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Chromophoric dissolved organic matter (CDOM) is a ubiquitous component in marine environments, and substantial changes in its sources and distribution, related to the carbon cycle in the Arctic Ocean, are expected due to Arctic warming. In this study, we present unique CDOM data in the Eurasian Arctic Ocean derived from the year‐round MOSAiC expedition. We used CDOM absorbance spectra and fluorescence excitation‐emission matrices in combination with parallel factor analysis to characterize differences in DOM sources and composition. Our results suggested that terrestrial DOM was less sensitive to seasonal changes but controlled by regionality in hydrography. Elevated dissolved organic carbon (DOC) levels in polar surface water were primarily derived from terrigenous sources as identified by CDOM absorption and fluorescence characteristics. In the Amundsen Basin and western Fram Strait surface waters, to which terrestrial DOM is primarily transported by the Transpolar Drift, we found, on average, a 188% larger meteoric water fraction and a 40% higher DOC concentration compared to the Atlantic water that dominated western Nansen Basin and Yermak Plateau. In the Amundsen Basin, the DOC concentration in summer of surface water was only 13% higher compared to winter season. Additionally, autochthonous DOM and chlorophyll‐a concentrations were relatively low in surface water and exhibited significant differences compared to those observed in summer, while there were significant differences between autochthonous DOM and chlorophyll‐a. We also observed that sea ice melt contributed to autochthonous DOM in summer, while storms in winter affected the vertical distribution of terrestrial and autochthonous DOM in the subsurface. Plain Language Summary: In the Arctic Ocean, dissolved organic matter (DOM) is important because it represents a large carbon reservoir that cycles through the environment and supports the growth of marine life in the Arctic. DOM originates from various sources and its composition changes depending on factors like location and time of year, largely influenced by ocean currents. Microbes in the water break down DOM, but this process is hindered during the winter months and in areas with limited sunlight. Due to the very limited and challenging accessibility of the Central Arctic Ocean, there is a lack of DOM data, especially during winter in the Eurasian Arctic Ocean. Scientists conducted a comprehensive year‐round study during the MOSAiC expedition and discovered that much of the DOM in the Arctic comes from land in Siberia. Additionally, seasonal changes were observed to alter the composition of locally produced chromophoric DOM (CDOM) in surface waters of the Amundsen Basin. In contrast, terrestrial CDOM is less responsive to seasonal shifts but more influenced by regional variations in hydrography. This highlights the substantial role of regional hydrographic differences in shaping the characteristics of DOM in surface water across the Eurasian Arctic Ocean. Key Points: Elevated dissolved organic carbon levels in polar surface water are primarily derived from terrigenous sources in the Eurasian basinContrasting dissolved organic matter characteristics in Amundsen and Nansen basins caused by Transpolar Drift watersAutochthonous dissolved organic matter in polar surface water is low in winter and significantly different from that in summer [ABSTRACT FROM AUTHOR]

Some bacteria can degrade organic micropollutants (OMPs) as primary carbon sources. Due to typically low OMP concentrations, these bacteria may benefit from supplemental assimilation of natural substrates present in the pool of dissolved organic matter (DOM). The biodegradability of such auxiliary substrates and the impacts on OMP removal are tightly linked to biotransformation pathways. Here, we aimed to elucidate the biodegradability and effect of different DOM constituents for the carbofuran degrader Novosphingobium sp. KN65.2, using a novel approach that combines pathway prediction, laboratory experiments, and fluorescence spectroscopy. Pathway prediction suggested that ring hydroxylation reactions catalysed by Rieske‐type dioxygenases and flavin‐dependent monooxygenases determine the transformability of the 11 aromatic compounds used as model DOM constituents. Our approach further identified two groups with distinct transformation mechanisms amongst the four growth‐supporting compounds selected for mixed substrate biodegradation experiments with the pesticide carbofuran (Group 1: 4‐hydroxybenzoic acid, 4‐hydroxybenzaldehyde; Group 2: p‐coumaric acid, ferulic acid). Carbofuran biodegradation kinetics were stable in the presence of both Group 1 and Group 2 auxiliary substrates. However, Group 2 substrates would be preferable for bioremediation processes, as they showed constant biodegradation kinetics under different experimental conditions (pre‐growing KN65.2 on carbofuran vs. DOM constituent). Furthermore, Group 2 substrates were utilisable by KN65.2 in the presence of a competitor (Pseudomonas fluorescens sp. P17). Our study thus presents a simple and cost‐efficient approach that reveals mechanistic insights into OMP‐DOM biodegradation. [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]

Analysis of lignin in seawater is essential to understanding the fate of terrestrial dissolved organic matter (DOM) in the ocean and its role in the carbon cycle. Lignin is typically quantified by gas or liquid chromatography, coupled with mass spectrometry (GC‐MS or LC‐MS). MS instrumentation can be relatively expensive to purchase and maintain. Here we present an improved approach for quantification of lignin phenols using LC and absorbance detection. The approach applies a modified version of parallel factor analysis (PARAFAC2) to 2nd derivative absorbance chromatograms. It is capable of isolating individual elution profiles of analytes despite co‐elution and overall improves sensitivity and specificity, compared to manual integration methods. For most lignin phenols, detection limits below 5 nmol L−1 were achieved, which is comparable to MS detection. The reproducibility across all laboratory stages for our reference material showed a relative standard deviation between 1.47% and 16.84% for all 11 lignin phenols. Changing the amount of DOM in the reaction vessel for the oxidation (dissolved organic carbon between 22 and 367 mmol L−1), did not significantly affect the final lignin phenol composition. The new method was applied to seawater samples from the Kattegat and Davis Strait. The total concentration of dissolved lignin phenols measured in the two areas was between 4.3–10.1 and 2.1–3.2 nmol L−1, respectively, which is within the range found by other studies. Comparison with a different oxidation approach and detection method (GC‐MS) gave similar results and underline the potential of LC and absorbance detection for analysis of dissolved lignin with our proposed method. [ABSTRACT FROM AUTHOR]

This study traces dissolved organic matter (DOM) in different water masses of the Arctic Ocean and its effect on the distributions of trace elements (TEs; Fe, Cu, Mn, Ni, Zn, Cd) using fluorescent properties of DOM and the terrigenous biomarker lignin. The Nansen, Amundsen, and Makarov Basins were characterized by the influence of Atlantic water and the fluvial discharge of the Siberian Rivers with high concentrations of terrigenous DOM (tDOM). The Canada Basin and the Chukchi Sea were characterized by Pacific water, modified through contact with productive shelf sediments with elevated levels of marine DOM. Within the surface layer of the Beaufort Gyre, meteoric water (river water and precipitation) was characterized by low concentrations of lignin and tDOM fluorescence proxies as DOM is removed during freezing. High‐resolution in situ fluorescence profiles revealed that DOM distribution closely followed isopycnals, indicating the strong influence of sea‐ice formation and melt, which was also reflected in strong correlations between DOM fluorescence and brine contributions. The relationship of DOM and hydrography to TEs showed that terrigenous and marine DOM were likely carriers of dissolved Fe, Ni, Cu from the Eurasian shelves into the central Arctic Ocean. Chukchi shelf sediments were important sources of dCd, dZn, and dNi, as well as marine ligands that bind and carry these TEs offshore within the upper halocline in the Canada Basin. Our data suggest that tDOM components represent stronger ligands relative to marine DOM components, potentially facilitating the long‐range transport of TE to the North Atlantic. Plain Language Summary: The Arctic Ocean receives a disproportionate large amount of global river discharge and has limited but well‐constrained exchanges with other oceans. This makes the Arctic Ocean unique in terms of dissolved organic matter (DOM) and trace element (TE) sources and distribution. We used data collected during two expeditions spanning the entire Arctic Ocean to characterize the unique distribution of DOM and to study its potential as a water mass tracer and its role in the transport of TE. While the Atlantic‐dominated Nansen Basin was characterized by low levels of the DOM and TE, the central Arctic was dominated by the Transpolar Drift, a current that connects the Eurasian shelves to the Fram Strait and transports DOM from the Siberian Rivers toward the North Atlantic. In contrast, the Chukchi shelf‐Canada Basin region was characterized by the dominance of Pacific water that is enriched by marine DOM from the shallow and productive Chukchi shelf. The distribution of DOM from these different sources was affected by freezing and thawing processes and, therefore, can be used to study water mass transformations and pathways in the Arctic Ocean. Key Points: Dissolved Organic Matter (DOM) distribution in the Arctic Ocean is largely controlled by sea ice formation and melt processesDOM distribution in the Arctic Ocean reveals its potential as a tracer for halocline formation and freshwater source assignmentsTerrigenous and marine DOM are carriers of trace elements from shelves to the open Arctic Ocean, but terrigenous DOM represent stronger ligands [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) in the Northeastern North Atlantic and Nordic Seas exhibits pronounced variability across seasonal to decadal time scales. These changes can be expected to be driven by a combination of altered local conditions, shifts in seasonality and large‐scale regional oceanographic change. Separating the contribution from each of these offers insight into how the region is changing. Here, we present the result of an analysis of weekly satellite‐derived SST data from 1979 to 2020. An empirical orthogonal function (EOF) analysis allows us to separate observed changes in SST into independent underlying timeseries. Each timeseries explains part of the variability in SST. EOF1 can be allocated with changes in seasonality and a long‐term warming trend, with summer maxima warming with twice the rate (0.043°C year−1) compared to winter minima (0.023°C year−1). EOF2 is associated with the North Atlantic subpolar gyre and the North Atlantic Oscillation, affecting the Atlantic Water flow across the Greenland‐Scotland Ridge, imposing a dipole cooling/warming pattern. Local sea‐ice melt along the southeast Greenland shelf is represented by EOF3, and finally the influx of warmer water with the North Icelandic Irminger Current is captured by EOF4. Each of these disaggregated signals differ considerably in their contribution to driving temporal and spatial trends in SST. The isolated signals offer a high‐resolution long‐time series of valuable indicators of oceanographic change which will likely be reflected in biogeochemistry, plankton, fish, mammals, and seabirds in the region. Plain Language Summary: Sea surface temperature (SST) can be measured by sensors mounted on satellites and this provides a data set with exceptional regional and temporal coverage from 1979 to present. Here, our focus is on examining oceanographic change in the Northeastern Atlantic and Nordic Seas, bordering Greenland, Iceland, and Norway. We apply a data analysis technique that allows us to separate the variability in SST in the region into a series of underlying factors. With this we can resolve: how the seasonal winter minimum temperatures and summer maximum temperatures have been increasing; how conditions in the North Atlantic are driving changes in the region; how increased sea‐ice melt is influencing SST; and finally trace the occurrence and impact of an abrupt inflow of warmer water northwards along the west coast of Iceland. Combined these disaggregated factors may help explain changes in the distribution and structure of the marine ecosystem in the region. Key Points: Seasonal fluctuations in Nordic Seas and Northeastern Atlantic sea surface temperatures explains 90% of the variabilityBoth summer maxima and winter minima are warming, with summer temperatures warming twice as fast (0.4 and 0.2°C per decade)Other sources of variability include sea‐ice melt and switches in large‐scale oceanographic conditions in the Northeastern Atlantic [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]

The export of dissolved organic carbon (DOC) from the Arctic Ocean is expected to change due to warming and increased river runoff. Here we present a method to quantify DOC transport with the East Greenland Current combining synoptic and year‐round data (2009–2018). An algorithm based on quantitative and qualitative aspects of colored dissolved organic matter (CDOM) was developed to provide DOC estimates. Combined with mooring‐derived monthly Polar Surface Water (PSW) volume transports, we estimate DOC exports for the period from 2003–2017. For much of the period DOC exports have been reasonably constant at 46.8 (±6.2) Tg yr−1, while the reduction in PSW export in recent years has resulted in lower annual DOC exports (below 39 Tg C yr−1). We now have a technique to resolve seasonal and annual fluctuations in Arctic carbon export, offering a significant improvement over earlier bulk estimates and represents a baseline to detect future change. Plain Language Summary: The Arctic Ocean has higher concentrations of dissolved organic carbon (DOC) compared to the other oceans due to the supply from large Siberian rivers that drain expansive carbon‐rich watersheds. Climate change in the Arctic is altering the supply of river water and DOC to the Arctic Ocean. This is eventually expected to also be reflected in the subsequent export of DOC from the Arctic to the North Atlantic via a major gateway, the Fram Strait. Here we present a new approach to quantify the transport of DOC through the Fram Strait based on year‐round mooring data and spectroscopic measurements of colored dissolved organic matter (CDOM). We calculate DOC transports using our measurements over a 10‐yr period, and the results provide a baseline from which to observe future changes in the Arctic Ocean that were not resolved with earlier estimates based on bulk concentrations and bulk volume transports. Key Points: Spectral characteristics of CDOM can predict DOC concentrations in the Fram StraitDOC transports are strongly correlated with Polar Surface Water volume transports from the Arctic Ocean with the East Greenland CurrentAnnual DOC transport estimates for 2009 to 2018 varied between 27 and 60 Tg C yr−1 [ABSTRACT FROM AUTHOR]

Arctic waters are often enriched with terrestrial dissolved organic matter (DOM) characterized by having elevated visible wavelength fluorescence (commonly termed humic‐like). Here, we have identified the sources of fluorescent DOM (FDOM) in a high Arctic fjord (Young Sound, NE Greenland) influenced by glacial meltwater. The biological transformation of FDOM was further investigated using plankton community size‐fractionation experiments. The intensity of ultraviolet fluorescence (commonly termed amino acid‐like) was highly variable and positively correlated to bacterial production and mesozooplankton grazing. The overall distribution of visible FDOM in the fjord was hydrographically driven by the high‐signal intrusion of Arctic terrestrial DOM from shelf waters and dilution with glacial runoff in the surface waters. However, the high‐intensity visible FDOM that accumulated in subsurface waters in summer was not solely linked to allochthonous sources. Our data indicate that microbial activity, in particular, protist bacterivory, to be a source. A decrease in visible FDOM in subsurface waters was concurrent with an increase in bacterial abundance, indicating an active bacterial uptake or modification of this DOM fraction. This was confirmed by net‐loss of visible FDOM in experiments during summer when bacterial activity was high. The degradation of visible FDOM appeared to be associated with bacteria belonging to the order Alteromonadales mainly the genus Glaciecola and the SAR92 clade. The findings provide new insight into the character of Arctic terrestrial DOM and the biological production and degradation of both visible and UV wavelength organic matter in the coastal Arctic. [ABSTRACT FROM AUTHOR]

Dissolved organic matter (DOM) concentrations in sediment porewaters are often orders of magnitude higher than in the overlying water column resulting in a diffusive flux of DOM from sediments. The intensity and fate of this DOM flux is poorly understood. The Gulf of Gdansk in the Southern Baltic Sea is dominated by the Vistula River, one of the largest and most anthropogenically impacted rivers in the Baltic Sea catchment. The sediment characteristics of the region are varied, from mixed sandy conditions near shore to mud sediments in the Gdansk Deep. We investigated the significance of sediment‐derived DOM in the Gulf of Gdansk in comparison to that supplied by the river. Sediment‐derived DOM in the region was found to have an organic matter fluorescence signature distinct from the DOM in the water column. The visible wavelength fluorescence could be used to distinguish organic matter from near shore sediments influenced by riverine sources and organic matter from deeper offshore sediments, influenced by more pelagic sources. Ultraviolet‐A wavelength fluorescence dominated the sediment flux but was rapidly removed in bottom waters suggesting that it may contribute to bottom water oxygen consumption. While there is potential for DOM fluxes from sediments in the Gulf of Gdansk, the high background pelagic concentration of DOM in these waters and the much stronger influence of the Vistula River can mask the contributions from sediments. Key Points: The Vistula river supplies large amounts of terrestrial dissolved organic matter to the Gulf of GdanskSediment‐derived dissolved organic matter has a unique fluorescent signatureFluorescent‐dissolved organic matter in the Gulf of Gdansk is dominated by pelagic sources [ABSTRACT FROM AUTHOR]

A portion of the freshwater transport through Fram Strait consists of low‐salinity Pacific‐derived Arctic water flowing southward along the east coast of Greenland. The pathways of this water are currently unclear. An Ice Tethered Profiler deployed over the southeastern Wandel Sea shelf (northeast Greenland) in May 2015 collected a profile every 3 hr for a year recording conductivity‐temperature‐depth (CTD) and Colored Dissolved Organic Matter (CDOM) fluorescence. This was accompanied by velocity observations. The CTD data revealed that the subsurface water (~15–85 m depth) characterized by high CDOM resembles the "cold Halostad" in the Canada Basin formed by the injection of Pacific water. A coastal branch of the Pacific water outflow from the Arctic Ocean supplies the Wandel Sea halostad, which shows a clear seasonal pattern. From July to October–November, the halostad is shallow, more saline, warmer, and with less CDOM. Conversely, from November to April, the halostad deepens, cools, freshens and CDOM increases, likely indicating a higher fraction of Pacific winter water. The CTD surveys, wind and current data, and numerical simulations show that the seasonal variation of wind over the continental slope likely controls seasonal changes of this intermediate water layer. Over northeast Greenland, winter winds have a northerly component from November to April, favoring Ekman transport of the Pacific‐derived water to the Wandel Sea shelf. In contrast, the prevailing southerly summer winds result in retreat of the Pacific‐derived water off the shelf. The landfast ice off‐slope extension modifies wind‐forcing disrupting seasonal patterns. Plain Language Summary: Arctic climate change is manifested in increased freshening of the surface seawater over recent decades due to increased precipitation, river runoff, and sea‐ice and glacier melt. Over the coastal domains of northern Greenland, contributions from glacial melt are superimposed on the low‐salinity surface water comprised by river runoff and low‐salinity water of Pacific origin, while intermediate water is primarily of Atlantic origin. The glacier meltwater fraction can be significant, but its quantification requires differentiation from the Pacific water, the second largest source of the fresh water in the Arctic Ocean. To date, the Pacific‐derived Arctic water component over the Greenland shelves remains poorly studied. The ice‐tethered oceanographic mooring deployed over the southeastern Wandel Sea shelf (northeast Greenland) in May 2015 collected oceanographic data for a year. The obtained data and model simulations suggest that the Wandel Sea subsurface layer down to about 80 m depth is supplied by the coastal branch of the Pacific water outflow from the Arctic Ocean. This layer also shows a clear seasonal pattern likely indicating a higher fraction of the Pacific‐derived water during winter. The seasonal variation of wind over the continental slope seems to control the Pacific water on‐shelf inflow. Key Points: The Pacific‐derived Arctic water modifies structure of the Wandel Sea haloclineDuring winter, northerly winds force the on‐shelf Ekman transport of Pacific water (PW)During summer, southerly winds favor coastal upwelling forcing PW off‐shelf [ABSTRACT FROM AUTHOR]

Abstract: The size‐dependent optical properties of dissolved organic matter (DOM) from four Swedish lakes were investigated using High Performance Size Exclusion Chromatography (HPSEC) in conjunction with online characterization of absorbance (240–600 nm) and fluorescence (excitation: 275 nm, emission: 300–600 nm). The molecular size of chromophoric DOM (CDOM) was consistently higher than that of fluorescent DOM (FDOM), with an average difference of 0.37 kDa. The relative abundance of FDOM vs. CDOM ranged from 0.3 to 0.7 across lakes, and increased with decreasing average molecular size. Across sites, the CDOM spectral slopes of the large molecular size fraction were highly similar while the low molecular size fraction differed and contributed to different bulk spectral slopes. Our results indicate structural congruence of high molecular size DOM across systems while lake trophic status determined the characteristics of the low size range. Furthermore, the combination of HPSEC and parallel factor analysis (HPSEC‐PARAFAC2) allowed the decomposition of DOM fluorescence chromatograms. Three humic‐like components and one protein‐like component with broadly overlapping molecular size distributions were identified. This overlap provides further evidence for the supramolecular assembly hypothesis since fluorophores, as revealed by PARAFAC2, occur in aggregates of overlapping molecular size. Our results further suggest a link between the molecular size of these fluorophores and the associated supramolecular assemblies. This study demonstrates the potential for HPSEC and novel mathematical approaches to provide unprecedented insights into the relationship between optical and chemical properties of DOM in aquatic systems. [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]

The Arctic Ocean receives a large amount of terrestrial dissolved organic matter (DOM) from rivers and more than half of this is removed during its passage through the Arctic Ocean. Terrestrial DOM is generally believed to have a low bioavailability and recent studies point to physicochemical processes such as sea ice formation as the source of the significant DOM removal in the Arctic Ocean. We present the results of a mesocosm experiment designed to investigate how sea ice formation affects DOM composition and bioavailability. We measured the change in different fluorescent dissolved organic matter (FDOM) fractions in sea ice, brines (contained in small pores between the ice crystals), and the underlying seawater during a 14 d experiment. Two series of mesocosms were used: one with seawater alone and one with seawater enriched with humic-rich river water. Abiotic processes increased the humic-like FDOM signal in the seawater below the ice during the initial ice formation. Humic-like FDOM fractions with a marine signal were preferentially retained in sea ice (relative to salinity), whereas humic-like FDOM with a terrestrial signal behaved more conservatively with respect to salinity. Amino acid-like FDOM and an unknown FDOM component, only previously found in Antarctic brines, were associated with biological activity and possibly extracellular polymeric substances in sea ice. An additional long-term (226-228 d) bioassay experiment with seawater collected from the mesocosm experiment revealed that 11% ± 2% of the bulk dissolved organic carbon (DOC) was bioavailable. However, 16% ± 12% of DOC expelled from the ice into the seawater below was bioavailable and the bioavailability of DOC in brine was even higher at 45%. DOM is highly susceptible to physicochemical changes during sea ice formation, leading to modifications in composition and increased bioavailability, which can in part explain terrestrial DOC removal in the Arctic Ocean. [ABSTRACT FROM AUTHOR]

Inland waters transport large amounts of dissolved organic matter ( DOM) from terrestrial environments to the oceans, but DOM also reacts en route, with substantial water column losses by mineralization and sedimentation. For DOM transformations along the aquatic continuum, lakes play an important role as they retain waters in the landscape allowing for more time to alter DOM. We know DOM losses are significant at the global scale, yet little is known about how the reactivity of DOM varies across landscapes and climates. DOM reactivity is inherently linked to its chemical composition. We used fluorescence spectroscopy to explore DOM quality from 560 lakes distributed across Sweden and encompassed a wide climatic gradient typical of the boreal ecozone. Six fluorescence components were identified using parallel factor analysis ( PARAFAC). The intensity and relative abundance of these components were analyzed in relation to lake chemistry, catchment, and climate characteristics. Land cover, particularly the percentage of water in the catchment, was a primary factor explaining variability in PARAFAC components. Likewise, lake water retention time influenced DOM quality. These results suggest that processes occurring in upstream water bodies, in addition to the lake itself, have a dominant influence on DOM quality. PARAFAC components with longer emission wavelengths, or red-shifted components, were most reactive. In contrast, protein-like components were most persistent within lakes. Generalized characteristics of PARAFAC components based on emission wavelength could ease future interpretation of fluorescence spectra. An important secondary influence on DOM quality was mean annual temperature, which ranged between −6.2 and +7.5 °C. These results suggest that DOM reactivity depends more heavily on the duration of time taken to pass through the landscape, rather than temperature. Projected increases in runoff in the boreal region may force lake DOM toward a higher overall amount and proportion of humic-like substances. [ABSTRACT FROM AUTHOR]

1. Dissolved organic carbon (DOC) concentration was determined for a range of lakes of varying conductivity (30–4000 μS cm−1) in the low Arctic of SW Greenland. DOC concentration range from <1 to >100 mg C L−1, occasionally approaching 200 mg C L−1 in meromictic, oligosaline lakes. DOC concentration was strongly related to [log10] conductivity and total nitrogen. 2. Peak DOC concentrations (>80 mg L−1) occur in lakes located approximately 50 km from the present ice sheet margin, a zone of low effective precipitation; evaporative concentration is the first-order control on DOC concentration. Lakes at the coast and closer to the ice margin had lower DOC concentrations (<20 mg C L−1). Local factors, notably the presence or absence of an outflow and catchment morphometry, resulted in considerable variability in concentration (20–100 mg C L−1) within the area of maximum concentration around 51°W. 3. Despite their high DOC concentration, these lakes are essentially colourless. Dissolved organic matter (DOM) absorption ( a375) was low in most lakes (<10 m−1) with maximum values (approximately 20 m−1) occurring in one humic-stained lake in the area. Absorption values corrected for DOC concentration ( ) were very low (<0.6 m2 g−1 C) for all lakes apart from those at the coast, perhaps reflecting greater allochthonous inputs at these sites. 4. S, the spectral slope coefficient, ranged from 16 to 27 μm−1 and was weakly correlated with DOC concentration. Both a375 and S showed similar distribution patterns along the sampling gradient as did DOC, with maximum values at approximately 51°W. High and low S may indicate fresher, more rapidly flushed, systems with less degraded DOM or greater inputs from their catchments. 5. The lakes closer to the head of the fjord with higher conductivity, had low (<0.2 m2 g−1 C) and high S (>21 μm−1) and this may reflect increasingly longer lake water residence times, greater DOM age and photochemical degradation. [ABSTRACT FROM AUTHOR]

We examined how variations in algal-bacterial community structure relate to Cu, Zn, and Mn speciation during a diatom-rich bloom that was induced by daily additions of inorganic macronutrients to fjord waters in August 2002. The experiments were carried out in 11-m³ floating mesocosm bags deployed in the Raunefjord, near Bergen, Norway, and operated in a chemostat (flow-through) mode. Copper speciation was controlled by the formation of very strong organic complexes (log K′1 = 15.2-15.8; log K′2 = 13.0-13.4) whose likely source was the cyanobacterium Synechococcus sp. Strong ligand concentrations were comparable to dissolved Cu levels. This covariation kept the free Cu2+ concentration within the range of l0-12.4 to 10-11.2 mol L-1, i.e., below the toxicity threshold for Synechococcus. Weaker ligands (log K′3 = 8.2-9.4) were released during--and up to 4 d following--the exponential growth of algae. During this period, the weaker Cu-binding ligands appeared to have the same source or production process as the proteinlike fluorophores detected in these coastal waters. Zinc speciation was controlled by complexation with a single class of organic ligands that appeared to be released inadvertently upon the death and/or grazing of phytoplankton. Labile manganese fluctuations were inversely synchronized with the abundance of heterotrophic bacteria until the coastal waters experienced a massive rain event on day 17 of the experiment. The rainfall, which was a source of nitrogen and micronutrients, appeared to stimulate the growth of larger cells (diatoms) but to inhibit that of the smaller cells (heterotrophic bacteria and cyanobacteria). [ABSTRACT FROM AUTHOR]

We present the results of a mesocosm experiment investigating the production and utilization of autochthonous dissolved organic matter (DOM) by the plankton community under different inorganic nutrient regimes. Fluorescence spectroscopy combined with parallel factor analysis was applied to study the dynamics of autochthonous DOM. Seven independent fluorescent fractions were identified, differing in their spectral characteristics, production rates, and sensitivity to photochemical and microbial degradation processes. Five different humic fractions, a marine protein, and a peptide fluorescence were found. The five humic fractions were produced microbially, with the greatest production occurring under combined Si- and P-limiting conditions. The two proteinaceous fractions were produced during exponential growth of phytoplankton, irrespective of biomass composition. Photodegradation was an important sink for the microbially derived humic material, and the marine protein material was susceptible to both photo-and microbial degradation. [ABSTRACT FROM AUTHOR]

Excitation emission matrix fluorescence spectroscopy combined with PARAFAC analysis provides a fast and effective method of characterizing the fluorescent fraction of dissolved organic matter (DOM). Fluorescence measurements can be used as a tracer for quantitative and qualitative changes occurring in the DOM pool as a whole. An earlier study found that the fluorescence signal could be modeled by five fractions. This study presents an analysis on a considerably larger data set (>1,200 samples) resulting from a 1-yr sampling program in Horsens Estuary, Denmark. Eight fluorescent fractions were identified. Four biogenic terrestrial, two anthropogenic, and two protein-like fractions were identified. Analysis of covariation between the components identified source-specific fractions and the presence of common factors controlling the composition of terrestrial DOM exported from different catchments. [ABSTRACT FROM AUTHOR]

Freshwater from the Arctic participates in the globally important Atlantic Meridional Overturning Circulation (AMOC). We use high‐resolution, in situ observations of dissolved organic matter (DOM) fluorescence to trace the origins of freshwater and organic carbon in the densest component of the AMOC, namely Denmark Strait Overflow Water (DSOW). We find a distinct terrestrial DOM signal in DSOW and trace it upstream to the Siberian shelves in the Arctic Ocean. This implies a riverine origin of freshwater in DSOW. We estimate that the Siberian Shelf water contribution constitutes approximately 1% of DSOW. Ocean circulation modeling confirms the inferred pathway and highlights Denmark Strait as an important location for the entrainment of the riverine signal into DSOW. Our proposed method can be deployed on a range of observing systems to elucidate freshwater dispersion across the Arctic and subarctic, thereby contributing to the broader discussion on freshwater impacts and organic carbon sequestration in the AMOC. [ABSTRACT FROM AUTHOR]

Data from a long time series of temperature, salinity, and nutrient measurements in Disko Bay (West Greenland) reveal a marked change in the water characteristics during recent years. Seasonal dynamics in the upper 150 m of the water column were highly affected by the seasonality in meteorological conditions, while the deeper water strata were more stable and were primarily influenced by large-scale circulation patterns. There was a marked increase in the average water temperatures at 200-m depth in spring 1997, with the long-term average increasing from 1.30°C to 2.25°C. Weekly data from 1996 to 1997 show that the sudden change in bottom water occurred in April 1997, due to the inflow of a larger proportion of North Atlantic water, which propagated north along the coast before entering the bay. Further support for this inflow was found when tracing the relative proportion of Atlantic water in the bay, using inorganic nutrients. These changes in the oceanography of the bay will not only lead to further glacial retreat but will also affect the local marine ecosystem by changing the relative dominance of major copepod species that overwinter in bottom waters of the bay. [ABSTRACT FROM AUTHOR]