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The Ruki is a pristine blackwater tributary in the Congo Basin draining tropical lowland forest. Daily discharge and fortnightly concentrations, isotopic ratios, and molecular composition of carbon and organic matter were measured for 1 yr (2019–2020). Like the Congo River, discharge peaked from November–January, with a smaller secondary peak in June. Dissolved organic carbon (DOC), inorganic carbon (DIC), carbon dioxide (pCO2), and methane (pCH4) concentrations were high (21.3 ± 4.8 mg C L−1, 5.8 ± 0.9 mg C L−1, 6370 ± 1740 ppm, and 250 ± 100 ppm, respectively) and positively correlated with discharge, indicating transport limitation. Total suspended solids and particulate organic carbon (POC) concentrations were generally low (3.68 ± 1.61 mg L−1, 0.88 ± 0.33 mg C L−1, respectively) and varied inversely with discharge, indicating source limitation. The Ruki exported a total of 3.25 Tg C yr−1, of which DOC, DIC, and POC comprised 76%, 20%, and 3%, respectively. This DOC flux represents ~ 20% of the annual Congo Basin flux from about 5% of its area, highlighting the high yield. Isotopic ratios of DOC and POC indicate modern C3 forest vegetation as a source, except for a few older samples potentially indicating peat inputs. The bulk molecular composition of dissolved organic matter was seasonally consistent; however, a more oxidized and aromatic assemblage occurred at high discharge, corresponding with forest vegetation, while a more aliphatic, nitrogen‐, and sulfur‐enriched assemblage was found during low discharge, corresponding with soil‐derived organic matter. Overall, these results underscore how hydrology controls C concentrations in the Ruki River and how this blackwater river contributes disproportionately to C export per unit area within the Congo Basin hydrosystem.

Wetlands are integral to the global carbon cycle, serving as both a source and a sink for organic carbon. Their potential for carbon storage will likely change in the coming decades in response to higher temperatures and variable precipitation patterns. We characterized the dissolved organic carbon (DOC) and dissolved organic matter (DOM) composition from 12 different wetland sites across the USA spanning gradients in climate, landcover, sampling depth, and hydroperiod for comparison to DOM in other inland waters. Using absorption spectroscopy, parallel factor analysis modeling, and ultra‐high resolution mass spectroscopy, we identified differences in DOM sourcing and processing by geographic site. Wetland DOM composition was driven primarily by differences in landcover where forested sites contained greater aromatic and oxygenated DOM content compared to grassland/herbaceous sites which were more aliphatic and enriched in N and S molecular formulae. Furthermore, surface and porewater DOM was also influenced by properties such as soil type, organic matter content, and precipitation. Surface water DOM was relatively enriched in oxygenated higher molecular weight formulae representing HUPHigh O/Ccompounds than porewaters, whose DOM composition suggests abiotic sulfurization from dissolved inorganic sulfide. Finally, we identified a group of persistent molecular formulae (3,489) present across all sites and sampling depths (i.e., the signature of wetland DOM) that are likely important for riverine‐to‐coastal DOM transport. As anthropogenic disturbances continue to impact temperate wetlands, this study highlights drivers of DOM composition fundamental for understanding how wetland organic carbon will change, and thus its role in biogeochemical cycling. Dissolved organic matter (DOM) is often the most reactive form of organic carbon in wetlands, but its molecular characteristics and distribution are not well defined across different wetland types. We characterized DOM and analyzed dissolved organic carbon (DOC) concentrations across 12 temperate USA wetlands during wet and dry seasons from both surface‐ and porewaters. Wetland DOM primarily originates from the landscapes with greater DOC concentrations than similar inland waters such as lakes and rivers. DOM composition differs mostly by geographic site, suggesting that forested wetlands contain more aromatic DOM compounds from vegetation and soil than grassland/herbaceous wetlands, which contain DOM that is more processed. DOM composition between surface and porewaters is influenced by local ecological properties such as soil content and precipitation, with porewater compositions heavily impacted by mineral interactions. Finally, we identified common molecular signatures across all wetlands that have also been found in the largest arctic rivers, highlighting the role of wetlands as potential organic carbon sources to rivers and coastal systems. As precipitation and temperature patterns continue to change across temperate regions, the balance between different carbon pools will likely respond, particularly between the distribution and composition of wetland surface and porewater organic matter. Wetlands contain higher dissolved organic carbon (DOC) concentrations and greater aromatic dissolved organic matter (DOM) composition than other inland waters across the USAWetland DOM composition differs spatially between forested and grassland/herbaceous landcover as well as between surface and porewatersPersistent molecular formulae are observed across all wetlands that may be important for riverine‐to‐coastal DOM cycling Wetlands contain higher dissolved organic carbon (DOC) concentrations and greater aromatic dissolved organic matter (DOM) composition than other inland waters across the USA Wetland DOM composition differs spatially between forested and grassland/herbaceous landcover as well as between surface and porewaters Persistent molecular formulae are observed across all wetlands that may be important for riverine‐to‐coastal DOM cycling

Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) has been increasingly employed to characterize dissolved organic matter (DOM) across a range of aquatic environments highlighting the role of DOM in global carbon cycling. DOM analysis commonly utilizes electrospray ionization (ESI), while some have implemented other techniques, including dopant-assisted atmospheric pressure photoionization (APPI). We compared various extracted DOM compositions analyzed by negative ESI and positive APPI doped with both toluene and tetrahydrofuran (THF), including a fragmentation study of THF-doped riverine DOM using infrared multiple photon dissociation (IRMPD). DOM compositions followed the same trends in ESI and dopant-assisted APPI with the latter presenting saturated, less oxygenated, and more N-containing compounds than ESI. Between the APPI dopants, THF-doping yielded spectra with more aliphatic-like and N-containing compounds than toluene-doping. We further demonstrate how fragmentation of THF-doped DOM in APPI resolved subtle differences between riverine DOM that was absent from ESI. In both ionization methods, we describe a linear relationship between atomic and formulaic N-compositions from a range of DOM extracts. This study highlights that THF-doped APPI is useful for uncovering low-intensity aliphatic and peptide-like components in autochthonous DOM, which could aid environmental assessments of DOM across biolability gradients.

Northern high‐latitude lakes are hotspots for cycling dissolved organic carbon (DOC) inputs from allochthonous sources to the atmosphere. However, the spatial distribution of lake dissolved organic matter (DOM) is largely unknown across Arctic‐boreal regions with respect to the surrounding landscape. We expand on regional studies of northern high‐latitude DOM composition by integrating DOC concentrations, optical properties, and molecular‐level characterization from lakes spanning the Canadian Taiga to the Alaskan Tundra. Lakes were sampled during the summer from July to early September to capture the growing season. DOM became more optically processed and molecular‐level aromaticity increased northward across the Canadian Shield to the southern Arctic and from interior Alaska to the Tundra, suggesting relatively greater DOM incorporation from allochthonous sources. Using water isotopes (δ18O‐H2O), we report a weak overall trend of increasing DOC and decreasing aromaticity in lakes that were hydrologically isolated from the landscape and enriched in δ18O‐H2O, while within‐region trends were stronger and varied depending on the landscape. Finally, DOC correlated weakly with chromophoric dissolved organic matter (CDOM) across the study sites, suggesting that autochthonous and photobleached DOM were a major component of the DOC in these regions; however, some of the northernmost and wetland‐dominated lakes followed pan‐Arctic riverine DOC‐CDOM relationships, indicating strong contributions from allochthonous inputs. As many lakes across the North American Arctic are experiencing changes in temperature and precipitation, we expect the proportions of allochthonous and autochthonous DOM to respond with aquatic optical browning with greater landscape connectivity and more internally produced DOM in hydrologically isolated lakes. As the Arctic responds to warming, permafrost thaw, and variations in precipitation, the distribution of carbon pools within northern high‐latitude lakes will also change. Specifically, the composition of dissolved organic matter (DOM) and how it is altered and moved from the landscape to the atmosphere will be highly dependent on local precipitation patterns and hydrology, but these relationships are not well constrained across large regions. We sampled over 70 individual lakes during the summer spanning various ecoregions from interior Canada to the Alaskan Tundra and characterized their dissolved organic carbon (DOC) concentrations and DOM composition using bulk and molecular‐level analysis. Overall, DOM from these lakes was highly influenced by aquatic primary production but increased in the relative proportion of terrestrially derived organic matter as lake setting transitioned from forests to shrublands above the tree line. We also report a weak relationship between increasing DOC and decreasing terrestrial DOM as lakes become more hydrologically isolated across the pan‐Arctic; however, regional trends were stronger within forested sampling areas and weaker in shrublands. With the hydrologic setting of many northern high‐latitude lakes predicted to change in the coming decades, we expect the proportions of land‐ and aquatic‐derived DOM to respond as well. Dissolved organic matter in high‐latitude Canadian lakes increases in aromaticity northward as forests transition to shrublandsHigh‐latitude lakes that are hydrologically connected to the landscape have lower dissolved organic carbon (DOC) and are more allochthonous than isolated lakesDOC in many northern high‐latitude lakes was influenced by autochthonous production and correlated weakly with absorbance Dissolved organic matter in high‐latitude Canadian lakes increases in aromaticity northward as forests transition to shrublands High‐latitude lakes that are hydrologically connected to the landscape have lower dissolved organic carbon (DOC) and are more allochthonous than isolated lakes DOC in many northern high‐latitude lakes was influenced by autochthonous production and correlated weakly with absorbance

Redox-active functional groups in dissolved organic matter (DOM) are crucial for microbial electron transfer and methane emissions. However, the extent of aquatic DOM redox properties across northern high-latitude lakes and their relationships with DOM composition have not been thoroughly described. We quantified electron donating capacity (EDC) and electron accepting capacity (EAC) in lake DOM from Canada to Alaska and assessed their relationships with parameters from absorbance, fluorescence, and ultrahigh resolution mass spectrometry (FT-ICR MS) analyses. EDC and EAC are strongly tied to aromaticity and negatively related to aliphaticity and protein-like content. Redox-active formulae spanned a range of aromaticity, including highly unsaturated phenolic formulae, and correlated negatively with many aliphatic N and S-containing formulae. This distribution illustrates the compositional diversity of redox-sensitive functional groups and their sensitivity to ecosystem properties such as local hydrology and residence time. Finally, we developed a reducing index (RI) to predict EDC in aquatic DOM from FT-ICR MS spectra and assessed its robustness using riverine DOM. As the hydrology of the northern high-latitudes continues to change, we expect differences in the quantity and partitioning of EDC and EAC within these lakes, which have implications for local water quality and methane emissions.

The Congo and Amazon are the two largest rivers on Earth and serve as major sources of dissolved organic carbon to the ocean. We compared the dissolved organic matter (DOM) composition of both rivers using Fourier‐transform ion cyclotron resonance mass spectrometry to investigate seasonal and regional differences in DOM composition exported to the ocean. We found that over a 15‐month observational period in the Congo River, molecular aromaticity and oxygenation between the wet and dry periods varied slightly, but most of the relative abundance of DOM formulae (∼90%) were present in all samples, suggesting that Congo River DOM quality is stable across different hydrological conditions. In contrast, the multi‐year DOM composition in the Amazon River was highly susceptible to changes in hydrology, with clear differences in molecular aromaticity, oxygenation, and heteroatom (N, S, P) content between the wet and dry seasons. Overall, the DOM composition of the Congo River was more terrestrial than Amazon River DOM, which was more characteristic of aquatic DOM. Finally, we compared the relative contribution of island of stability (IOS) formulae between the rivers and found that both rivers export similar amounts of these formulae annually, more than several major rivers combined, and that the Congo is more than twice as efficient in exporting these IOS formulae. With changing precipitation and land use, the quantity and composition of exported DOM will likely reflect the mobilization of additional terrestrial and anthropogenic sources that will also be subjected to downstream land‐to‐ocean cycling. Almost 16% of the global riverine dissolved organic carbon (DOC) export is delivered by just two rivers: Amazon and Congo. Although both rivers drain immense tropical rainforests, differences in landscape, precipitation, and riverine discharge all contribute to the distinct molecular composition of their dissolved organic matter (DOM). We compared DOM composition of both rivers over several years and found that hydrology is an important driver for determining the DOM composition of both rivers; however, it is more important for the Amazon than Congo, which had DOM that was more similar across the wet and dry seasons. Overall, both rivers had more aromatic and oxygen‐rich DOM than temperate or arctic rivers, but DOM from the Congo had a more terrestrial signature than the Amazon River DOM. Finally, we found that DOM molecular formulae thought to be stable over long periods of time in marine systems were exported in similar amounts by both rivers, even though the Amazon exports about twice as much DOC annually compared to the Congo, suggesting that the Congo is more than twice as efficient per unit volume at exporting molecular formulae that is thought to be stable in the ocean for long periods of time. Congo River dissolved organic matter (DOM) composition remains mostly stable across the year, while Amazon River DOM varies seasonally with dischargeAmazon River DOM is less aromatic, less oxygenated, and more heteroatom‐rich than Congo River DOMBoth rivers export similar amounts of island of stability molecular formulae but the Congo is over twice as efficient Congo River dissolved organic matter (DOM) composition remains mostly stable across the year, while Amazon River DOM varies seasonally with discharge Amazon River DOM is less aromatic, less oxygenated, and more heteroatom‐rich than Congo River DOM Both rivers export similar amounts of island of stability molecular formulae but the Congo is over twice as efficient

Arctic lakes store, modify, and transport large quantities of carbon from terrestrial environments to the atmosphere; however, the spatial and temporal relationships between quantity and composition of dissolved organic matter (DOM) have not been well characterized across broad arctic regions. Moreover, most arctic lake DOM compositions have been examined during the ice‐free summer, whereas DOM cycling between the ice‐covered winter months and summer have not been addressed. To resolve these spatial and seasonal uncertainties in DOM cycling, we sampled a series of arctic lakes from the North Slope of Alaska across a latitudinal gradient in the winter and summer over 3 years. Samples were analyzed for dissolved organic carbon concentration and DOM composition was characterized using optical and fluorescence properties combined with molecular‐level analysis using Fourier transform‐ion cyclotron resonance mass spectrometry. Tundra lake DOM properties including aromaticity and molecular stoichiometries were similar to other northern high‐latitude lakes, but optical parameters related to aromaticity and molecular weight were greater in major arctic rivers and in coastal lakes in the North Slope region. DOM composition was highly seasonal, with ice exclusion concentrating microbially processed DOM in the winter water columns, potentially influencing DOM cycling the following summer. However, the greatest variations in DOM composition were related to lake depth and likely other physical features including morphology and bathymetry. As the Arctic warms, we expect changes in hydrology and ice cover to enhance under‐ice microbial DOM processing, early summer photodegradation, and ultimately carbon fluxes to the atmosphere after ice‐out. Arctic lakes contain large amounts of both inorganic and dissolved organic carbon (DOC) that is stored, processed, and transferred to the atmosphere. This cycling has been enhanced by recent arctic warming via anthropogenic climate change, making the fate and distribution of these different pools uncertain on both seasonal and spatial scales. We demonstrate that the amount of DOC and the composition of dissolved organic matter (DOM) vary significantly across tundra lakes between summer and winter months when the lakes are entirely covered by ice. Furthermore, these seasonal trends are consistent across the North Slope of Alaska with lakes closer to the coast of the Arctic Ocean differing in bulk optical and molecular‐level properties, suggesting DOM is supplied by different sources and influenced by maritime effects. Finally, while seasonality was an important factor in explaining DOM compositions across the tundra, the greatest differences between lakes were attributed to other factors, likely related to the variations in lake geomorphology including lake depth and area across the region. As temperatures and precipitation change across the Arctic are expected, lake morphology will respond to a thawing landscape and influence the composition of DOM as well as how it is cycled from lakes to the atmosphere. Dissolved organic carbon concentration and dissolved organic matter (DOM) composition differ seasonally due to ice exclusion and microbial processing in the winter monthsLake geomorphology is likely a major control for the overall lake DOM composition across the arctic tundraTundra lake DOM composition differs in optical properties and heteroatomic content between coastal and inland lakes Dissolved organic carbon concentration and dissolved organic matter (DOM) composition differ seasonally due to ice exclusion and microbial processing in the winter months Lake geomorphology is likely a major control for the overall lake DOM composition across the arctic tundra Tundra lake DOM composition differs in optical properties and heteroatomic content between coastal and inland lakes

As climate‐driven El Niño Southern Oscillation (ENSO) events are projected to increase in frequency and severity, much attention has focused on impacts regarding ecosystem productivity and carbon balance in Amazonian rainforests, with comparatively little attention given to carbon dynamics in fluvial ecosystems. In this study, we compared the wet 2012 La Niña period to the following normal hydrologic period in the Amazon River. Elevated water flux during the La Niña period was accompanied by dilution of inorganic ion concentrations. Furthermore, the La Niña period exported 2.77 Tg C yr−1more dissolved organic carbon (DOC) than the normal period, an increase greater than the annual amount of DOC exported by the Mississippi River. Using ultra‐high‐resolution mass spectrometry, we detected both intra‐ and interannual differences in dissolved organic matter (DOM) composition, revealing that DOM exported during the dry season and the normal period was more aliphatic, whereas compounds in the wet season and following the La Niña event were more aromatic, with ramifications for its environmental role. Furthermore, as this study has the highest temporal resolution DOM compositional data for the Amazon River to‐date we showed that compounds were highly correlated to a 6‐month lag in Pacific temperature and pressure anomalies, suggesting that ENSO events could impact DOM composition exported to the Atlantic Ocean. Therefore, as ENSO events increase in frequency and severity into the future it seems likely that there will be downstream consequences for the fate of Amazon Basin‐derived DOM concurrent with lag periods as described here. Increases in atmospheric carbon concentrations originate from many sources and pose a serious threat to global ecosystem health and humanity. The Amazon River delivers one‐fifth of global discharge and represents the largest single flux of dissolved organic carbon (DOC) from land to ocean. As climate change is projected to increase precipitation anomalies throughout the Amazon, flooding and droughts will become more frequent and severe, disrupting the natural seasonal rhythm of the Amazon River. We demonstrate that precipitation anomalies in South America (caused by La Niña) exported an additional amount of DOC from the Amazon River to the Atlantic Ocean than the Mississippi River exports annually. Organic compounds mobilized during the La Niña were more aromatic, presumably from terrestrial sources. These compositions, measured near the mouth of the Amazon River, arrived six months after Pacific sea‐surface temperature and pressure anomalies indicated the onset of La Niña, highlighting the lag time that events in the Pacific take to impact the Atlantic Ocean. The Amazon River during a La Niña year exports an additional amount of dissolved organic carbon than the Mississippi exports annuallyThe Amazon River dissolved organic matter composition in a La Niña year was more oxidized and aromatic than during a non‐ENSO yearThe dissolved organic matter composition in the Amazon River correlates to a 6‐month lag with La Niña indices The Amazon River during a La Niña year exports an additional amount of dissolved organic carbon than the Mississippi exports annually The Amazon River dissolved organic matter composition in a La Niña year was more oxidized and aromatic than during a non‐ENSO year The dissolved organic matter composition in the Amazon River correlates to a 6‐month lag with La Niña indices

The Amazon River drains a diverse tropical landscape greater than 6 million km2, culminating in the world's largest export of freshwater and dissolved constituents to the ocean. Here, we present dissolved organic carbon (DOC), organic and inorganic nitrogen (DON, DIN), orthophosphate (PO43−), and major and trace ion concentrations and fluxes from the Amazon River using 26 samples collected over three annual hydrographs. Concentrations and fluxes were predominantly controlled by the annual wet season flood pulse. Average DOC, DON, DIN, and PO43−fluxes (±1 s.d.) were 25.5 (±1.0), 1.14 (±0.05), 0.82 (±0.03), and 0.063 (±0.003) Tg yr−1, respectively. Chromophoric dissolved organic matter absorption (at 350 nm) was strongly correlated with DOC concentrations, resulting in a flux of 74.8 × 106m−2yr−1. DOC and DON concentrations positively correlated with discharge while nitrate + nitrite concentrations negatively correlated, suggesting mobilization and dilution responses, respectively. Ammonium, PO43−, and silica concentrations displayed chemostatic responses to discharge. Major and trace ion concentrations displayed clockwise hysteresis (except for chloride, sodium, and rubidium) and exhibited either dilution or chemostatic responses. The sources of weathered cations also displayed seasonality, with the highest proportion of carbonate‐ and silicate‐derived cations occurring during peak and baseflow, respectively. Finally, our seasonally resolved weathering model resulted in an average CO2consumption yield of (3.55 ± 0.11) × 105mol CO2km−2yr−1. These results represent an updated and temporally refined quantification of dissolved fluxes that highlight the strong seasonality of export from the world's largest river and set a robust baseline against which to gauge future change. Concentrations/fluxes of dissolved carbon, nitrogen, phosphorus, and major/trace ions were predominantly controlled by annual flood pulseMean Amazon River dissolved organic carbon flux was 25.5 teragrams per yearAnnual carbonate + silicate weathering CO2consumption yield was 3.55 × 105moles carbon dioxide per square kilometer Concentrations/fluxes of dissolved carbon, nitrogen, phosphorus, and major/trace ions were predominantly controlled by annual flood pulse Mean Amazon River dissolved organic carbon flux was 25.5 teragrams per year Annual carbonate + silicate weathering CO2consumption yield was 3.55 × 105moles carbon dioxide per square kilometer