Terrestrial Dissolved Organic Matter Research Articles

Karenia brevis and Pyrodinium bahamense Utilization of Dissolved Organic Matter in Urban Stormwater Runoff and Rainfall Entering Tampa Bay, Florida

This study investigated how nitrogen and dissolved organic matter (DOM) from stormwater runoff and rainfall support the growth of Karenia brevis and Pyrodinium bahamense. Excitation–emission matrix spectroscopy coupled with parallel factor analysis tracked changes in the optical properties of DOM in each bioassay, revealing greater reactivity of terrestrial humic-like DOM. Significant increases in cell yield and specific growth rates were observed upon additions of runoff for both species, with significant increases in specific growth rates upon the addition of a 2 in simulated rain event for P. bahamense only. By hour 48, 100% of the dissolved organic nitrogen (DON) in each treatment was utilized by P. bahamense, and by hour 72, over 50% of the DON was utilized by K. brevis. The percentage of bioavailable dissolved organic carbon (DOC) was greater for P. bahamense compared to K. brevis, suggesting a greater affinity for DOC compounds by P. bahamense. However, the bioavailability of DOM for each species could be owed to distinct chemical characteristics of labile DOM conveyed from each site. This study demonstrates that stormwater runoff and rainfall are both sources of labile DOM and DON for K. brevis and P. bahamense, which has implications for blooms of these species in Tampa Bay waters.

Storm-driven hydrological, seasonal, and land use/land cover impact on dissolved organic matter dynamics in a mid-Atlantic, USA coastal plain river system characterized by 21 T FT-ICR mass spectrometry

Introduction: Dissolved organic matter (DOM) as primary and secondary energy sources can be transported via rivers to estuaries and impact coastal biogeochemical cycles. Storm-induced high discharge events can alter the dominant river flow paths and enhance leaching of shallow organic-rich soil layers, leading to elevated terrestrial DOM export. Land use/land cover (LULC) and associated anthropogenic interventions (including artificial reservoirs and agricultural irrigation) can control sources and transformation processes of exported DOM along with hydrologic factors. The relative significance of LULC, hydrological factors, and temperature variations with seasons will differ depending on geographical locations and complicate their incorporation in biogeochemical models of DOM dynamics. This study investigates the role of LULC, seasonality, and storm events on DOM concentrations and molecular composition in the Murderkill River system.Method: Surface water samples were collected seasonally and before/after storm events from 6 sites representing forested, agricultural, and developed LULC units. The DOM was characterized via parallel factor analysis of excitation-emission matrix data and electrospray ionization 21 T Fourier-transform ion cyclotron resonance mass spectrometry to determine potential DOM sources and enable the development of a conceptual model for DOM dynamics in rivers impacted by anthropogenic reservoirs.Result and Discussion: Our results suggest that storm-induced shallow and overland flow paths can increase surface-vegetation/plant-litter derived DOM based on atomic ratios associated with specific biogenic precursors (i.e., lignin, tannins, and/or oxygenated aromatic DOM), particularly in winter when autochthonous production was suppressed due to reduced temperatures. We further demonstrate that the damming effects of artificial reservoirs enhance the role of seasonal patterns of autochthonous production, disrupting storm-shunt process and stimulating significantly more bio-produced DOM export during spring and summer (i.e., tryptophan/tyrosine-like. N- and S- containing, phytoplankton-derived compounds). Collectively, these results demonstrate how artificial reservoirs alter the characteristics of DOM exported from rivers with implications for understanding carbon export and fate at river-estuary interfaces.

Terrestrial dissolved organic matter inputs accompanied by dissolved oxygen depletion and declining pH exacerbate CO2 emissions from a major Chinese reservoir

Terrestrial inputs and subsequent degradation of dissolved organic matter (DOM) in lake ecosystems can result in rapid depletion of dissolved oxygen (DO). Inputs of terrestrial DOM including organic acids can also lead to decreases in pH. However, to date, few studies have investigated the linkages between terrestrial DOM inputs, DO and pH levels in the water column, and carbon dioxide (CO2) emissions from lake ecosystems. Based on monthly field sampling campaigns across 100 sites in Lake Qiandao, a major man-made drinking water reservoir in China, from May 2020 to April 2021, we estimated an annual CO2 efflux (FCO2) of 37.2 ± 29.0 gC m−2 yr−1, corresponding to 0.02 ± 0.02 TgC yr−1 from this lake. FCO2 increased significantly with decreasing DO, chlorophyll-a (Chl-a) and δ2H-H2O, while FCO2 increased with increasing specific UV absorbance (SUVA254) and a terrestrial humic-like component (C2). We found that DO concentration and pH declined with increasing terrestrial DOM inputs, i.e. increased SUVA254 and terrestrial humic-like C2 levels. Vertical profile sampling revealed that the partial pressure of CO2 (pCO2) increased with increasing terrestrial DOM fluorescence (FDOM), while DO, pH, and δ13C-CO2 declined with increasing terrestrial FDOM. These results highlight the importance of terrestrial DOM inputs in altering physico-chemical environments and fueling CO2 emissions from this lake and potentially other aquatic ecosystems.

Proton Binding Characteristics of Dissolved Organic Matter Extracted from the North Atlantic.

Marine dissolved organic matter (DOM) presents key thermodynamic properties that are not yet fully constrained. Here, we report the distribution of binding sites occupied by protons (i.e., proton affinity spectra) and parametrize the median intrinsic proton binding affinities (log K̅H) and heterogeneities (m), for DOM samples extracted from the North Atlantic. We estimate that 11.4 ± 0.6% of C atoms in the extracted marine DOM have a functional group with a binding site for ionic species. The log K̅H of the most acidic groups was larger (4.01-4.02 ± 0.02) than that observed in DOM from coastal waters (3.82 ± 0.02), while the chemical binding heterogeneity parameter increased with depth to values (m1= 0.666 ± 0.009) ca. 10% higher than those observed in surface open ocean or coastal samples. On the contrary, the log K̅H for the less acidic groups shows a difference between the surface (10.01 ± 0.08) and deep (9.22 ± 0.35) samples. The latter chemical groups were more heterogeneous for marine than for terrestrial DOM, and m2 decreased with depth to values of 0.28 ± 0.03. Binding heterogeneity reflects aromatic carbon compounds' persistence and accumulation in diverse, low-abundance chemical forms, while easily degradable low-affinity groups accumulate more uniformly in the deep ocean.

DOM Associates with Greenhouse Gas Emissions in Chinese Rivers under Diverse Land Uses.

Growing evidence indicates that rivers are hotspots of greenhouse gas (GHG) emissions and play multiple roles in the global carbon budget. However, the roles of terrestrial carbon from land use in river GHG emissions remain largely unknown. We studied the microbial composition, dissolved organic matter (DOM) properties, and GHG emission responses to different landcovers in rivers (n = 100). The bacterial community was mainly constrained by land-use intensity, whereas the fungal community was mainly controlled by DOM chemical composition (e.g., terrestrial DOM with high photoreactivity). Anthropogenic stressors (e.g., land-use intensity, gross regional domestic product, and total population) were the main factors affecting chromophoric DOM (CDOM). DOM biodegradability exhibited a positive correlation with CDOM and contributed to microbial activity for DOM transformation. Variations in CO2 and CH4 emissions were governed by the biodegradation or photomineralization of dissolved organic carbon derived from autotrophic DOM and were indirectly affected by land use via changes in DOM properties and water chemistry. Because the GHG emissions of rivers offset some of the climatic benefits of terrestrial carbon (or ocean) sinks, intensified urban land use inevitably alters carbon cycling and changes the regional microclimate.

Bioactivity profile of dissolved organic matter and its relation to molecular composition

Dissolved organic matter (DOM) occupies a huge and uncharted molecular space. Given its properties, DOM can be presented as a promising biotechnological resource. However, research into bioactivities of DOM is still in early stages. In this study, the biotechnological potential of terrestrial and marine DOM, its molecular composition and their relationships are investigated. Samples were screened for their in vitro antibacterial, antifungal, anticancer and antioxidant activities. Antibacterial activity was detected against Staphylococcusaureus in almost all DOM samples, with freshwater DOM showing the lowest IC50 values. Most samples also inhibited Staphylococcusepidermidis, and four DOM extracts showed up to fourfold higher potency than the reference drug. Antifungal activity was limited to only porewater DOM towards human dermatophyte Trichophytonrubrum. No significant in vitro anticancer activity was observed. Low antioxidant potential was exerted. The molecular characterization by FT-ICR MS allowed a broad compositional overview. Three main distinguished groups have been identified by PCoA analyses. Antibacterial activities are related to high aromaticity content and highly-unsaturated molecular formulae (O-poor). Antifungal effect is correlated with highly-unsaturated molecular formulae (O-rich). Antioxidant activity is positively related to the presence of double bonds and polyphenols. This study evidenced for the first time antibacterial and antifungal activity in DOM with potential applications in cosmeceutical, pharmaceutical and aquaculture industry. The lack of cytotoxicity and the almost unlimited presence of this organic material may open new avenues in future marine bioprospecting efforts.Graphical abstract

Mercury Sources and Fate in a Large Brackish Ecosystem (the Baltic Sea) Depicted by Stable Isotopes.

Identifying Hg sources to aquatic ecosystems and processes controlling the levels of monomethylmercury (MMHg) is critical for developing efficient policies of Hg emissions reduction. Here we measured Hg concentrations and stable isotopes in sediment, seston, and fishes from the various basins of the Baltic Sea, a large brackish ecosystem presenting extensive gradients in salinity, redox conditions, dissolved organic matter (DOM) composition, and biological activities. We found that Hg mass dependent fractionation (Hg-MDF) values in sediments mostly reflect a mixing between light terrestrial Hg and heavier industrial sources, whereas odd Hg isotope mass independent fractionation (odd Hg-MIF) reveals atmospheric inputs. Seston presents intermediate Hg-MDF and odd Hg-MIF values falling between sediments and fish, but in northern basins, high even Hg-MIF values suggest the preferential accumulation of wet-deposited Hg. Odd Hg-MIF values in fish indicate an overall low extent of MMHg photodegradation due to limited sunlight exposure and penetration but also reveal large spatial differences. The photodegradation extent is lowest in the central basin with recurrent algal blooms due to their shading effect and is highest in the northern, least saline basin with high concentrations of terrestrial DOM. As increased loads of terrestrial DOM are expected in many coastal areas due to global changes, its impact on MMHg photodegradation needs to be better understood and accounted for when predicting future MMHg concentrations in aquatic ecosystems.

Assessing spatiotemporal variability in the concentration and composition of dissolved organic matter and its impact on iron solubility in tropical freshwater systems through a machine learning approach

Dissolved organic matter (DOM) plays important roles not only in maintaining the productivity and functioning of aquatic ecosystems but also in the global carbon cycle, although the sources and biogeochemical functions of terrestrially derived DOM have not been fully elucidated, particularly in the tropics and subtropics. This study aimed to evaluate the factors influencing spatiotemporal variability in (i) the concentration and composition of DOM, including dissolved organic carbon (DOC), ultraviolet absorption coefficient at 254-nm wavelength (a254), and components identified by fluorescence excitation–emission matrix coupled with parallel factor analysis (EEM-PARAFAC), and (ii) the concentration of dissolved iron (DFe) across freshwater systems (rivers, forested streams, and dam reservoirs) on a tropical island (Ishigaki Island, Japan) based on the results of water quality monitoring at 2-month intervals over a 2-year period. Random forests (RF) machine learning algorithm was employed, with the catchment characteristics (land use, soil type) and water temperature as the predictor variables for DOM and the composition of DOM (EEM-PARAFAC components) and hydrochemistry (water temperature, pH, and concentrations of divalent cations) as the predictor variables for DFe. The RF models for DOC, a254, and three humic-like components exhibited excellent predictive performance, indicating that these DOM properties are not only seasonally variable but also strongly influenced by the compositions of land uses and soil types in the upstream watershed. Poorly drained riparian lowland soil (Gleyic Fluvisols) was identified as the most important catchment parameter that positively influences these DOM variables. The RF model also explained a large portion of the variation in DFe, while terrestrial humic-like components were the most important parameters, emphasizing their significance as organic ligands for iron. These results improve our understanding of the impacts of terrestrial DOM and iron loadings on tropical and subtropical coastal ecosystems as well as on regional and global carbon budgets.

Rethinking terrestrial dissolved organic matter in dam reservoirs before mixing: Linking photodegradation and biodegradation and the phenanthrene binding behavior

With the increased construction of dam reservoirs and the demand for water security, terrestrial dissolved organic matter (DOM) has received attention because of its role in regulating water quality, ecological functions, and the fate and transport of pollutants in dam reservoirs. This study investigated the transformations of soil DOM and vegetation DOM of dam reservoirs following photodegradation and biodegradation before conservative mixing, as well as the resultant effects on phenanthrene binding. Based on the results, terrestrial DOM could undergo transformation via photodegradation and biodegradation before conservative mixing in dam reservoirs. Although both processes resulted in substantial decreases in DOM concentrations, the changes in chromophoric DOM and fluorescent DOM depended on the original DOM sources. Furthermore, the photodegradation of terrestrial DOM resulted in more pronounced photobleaching than photomineralization. In addition, photodegradation of terrestrial DOM resulted in the generation of DOM-derived by-products with low molecular weight and low aromaticity, whereas the biodegradation of terrestrial DOM resulted in DOM-derived by-products with low molecular weight and high aromaticity. Subsequently, the photodegradation and biodegradation of terrestrial DOM substantially enhanced the binding affinity of phenanthrene. Soil DOM is prior to vegetation DOM when predicting the ecological risk of HOCs. These results indicate that the terrestrial DOM in dam reservoirs should be reconsidered before conservative mixing. Further studies on the coupling effects of both biogeochemical processes, as well as on the relative contributions of soil DOM and vegetation DOM after transformation to the aquatic DOM in dam reservoirs, are required. This study provides information on the environmental effects of dam construction from the perspective of biogeochemical processes.

Photochemical transformation of terrestrial dissolved organic matter derived from multiple sources in tropical plantations

Photochemical transformation is a critical geochemical fate of terrestrial dissolved organic matter (DOM) that drains from forests to downstream water bodies. However, the photo-degradability and photo-transformation of terrestrial DOM from various sources are yet to be explored. Here, optical spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry were used to analyze photochemical change in spectroscopic and molecular-level signatures of DOM derived from throughfall, stemflow, runoff, and soil pore water at depths of 20 and 40 cm in tropical typical Eucalyptus urophylla (EU) and Acacia auriculiformis (AA) plantations. The dissolved organic carbon concentrations were generally higher in the EU plantation than in the AA plantation and higher in stemflow than in other sample types. The greatest DOM degradation occurred in soil pore water and the least degradation occurred in stemflow. Similarly, the common photo-transformation of DOM, including decreased aromaticity and unsaturation degrees, was greatest for soil pore water and the least for stemflow DOM. Most of the photochemical products were high H/C and O/C molecules for throughfall, stemflow, and pore water, but high H/C and low O/C molecules for runoff because of strong decarboxylation. These findings reveal the highly varied molecular composition and photo-degradability of DOM from different terrestrial sources and highlight that stemflow is an important contributor to photo-resistant DOM among terrestrial DOM sources.

Linking the source, molecular composition, and reactivity of dissolved organic matter in major rivers across the pearl river delta

Dissolved organic matter (DOM) constitutes a substantial portion of the global carbon cycle and serves as the largest reservoir of reactive carbon on Earth. Studying DOM's sources, molecular compositions, and reactivity is central to understanding the fate and environmental effects of organic carbon, yet, links between DOM composition and reactivity remain to be established. In this study, stable carbon isotopic composition and ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) were combined with degradation experiments to comprehensively study DOM's properties in the major rivers across the Pearl River Delta (PRD) in South China. The increasing δ13C-DOC along the Pearl River at the PRD suggests that terrestrial DOM from upstream was gradually diluted by local processes. Although affected by different interaction patterns of multiple sources, a major component of DOM, enriched in CHO and lignins, is molecularly indistinguishable in different rivers (molecular Bray-Curtis dissimilarity of 15%–31%). The photo-degradation removed 34% (PDOC%) of initial DOM within 14 days, twice that by biodegradation (BDOC%). In contrast to earlier research, the distinctive integration of molecular characterization with DOM degradation kinetics indicates that both regional (common shared molecules) and local (partial shared and unique molecules) derived DOM play a collective role in governing the bio-degradation kinetics of DOM, while the photo-degradation kinetics of DOM are barely correlated to molecular composition. The thorough investigation of DOM molecular composition and its correlation with bulk DOM properties in the PRD region, though requiring further investigation into its mechanisms, offers valuable insights into the role of riverine DOM in coastal hypoxia and the global carbon cycle.

Composition, variation and contribution of chromophoric dissolved organic matter in Laizhou Bay estuaries, North China

To address the complex biogeochemical processes and potential function of dissolved organic matter (DOM) in estuaries before it enters the sea, water samples were collected seasonally in a dozen Laizhou Bay estuaries and in the Yellow River estuary, North China. The results showed that chromophoric dissolved organic matter (CDOM) varied in abundance but had consistent spectral slopes at the same sampling time. Gradually, CDOM decreased while spectral slopes increased from freshwater to seawater. The spectral slope of CDOM varied temporally (higher in October and lower in August) more than it varied spatially. The fluorescent components and biological indices of CDOM indicated that in situ biological activities were the dominant sources in the waters examined in this study. The CDOM was generally more variable than the dissolved organic carbon (DOC) in most of the estuaries. However, marine CDOM may obviously increase in estuaries when indirectly stimulated by rapidly decomposing terrestrial DOM.

Linkages between fluorescent dissolved organic matter and lignin phenols in the Canada Basin, Arctic Ocean

Lignin is one of the more refractory components of dissolved organic matter (DOM) and its concentration and composition are used to trace terrigenous DOM in the ocean. In this study, lignin oxidation products (LOP), and chromophoric and fluorescent DOM were measured in Polar Mixed Layer (PML), Atlantic water (AW), and Canada Basin Deep Water (CBDW) layers in the Canada Basin to elucidate the composition and source of terrestrial DOM. Significant differences in total lignin phenol concentration (Σ11) and cinnamyl to vanillyl ratio (C/V) measured with CuO oxidation in PML, AW, and CBDW samples were found. The fluorescence intensities of humic-like C1 were correlated with Σ11 in PML, congruent with the humic-like terrestrial origin. However, no significant correlation was found when all layers are considered (p > 0.05), suggesting a change in the molecular composition of C1. The protein-like C3 and C5 intensities were strongly correlated with Σ11 (rs = 0.55–0.64) corroborating that lignins can contribute to the protein-like fluorescence in the water column. The principal component analysis (PCA) confirmed the origin and composition of DOM vary between the different water masses, especially in terms of C/V, DOC, and fluorescent components. Significant relationships between humic-like fluorescence intensities and C/V reveal the significant contribution of woody sources to humic material in the Canada Basin. Elevated S/V and humic-like C6 levels were associated with a cold eddy DOM sample but further studies are required to validate the use of these DOM characteristics to trace deep eddy.

Reverse osmosis pretreatment by a fluidized magnetic ion exchange resin for application in a power plant: Pilot-scale and high-rate operations and foulant characterization

Biofouling is one of the most serious problems in reverse osmosis (RO) operations. Here, we operated a pilot-scale MIEX® system with a high-rate configuration as a pretreatment for an RO system in a water purification plant. The goal was to investigate the RO foulants obtained with and without pretreatment by MIEX®. The MIEX® resin (10 to 300 mL/L) removed dissolved organic matter (DOM) within 5 min. A pilot-scale fluidized reactor with 100 mL/L of MIEX® removed 35.6%, 49.4%, and 56.2% of dissolved organic carbon (DOC) for effective resin doses of 0.03, 0.06, and 0.12 mL/L, respectively. Unknown screening analysis by Orbitrap mass spectrometry revealed that the MIEX® pretreatment decreased or removed 165–363 DOM features with carbon, hydrogen, and oxygen (CHO features) and 145–220 CHO features with N heteroatoms (CHON features) with effective resin doses of 0.03–0.12 mL/L. Moreover, fluorescence EEM revealed the preferential removal of terrestrial DOM, resulting in over half of the protein-like substances remaining in the RO feed. MIEX® selectively removed highly oxygenated DOM features within a short contact time, while condensed aromatic structures and lignin-like compounds were removed after 30 min. Increasing the MIEX® dose resulted in the additional removal of DOM features with less oxygenated characters and longer alkyl chains. The RO system with and without MIEX® pretreatment was similarly fouled by protein-like substances and CHON features; however, the MIEX® pretreatment decreased CHO features and terrestrial DOM foulants. The MIEX® pretreatment decreased organic and irreversible fouling and doubled the RO system lifetime, resulting in lower chemical cleaning and pressure requirements for an identical permeate volume.

Improved Predictions of Dissolved Organic Carbon–Water Partitioning and Unveiling the Chemodiversity of Dissolved Organic Matter

Dissolved organic matter (DOM) is a complex mixture of organic compounds with chemical heterogeneity in natural soils, sediments, and waters. Predicting the dissolved organic carbon (DOC)–water partition coefficients (KDOC) of organic pollutants to DOMs from various sources is important for assessing their fate and bioavailability. However, the accuracy of the recently developed polyparameter linear free energy relationship (pp-LFER) KDOC models is generally restricted by the small data set, inclusion of experimental data from unreliable measurement methods, or undefined selection standards. The aim of this study was to establish pp-LFER models for predicting the KDOC of nonionic organic chemicals to a variety of sources of DOMs and to get an understanding of DOM chemodiversity. A reliable and expanded experimental KDOC data set was compiled. Improved pp-LFER models were developed and assessed for All DOM (DOM from all sources), Aldrich humic acid (HA), Roth HA, soil porewater DOM, sediment porewater DOM, natural aquatic DOM, natural terrestrial DOM, natural DOM, and commercial DOM. The models developed in this study were reasonably robust and accurate with a root mean square error (RMSE) of 0.538 for the All DOM model and RMSE from 0.196 to 0.860 for the specific DOMs. Also, the models generally performed better than previously published ones. Moreover, the system parameters of the models well described the chemical variabilities between soil porewater DOM and sediment porewater DOM, between natural aquatic DOM and natural terrestrial DOM, and between natural DOM and commercial DOM regarding polarizability, dipolarity, H-bond-accepting and -donating ability, and cavity formation energy. This study provides effective tools to assess the tendencies of organic chemicals to DOMs and improves our understanding of the chemical heterogeneity of DOMs from different sources.

Overlooked great role of wind erosion in terrestrial dissolved organic matter input to lake ecosystem in cold and arid regions

Lakes in cold and arid regions have weak hydrological connectivity to their watersheds and serious soil erosion by wind, and are particularly sensitive to changes in underlying surface process and global climate changes, which may result in unique carbon cycles across the terrestrial-aquatic interface and ecological impacts. However, the roles of input pathways of terrestrial dissolved organic matter (TDOM) to lakes in cold and arid regions, especially the effects of possible TDOM input caused by wind erosion, has not been fully understood. Taking a typical lake in cold and arid regions as an example, this study comprehensively investigated the characteristics and contributions of dissolved organic matter (DOM) input from different TDOM input pathways, ultimately highlighted the impacts of wind erosion from composition characteristics, historical changes and universal proofs. The results showed that DOM introduced by wind erosion contributed 37.34 % to all TDOM input, and exhibited the strongest humification and aromaticity, and greatest molecular weight and stability. The considerable input and refractory characteristics led to the differences in the TDOM distribution and DOM composition between the near-wind and far-wind shores of the lake. Moreover, historical analysis showed that wind erosion became the main way driving the changes in buried terrestrial organic matter in the lake through a combined action of precipitation and land cover changes after 2008. The pervasive importance of wind erosion pathways on TDOM inputs in the cold and arid regions was further demonstrated through the proof from two other representative lakes. The findings also shed light on the possible impacts of wind erosion on material distribution, aquatic productivity, and energy input in lake ecosystems. The study provides new insights to broaden the content of global lake-landscape interactions and regional ecosystem conservation.

Source characterization of dissolved organic matter in the eastern Beagle Channel from a spring situation

Dissolved organic matter (DOM) was analyzed by its optical and fluorescent properties in order to study its distribution, source and transformation in relation to microbial abundances, chlorophyll and physicochemical gradients along a transect in the eastern Beagle Channel (BC) during austral spring 2019. Moreover, DOM evolution was followed during three tidal cycles in two fixed stations (F1, F2) separated by the Mackinlay Strait, a hydrographic frontier between water masses under stronger continental influence (inner sector) and that of less modified Sub-Antarctic waters (outer sector). The fluorescence signals of protein- and humic-like compounds were used as a proxy for labile and non-labile material, respectively, while the concentration of nano-, pico- and virioplankton populations, chlorophyll-a (Chl-a) and dissolved organic carbon (DOC) were used as a proxy for biological activity. Moreover, several spectroscopic indices, such as the indicator of microbially produced DOM (BIX), the Freshness index (FIX), an indicator of molecular weight, the Slope Ratio (SR) and an indicator of chromophoric DOM (CDOM), were used to trace the quality and origin of DOM. Chl-a concentration was generally low in the sampling area (< 1 μg L−1) but increased towards the end of the campaign and especially during the visit of F1 along with the concentration of picoeukaryotes and the protein-like fluorophores evidencing the onset of a bloom event. Protein-like fluorophores peaked in the euphotic zone of the inner sector of the BC while humic-like material of terrestrial origin was homogeneously distributed throughout the water column and showed a decreasing gradient towards the outer sector and Atlantic waters. Furthermore, the inner sector was characterized by higher amounts of CDOM, high molecular weight material and more recalcitrant DOM. Accumulation of autochthonous humic-like material in bottom waters of the inner sector along with enhanced values of FIX and BIX preceded by colored organic material suggest an efficient functioning of the microbial carbon pump fueled by terrestrial DOM sources in these waters.

Characterizing Uncertainty in Pan‐Arctic Land‐Ocean Dissolved Organic Carbon Flux: Insights From the Onega River, Russia

AbstractDissolved organic carbon (DOC) flux from rivers in the pan‐Arctic watershed represents an important connection between major terrestrial carbon stocks and the Arctic Ocean. Previous estimates of Arctic carbon flux and dissolved organic matter (DOM) seasonal dynamics have relied predominantly on measurements from the six major Arctic rivers, yet these may not be representative of northern high‐latitude constrained smaller watersheds. Here, we evaluate DOC concentration and DOM composition in the Onega River, a small Arctic watershed, using optical measurements and ultrahigh resolution mass spectrometry. Compared to the six largest Arctic rivers, DOC, absorbance at a350, and indicators of terrestrial DOM (e.g., specific UV absorbance at 254 nm, modified aromaticity index, relative abundance of condensed aromatics and polyphenolics) were elevated in the Onega throughout the year. Seasonality was also generally muted in comparison to the major Arctic rivers with relatively elevated DOC and terrestrial markers in both spring and fall seasons. The Onega exhibits a strong relationship between a350 and DOC, and its organic‐rich nature is apparent in its high DOC yield (4.85 g m2yr−1), and higher chromophoric DOM per unit DOC than the six largest Arctic rivers. As DOC yield from the Onega may be more representative of smaller northern high‐latitude rivers, we derived a new pan‐Arctic DOC flux scaling estimate which is over 50% higher than previous estimates scaled solely from the six major Arctic rivers. These observations suggest that smaller northern high‐latitude rivers may be underrepresented in Arctic carbon flux models and highlights uncertainty around constraining the export of DOC to the Arctic Ocean.

Trees in the Stream: Determining Patterns of Terrestrial Dissolved Organic Matter Contributions to the Northeast Pacific Coastal Temperate Rainforest

AbstractDissolved organic matter (DOM) composition in small watersheds depends on complex antecedent conditions that ultimately influence DOM generation, processing, and stability downstream. Here, we used ultrahigh resolution Fourier‐transform ion cyclotron resonance mass spectrometry and total dissolved nitrogen and dissolved organic carbon concentrations to investigate how DOM is produced in distinct sub‐catchment types (poor fen, forested wetland, and upland forest) and transported through a watershed in the northeast Pacific coastal temperate rainforest (NPCTR). We traced a suite of previously identified source‐specific marker formulae from vegetation and soil downstream and used them to test models of terrestrial DOM inputs. Marker formulae escaped microbial degradation and were exported from the watershed, demonstrating strong land‐to‐ocean connectivity through the transfer of unmodified tree DOM from specific tree species into the marine environment. Simple hydrologic and temperature variables were better able to predict inputs of soil‐sourced DOM into the stream network than tree‐sourced DOM, highlighting the role of antecedent conditions (e.g., plant growth stage) in DOM source availability and hydrologic flow connectivity, particularly for plant‐derived material. Forested wetland pore waters featured thousands of nitrogen‐containing molecular formulae that potentially provide a path of direct organic nitrogen uptake to organisms. The modified aromaticity index peaked in midsummer (up to 0.55 for fen headwaters) suggesting DOM inputs from freshly produced vegetation provide a strong summertime terrestrial signal. As the climate changes, new watershed‐scale conditions may further complicate predictions of DOM source availability, flow connectivity, and downstream fate in NPCTR watersheds.

Sunlight irradiation promotes both the chemodiversity of terrestrial DOM and the biodiversity of bacterial community in a subalpine lake

Alpine lake habitats are evolving into subalpine lakes under the scenario of climate change, where the vegetation are promoted due to increasing temperature and precipitation. The abundant terrestrial dissolved organic matter (TDOM) leached from watershed soil into subalpine lakes would undergo strong photochemical reaction due to the high altitude, with the potential to alter DOM composition and affect the bacterial communities. To reveal the transformation of TDOM by both photochemical and microbial processes in a typical subalpine lake, Lake Tiancai (located 200 m below the tree line) was chosen. TDOM was extracted from the surrounding soil of Lake Tiancai and then subjected to the photo/micro-processing for 107 days. The transformation of TDOM was analyzed by Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and fluorescence spectroscopy, and the shift of bacterial communities was analyzed using 16s rRNA gene sequencing technology. Dissolved organic carbon and light-absorbing components (a350) decay accounted for approximately 40% and 80% of the original, respectively, in the sunlight process, but both less than 20% in the microbial process for 107 days. The photochemical process promoted the chemodiversity as there were ∼7000 molecules after sunlight irradiation, compared to ∼3000 molecules in the original TDOM. Light promoted the production of highly unsaturated molecules and aliphatics, which were significantly associated with Bacteroidota, suggesting that light may influence bacterial communities by regulating the DOM molecules. Carboxylic-rich alicyclic molecules were generated in both photochemical and biological processes, suggesting TDOM was converted to a stable pool over time. Our finding on the transformation of terrestrial DOM and the alternation of bacterial community under the simultaneously photochemical and microbial processes will help to reveal the response of the carbon cycle and lake system structure to climate change for high-altitude lakes.