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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]

The Arctic Ocean receives a large supply of dissolved organic matter (DOM) from its catchment and shelf sediments, which can be traced across much of the basin's upper waters. This signature can potentially be used as a tracer. On the shelf, the combination of river discharge and sea‐ice formation, modifies water densities and mixing considerably. These waters are a source of the halocline layer that covers much of the Arctic Ocean, but also contain elevated levels of DOM. Here we demonstrate how this can be used as a supplementary tracer and contribute to evaluating ocean circulation in the Arctic. A fraction of the organic compounds that DOM consists of fluoresce and can be measured using in‐situ fluorometers. When deployed on autonomous platforms these provide high temporal and spatial resolution measurements over long periods. The results of an analysis of data derived from several Ice Tethered Profilers (ITPs) offer a unique spatial coverage of the distribution of DOM in the surface 800 m below Arctic sea‐ice. Water mass analysis using temperature, salinity and DOM fluorescence, can clearly distinguish between the contribution of Siberian terrestrial DOM and marine DOM from the Chukchi shelf to the waters of the halocline. The findings offer a new approach to trace the distribution of Pacific waters and its export from the Arctic Ocean. Our results indicate the potential to extend the approach to separate freshwater contributions from, sea‐ice melt, riverine discharge and the Pacific Ocean. Plain Language Summary: The Arctic Ocean has a layered structure with comparatively warm Atlantic waters (above freezing temperatures) separated from the surface by a cold layer of water called the halocline. This layer allows the surface waters to be cooled sufficiently for sea ice to form. Here we present a new approach to trace the origins of the halocline using organic matter fluorescence as a tracer. These measurements are made from autonomous profiling systems which offer an excellent opportunity to vastly expand our sampling coverage temporally and spatially. Key Points: Arctic surface waters with comparable temperature and salinity have contrasting in situ dissolved organic matter fluorescenceOrganic matter fluorescence can track low salinity waters feeding into the Transpolar Drift and halocline layersSiberian and Chukchi shelf waters can be separated based on their fluorescence to salinity relationship [ABSTRACT FROM AUTHOR]

Abstract: Dissolved organic matter (DOM) concentration and quality were examined from Arctic lakes located in three clusters across south‐west (SW) Greenland, covering the regional climatic gradient: cool, wet coastal zone; dry inland interior; and cool, dry ice‐marginal areas. We hypothesized that differences in mean annual precipitation between sites would result in a reduced hydrological connectivity between lakes and their catchments and that this concentrates degraded DOM. The DOM in the inland lake group was characterized by a lower aromaticity and molecular weight, a low soil‐like fluorescence, and carbon stable isotope (δ13C‐DOC) values enriched by ~2‰ relative to the coastal group. DOC‐specific absorbance (SUVA254) and DOC‐specific soil‐like fluorescence (SUVFC1) revealed seasonal and climatic gradients across which DOM exhibited a dynamic we term “pulse‐process”: Pulses of DOM exported from soils to lakes during snow and ice melt were followed by pulses of autochthonous DOM inputs (possibly from macrophytes), and their subsequent photochemical and microbial processing. These effects regulated the dynamics of DOM in the inland lakes and suggested that if circumpolar lakes currently situated in cool wetter climatic regimes with strong hydrological connectivity have reduced connectivity under a drier future climate, they may evolve toward an end‐point of large stocks of highly degraded DOC, equivalent to the inland lakes in the present study. The regional climatic gradient across SW Greenland and its influence on DOM properties in these lakes provide a model of possible future changes to lake C cycling in high‐latitude systems where climatic changes are most pronounced. [ABSTRACT FROM AUTHOR]

Fine submicron organic particles can represent an important fraction of the dissolved organic matter (DOM) pool in aquatic ecosystems and their optical properties differ from those normally considered dissolved (< 0.2 μm), which means that the choice of filter type/pore size can influence the light absorption characteristics. In this study, a total of 867 paired CDOM absorption spectra ( n = 1734) from different ecosystems (lakes, streams, sewages and estuaries) were measured on 0.2 μm and GF/F (nominal pore size 0.7 μm) filters. The aims were to evaluate how fine organic particles influence the spectral signature of the DOM pool and to quantify the effects of choice of filter type. In aquatic ecosystems influenced by terrestrial DOM (rivers and lakes), the dissolved fraction (here defined as < 0.2 μm) overwhelmed the fine particulate signal (0.2–0.7 μm) which did not contribute significantly to the absorption signal. In contrast, freshly-produced fine particles released by phytoplankton significantly increased measured CDOM absorption in productive environments with low terrestrial background. Our results demonstrate that the choice of filter pore size can have a significant impact on the outcome of spectral metrics often used to characterise CDOM such as the spectral slope (S) or the slope ratio (SR). Hence, this may complicate the combining of CDOM absorption measurements from different studies where different pore sizes were used as fine particulate material may significantly influence the spectral signature, particularly in situations where phytoplankton is the dominating source of DOM. [ABSTRACT FROM AUTHOR]

Treated drinking water may become contaminated while travelling in the distribution system on the way to consumers. Elevated dissolved organic matter (DOM) at the tap relative to the water leaving the treatment plant is a potential indicator of contamination, and can be measured sensitively, inexpensively and potentially on-line via fluorescence and absorbance spectroscopy. Detecting elevated DOM requires potential contamination events to be distinguished from natural fluctuations in the system, but how much natural variation to expect in a stable distribution system is unknown. In this study, relationships between DOM optical properties, microbial indicator organisms and trace elements were investigated for households connected to a biologically-stable drinking water distribution system. Across the network, humic-like fluorescence intensities showed limited variation (RSD = 3.5–4.4%), with half of measured variation explained by interactions with copper. After accounting for quenching by copper, fluorescence provided a very stable background signal (RSD < 2.2%) against which a ∼2% infiltration of soil water would be detectable. Smaller infiltrations would be detectable in the case of contamination by sewage with a strong tryptophan-like fluorescence signal. These findings indicate that DOM fluorescence is a sensitive indicator of water quality changes in drinking water networks, as long as potential interferents are taken into account. [ABSTRACT FROM AUTHOR]

Abstract: Optical and chemical properties of dissolved organic matter (DOM) were resolved in the mixing zone between the North Sea and the Baltic Sea. The results from four cruises undertaken from August 2006 to February 2007 are presented. Relationships between the optical (absorption and excitation–emission matrix (EEM) fluorescence), dissolved organic carbon (DOC), and dissolved lignin concentrations were determined and carbon stable isotope values (δ13C) of DOM were also used to evaluate DOM sources. Chromophoric DOM (CDOM), measured as absorption at 300nm, was a strong predictor for DOC and lignin concentrations. The DOM fluorescence (FDOM) characteristics were modeled by parallel factor analysis (PARAFAC) and a six component model was derived. The intensity of one terrestrial humic-like fluorescent component (Fmax3) was strongly correlated to dissolved lignin concentrations. DOC concentrations were best modeled by the combination of this component with an amino acid-like fluorescent component. A three end-member mixing model developed for the region using CDOM–salinity relationships was used to estimate end-member lignin and DOC concentrations, which produced results consistent with other published results from the region. The Baltic Sea outflow was determined to be the dominant source of dissolved lignin in these waters. Despite the high riverine influence in the Jutland Coastal Current, the DOM originating from these waters contained little lignin and was predominantly autochthonous. Additionally CDOM and carbon stable isotope measurements revealed substantial autochthonous production in the Kattegat. Finally, we used the fluorescence–DOC relationships to estimate the net export of terrestrial C from the Baltic Sea at 0.8Tgyear−1, which represents 45% of the calculated net DOC flux (1.7Tgyear−1). [Copyright &y& Elsevier]

Abstract: The spectral absorption properties of chromophoric dissolved organic matter (CDOM) and their distributions in two Chinese estuaries, the Yangtze River Estuary and the Jiulong River Estuary, were studied during August 2003 (wet season) and during different seasons between 2003−2005, respectively. The CDOM concentrations (a 355) of fresh end members in the Jiulong River Estuary varied seasonally, while its quality remained relatively stable. However, the a 355 of the marine end members exhibited less variability. Application of a conservative mixing model indicated that CDOM behaved conservatively in the Yangtze River Estuary. No photobleaching removal was observed at high salinity region of this estuary. Although CDOM showed conservative behavior for many cruises in the Jiulong River Estuary, there was evidence for removal in the low salinity regions during some cruises. Laboratory mixing experiments and a salt addition experiment suggested that particle sorption of CDOM maybe the possible reason for the removal. These results showed that absorption properties of CDOM can be used as a tool to observe the quantitative and qualitative dynamics of DOM during estuarine mixing. [Copyright &y& Elsevier]

Abstract Advancing our understanding of the behaviour of dissolved organic matter (DOM) in aquatic environments necessitates efforts to combine complementary analytical data sets. However, some analytical measurements require sample pre-treatment, while others are carried out on bulk water samples, and it remains unclear if the resulting data sets can be compared. Here, we investigated the impact of solid-phase extraction with PPL resins on DOM optical properties. In samples from contrasting Arctic fjords, extraction efficiencies based on optical properties varied spectrally with averages between 31 ± 13% at 411 nm and 40 ± 12% at 363 nm for chromophoric DOM. Similarly, the extraction efficiency for specific fluorescence components varied between 37 ± 16% and 58 ± 18%. Solid-phase extraction also decreased S 275–295 , fluorescence index, and the freshness index, but increased S 350–400 , and apparent fluorescence quantum yields, indicating that the extraction process was qualitatively selective. Six fluorescence components identified independently in bulk water samples and extracted DOM using parallel factor analysis exhibited different behaviours. Three had identical spectral properties before and after extraction, although their extraction efficiencies varied with water mass characteristics and DOM composition, whereas three other components appeared to change after extraction. With the exception of one fluorescence component, the dynamics of optical properties in bulk water samples were not accurately reflected by DOM extracts. These results indicate that solid-phase extraction imparts a qualitative selectivity that leads to the homogenization of DOM extracts relative to their original samples. Efforts to integrate chemical information from different analytical methods should prioritize comparisons of measurements obtained on the same samples. Graphical abstract Unlabelled Image Highlights • Solid-phase extraction occured at spectrally-variable recoveries for CDOM and FDOM. • Extraction changed widely-used fluorescence indicies and CDOM slopes. • Three of the six whole-water fluorescence components changed upon extraction. • Unchanged FDOM components were extracted with efficiencies between 37 and 58%. • Extracts did not accurately reflect changes of most optical properties in bulk samples. [ABSTRACT FROM AUTHOR]

Dissolved lignin phenols, chromophoric dissolved organic matter (DOM), and in situ fluorescence were determined in waters of the Laptev Sea and major Arctic basins, and they were compared with dissolved iron (dFe) distributions to elucidate the sources, molecular characteristics and distributions of iron-binding ligands in the Arctic Ocean. In the Transpolar Drift region (TPD), concentrations of dFe were positively correlated with concentrations of lignin phenols and multiple optical proxies of DOM composition and source. Strong relationships between dFe and visible and ultraviolet wavelength fluorescent DOM indicated that vascular plant and algal-derived DOM contributed to the dFe-ligand pool. These observations are consistent with previous studies suggesting the association of dFe with humic terrigenous and marine organic ligands. The primary sources of iron-binding ligands appear to be the riverine discharge of terrigenous DOM, marine organic matter produced on the shelves, and degradation products of plankton-derived organic matter in the shelf sediments. A stronger relationship between dFe and visible wavelength CDOM fluorescence than with lignin phenols suggested the presence of multiple terrigenous ligands, such as aromatic tannins. The aromatic nature of these terrigenous ligands was indicated by a strong relationship between dFe and the absorption coefficient at 254 nm. A strong negative correlation between the p-hydroxyl to vanillyl lignin phenols ratio and dissolved iron concentrations indicated recently-discharged terrigenous DOM (tDOM) was an important source of iron-binding ligands. Given the strong relationships of marine and terrigenous DOM with dissolved iron, iron-binding functional groups appear to occur in diverse molecules of multiple sources. Examples of such iron-binding functional groups included catechols and carboxylates found in lignins and tannins of terrigenous origins and carboxyl-rich alicyclic molecules (CRAM) of terrigenous and marine origins. The observed dFe distributions in the Arctic Ocean could not be explained by the presence of a single ligand type, but rather by a potpourri of ligand molecules of varying concentrations and binding strengths. This molecular diversity of ligands and associated binding strengths ultimately controls the distribution and transport of dFe in the Arctic Ocean and beyond. • The Arctic Ocean receives disproportional loads of terrigenous organic matter that can bind dissolved iron (dFe) • Terrigenous organic matter can and keep dFe in solution over long spatial and temporal scales • Binding sites and functional group composition capable of binding dFe are found in a diverse group of organic molecules from various sources • The sources include vascular plants and marine phytoplankton in the Arctic Ocean • dFe-binding organic molecules in the Arctic Ocean likely include tannins, lignins, carbohydrates and carboxyl-rich alicyclic molecules (CRAM) • dFe distribution patterns in the Arctic Ocean depend on the concentrations as well as the average binding strength of the organic ligands in solution • Optical properties indicate that dFe distribution and transport is related to major hydrographic features of the Arctic Ocean [ABSTRACT FROM AUTHOR]