Amazon River carbon dioxide outgassing fuelled by wetlands

The shifting pattern of CO2 source sink in a subtropical urbanizing lightly eutrophic lake

Globally, numerous freshwater lakes exist, and rapid urbanization has impacted carbon biogeochemical cycling at the interface where water meets air in these bodies. However, there is still a limited understanding of CO2 absorption/emission in eutrophic urbanizing lakes. This study therefore involved biweekly in-situ monitoring to evaluate fluctuations in the partial pressure (pCO2) and flux (fCO2) of CO2 and associated parameters from January to September 2020 (7:00-17:00 CST) in an urbanizing lake in southwestern China. Our study revealed that during the daylight hours of the 11 sampling days, both pCO2 and fCO2 consistently demonstrated decreasing trends from the early morning period to the late afternoon period, with notable increases on May 7th and August 15th, respectively. Interestingly, unlike our previous findings, an nonsignificant difference (p > 0.05) in mean pCO2 and fCO2 was observed between the morning period and the afternoon period (n = 22). Furthermore, the mean pCO2 in January (~105 μatm; n = 4) and April (133-212 μatm; n = 8) was below the typical atmospheric CO2 level (C-sink), while that in the other months surpassed 410 μatm (C-source), although the average values (n = 44) of pCO2 and fCO2 were 960 ± 841 μatm and 57 ± 85 mmol m-2 h-1, respectively. Moreover, the pCO2 concentration was significantly greater in summer (May to August, locally reaching 1087 μatm) than in spring (January to April at 112 μatm), indicating a seasonal shift between the C-sink (spring) and the C-source (summer). In addition, a significant positive correlation in pCO2/fCO2 with chlorophyll-a/nitrate but a negative correlation in dissolved oxygen and total phosphorus were recorded, suggesting that photosynthesis and respiration were identified as the main drivers of CO2 absorption/emissions, while changes in nitrate and phosphorus may be attributed to urbanization. Overall, we hypothesize that this lightly eutrophic lake demonstrated a distinct shifting pattern of CO2 source-sink variability at daily and seasonal scales.

Connectivity of floodplain influences riverine carbon outgassing and dissolved carbon transport

Rivers not only function as a conduit for the delivery of terrestrial constituents to oceans, but they also serve as an essential medium for biogeochemical processing of the constituents. While extensive research has been conducted on carbon transport in many rivers, little is known about carbon transformation in engineered rivers reconnected with their floodplain network. Being the largest distributary of the levee-confined Mississippi River (MR), the Atchafalaya River (AR) carries 25 % of the MR water, flowing through North America's largest freshwater swamp basin and emptying into the Gulf of Mexico. Previous studies reported that this 200-km long, 5-30-km wide river basin can remove a substantial amount of riverine nutrients and organic carbon. This study aimed to test the hypothesis that the AR emits significantly higher CO2 into the atmosphere as it flows through its extensive floodplain network than the levee-confined MR does. From January 2019 to December 2021, we conducted biweekly - monthly in-situ measurements in the lower AR at Morgan City and in the lower Mississippi River at Baton Rouge. Field measurements included partial pressure of dissolved CO2 (pCO2), water temperature, chlorophyll a, colored dissolved organic matter, dissolved oxygen, pH, and turbidity. During each field sampling, water samples were collected and analyzed for concentrations of dissolved organic and inorganic carbon (DOC and DIC). Mass transport of DOC and DIC and outgassing of CO2 were quantified for the two rivers. We found that pCO2 levels were significantly higher in the AR (mean: 3563 μatm; min-max: 1130-8650 μatm) than those in the MR (1931 μatm, 836-3501 μatm), resulting in a doubled CO2 outgassing rate in the AR (486 mmol m2 d-1) than in the MR (241 mmol m2 d-1). The AR had higher DOC (8.5 mg L-1) but lower chlorophyll a (153.9 AFU) when compared with the MR (7.5 mg L-1 and 164.0 AFU). Water temperature was constantly higher in the AR than in the MR, especially during the wintertime. Since the Mississippi-Atchafalaya River system is among the world's largest and most engineered river systems, our assessment offers a field case study to inform on the potential implications of reconnecting rivers with their floodplains networks.

Labile dissolved organic matter (DOM) and nitrogen inputs modified greenhouse gas dynamics: A source-to-estuary study of the Yangtze River

Although rivers are increasingly recognized as essential sources of greenhouse gases (GHG) to the atmosphere, few systematic efforts have been made to reveal the drivers of spatiotemporal variations of dissolved GHG (dGHG) in large rivers under increasing anthropogenic stress and intensified hydrological cycling. Here, through a source-to-estuary survey of the Yangtze River in March (spring) and October (autumn) of 2018, we revealed that labile dissolved organic matter (DOM) and nitrogen inputs remarkably modified the spatiotemporal distribution of dGHG. The average partial pressure of CO2 (pCO2), CH4 and N2O concentrations of all sampling sites in the Yangtze River were 1015 ± 225 μatm, and 87.5± 36.5 nmol L-1, and 20.3 ± 6.6 nmol L-1, respectively, significantly lower than the global average. In terms of longitudinal and seasonal variations, higher GHG concentrations were observed in the middle-lower reach in spring. The dominant drivers of spatiotemporal variations in dGHG were labile, protein-like DOM components and nitrogen level. Compared with the historical data of dGHG from published literature, we found a significant increase in N2O concentrations in the Yangtze River during 2004-2018, and the increasing trend was consistent with the rising riverine nitrogen concentrations. Our study emphasized the critical roles of labile DOM and nitrogen inputs in driving the spatial hotspots, seasonal variations and annual trends of dGHG. These findings can contribute to constraining the global GHG budget estimations and controls of GHG emission in large rivers in response to global change.

Dearomatization drives complexity generation in freshwater organic matter

Dissolved organic matter (DOM) is one of the most complex, dynamic and abundant sources of organic carbon, but its chemical reactivity remains uncertain1-3. Greater insights into DOM structural features could facilitate understanding its synthesis, turnover and processing in the global carbon cycle4,5. Here we use complementary multiplicity-edited 13C nuclear magnetic resonance (NMR) spectra to quantify key substructures assembling the carbon skeletons of DOM from four main Amazon rivers and two mid-size Swedish boreal lakes. We find that one type of reaction mechanism, oxidative dearomatization (ODA), widely used in organic synthetic chemistry to create natural product scaffolds6-10, is probably a key driver for generating structural diversity during processing of DOM that are rich in suitable polyphenolic precursor molecules. Our data suggest a high abundance of tetrahedral quaternary carbons bound to one oxygen and three carbon atoms (OCqC3 units). These units are rare in common biomolecules but could be readily produced by ODA of lignin-derived and tannin-derived polyphenols. Tautomerization of (poly)phenols by ODA creates non-planar cyclohexadienones, which are subject to immediate and parallel cycloadditions. This combination leads to a proliferation of structural diversity of DOM compounds from early stages of DOM processing, with an increase in oxygenated aliphatic structures. Overall, we propose that ODA is a key reaction mechanism for complexity acceleration in the processing of DOM molecules, creation of new oxygenated aliphatic molecules and that it could be prevalent in nature.

Characteristics of soil organic carbon fractions in four vegetation communities of an inland salt marsh

BACKGROUND

The study of soil organic carbon characteristics and its relationship with soil environment and vegetation types is of great significance to the evaluation of soil carbon sink provided by inland salt marshes. This paper reports the characteristics of soil organic carbon fractions in 0-50 cm soil layers at four vegetation communities of the Qinwangchuan salt marsh.

RESULTS

(1) The soil organic carbon content of Phragmites australis community (9.60 ± 0.32 g/kg) was found to be higher than that of Salicornia europae (7.75 ± 0.18 g/kg) and Tamarix ramosissima (4.96 ± 0.18 g/kg) and Suaeda corniculata community (4.55 ± 0.11 g/kg). (2) The soil dissolved organic carbon, particulate organic carbon and soil microbial biomass carbon in 0-50 cm soil layer of Phragmites australis community were higher, which were 0.46 ± 0.01 g/kg, 2.81 ± 0.06 g/kg and 0.31 ± 0.01 g/kg, respectively. (3) Soil organic carbon was positively correlated with dissolved organic carbon, particulate organic carbon, and microbial biomass carbon, and negatively correlated with easily oxidized organic carbon. (4) Above-ground biomass has a strong direct positive effect on soil organic carbon, total nitrogen and pH have a strong direct positive effect on microbial biomass carbon content, pH and average density have a strong direct negative effect on easily oxidized organic carbon, and particulate organic carbon.

CONCLUSIONS

The interaction between plant community characteristics and soil factors is an important driving factor for soil organic carbon accumulation in inland salt marshes.

Revealing the hidden carbon in forested wetland soils

Inland wetlands are critical carbon reservoirs storing 30% of global soil organic carbon (SOC) within 6% of the land surface. However, forested regions contain SOC-rich wetlands that are not included in current maps, which we refer to as 'cryptic carbon'. Here, to demonstrate the magnitude and distribution of cryptic carbon, we measure and map SOC stocks as a function of a continuous, upland-to-wetland gradient across the Hoh River Watershed (HRW) in the Pacific Northwest of the U.S., comprising 68,145 ha. Total catchment SOC at 30 cm depth (5.0 TgC) is between estimates from global SOC maps (GSOC: 3.9 TgC; SoilGrids: 7.8 TgC). For wetland SOC, our 1 m stock estimates are substantially higher (Mean: 259 MgC ha-1; Total: 1.7 TgC) compared to current wetland-specific SOC maps derived from a combination of U.S. national datasets (Mean: 184 MgC ha-1; Total: 0.3 TgC). We show that total unmapped or cryptic carbon is 1.5 TgC and when added to current estimates, increases the estimated wetland SOC stock to 1.8 TgC or by 482%, which highlights the vast stores of SOC that are not mapped and contained in unprotected and vulnerable wetlands.

Unexpected low CO2 emission from highly disturbed urban inland waters

Constituents and functionality of urban inland waters are significantly perturbed by municipal sewage inputs and tailwater discharge from wastewater treatment plants. However, large knowledge gaps persist in understanding greenhouse gas dynamics in urban inland waters due to a lack of in situ measurements. Herein, via a 3-year field campaign (2018-2020), we report river and lake CO2 emission and related aquatic factors regulating the emission in the municipality of Beijing. Mean pCO2 (546 ± 481 μatm) in the two urban lakes was lower than global non-tropical freshwater lakes and CO2 flux in 47% of the lake observations was negative. Though average pCO2 in urban rivers (3124 ± 3846 μatm) was among the higher range of global rivers (1300-4300 μatm), average CO2 flux was much lower than the global river average (99.7 ± 147.5 versus 358.4 mmol m-2 d-1). The high pCO2 cannot release to the atmosphere due to the low gas exchange rate in urban rivers (average k600 of 1.3 ± 1.3 m d-1), resulting in low CO2 flux in urban rivers. Additionally, eutrophication promotes photosynthetic uptake and aquatic organic substrate production, leading to no clear relationships observed between pCO2 and phytoplankton photosynthesis or dissolved organic carbon. In consistence with the findings, CO2 emission accounted for only 32% of the total greenhouse gas (GHG) emission equivalence (CO2, CH4 and N2O) in Beijing waters, in contrast to a major role of anthropogenic CO2 to anthropogenic GHG in the atmosphere in terms of radiative forcing (66%). These results pointed to unique GHG emission profiles and the need for a special account of urban inland waters in terms of aquatic GHG emissions.

Microbially mediated climate feedbacks from wetland ecosystems

Wetlands are crucial nodes in the carbon cycle, emitting approximately 20% of global CH4 while also sequestering 20%-30% of all soil carbon. Both greenhouse gas fluxes and carbon storage are driven by microbial communities in wetland soils. However, these key players are often overlooked or overly simplified in current global climate models. Here, we first integrate microbial metabolisms with biological, chemical, and physical processes occurring at scales from individual microbial cells to ecosystems. This conceptual scale-bridging framework guides the development of feedback loops describing how wetland-specific climate impacts (i.e., sea level rise in estuarine wetlands, droughts and floods in inland wetlands) will affect future climate trajectories. These feedback loops highlight knowledge gaps that need to be addressed to develop predictive models of future climates capturing microbial contributions. We propose a roadmap connecting environmental scientific disciplines to address these knowledge gaps and improve the representation of microbial processes in climate models. Together, this paves the way to understand how microbially mediated climate feedbacks from wetlands will impact future climate change.

Distinct Non-conservative Behavior of Dissolved Organic Matter after Mixing Solimões/Negro and Amazon/Tapajós River Waters

Positive and negative electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry and 1H NMR revealed major compositional and structural changes of dissolved organic matter (DOM) after mixing two sets of river waters in Amazon confluences: the Solimões and Negro Rivers (S + N) and the Amazon and Tapajós Rivers (A + T). We also studied the effects of water mixing ratios and incubation time on the composition and structure of DOM molecules. NMR spectra demonstrated large-scale structural transformations in the case of S + N mixing, with gain of pure and functionalized aliphatic units and loss of all other structures after 1d incubation. A + T mixing resulted in comparatively minor structural alterations, with a major gain of small aliphatic biomolecular binding motifs. Remarkably, structural alterations from mixing to 1d incubation were in essence reversed from 1d to 5d incubation for both S + N and A + T mixing experiments. Heterotrophic bacterial production (HBP) in endmembers S, N, and S + N mixtures remained near 0.03 μgC L-1 h-1, whereas HBP in A, T, and A + T were about five times higher. High rates of dark carbon fixation took place at S + N mixing in particular. In-depth biogeochemical characterization revealed major distinctions between DOM biogeochemical changes and temporal evolution at these key confluence sites within the Amazon basin.

Diurnal and seasonal variations of pCO2 and fluorescent dissolved organic matter (FDOM) in different polluted lakes

This study aimed to assess pollution and daily-to-seasonal dynamics of the partial pressure of CO2 (pCO2) and CO2 degassing flux concerning the fluorescent dissolved organic matter (FDOM) from tropical lakes. A membrane-enclosed pCO2 sensor and water quality multimeter analyzer was deployed to continuously record daily and seasonal variations in pCO2 and CO2 degassing flux in three lakes in Savar, Dhaka. During both wet and dry seasons, all lake water was supersaturated with CO2 in contrast to the atmospheric equilibrium (~400 μatm). The pCO2 values in the lake water during the dry season were relatively low in comparison, and the pCO2 levels in the wet season were much higher due to external inputs of organic matter from watersheds and direct inputs of CO2 from soils or wetlands. The estimated water-to-air CO2 degassing flux in the different levels of polluted lakes varies with the pollution context. Study areas calculated the carbon flux and three lakes released respectively 86.75×107g CO2 year-1, 13.8×107g CO2 year-1, and 9.17×107g CO2 year-1. Three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy combined with parallel factor (PARAFAC) analysis was used to investigate the distributions of fluorescent components in DOM. EEM-PARAFAC analysis identified humic-like, fulvic-like, protein-like, and more tyrosine-like FDOM components and their environmental dynamics. Terrestrial DOM may provide inputs to the terrestrial humic-like component in the lake water. In contrast, the biological activity of plankton-derived FDOM is the most likely source for the autochthonous humic-like component. FDOM and DO concentrations have negative correlations with pCO2, indicating that when the FDOM and DO level is decreased, the amount of pCO2 values increases.

The climatic risk of Amazonian protected areas is driven by climate velocity until 2050

Changes in species distribution in response to climate change might challenge the territorial boundaries of protected areas. Amazonia is one of the global regions most at risk of developing long distances between current and future analogous climates and the emergence of climate conditions without analogs in the past. As a result, species present within the network of Protected Areas (PAs) of Amazonia may be threatened throughout the 21st century. In this study, we investigated climate velocity based on future and past climate-analogs using forward and backward directions in the network of PAs of Amazonia, in order to assess the climatic risk of these areas to climate change and verify their effectiveness in maintaining the current climate conditions. Using current (1970-2000) and future (2041-2060) average annual air temperature and precipitation data with a resolution of 10 km, climate velocities across the entire Amazon biome and average climate velocities of PAs and Indigenous Lands (ILs) were evaluated. The results show that the effects of backward velocity will be greater than that of forward velocity in the Amazon biome. However, the PA network will be less exposed to backward velocity impacts than unprotected areas (UAs)-emphasizing the importance of these areas as a conservation tool. In contrast, for the forward velocity impacts, the PA network will be slightly more exposed than UAs-indicating that the current spatial arrangement of the PA network is still not the most suitable to minimize impacts of a possible climate redistribution. In addition, a large extent of no-analog climates for backward velocities was found in central Amazonia, indicating that high temperatures and changes in precipitation patterns in this region will surpass the historical variability of the entire biome, making it a potentially isolated and unsuitable climatic envelope for species in the future. Most of the no-analog climates are in PAs, however the climate risks in ILs should also be highlighted since they presented higher climate velocities than PAs in both metrics. Our projections contrast with the median latitudinal migration rate of 2 km/year observed in most ecosystems and taxonomic groups studied so far and suggest the need for median migration rates of 7.6 km/year. Thus, despite the important role of PAs and ILs as conservation tools, they are not immune to the effects of climate change and new management strategies, specific to each area and that allow adaptation to global changes, will be necessary.

Catchment-scale carbon fluxes and processes in major rivers of northern Québec, Canada

Boreal rivers transport and process large amounts of organic and inorganic materials derived from their catchments, yet quantitative estimates and patterns of carbon (C) transport and emissions in these large rivers are scarce relative to those of high-latitude lakes and headwater streams. Here, we present the results of a large-scale survey of 23 major rivers in northern Québec sampled during the summer period of 2010, which aimed to determine the magnitude and spatial variability of different C species (carbon dioxide - CO₂, methane - CH₄, total carbon - TC, dissolved organic carbon - DOC and inorganic carbon - DIC), as well as to identify their main drivers. In addition, we constructed a first order mass balance of total riverine C emissions to the atmosphere (outgassing from the main river channel) and export to the ocean over summer. All rivers were supersaturated in pCO₂ and pCH₄ (partial pressure of CO₂ and CH₄), and the resulting fluxes varied widely among rivers, especially the CH₄. There was a positive relationship between DOC and gas concentrations, suggesting a common watershed source of these C species. DOC concentrations declined as a function of % land surface covered by water (lentic + lotic systems) in the watershed, suggesting that lentic systems may act as a net sink of organic matter in the landscape. The C balance suggests that the export component is higher than atmospheric C emissions in the river channel. However, for heavily dammed rivers, C emissions to the atmosphere approaches the C export component. Such studies are highly important for the overall efforts to effectively quantify and incorporate major boreal rivers into whole-landscape C budgets, to determine the net role of these ecosystems as C sinks or sources, and to predict how these might shift under anthropogenic pressures and dynamic climate conditions.

Eco-morphodynamic carbon pumping by the largest rivers in the Neotropics

The eco-morphodynamic activity of large tropical rivers in South and Central America is analyzed to quantify the carbon flux from riparian vegetation to inland waters. We carried out a multi-temporal analysis of satellite data for all the largest rivers in the Neotropics (i.e, width > 200 m) in the period 2000-2019, at 30 m spatial resolution. We developed a quantification of a highly efficient Carbon Pump mechanism. River morphodynamics is shown to drive carbon export from the riparian zone and to promote net primary production by an integrated process through floodplain rejuvenation and colonization. This pumping mechanism alone is shown to account for 8.9 million tons/year of carbon mobilization in these tropical rivers. We identify signatures of the fluvial eco-morphological activity that provide proxies for the carbon mobilization capability associated with river activity. We discuss river migration-carbon mobilization nexus and effects on the carbon intensity of planned hydroelectric dams in the Neotropics. We recommend that future carbon-oriented water policies on these rivers include a similar analysis.

Spatial and temporal distribution characteristics of pCO2 and CO2 evasion in karst rivers under the influence of urbanization

Surface rivers play an essential role in carbon cycle processes in karst regions. However, the CO₂ diffusion flux from karst rivers under the influence of urbanization has been scarcely examined in the literature. Along these lines, in this work, the CO₂ partial pressure (pCO₂) and its degassing in a typical karst river (Nanming River and its tributaries) were thoroughly investigated, which are significantly affected by urbanization in Southwest China. From the acquired results, it was demonstrated that the average values of pCO₂ in the main stream of the Nanming River in the wet season, dry season, and flat season were 1975.77 ± 714.45 μatm, 1116.08 ± 454.24 μatm, and 976.89 ± 746.37 μatm, respectively. On the other hand, the tributary showed mean pCO₂ values of 1770.46 ± 1120.79 μatm, 1638.13 ± 1121.82 μatm, and 1107.74 ± 824.03 μatm in the three different hydrographic periods. Overall, the pCO₂ of the Nanming River basin decreased in the following order: wet season > dry season > flat season, while the mainstream of the Nanming River was slightly higher than that of the tributaries in the wet season. However, it was lower than that of the tributaries in the dry and flat seasons. Additionally, more than 90% of the samples displayed a supersaturated state of CO₂, which acted as an important source of CO₂ in the atmosphere. From a spatial point of view, pCO₂ tended to be higher in the western region than in the eastern region, higher in the middle than in the immediate vicinity, and higher in the south during the three seasons. The higher urban areas showed also relatively higher pCO₂ than the lower urban areas. The urban land along with the Nanming River's mainstream exhibited a weaker correlation with pCO₂ than the urban land along with the main tributaries due to the mainstream's regular management in recent years. Moreover, the pCO₂ was mainly influenced by the carbonate rocks dissolution, aquatic organism metabolic processes, and human activities. In the Nanming River basin, the average CO₂ diffusion fluxes in the wet season, dry season, and flat season were 147.02 ± 100.3 mmol·m^-2·d^-1, 76.02 ± 67.45 mmol·m^-2·d^-1, and 119.28 ± 168.22 mmol·m^-2·d^-1, respectively, which indicates high potential CO₂ emissions. In addition, it was found that urban construction could increase the pCO₂ of karst rivers and cause an increase in the CO₂ release flux during the regional urban expansion. In view of the increasing intensive and extensive urbanization in karst regions, our findings are helpful to elucidate the characteristics of carbon dioxide emissions from karst rivers under the disturbance of human activities and further deepen the understanding of the carbon balance in karst river basins.

Land use and hydrological factors control concentrations and diffusive fluxes of riverine dissolved carbon dioxide and methane in low-order streams

We analyzed the impacts of land use/land cover types on carbon dioxide (CO₂) and methane (CH₄) concentration and diffusion in 1^st to 4^th Strahler order tributaries of the Longchuan River to the upper Yangtze River in China by using headspace equilibration method and CO₂SYS program. Field sampling and measurements were conducted during the dry and wet seasons from 2017 to 2019. The average of calculated CO₂ partial pressure (pCO₂, mean ± SD: 2389 ± 3220 μatm) by CO₂SYS program was 1.9-fold higher than the value (mean ± SD: 1230 ± 1440 μatm) 10 years ago in the Longchuan River basin, where the urban land area increased by a factor of 7 times. Further analysis showed that corrected pCO₂ by headspace method and dissolved CH₄ (dCH₄) decrease as the stream order and flow velocity increase. The pCO₂ and dCH₄ in the wet season was lower than that in the dry season. The explanatory ability of land use types on the variation of corrected pCO₂ and dCH₄ was stronger at the reach scale than at the riparian and catchment scales in two seasons. Urban land at reach scale further showed much higher explanation on corrected pCO₂ and dCH₄ than cropland, grassland and forest land in the wet season. The Longchuan River emits approximately 112.5 kt CO₂-C and 1.0 kt CH₄-C per year, being 1.7-fold of the total lateral export of dissolved inorganic and dissolved organic carbon (68.3 kt C y^-1). The findings highlight the scale effects of land use on the observed seasonality in dissolved carbon gases in low-order streams.

Comprehensive assessment of dissolved organic matter processing in the Amazon River and its major tributaries revealed by positive and negative electrospray mass spectrometry and NMR spectroscopy

Rivers are natural biogeochemical systems shaping the fates of dissolved organic matter (DOM) from leaving soils to reaching the oceans. This study focuses on Amazon basin DOM processing employing negative and positive electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI[±] FT-ICR MS) and nuclear magnetic resonance spectroscopy (NMR) to reveal effects of major processes on the compositional space and structural characteristics of black, white and clear water systems. These include non-conservative mixing at the confluences of (1) Solimões and the Negro River, (2) the Amazon River and the Madeira River, and (3) in-stream processing of Amazon River DOM between the Madeira River and the Tapajós River. The Negro River (black water) supplies more highly oxygenated and high molecular weight compounds, whereas the Solimões and Madeira Rivers (white water) contribute more CHNO and CHOS molecules to the Amazon River main stem. Aliphatic CHO and abundant CHNO compounds prevail in Tapajos River DOM (clear water), likely originating from primary production. Sorption onto particles and heterotrophic microbial degradation are probably the principal mechanisms for the observed changes in DOM composition in the Amazon River and its tributaries.

River ecosystem metabolism and carbon biogeochemistry in a changing world

River networks represent the largest biogeochemical nexus between the continents, ocean and atmosphere. Our current understanding of the role of rivers in the global carbon cycle remains limited, which makes it difficult to predict how global change may alter the timing and spatial distribution of riverine carbon sequestration and greenhouse gas emissions. Here we review the state of river ecosystem metabolism research and synthesize the current best available estimates of river ecosystem metabolism. We quantify the organic and inorganic carbon flux from land to global rivers and show that their net ecosystem production and carbon dioxide emissions shift the organic to inorganic carbon balance en route from land to the coastal ocean. Furthermore, we discuss how global change may affect river ecosystem metabolism and related carbon fluxes and identify research directions that can help to develop better predictions of the effects of global change on riverine ecosystem processes. We argue that a global river observing system will play a key role in understanding river networks and their future evolution in the context of the global carbon budget.

The Impacts of Nitrogen Pollution and Urbanization on the Carbon Dioxide Emission from Sewage-Draining River Networks

Carbon dioxide (CO₂) emissions from river water have sparked worldwide concerns due to supersaturate CO₂ levels in the majority of global rivers, while the knowledge on the associations among nitrogen pollution, urbanization, and CO₂ emissions is still limited. In this study, the CO₂ partial pressure (pCO₂), carbon and nitrogen species, and water parameters in sewage-draining river networks were investigated. Extremely high pCO₂ levels were observed in sewage and drainage river waters, such as Longfeng River, Beijing-drainage River, and Beitang-drainage River, which were approximately 4 times higher than the averaged pCO₂ in worldwide rivers. Correlations of carbon/nitrogen species and pCO₂ indicated that carbon dioxide in rural rivers and sewage waters primarily originated from soil aeration zones and biological processes of organic carbon/nitrogen input from drainage waters, while that in urban rivers and lakes was mainly dominated by organic matter degradation and biological respiration. Enhanced internal primary productivity played critical roles in absorbing CO₂ by photosynthesis in some unsaturated pCO₂ sampling sites. Additionally, higher pCO₂ levels have been observed with higher NH₄⁺-N and lower DO. CO₂ fluxes in sewage waters exhibited extremely high levels compared with those of natural rivers. The results could provide implications for assessing CO₂ emissions in diverse waters and fulfilling water management polices when considering water contamination under intense anthropogenic activities.

Anthropogenic land use substantially increases riverine CO2 emissions

Carbon dioxide (CO₂) emissions from inland waters to the atmosphere are a pivotal component of the global carbon budget. Anthropogenic land use can influence riverine CO₂ emissions, but empirical data exploring cause-effect relationships remain limited. Here, we investigated CO₂ partial pressures (pCO₂) and degassing in a monsoonal river (Yue River) within the Han River draining to the Yangtze in China. Almost 90% of river samples were supersaturated in CO₂ with a mean ± standard deviation of 1474 ± 1614 µatm, leading to emissions of 557 - 971 mmol/m²/day from river water to the atmosphere. Annual CO₂ emissions were 1.6 - 2.8 times greater than the longitudinal exports of riverine dissolved inorganic and organic carbon. pCO₂ was positively correlated to anthropogenic land use (urban and farmland), and negatively correlated to forest cover. pCO₂ also had significant and positive relationships with total dissolved nitrogen and total dissolved phosphorus. Stepwise multiple regression models were developed to predict pCO₂. Farmland and urban land released nutrients and organic matter to the river system, driving riverine pCO₂ enrichment due to enhanced respiration in these heterotrophic rivers. Overall, we show the crucial role of land use driving riverine pCO₂, which should be considered in future large-scale estimates of CO₂ emissions from streams. Land use change can thus modify the carbon balance of urban-river systems by enhancing river emissions, and reforestation helps carbon neutral in rivers.

Greenhouse gas emissions from African lakes are no longer a blind spot

Natural lakes are thought to be globally important sources of greenhouse gases (CO₂, CH₄, and N₂O) to the atmosphere although nearly no data have been previously reported from Africa. We collected CO₂, CH₄, and N₂O data in 24 African lakes that accounted for 49% of total lacustrine surface area of the African continent and covered a wide range of morphology and productivity. The surface water concentrations of dissolved CO₂ were much lower than values attributed in current literature to tropical lakes and lower than in boreal systems because of a higher productivity. In contrast, surface water-dissolved CH₄ concentrations were generally higher than in boreal systems. The lowest CO₂ and the highest CH₄ concentrations were observed in the more shallow and productive lakes. Emissions of CO₂ may likely have been substantially overestimated by a factor between 9 and 18 in African lakes and between 6 and 26 in pan-tropical lakes.

Fluvial CO2 and CH4 in a lowland agriculturally impacted river network: Importance of local and longitudinal controls

Despite streams and rivers play a critical role as conduits of terrestrially produced organic carbon to the atmosphere, fluvial CO₂ and CH₄ are seldom integrated into regional carbon budgets. High spatial variability hinders our ability to understand how local and longitudinal controls affect underlying processes of riverine CO₂ and CH₄ and challenge the prediction and upscaling across large areas. Here, we conducted a survey of fluvial CO₂ and CH₄ concentrations spanning multiple stream orders within an agriculturally impacted region, the North China Plain. We explored the spatial patterns of fluvial CO₂ and CH₄ concentrations, and then examined whether catchment and network properties and water chemical parameters can explain the variations in both carbon gases. Streams and rivers were systematically supersaturated with CO₂ and CH₄ with the mean concentrations being 111 and 0.63 μmol L^-1, respectively. Spatial variability of both gases was regulated by network properties and catchment features. Fluvial CO₂ and CH₄ declined longitudinally and could be modeled as functions of stream order, dissolved oxygen, and water temperature. Both models explained about half of the variability and reflected longitudinal and local drivers simultaneously, albeit CO₂ was more local-influenced and CH₄ more longitudinal-influenced. Our empirical models in this work contribute to the upscaling and prediction of CO₂ and CH₄ emissions from streams and rivers and the understanding of proximal and remote controls on spatial patterns of both gases in agriculturally impacted regions.

Unravelling the spatiotemporal variation of pCO2 in low order streams: Linkages to land use and stream order

Headwater streams make the majority of cumulative stream length in a river basin, carbon dioxide (CO₂) emission from headwater (low order) streams is thus an essential component. Anthropogenic activities in headwater areas such as land use change and land use practices can strongly modify terrestrial carbon and nutrient input, which could affect the level of partial pressure of dissolved carbon dioxide (pCO₂) and CO₂ degassing from streams. However, there are large uncertainties in estimates due to the lack of data in subtropical rivers of rapidly developing rural regions. The spatiotemporal variation and driving factors of the pCO₂ and CO₂ degassing from low-order streams remain to be explored. In this study, we assess multi-spatial scale effects of land use on pCO₂ dynamics in seven headwater tributary rivers in Central China during 2016, 2017 and 2018 in rainy and dry seasons. Our results reveal that the stream pCO₂ level consistently increases as the stream order increases from 1 to 3 under apparent seasonal variations. Riverine pCO₂ is positively related to the percentage of urban land and cropland surrounding the river segments, but is negatively related to the percentage of forest land. The stream pCO₂ is more closely correlated with the 1000 and 2000 m diameters of circular buffers at upstream sampling sites than the circular buffers with 100 and 500 m diameters. There exist significant relationships of pCO₂ with the concentrations of TN, TP, DO, and DOC in the low-order streams. The partial redundancy analysis quantifies the relative importance of anthropogenic land uses, natural factors and water chemical variables in mediating stream pCO₂, showing that influences of anthropogenic land uses (urban and cropland) on pCO₂ decrease, with a percentage role of 34%, 14%, and 4% in the 1st-, 2nd- and 3rd-order streams, respectively. The impact of nutrients on pCO₂, however, increases as the stream order increases. Urban influence on stream pCO₂ also decreases as stream order increases. Our study highlights the effect of land use/land cover types and stream order on riverine pCO₂ and provides new insight into estimating CO₂ emission in headwater streams. Future studies are needed on the linkage between riverine CO₂ degassing and stream orders under changing land use conditions.

The importance of hydrology in routing terrestrial carbon to the atmosphere via global streams and rivers

Stream/river carbon dioxide (CO₂) emission has significant spatial and seasonal variations critical for understanding its macroecosystem controls and plumbing of the terrestrial carbon budget. We relied on direct fluvial CO₂ partial pressure measurements and seasonally varying gas transfer velocity and river network surface area estimates to resolve reach-level seasonal variations of the flux at the global scale. The percentage of terrestrial primary production (GPP) shunted into rivers that ultimately contributes to CO₂ evasion increases with discharge across regions, due to a stronger response in fluvial CO₂ evasion to discharge than GPP. This highlights the importance of hydrology, in particular water throughput, in terrestrial–fluvial carbon transfers and the need to account for this effect in plumbing the terrestrial carbon budget.

The magnitude of stream and river carbon dioxide (CO₂) emission is affected by seasonal changes in watershed biogeochemistry and hydrology. Global estimates of this flux are, however, uncertain, relying on calculated values for CO₂ and lacking spatial accuracy or seasonal variations critical for understanding macroecosystem controls of the flux. Here, we compiled 5,910 direct measurements of fluvial CO₂ partial pressure and modeled them against watershed properties to resolve reach-scale monthly variations of the flux. The direct measurements were then combined with seasonally resolved gas transfer velocity and river surface area estimates from a recent global hydrography dataset to constrain the flux at the monthly scale. Globally, fluvial CO₂ emission varies between 112 and 209 Tg of carbon per month. The monthly flux varies much more in Arctic and northern temperate rivers than in tropical and southern temperate rivers (coefficient of variation: 46 to 95 vs. 6 to 12%). Annual fluvial CO₂ emission to terrestrial gross primary production (GPP) ratio is highly variable across regions, ranging from negligible (<0.2%) to 18%. Nonlinear regressions suggest a saturating increase in GPP and a nonsaturating, steeper increase in fluvial CO₂ emission with discharge across regions, which leads to higher percentages of GPP being shunted into rivers for evasion in wetter regions. This highlights the importance of hydrology, in particular water throughput, in routing terrestrial carbon to the atmosphere via the global drainage networks. Our results suggest the need to account for the differential hydrological responses of terrestrial–atmospheric vs. fluvial–atmospheric carbon exchanges in plumbing the terrestrial carbon budget.

Coupled CH4 production and oxidation support CO2 supersaturation in a tropical flood pulse lake (Tonle Sap Lake, Cambodia)

Freshwaters inextricably link flows of carbon between the land, oceans, and atmosphere. Resulting carbon dioxide supersaturation relative to the atmosphere in most of the world’s lakes and rivers has long been assumed to come from aerobic respiration. Although carbon dioxide also comes from the oxidation of anaerobically produced methane, this has been largely ignored within freshwaters. Here, we use stable carbon isotopes of carbon dioxide and methane to show that a nontrivial proportion of the total dissolved carbon dioxide in a tropical flood pulse lake comes from methane oxidation. Seasonal pulses of flooding are common in the tropics, suggesting that coupled methane production and oxidation likely contribute more broadly to flows of carbon between the land, understudied tropical freshwaters, and atmosphere.

Carbon dioxide (CO₂) supersaturation in lakes and rivers worldwide is commonly attributed to terrestrial–aquatic transfers of organic and inorganic carbon (C) and subsequent, in situ aerobic respiration. Methane (CH₄) production and oxidation also contribute CO₂ to freshwaters, yet this remains largely unquantified. Flood pulse lakes and rivers in the tropics are hypothesized to receive large inputs of dissolved CO₂ and CH₄ from floodplains characterized by hypoxia and reducing conditions. We measured stable C isotopes of CO₂ and CH₄, aerobic respiration, and CH₄ production and oxidation during two flood stages in Tonle Sap Lake (Cambodia) to determine whether dissolved CO₂ in this tropical flood pulse ecosystem has a methanogenic origin. Mean CO₂ supersaturation of 11,000 ± 9,000 μatm could not be explained by aerobic respiration alone. ¹³C depletion of dissolved CO₂ relative to other sources of organic and inorganic C, together with corresponding ¹³C enrichment of CH₄, suggested extensive CH₄ oxidation. A stable isotope-mixing model shows that the oxidation of ¹³C depleted CH₄ to CO₂ contributes between 47 and 67% of dissolved CO₂ in Tonle Sap Lake. ¹³C depletion of dissolved CO₂ was correlated to independently measured rates of CH₄ production and oxidation within the water column and underlying lake sediments. However, mass balance indicates that most of this CH₄ production and oxidation occurs elsewhere, within inundated soils and other floodplain habitats. Seasonal inundation of floodplains is a common feature of tropical freshwaters, where high reported CO₂ supersaturation and atmospheric emissions may be explained in part by coupled CH₄ production and oxidation.

pCO2 and CO2 evasion from two small suburban rivers: Implications of the watershed urbanization process

Rivers are widely reported as CO₂-emitting hotpots and are attracting increasing attention worldwide. However, less attention has been given to the CO₂ emission from the suburban rivers which are experiencing rapid watershed urbanization and increasing anthropogenic stress. Here, two small suburban rivers in Southwest China were studied, and seasonal sampling campaigns with high spatial resolution were carried out to explore the characterization of partial pressure (pCO₂) and CO₂ efflux and their possible controls. The results showed that, the pCO₂ and estimated CO₂ fluxes from the two suburban rivers ranged from 37 to 6466 μatm (mean of 1293 ± 1126 μatm) and -72-1569 mmol·m^-2·d^-1 (mean of 185 ± 240 mmol·m^-2·d^-1), respectively. And, both of them exhibited disproportionately high variability and acted as strong CO₂ emitters to the atmosphere. The pCO₂ in the two suburban rivers showed significant spatial variability, with urban sections having 2-2.5 times higher values than exurban sections, and, the urban land use proportion in the basins accounted for 35%-67% of such spatial variation in pCO₂. The sewage-dominated urban tributaries had much higher pCO₂ and acted as an obvious exciter to the high pCO₂ in urban sections of suburban rivers. Carbon and nutrients concentrations also accounted for the spatial variation in pCO₂ and fCO2 in the two suburban rivers, and acted as good indicators. The seasonal variation in pCO₂, with the highest values in autumn and lowest values in spring, was controlled by the precipitation dilution effect and seasonal temperature as well as the boosted primary production at several urban sites. We highlighted that small suburban rivers showed disproportionally high spatial variability in pCO₂ and CO₂ fluxes in their limited basin areas due to the development of urbanization, and could be used as a good model for studying the complex impacts of anthropogenic disturbances on river carbon biogeochemical processes.

Eutrophication effects on CH4 and CO2 fluxes in a highly urbanized tropical reservoir (Southeast, Brazil)

Shallow urban polluted reservoirs at tropical regions can be hotspots for CO2 and CH4 emissions. In this study, we investigated the relationships between eutrophication and GHG emissions in a highly urbanized tropical reservoir in São Paulo Metropolitan Area (Brazil). CO2 and CH4 fluxes and limnological variables (water and sediment) were collected at three sampling stations classified as hypereutrophic and eutrophic. Analysis of variance (ANOVA) and the principal component analysis (PCA) determined the most significant parameters to CO2 and CH4 fluxes. ANOVA showed significant differences of CO2 and CH4 fluxes between sampling stations with different trophic state. The hypereutrophic station showed higher mean fluxes for both CO2 and CH4 (5.43 ± 1.04 and 0.325 ± 0.167 g m-2 d-1, respectively) than the eutrophic stations (3.36 ± 0.54 and 0.060 ± 0.005 g m-2 d-1). The PCA showed a strong relationship between nutrients in the water column (surface and bottom) and GHG fluxes. We concluded that GHG fluxes were higher whenever the trophic state increases as observed previously in temperate and tropical reservoirs. High concentrations of nutrients in the water column in the studied area support the high production of autotrophic biomass that, when sedimented, ends up serving as organic matter for CH4 producers. These outcomes reinforce the necessity of water quality improvement and eutrophication mitigation in highly urbanized reservoirs in tropical regions.

Carbon dioxide hydrodynamics along a wetland-lake-stream-waterfall continuum (Blue Mountains, Australia)

The small-scale spatial variability in dissolved carbon dioxide (CO₂) and water-air CO₂ flux dynamics were investigated within first-order catchments of the upper Blue Mountains Plateau (New South Wales, Australia). Water samples were collected at 81 locations during winter and summer over two consecutive years across seven aquatic ecosystem types: wetland, impoundment, lake, tributary stream, mainstem, escarpment complex, and urban-aquatic interface. Dissolved [CO₂] ranged from 15 to 880 μM (94 to 4760%Sat), and dissolved [O₂] from 0 to 350 μM (0 to 101%Sat). CO₂ supersaturation was typically highest in wetlands and vegetated impoundments of the upper plateau, and decreased downstream approaching atmospheric equilibrium at the escarpment waterfalls. Gas transfer velocities ranged from 0.18 m d^-1 in lentic waters to 292 m d^-1 at the bottom of waterfalls due to bubble-mediated transfer. The first- and second-order streams represented only 4.8% of the total open water area yet contributed to 61% of the total water-air CO₂ outgassing. The lake, escarpment and mainstem group systems had narrow diel and seasonal CO₂ concentration variability, while wetlands and vegetated impoundments had the widest ranges. Our high resolution spatio-temporal sampling was essential to identifying CO₂ outgassing hotspots in these geomorphically diverse catchments. Overall, >95% of excess dissolved CO₂ traversing the upper Blue Mountains Plateau was outgassed to the atmosphere.

A highly agricultural river network in Jurong Reservoir watershed as significant CO2 and CH4 sources

Freshwaters are receiving growing concerns on atmospheric carbon dioxide (CO₂) and methane (CH₄) budget; however, little is known about the anthropogenic sources of CO₂ and CH₄ from river network in agricultural-dominated watersheds. Here, we chose such a typical watershed and measured surface dissolved CO₂ and CH₄ concentrations over 2 years (2015-2017) in Jurong Reservoir watershed for different freshwater types (river network, ponds, reservoir, and ditches), which located in Eastern China and were impacted by agriculture with high fertilizer N application. Results showed that significantly higher gas concentrations occurred in river network (CO₂: 112 ± 36 μmol L^-1; CH₄: 509 ± 341 nmol L^-1) with high nutrient concentrations. Dissolved CO₂ and CH₄ concentrations were supersaturated in all of the freshwater types with peak saturation ratios generally occurring in river network. Temporal variations in the gas saturations were positively correlated with water temperature. The saturations of CO₂ and CH₄ were positively correlated with each other in river network, and both of these saturations were also positively correlated with nutrient loadings, and negatively correlated with dissolved oxygen concentration. The highly agricultural river network acted as significant CO₂ and CH₄ sources with estimated emission fluxes of 409 ± 369 mmol m^-2 d^-1 for CO₂ and 1.6 ± 1.2 mmol m^-2 d^-1 for CH₄, and made a disproportionately large, relative to the area, contribution to the total aquatic carbon emission of the watershed. Our results suggested the aquatic carbon emissions accounted for 6% of the watershed carbon budget, and fertilizer N and watersheds land use played a large role in the aquatic carbon emission.

Spatiotemporal Dynamics of Suspended Sediments in the Negro River, Amazon Basin, from In Situ and Sentinel-2 Remote Sensing Data

Monitoring suspended sediments through remote sensing data in black-water rivers is a challenge. Herein, remote sensing reflectance (Rrs) from in situ measurements and Sentinel-2 Multi-Spectral Instrument (MSI) images were used to estimate the suspended sediment concentration (SSC) in the largest black-water river of the Amazon basin. The Negro River exhibits extremely low Rrs values (<0.005 sr−1 at visible and near-infrared bands) due to the elevated absorption of coloured dissolved organic matter (aCDOM at 440 nm > 7 m−1) caused by the high amount of dissolved organic carbon (DOC > 7 mg L−1) and low SSC (<5 mg L−1). Interannual variability of Rrs is primarily controlled by the input of suspended sediments from the Branco River, which is a clear water river that governs the changes in the apparent optical properties of the Negro River, even at distances that are greater than 90 km from its mouth. Better results were obtained using the Sentinel-2 MSI Red band (Band 4 at 665 nm) in order to estimate the SSC, with an R2 value greater than 0.85 and an error less than 20% in the adjusted models. The magnitudes of water reflectance in the Sentinel-2 MSI Red band were consistent with in situ Rrs measurements, indicating the large spatial variability of the lower SSC values (0 to 15 mg L−1) in a complex anabranching reach of the Negro River. The in situ and satellite data analysed in this study indicates sedimentation processes in the lower Negro River near the Amazon River. The results suggest that the radiometric characteristics of sensors, like sentinel-2 MSI, are suitable for monitoring the suspended sediment concentration in large tropical black-water rivers.

Major Elements in the Upstream of Three Gorges Reservoir: An Investigation of Chemical Weathering and Water Quality during Flood Events

Rivers transport terrestrial matter into the ocean, constituting a fundamental channel between inland and oceanic ecosystem and affect global climate change. To reveal chemical weathering processes and environmental health risks during flood periods, water samples were collected in the upper reaches of Three Gorges Reservoir (TGR) in 2020. HCO3− and Ca2+ were the most abundant anions and cations of the river water, respectively. The range of HCO3− concentration was between 1.81 and 3.02 mmol/L, while the mean content of Ca2+ was 1.03 mmol/L. The results of the Piper diagram and element ratios revealed that the river solutes were mainly contributed by carbonate weathering and gypsum-rich evaporite dissolution. A mass balance model indicated that the contribution order of sources to cations in the main channel (Yibin-Luzhou) was evaporites > carbonates > atmospheric input > silicates. The order in the Chongqing—Three Gorges Dam was carbonates > atmospheric input > evaporites > silicates. These results showed a lithologic control on hydrochemical characteristics. Most sampling sites were suitable for agricultural irrigation according to the water quality assessment. However, indexes sodium adsorption ratio (SAR) and soluble sodium percentage (Na%) were higher than 1.0 in Yibin-Luzhou and 30% in Yibin–Chongqing, respectively, suggesting a potential sodium hazard. In addition, except Tuojiang River and Shennong River, the risk of sodium hazard in tributaries was relatively low. High Na+ concentration in irrigation water can damage soil structure and function and ultimately affect agricultural production. Water quality in the upstream of a Piper diagram should attract enough attention.

Greenhouse gas emissions from the water-air interface of a grassland river: a case study of the Xilin River

Greenhouse gas (GHG) emissions from rivers and lakes have been shown to significantly contribute to global carbon and nitrogen cycling. In spatiotemporal-variable and human-impacted rivers in the grassland region, simultaneous carbon dioxide, methane and nitrous oxide emissions and their relationships under the different land use types are poorly documented. This research estimated greenhouse gas (CO₂, CH₄, N₂O) emissions in the Xilin River of Inner Mongolia of China using direct measurements from 18 field campaigns under seven land use type (such as swamp, sand land, grassland, pond, reservoir, lake, waste water) conducted in 2018. The results showed that CO₂ emissions were higher in June and August, mainly affected by pH and DO. Emissions of CH₄ and N₂O were higher in October, which were influenced by TN and TP. According to global warming potential, CO₂ emissions accounted for 63.35% of the three GHG emissions, and CH₄ and N₂O emissions accounted for 35.98% and 0.66% in the Xilin river, respectively. Under the influence of different degrees of human-impact, the amount of CO₂ emissions in the sand land type was very high, however, CH₄ emissions and N₂O emissions were very high in the artificial pond and the wastewater, respectively. For natural river, the greenhouse gas emissions from the reservoir and sand land were both low. The Xilin river was observed to be a source of carbon dioxide and methane, and the lake was a sink for nitrous oxide.

Small streams dominate US tidal reaches and will be disproportionately impacted by sea-level rise

Rivers and streams represent <0.6% of the Earth's land surface but play a disproportionately large role in global biogeochemical cycles and provide locally relevant ecosystem services. However, knowledge of how rivers influence material budgets and ecosystem services has major gaps due to the lack of explicit consideration of tidally-influenced reaches. Focusing on the conterminous US, we provide a foundation for understanding the role of tidal streams. We find that 66% of tidal stream length is contributed from low order streams (< 4th order), and that terrestrial ecosystem production in low-lying coastal zones is 30% greater than in adjacent terrestrial ecosystems. This prevalence of small streams indicates that small coastal watersheds dominate tidally influenced spatial domains. Furthermore, we find that relative sea-level rise (RSLR) will have a disproportionate impact on low order tidal streams and their terrestrial interfaces - 1 m RSLR will decrease the tidal stream land-water interface by 17% and the total surface area of US tidal streams by 31%. Upstream reaches of tidal zones will be extended in response to RSLR, but gains will be more than offset by coastal losses because topographic gradients become steeper moving inland, and accretion rates may not keep pace with RSLR. These results highlight previously unrecognized dominance, high productivity, and disproportionate future loss of low-order coastal ecosystems. This indicates a critical need to focus research on small tidal stream systems under contemporary and future conditions.

Net landscape carbon balance of a tropical savanna: Relative importance of fire and aquatic export in offsetting terrestrial production

The magnitude of the terrestrial carbon (C) sink may be overestimated globally due to the difficulty of accounting for all C losses across heterogeneous landscapes. More complete assessments of net landscape C balances (NLCB) are needed that integrate both emissions by fire and transfer to aquatic systems, two key loss pathways of terrestrial C. These pathways can be particularly significant in the wet-dry tropics, where fire plays a fundamental part in ecosystems and where intense rainfall and seasonal flooding can result in considerable aquatic C export (ΣF_aq ). Here, we determined the NLCB of a lowland catchment (~140 km² ) in tropical Australia over 2 years by evaluating net terrestrial productivity (NEP), fire-related C emissions and ΣF_aq (comprising both downstream transport and gaseous evasion) for the two main landscape components, that is, savanna woodland and seasonal wetlands. We found that the catchment was a large C sink (NLCB 334 Mg C km^-2  year^-1 ), and that savanna and wetland areas contributed 84% and 16% to this sink, respectively. Annually, fire emissions (-56 Mg C km^-2  year^-1 ) and ΣF_aq (-28 Mg C km^-2  year^-1 ) reduced NEP by 13% and 7%, respectively. Savanna burning shifted the catchment to a net C source for several months during the dry season, while ΣF_aq significantly offset NEP during the wet season, with a disproportionate contribution by single major monsoonal events-up to 39% of annual ΣF_aq was exported in one event. We hypothesize that wetter and hotter conditions in the wet-dry tropics in the future will increase ΣF_aq and fire emissions, potentially further reducing the current C sink in the region. More long-term studies are needed to upscale this first NLCB estimate to less productive, yet hydrologically dynamic regions of the wet-dry tropics where our result indicating a significant C sink may not hold.

Environmental investments decreased partial pressure of CO2 in a small eutrophic urban lake: Evidence from long-term measurements

Inland waters emit large amounts of carbon dioxide (CO₂) to the atmosphere, but emissions from urban lakes are poorly understood. This study investigated seasonal and interannual variations in the partial pressure of CO₂ (pCO₂) and CO₂ flux from Lake Wuli, a small eutrophic urban lake in the heart of the Yangtze River Delta, China, based on a long-term (2000-2015) dataset. The results showed that the annual mean pCO₂ was 1030 ± 281 μatm (mean ± standard deviation) with a mean CO₂ flux of 1.1 ± 0.6 g m^-2 d^-1 during 2000-2015, suggesting that compared with other lakes globally, Lake Wuli was a significant source of atmospheric CO₂. Substantial interannual variability was observed, and the annual pCO₂ exhibited a decreasing trend due to improvements in water quality driven by environmental investment. Changes in ammonia nitrogen and total phosphorus concentrations together explained 90% of the observed interannual variability in pCO₂ (R² = 0.90, p < 0.01). The lake was dominated by cyanobacterial blooms and showed nonseasonal variation in pCO₂. This finding was different from those of other eutrophic lakes with seasonal variation in pCO₂, mostly because the uptake of CO₂ by algal-derived primary production was counterbalanced by the production of CO₂ by algal-derived organic carbon decomposition. Our results suggested that anthropogenic activities strongly affect lake CO₂ dynamics and that environmental investments, such as ecological restoration and reducing nutrient discharge, can significantly reduce CO₂ emissions from inland lakes. This study provides valuable information on the reduction in carbon emissions from artificially controlled eutrophic lakes and an assessment of the impact of inland water on the global carbon cycle.

Empirical estimates of regional carbon budgets imply reduced global soil heterotrophic respiration

Resolving regional carbon budgets is critical for informing land-based mitigation policy. For nine regions covering nearly the whole globe, we collected inventory estimates of carbon-stock changes complemented by satellite estimates of biomass changes where inventory data are missing. The net land–atmospheric carbon exchange (NEE) was calculated by taking the sum of the carbon-stock change and lateral carbon fluxes from crop and wood trade, and riverine-carbon export to the ocean. Summing up NEE from all regions, we obtained a global ‘bottom-up’ NEE for net land anthropogenic CO₂ uptake of –2.2 ± 0.6 PgC yr⁻¹ consistent with the independent top-down NEE from the global atmospheric carbon budget during 2000–2009. This estimate is so far the most comprehensive global bottom-up carbon budget accounting, which set up an important milestone for global carbon-cycle studies. By decomposing NEE into component fluxes, we found that global soil heterotrophic respiration amounts to a source of CO₂ of 39 PgC yr⁻¹ with an interquartile of 33–46 PgC yr⁻¹—a much smaller portion of net primary productivity than previously reported.

Urbanization increases carbon concentration and pCO2 in subtropical streams

Urbanization growth may alter the hydrologic conditions and processes driving carbon concentrations in aquatic systems through local changes in land use. Here, we explore dissolved carbon concentrations (DIC and DOC) along urbanization gradient in Santa Catarina Island to evaluate potential increase of CO₂ in streams. Additionally, we assessed chemical, physical, and biotic variables to evaluate direct and indirect effects of urbanization in watersheds. We defined 3 specific urbanization levels: high (> 15% urbanized area), medium (15-5% urbanized area), and low (< 5% urbanized area) urbanization. The results showed that local changes due to growth of urban areas into watersheds altered the carbon concentrations in streams. DOC and DIC showed high concentrations in higher urbanization levels. The watersheds with an urban building area above 5% showed pCO₂ predominantly above the equilibrium with the atmosphere. These findings reveal that local modifications in land use may contribute to changes in global climate by altering the regional carbon balance in streams.

Biological index based on epiphytic diatom assemblages is more restrictive than the physicochemical index in water assessment on an Amazon floodplain, Brazil

Canadian Water Quality Index (CWQI) provides protection for freshwater life promoting healthy ecosystems and safeguarding human health. Biological Diatom Index (BDI) was developed to indicate the ecological status and water quality of freshwater systems. This paper evaluates the relations between the two different indices. During rising and falling, water samples were taken in the Curuai Floodplain, Brazil. CWQI was calculated using 14 physicochemical parameters and 1 microbiological parameter. The limits were established according to freshwater quality conditions and standards based on water use classes 1 and 2 determined in CONAMA 357 legislation and British Columbia. Canadian Water Quality Index categorization ranged from "marginal" to "excellent," most sampling units were "good" (71%), followed by "fair" (12%) and "excellent" (12%) water quality. Total phosphorus (38 times), chlorophyll a (20), dissolved oxygen (10), and total organic carbon (10) were the parameters that presented the most non-compliance values. Encyonema silesiacum (14%), Gomphonema parvulum (13%), and Navicula cryptotenella (12%) were the main taxa in the rising period, while G. lagenula, E. silesiacum, and Fragilaria capucina were the main taxa during the falling period. BDI ranges from I to V water quality classes. We observed "poor" to "very good" ecological status, with most sampling units "moderate" (52%) and "good" (29%). Water quality for class 2 was better than water quality for class 1, as the limits of the parameters evaluated were more restrictive in class 1 than in class 2 and the predominant uses of water require a higher degree of water purity. The biological index based on diatoms was the most restrictive index whose water classes and categorizations have shown an ecological status that could threaten the protection of aquatic communities on the Curuai floodplain. We suggest the combined use of both indices-physicochemical and biological for water quality assessment in this type of environment.

Linking riverine partial pressure of carbon dioxide to dissolved organic matter optical properties in a Dry-hot Valley Region

The mineralization of dissolved organic matter (DOM) can partially explain riverine carbon dioxide (CO₂) emissions to the atmosphere. However, little is known about how the DOM origin and composition drive CO₂ partial pressures (pCO₂). Here, we reveal links between aquatic pCO₂, DOM optical parameters (a₂₅₄, a₃₅₀ and S_275-295 and S_350-400) and nutrients in a subtropical river in China's Dry-hot Valley Region. Biodegradation preferentially decomposed low molecular weight (LMW) DOMs, increasing high molecular weight (HMW) DOMs along the main stem. pCO₂ was positively correlated with aromatic and lignin compounds, but negatively correlated with DOM molecular weight. Aquatic respiration of DOMs largely explained the pCO₂ levels in the drought period, while terrestrial inputs were a pCO₂ source in the initial-wet period. Our results illustrate how both DOM concentrations and speciation can explain pCO₂ distribution and sources in rivers.

Ichthyofauna from tributaries of Urubu and Amazonas rivers, Amazonas State, Brazil

Abstract: The Amazonas River basin comprises the world’s highest fish species diversity. Anthropogenic interferences in aquatic environments represent a pressure over the maintenance of ecological stability and biodiversity. We inventoried the ichthyofauna of 13 disturbed/modified tributaries of Urubu and Amazonas rivers in the region of the middle Amazon River, between June 2018 and March 2019. A total of 164 species were captured, represented by 11 orders, 37 families and 96 genera. Characiformes was the richest order, followed by Cichliformes and Siluriformes. The most representative families in number of species were Cichlidae, Serrasalmidae, and Characidae. Hemigrammus levis was the most abundant species, and Acarichthys heckelii the most common, registered in all sampled sites. In the present study, species with economic interest were collected, as well as many species recently described and one still waiting for formal description, identified provisionally as Moenkhausia aff. colletii. Therefore, the high fish diversity registered, even in disturbed environments in Middle Amazonas River, denotes the makeable ecological importance of this region for fishes resources and supports the necessity of evaluation of other aquatic environments in the region, as well as the potential impacts on composition, maintenance, and survival of ichthyofauna in environments directly affected by human activities.

Impact of Land Cover Types on Riverine CO2 Outgassing in the Yellow River Source Region

Under the context of climate change, studying CO2 emissions in alpine rivers is important because of the large carbon storage in these terrestrial ecosystems. In this study, riverine partial pressure of CO2 (pCO2) and CO2 emission flux (FCO2) in the Yellow River source region (YRSR) under different landcover types, including glaciers, permafrost, peatlands, and grasslands, were systematically investigated in April, June, August, and October 2016. Relevant chemical and environmental parameters were analyzed to explore the primary controlling factors. The results showed that most of the rivers in the YRSR were net CO2 source, with the pCO2 ranging from 181 to 2441 μatm and the FCO2 ranging from −50 to 1574 mmol m−2 d−1. Both pCO2 and FCO2 showed strong spatial and temporal variations. The highest average FCO2 was observed in August, while the lowest average was observed in June. Spatially, the lowest FCO2 were observed in the permafrost regions while the highest FCO2 were observed in peatland. By integrating seasonal changes of the water surface area, total CO2 efflux was estimated to be 0.30 Tg C year−1. This indicates that the YRSR was a net carbon source for the atmosphere, which contradicts previous studies that conclude the YRSR as a carbon sink. More frequent measurements of CO2 fluxes, particularly through several diel cycles, are necessary to confirm this conclusion. Furthermore, our study suggested that the riverine dissolved organic carbon (DOC) in permafrost (5.0 ± 2.4 mg L−1) is possibly derived from old carbon released from permafrost melting, which is equivalent to that in peatland regions (5.1 ± 3.7 mg L−1). The degradation of DOC may have played an important role in supporting riverine CO2, especially in permafrost and glacier-covered regions. The percent coverage of corresponding land cover types is a good indicator for estimating riverine pCO2 in the YRSR. In view of the extensive distribution of alpine rivers in the world and their sensitivity to climate change, future studies on dynamics of stream water pCO2 and CO2 outgassing are strongly needed to better understand the global carbon cycle.

A spherical falling film gas-liquid equilibrator for rapid and continuous measurements of CO2 and other trace gases

Use of gas-liquid equilibrators to measure trace gases such as CO2, methane, and radon in water bodies is widespread. Such measurements are critical for understanding a variety of water quality issues such as acidification due to elevated CO2 or other processes related ecosystem metabolism and function. However, because gas-liquid equilibrators rely on generating sufficient surface area for gas exchange between liquid and gas phases, most traditional equilibrators pass water through small orifices or interstitial spaces that rapidly clog in highly productive or turbid waters, conditions that are common in estuaries, coastal bays, and riverine systems. Likewise, in cold temperatures, such equilibrators are subject to freezing. Both situations lead to failure and limit utility, especially for long term, continuous environmental monitoring. Here we describe and test a gas-liquid equilibrator that relies on a continuous falling film of water over a spherical surface to drive gas exchange. Our results demonstrate that this design is accurate in its ability to equilibrate fully to aqueous CO2 concentrations, is functional across a wide range of gas concentrations, and has a response time that is comparable with other equilibrator designs. Because this equilibrator uses free flowing, falling water to produce a surface for gas exchange, our field trials have shown it to be very resistant to clogging and freezing, and therefore well suited to long term deployment in highly productive waters like estuaries where CO2 concentrations fluctuate hourly, daily, and seasonally. When generated across a spherical surface, the falling film is not adversely affected by tilting off vertical, conditions that are common on a ship, small vessel, or buoy.

Spatial heterogeneity and hydrological fluctuations drive bacterioplankton community composition in an Amazon floodplain system

Amazonian floodplains form complex hydrological networks that play relevant roles in global biogeochemical cycles, and bacterial degradation of the organic matter in these systems is key for regional carbon budget. The Amazon undergoes extreme seasonal variations in water level, which produces changes in landscape and diversifies sources of organic inputs into floodplain systems. Although these changes should affect bacterioplankton community composition (BCC), little is known about which factors drive spatial and temporal patterns of bacterioplankton in these Amazonian floodplains. We used high-throughput sequencing (Illumina MiSeq) of the V3-V4 region of the 16S rRNA gene to investigate spatial and temporal patterns of BCC of two size fractions, and their correlation with environmental variables in an Amazon floodplain lake (Lago Grande do Curuai). We found a high degree of novelty in bacterioplankton, as more than half of operational taxonomic units (OTUs) could not be classified at genus level. Spatial habitat heterogeneity and the flood pulse were the main factors shaping free-living (FL) BCC. The gradient of organic matter from transition zone-lake-Amazon River was the main driver for particle-attached (PA) BCC. The BCC reflected the complexity of the system, with more variation in space than in time, although both factors were important drivers of the BCC in this Amazon floodplain system.

Dissolved Carbon Transport and Processing in North America’s Largest Swamp River Entering the Northern Gulf of Mexico

Transport and transformation of riverine dissolved carbon is an important component of global carbon cycling. The Atchafalaya River (AR) flows 189 kilometers through the largest bottomland swamp in North America and discharges ~25% of the flow of the Mississippi River into the Gulf of Mexico annually, providing a unique opportunity to study the floodplain/wetland impacts on dissolved carbon. The aim of this study is to determine how dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in the AR change spatially and seasonally, and to elucidate which processes control the carbon cycling in this intricate swamp-river system. From May 2015 to May 2016, we conducted monthly river sampling from the river’s inflow to its outflow, analyzing samples for concentrations and δ13C stable isotope composition of DOC and DIC. We found that DIC concentrations in the AR were three times higher than the DOC concentrations on average, and showed more pronounced downstream changes than the DOC. During the study period, the river discharged a total of 5.35 Tg DIC and a total of 2.34 Tg DOC into the Gulf of Mexico. Based on the mass inflow–outflow balance, approximately 0.53 Tg (~10%) of the total DIC exported was produced within the floodplain/wetland system, while 0.24 Tg (~10%) of the DOC entering the basin was removed. The AR’s water was consistently oversaturated with CO2 partial pressure (pCO2) above the atmospheric pCO2 (with pCO2 varying from 551 µatm to 6922 µatm), indicating a large source of DIC from river waters to the atmosphere as well as to the coastal margins. Largest changes in carbon constituents occurred during periods of greatest inundation of the swamp-river basin and corresponded with shifts in isotopic composition. This effect was particularly pronounced during the initial flood stages, supporting the hypothesis that subtropical floodplains can act as effective enhancers of the biogeochemical cycling of dissolved carbon.

Aquatic carbon fluxes dampen the overall variation of net ecosystem productivity in the Amazon basin: An analysis of the interannual variability in the boundless carbon cycle

The river-floodplain network plays an important role in the carbon (C) cycle of the Amazon basin, as it transports and processes a significant fraction of the C fixed by terrestrial vegetation, most of which evades as CO₂ from rivers and floodplains back to the atmosphere. There is empirical evidence that exceptionally dry or wet years have an impact on the net C balance in the Amazon. While seasonal and interannual variations in hydrology have a direct impact on the amounts of C transferred through the river-floodplain system, it is not known how far the variation of these fluxes affects the overall Amazon C balance. Here, we introduce a new wetland forcing file for the ORCHILEAK model, which improves the representation of floodplain dynamics and allows us to closely reproduce data-driven estimates of net C exports through the river-floodplain network. Based on this new wetland forcing and two climate forcing datasets, we show that across the Amazon, the percentage of net primary productivity lost to the river-floodplain system is highly variable at the interannual timescale, and wet years fuel aquatic CO₂ evasion. However, at the same time overall net ecosystem productivity (NEP) and C sequestration are highest during wet years, partly due to reduced decomposition rates in water-logged floodplain soils. It is years with the lowest discharge and floodplain inundation, often associated with El Nino events, that have the lowest NEP and the highest total (terrestrial plus aquatic) CO₂ emissions back to atmosphere. Furthermore, we find that aquatic C fluxes display greater variation than terrestrial C fluxes, and that this variation significantly dampens the interannual variability in NEP of the Amazon basin. These results call for a more integrative view of the C fluxes through the vegetation-soil-river-floodplain continuum, which directly places aquatic C fluxes into the overall C budget of the Amazon basin.

Importance of the vegetation-groundwater-stream continuum to understand transformation of biogenic carbon in aquatic systems - A case study based on a pine-maize comparison in a lowland sandy watershed (Landes de Gascogne, SW France)

During land-aquatic transfer, carbon (C) and inorganic nutrients (IN) are transformed in soils, groundwater, and at the groundwater-surface water interface as well as in stream channels and stream sediments. However, processes and factors controlling these transfers and transformations are not well constrained, particularly with respect to land use effect. We compared C and IN concentrations in shallow groundwater and first-order streams of a sandy lowland catchment dominated by two types of land use: pine forest and maize cropland. Contrary to forest groundwater, crop groundwater exhibited oxic conditions all-year round as a result of higher evapotranspiration and better lateral drainage that decreased the water table below the organic-rich soil horizon, prevented the leaching of soil-generated dissolved organic carbon (DOC) in groundwater, and thus limited consumption of dissolved oxygen (O₂). In crop groundwater, oxic conditions inhibited denitrification and methanogenesis resulting in high nitrate (NO₃^-; on average 1140 ± 485 μmol L^-1) and low methane (CH₄; 40 ± 25 nmol L^-1) concentrations. Conversely, anoxic conditions in forest groundwater led to lower NO₃^- (25 ± 40 μmol L^-1) and higher CH₄ (1770 ± 1830 nmol L^-1) concentrations. The partial pressure of carbon dioxide (pCO₂; 30,650 ± 11,590 ppmv) in crop groundwater was significantly lower than in forest groundwater (50,630 ± 26,070 ppmv), and was apparently caused by the deeper water table delaying downward diffusion of soil CO₂ to the water table. In contrast, pCO₂ was not significantly different in crop (4480 ± 2680 ppmv) and forest (4900 ± 4500 ppmv) streams, suggesting faster degassing in forest streams resulting from greater water turbulence. Although NO₃^-concentrations indicated that denitrification occurred in riparian-forest groundwater, crop streams nevertheless exhibited important signs of spring and summer eutrophication such as the development of macrophytes. Stream eutrophication favored development of anaerobic conditions in crop stream sediments, as evidenced by increased ammonia (NH₄⁺) and CH₄ in stream waters and concomitant decreased in NO₃^- concentrations as a result of sediment denitrification. In crop streams, dredging and erosion of streambed sediments during winter sustained high concentration of particulate organic C, NH₄⁺ and CH₄. In forest streams, dissolved iron (Fe²⁺), NH₄⁺ and CH₄ were negatively correlated with O₂ reflecting the gradual oxygenation of stream water and associated oxidations of Fe²⁺, NH₄⁺ and CH₄. The results overall showed that forest groundwater behaved as source of CO₂ and CH₄ to streams, the intensity depending on the hydrological connectivity among soils, groundwater, and streams. CH₄ production was prevented in cropland in soils and groundwater, however crop groundwater acted as a source of CO₂ to streams (but less so than forest groundwater). Conversely, in streams, pCO₂ was not significantly affected by land use while CH₄ production was enhanced by cropland. At the catchment scale, this study found substantial biogeochemical heterogeneity in C and IN concentrations between forest and crop waters, demonstrating the importance of including the full vegetation-groundwater-stream continuum when estimating land-water fluxes of C (and nitrogen) and attempting to understand their spatial and temporal dynamics.