagery. In the first part of the study, we evaluated the applicability of existing empirical algorithms for water quality (WQ) parameter retrieval using hyperspectral, simulated multispectral, and satellite multispectral datasets and in situ measurements. In particular, we focused on three optically active WQ parameters: chlorophyll a (Chl,a), turbidity (TUR), and colored dissolved organic matter (CDOM) in oligotrophic coastal waters. We observed that most existing algorithms performed poorly when applied to different reflectance datasets, similar to previous findings in small and optically complex water bodies. Hence, we proposed a novel set of locally based empirical algorithms tailored for determining water quality parameters, which constituted the second part of our study. The newly developed regression-based algorithms utilized all possible combinations of spectral bands derived from UAV-generated hyperspectral data and exhibited coefficients of determination exceeding 0.9 for the three considered WQ parameters. The presented two-part approach was demonstrated in the semi-enclosed area of Kaštela Bay and the Jadro River estuary in the Central Eastern Adriatic Sea. This study introduces a promising and efficient screening method for UAV-based water quality monitoring in coastal areas worldwide. Such an approach may support decision-making processes related to coastal management and ultimately contribute to the conservation of coastal water ecosystems.

In the past decades the Arctic has experienced stronger temperature increases than any other region globally. Shifts in hydrological regimes and accelerated permafrost thawing have been observed and are likely to increase mobilization of organic carbon and its transport through rivers into the Arctic Ocean. In order to better quantify changes to the carbon cycle, Arctic rivers such as the Lena River in Siberia need to be monitored closely. Since 2018, a sampling program provides frequent in situ observations of dissolved organic carbon (DOC) and colored dissolved organic matter (CDOM) of the Lena River. Here, we utilize this ground truth dataset and aim to test the potential of frequent satellite observations to spatially and temporally complement and expand these observations. We explored all available overpasses (~3250) of the Ocean and Land Colour Instrument (OLCI) on Sentinel-3 within the ice-free periods (May – October) for four years (2018 to 2021) to develop a new retrieval scheme to derive concentrations of DOC. OLCI observations with a spatial resolution of ~300 m were corrected for atmospheric effects using the Polymer algorithm. The results of this study show that using this new retrieval, remotely sensed DOC concentrations agree well with in situ DOC concentrations (MAPD=10.89%, RMSE=1.55 mg L−1, r²=0.92, n=489). The high revisit frequency and wide swath of OLCI allow it to capture the entire range of DOC concentrations and their seasonal variability. Estimated satellite-derived DOC export fluxes integrated over the ice-free periods of 2018 to 2021 show a high interannual variability and agree well with flux estimates from in situ data (RMSD=0.186 Tg C, MAPD=4.05%). In addition, 10-day OLCI composites covering the entire Lena River catchment revealed increasing DOC concentration and local sources of DOC along the Lena from south to north. We conclude that moderate resolution satellite imagers such as OLCI are very capable of observing DOC concentrations in large/wide rivers such as the Lena River despite the relatively coarse spatial resolution. The global coverage of remote sensing offers the expansion to more rivers in order to improve our understanding of the land-ocean carbon fluxes in a changing climate.

The validation of algorithms developed from in situ reflectance to estimate water quality variables has the challenge of atmospheric correction (AC) when applied to satellite images. Estimating water quality variables from satellite images requires an accurate estimation of remote sensing reflectances (Rrs) which vary according to the AC applied. Validation processes for both Rrs and water quality algorithms were carried out, relating the in situ Rrs (convoluted to Sentinel-2-MSI spectral response function) with the satellite Rrs coming from different ACs (C2RCC, C2X, C2XC, and Polymer), and also relating the in situ water quality variable data with estimated water quality variable values, applying the water quality algorithms to the Rrs obtained for each AC. Regarding the Rrs validation results, the best ACs tested in this work were C2XC and Polymer. Regarding the water quality algorithm validation, the best results were also obtained using C2XC and Polymer Rrs. The results demonstrate the usefulness of the water quality algorithms developed from in situ reflectances since they are not specific to an AC and can be used with any processor. [ABSTRACT FROM AUTHOR]

The riverine flux of terrigenous dissolved organic matter (tDOM) to the ocean is a significant contributor to the global carbon cycle. In response to anthropogenic drivers the flux is expected to increase. This may impact the availability of sunlight in coastal ecosystems, and the seawater carbonate system and coastal CO2 fluxes. Despite its significance, there are few long‐term and high‐resolution time series of tDOM parameters. Corals incorporate fluorescent tDOM molecules from the chromophoric dissolved organic matter (CDOM) pool in their skeletons. The resulting coral skeletal luminescence variability has traditionally been used to reconstruct hydroclimate variation. Here, we use two replicate coral cores and concurrent in‐situ biogeochemical data from the Sunda Shelf Sea in Southeast Asia, where peatlands supply high tDOM inputs, to show that variability in coral luminescence green‐to‐blue ratios (coral G/B) can be used to quantitatively reconstruct terrigenous dissolved organic carbon (tDOC) concentration. Moreover, coral G/B can be used to reconstruct the CDOM absorption spectrum from 230 to 550 nm, and the specific ultraviolet absorbance at 254 nm (SUVA254) of the DOM pool. Comparison to a core from Borneo shows that there may be site‐specific offsets in the G/B–CDOM absorption relationship, but that the slope of the relationship is very similar, validating the robustness of the proxy. By demonstrating that corals can be used to estimate past changes in coastal tDOC and CDOM, we establish a method to study drivers of land–ocean tDOM fluxes and their ecological consequences in tropical coastal seas over decadal to centennial time scales. Plain Language Summary: Understanding the different processes of the carbon cycle is a critical priority to make increasingly accurate predictions of future CO2 concentration. The riverine flux of dissolved organic carbon (DOC) to the coastal ocean may impact the availability of sunlight to coastal life forms, cause coastal water acidification and further degas to the atmosphere as CO2. This DOC flux is particularly significant in regions with peatland soils with a high carbon content and where the flux is being further exacerbated by anthropogenic land‐use changes. Despite this, there are few long‐term instrumental measurements for DOC, and these are largely restricted to North America and Europe. In order to increase our understanding of the fate of DOC, we calibrate a method to reconstruct DOC records using luminescence measurements of coral cores. Coral skeletal luminescence is caused by the incorporation of humic acids, an integral component of terrestrial DOC. We use two replicate coral cores from coastal Singapore and concurrent biogeochemical measurements of coastal Singapore waters to show that coral luminescence measurements are quantitatively related to terrestrial DOC concentration measurements. Our proposed method can be used to reconstruct terrestrial DOC variability over centennial timescales allowing us to better understand the processes and consequences of its variability. Key Points: Coral luminescence green‐to‐blue ratio (coral G/B) is a quantitative proxy for terrigenous dissolved organic carbon concentration in river‐influenced tropical coastal oceansCoral G/B can also be used to reconstruct the absorption spectrum of terrigenous chromophoric dissolved organic matter over the wavelength range 230–550 nmDowncore luminescence G/B variability can therefore be used to investigate temporal variability in terrigenous dissolved organic carbon input and its impacts on optical water quality [ABSTRACT FROM AUTHOR]

Environmental fluorescence measurements sometimes use water Raman scattering as an internal standard to compensate for path length, lensing effects, and turbidity. Fluorescent dissolved organic matter (FDOM) in water may interfere strongly with the measurement of this reference. However, fluorescence in fluid solution is largely unpolarized, while the OH stretching Raman band of water is always strongly polarized. Using an environmental sample from Lake Wateree in South Carolina, USA, we demonstrate that judicious use of this polarization allows for a significant level of improvement in the contrast or visibility of the water Raman band relative to FDOM., Competing Interests: Declaration of Conflicting InterestsThe authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Absorption coefficient partitioning algorithms (APAs) were developed to partition the total absorption coefficient (a(λ)) or total non-water absorption coefficient (anw(λ)) into the absorption subcomponents, i.e., absorption due to phytoplankton aph(λ), colored dissolved organic matter (CDOM) ag(λ) and non-algal particulate matter ad(λ), λ is the wavelength. Absorption coefficients of CDOM and non-algal particulate matter are generally combined due to a similarity in exhibited spectral shape and represented as colored detrital matter (CDM) absorption coefficient, adg(λ). This study focuses on the applicability of five APAs Schofield's, Lin's, Zhang's, Stacked Constraints Model (SCM) and Generalized Stacked Constraints Model (GSCM), in deriving the absorption subcomponents from anw(λ) in optically complex coastal waters of Kochi and Goa, India. The average spectral Mean Absolute Percentage Errors (MAPE) obtained for all models in the retrieval of aph(λ), ad(λ), ag(λ) and adg(λ) lie in the ranges of 26–44%, 37–45%, 34–65% and 42–56%. Slopes of adg(λ),ag(λ) and ad(λ) as indicated by Sdg,Sg and Sd are derivable from GSCM, Schofield and Lin's models only. GSCM model exhibited good retrieval capability of Sd with MAPE values of 22% and a correlation coefficient of 0.74. In retrieval of Sg parameter, none of the models demonstrated satisfactory performance. Overall, the GSCM and Schofield's models demonstrated good performance in the retrieval of absorption subcomponents, aph(λ),adg(λ), ad(λ) and Sd. Effect of applying baseline correction to ad(λ) on model performance is studied. Tuning with in situ data can further improve the absorption subcomponent and slope parameter retrieval capability of the models.

Abstract Dissolved 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.

This study examined the physical and biogeochemical processes that determine the spatial and summer/winter distributions of dissolved organic matter (DOM) and chromophoric dissolved organic matter (CDOM) in the Taiwan Strait (TS). The hydrological conditions in the TS varied between early summer and winter because of differences in terrestrial input as well as input from the poleward South China Sea Water (SCSW) during the warm summer and the strong, nutrient-rich, equatorward China Coastal Water (CCW) in winter. Concentrations of dissolved organic carbon (DOC) ranged from 60 to 100 μM-C in early summer and from 70 to 144 μM-C in winter. The average stoichiometric ratio of DOC : DON:DOP was high (353:49:1) in the land-source impacted zones and low (168:21:1) in the SCSW-laden areas, both of which are higher than the Redfield ratio. The concentrations of DOC and nutrients were higher in the western TS (on the coast of China) than in the eastern TS (on the coast of Taiwan), and the highest concentrations were mostly found in the Minjiang River plume, reflecting the profound effects of freshwater input in early summer and CCW inflow in winter. The absorption and emission characteristics of the CDOM varied to a large extent with locations because of the mixing of the shelf water with different source waters. Although the absorption coefficient (aCDOM(325)) and fluorescence intensity of the terrestrial humic-like CDOM (Ft: Ex/Em= 260/400-460) were both strongly inversely correlated with salinity, as an optical tracer, Ft may be more effective than aCDOM(325) in examining the mixing among source waters, given that aCDOM(325) may be subject to various photobleaching effects in the TS. Ft was also significantly correlated with the concentrations of terrestrially derived nitrate and nitrite (N+N) and silicate. The results of factor analysis elucidated the complexity of the hydrological and biological processes that affect the distributions of DOM and CDOM in the TS.

The seasonal and spatial variability of surface phytoplankton assemblages and associated environmental niches regarding major nutrients, physical (temperature and salinity), and optical characteristics (inherent and apparent optical properties) were investigated in an anthropized subarctic coastal bay, in the Gulf of St. Lawrence: the Bay of Sept-Îles (BSI), Québec, Canada. Seven major phytoplankton assemblages were identified by applying a combined Principal Component Analysis and Hierarchical Cluster Analysis procedures, using pigment concentrations and