Abstract. Due to their intense terrestrial-aquatic linkages and intense ecosystem metabolism, many river deltas are aquatic hot spots for carbon dioxide (CO2) emissions to the atmosphere. A patchwork of wetlands, lakes, channels, and river reaches often complicates the analysis of CO2 sources such as ecosystem respiration or lateral transfer. Sensing techniques offer the opportunity of measuring the paired CO2-O2 concentrations at high temporal resolution for periods from days to months. Such time-series allow quantifying diurnal and seasonal cycles of river metabolism and lateral exchange. This study documents paired O2-CO2 measurements at 15-minute time resolution obtained from deployment of sensor packages in river reaches and channels of the Danube Delta in Romania during a total observation period of three years. We show how to combine results from covariance analysis with insights from averaged 24-hour diurnal cycles. The time series reveal two orders of magnitude variability in daily CO2 fluctuations along the main Danube reaches with typical maxima in the early morning and minima in the afternoon. The amplitude of monthly averaged daily cycles was 2–2.5 times larger in delta channels compared to the river reaches and the parameter for metabolic intensity reacted on average four times more sensitive to changes in water temperature and cloud cover within the delta compared to the main river. Inflow of O2-depleted and CO2-rich wetland water to the downstream river stations was most intense during spring floods with estimated mixing rations of up to 5–20 % depending on the station. Stoichiometry of O2- CO2 changes pointed to strong contributions of methane oxidation and/or nitrification in 40 % of the analysed summer data, which was also evident from the frequently observed oxygen deficiency. During June–July, a monitored lake system and an adjacent channel showed clear evidence for calcite precipitation and a switch to photosynthetic uptake of HCO3-. The study illustrates the benefits of a combining covariance analysis with 24-hours averaging for identifying the timing, intensity and type of processes in river metabolism and lateral exchange.

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.

Global estimates see river deltas and estuaries contributing about equally to CO 2 and CH 4 emissions as lakes and reservoirs, despite a factor 6 smaller surface area. Assessing the horizontal gradients in dissolved gas concentrations from large river reaches to connecting canals and wetland lakes remains a challenge in many deltaic systems. To elucidate the processes affecting local CO 2 and CH 4 concentrations in the Romanian part of the Danube Delta, we mapped dissolved O 2 , N 2 , He and Ar using a portable gas-equilibration membrane-inlet mass spectrometer (GE-MIMS), along with CO 2 , CH 4 , water temperature and conductivity. We measured the concentrations along the aquatic continuum from a small houseboat during two campaigns, in spring and autumn, to capture different hydrological and plant growth conditions. Delta-scale concentration patterns were comparably stable across seasons. Small connecting channels were highly influenced by the riparian wetland, which was strongest in the eastern part of the biosphere reserve. These sites represented the delta’s CO 2 and CH 4 hotspots and showed clear signs of excess air, i.e., supersaturation of dissolved noble gases with respect to air-saturated water. As the adjacent wetland was permanently inundated, this signal was likely caused by root aeration of Phragmites australis , as opposed to traditional excess air formation via water table fluctuations in the unsaturated zone. The special vegetation setting with reed growing on floating peat coincided with the highest CO 2 and CH 4 concentrations (>700 μmol/L CO 2 and 13 μmol/L CH 4 , respectively) observed in an adjacent channel. Shallow lakes, on the other hand, were major sites of photosynthetic production with O 2 oversaturation reaching up to 150% in spring. The observed deficit in non-reactive gases (He, Ar and N 2 ) indicated that the lakes were affected by O 2 ebullition from macrophytes. According to our estimations, this ebullitive flux decreased O 2 concentrations by up to 2 mg/L. This study highlights the effect of plant-mediated gas transfer on dissolved gas concentrations and supports recent studies stressing the need to account for ebullitive gas exchange when assessing metabolism parameters from O 2 in shallow, productive settings.

River deltas, with their mosaic of ponds, channels and seasonally inundated areas, act as the last continental hot spots of carbon turnover along the land–ocean aquatic continuum. There is increasing evidence for the important role of riparian wetlands in the transformation and emission of terrestrial carbon to the atmosphere. The considerable spatial heterogeneity of river deltas, however, forms a major obstacle for quantifying carbon emissions and their seasonality. The water chemistry in the river reaches is defined by the upstream catchment, whereas delta lakes and channels are dominated by local processes such as aquatic primary production, respiration or lateral exchange with the wetlands. In order to quantify carbon turnover and emissions in the complex mosaic of the Danube Delta, we conducted monthly field campaigns over 2 years at 19 sites spanning river reaches, channels and lakes. Here we report on the greenhouse gas fluxes (CO2 and CH4) from the freshwater systems of the Danube Delta and present the first seasonally resolved estimates of its freshwater carbon emissions to the atmosphere. Furthermore, we quantify the lateral carbon transport of the Danube River to the Black Sea. We estimate the delta’s CO2 and CH4 emissions to be 65 GgC yr−1 (30–120 GgC yr−1, a range calculated using 25 to 75 percentiles of observed fluxes), of which about 8 % are released as CH4. The median CO2 fluxes from river branches, channels and lakes are 25, 93 and 5.8 mmol m−2 d−1, respectively. Median total CH4 fluxes amount to 0.42, 2.0 and 1.5 mmol m−2 d−1. While lakes do have the potential to act as CO2 sinks in summer, they are generally the largest emitters of CH4. Small channels showed the largest range in emissions, including a CO2 and CH4 hot spot sustained by adjacent wetlands. Thereby, the channels contribute disproportionately to the delta’s emissions, considering their limited surface area. In terms of lateral export, we estimate the net total export (the sum of dissolved inorganic carbon, DIC, dissolved organic carbon, DOC, and particulate organic carbon, POC) from the Danube Delta to the Black Sea to be about 160± 280 GgC yr−1, which only marginally increases the carbon load from the upstream river catchment (8490± 240 GgC yr−1) by about 2 %. While this contribution from the delta seems small, deltaic carbon yield (45.6 gC m−2 yr−1; net export load/surface area) is about 4 times higher than the riverine carbon yield from the catchment (10.6 gC m−2 yr−1).

The Zambezi River Basin in Southern Africa is undergoing rapid development and population growth. Agricultural intensification, urbanization and future development of hydropower dams will likely lead to a degradation of surface water quality, but there have been few formal assessments of where, how and why these changes impact specific water quality parameters based on in situ data spanning a large region. We sampled a large suite of biogeochemical water quality parameters at 14 locations in four field campaigns in central and southern Zambia in 2018 and 2019 to characterize seasonal changes in water quality in response to large hydropower dams and human landscape transformations. We find that the major rivers (Zambezi and Kafue) are very clean with extremely low concentrations of solutes, but suffer from thermal changes, hypoxia and loss of suspended sediment below dams. Smaller tributaries with a relatively large anthropogenic landcover footprint in their catchments show signs of pollution in the form of higher concentrations of nutrients and dissolved ions. We find significant relationships between crop and urban land cover metrics and selected water quality metrics (i.e. conductivity, phosphorus and nitrogen) across our data set. These results reflect a very high-quality waterscape exhibiting some hotspots of degradation associated with specific human activities. We anticipate that as agricultural intensification, urbanization and future hydropower development continue to accelerate in the basin, the number and extent of these hotspots of water quality degradation will grow in response. There is an opportunity for governments, managers and industry to mitigate water quality degradation via investment in sustainable infrastructure and practice, such as wastewater treatment, environmental dam operations, or riparian protection zones.

Recent studies show that tropical hydroelectric reservoirs may be responsible for substantial greenhouse gas emissions to the atmosphere, yet emissions from the surface of released water downstream of the dam are poorly characterized if not neglected entirely from most assessments. We found that carbon dioxide (CO2) emission downstream of Kariba Dam (southern Africa) varied widely over different timescales and that accounting for downstream emissions and their fluctuations is critically important to the reservoir carbon budget. Seasonal variation was driven by reservoir stratification and the accumulation of CO2 in hypolimnetic waters, while subdaily variation was driven by hydropeaking events caused by dam operation in response to daily electricity demand. This "carbopeaking" resulted in hourly variations of CO2 emission up to 200% during stratification. Failing to account for seasonal or subdaily variations in downstream carbon emissions could lead to errors of up to 90% when estimating the reservoir's annual emissions. These results demonstrate the critical need to include both limnological seasonality and dam operation at subdaily time steps in the assessment of carbon budgeting of reservoirs and carbon cycling along the aquatic continuum.

This dataset contains oxygen stable isotope data on surface water samples collected on the Kafué River (Zambia) between February 2012 and November 2013These data were collected within the framework of ERC-StG AFRIVAL: African river basins: catchment-scale carbon fluxes and transformations (2009-2014) Ancillary data and site information can be found in : Teodoru CR, Nyoni FC, Borges AV, Darchambeau F, Nyambe I, & Bouillon S (2015) Dynamics of greenhouse gases (CO2, CH4, N2O) along the Zambezi River and major tributaries, and their importance in the riverine carbon budget. Biogeosciences 12: 2431-2453 https://bg.copernicus.org/articles/12/2431/2015/

One prominent effect of nutrient pollution of surface waters is the mass invasion of floating plants, which can clog waterways, disrupting human use of aquatic systems. These plants are widely vilified and motivate expensive control campaigns, but their presence may be providing a poorly recognized function in the cycling of excess nutrients. The capacity for floating plants to absorb nutrients from surface water has been understood for decades, primarily from their use in constructed wetlands for wastewater treatment. Yet, in natural settings, there has not been to date any effort to quantify whether floating plant invasions represent important pools or fluxes of nutrients relative to those of the river catchments in which they occur. We found that seasonal hydrologic cycles in the Zambezi trap and flush floating plants from river choke points, such as dams and river confluences, on an annual basis. Peak plant biomass at such choke points constitutes a proxy for estimating annual plant-bound nutrient loads. We assessed the significance of floating vegetation as nutrient sinks by comparing annual plant-bound nutrient loading to conventional river nutrient loading (dissolved and particulate) for four tributaries of the Zambezi River in Zambia. We found that the relative importance of floating vegetation was greatest in the more urbanized catchments, such as the Maramba River draining the city of Livingstone, representing approximately 30% and 9% of annual digestible phosphorus and nitrogen flux respectively. We also found plant-bound phosphorus to be important in the Kafue River (19%), draining the industrial town of Kafue and extensive sugarcane plantations. These results demonstrate the great potential of floating plants to take up excess nutrients from natural river systems. Given the importance of hydrology in the life cycle of floating vegetation, controlled dam discharges may have an important role in managing them and their water quality treatment functions.

We carried out 10 field expeditions between 2010 and 2015 in the lowland part of the Congo River network in the eastern part of the basin (Democratic Republic of the Congo), to describe the spatial variations in fluvial dissolved carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) concentrations. We investigate the possible drivers of the spatial variations in dissolved CO2, CH4 and N2O concentrations by analyzing covariations with several other biogeochemical variables, aquatic metabolic processes (primary production and respiration), catchment characteristics (land cover) and wetland spatial distributions. We test the hypothesis that spatial patterns of CO2, CH4 and N2O are partly due to the connectivity with wetlands, in particular with a giant wetland of flooded forest in the core of the Congo basin, the “Cuvette Centrale Congolaise” (CCC). Two transects of 1650 km were carried out from the city of Kisangani to the city of Kinshasa, along the longest possible navigable section of the river and corresponding to 41 % of the total length of the main stem. Additionally, three time series of CH4 and N2O were obtained at fixed points in the main stem of the middle Congo (2013–2018, biweekly sampling), in the main stem of the lower Kasaï (2015–2017, monthly sampling) and in the main stem of the middle Oubangui (2010–2012, biweekly sampling). The variations in dissolved N2O concentrations were modest, with values oscillating around the concentration corresponding to saturation with the atmosphere, with N2O saturation level (%N2O, where atmospheric equilibrium corresponds to 100 %) ranging between 0 % and 561 % (average 142 %). The relatively narrow range of %N2O variations was consistent with low NH4+ (2.3±1.3 µmol L−1) and NO3- (5.6±5.1 µmol L−1) levels in these near pristine rivers and streams, with low agriculture pressure on the catchment (croplands correspond to 0.1 % of catchment land cover of sampled rivers), dominated by forests (∼70 % of land cover). The covariations in %N2O, NH4+, NO3- and dissolved oxygen saturation level (%O2) indicate N2O removal by soil or sedimentary denitrification in low O2, high NH4+ and low NO3- environments (typically small and organic matter rich streams) and N2O production by nitrification in high O2, low NH4+ and high NO3- (typical of larger rivers that are poor in organic matter). Surface waters were very strongly oversaturated in CO2 and CH4 with respect to atmospheric equilibrium, with values of the partial pressure of CO2 (pCO2) ranging between 1087 and 22 899 ppm (equilibrium ∼400 ppm) and dissolved CH4 concentrations ranging between 22 and 71 428 nmol L−1 (equilibrium ∼2 nmol L−1). Spatial variations were overwhelmingly more important than seasonal variations for pCO2, CH4 and %N2O as well as day–night variations for pCO2. The wide range of pCO2 and CH4 variations was consistent with the equally wide range of %O2 (0.3 %–122.8 %) and of dissolved organic carbon (DOC) (1.8–67.8 mg L−1), indicative of generation of these two greenhouse gases from intense processing of organic matter either in “terra firme” soils, wetlands or in-stream. However, the emission rate of CO2 to the atmosphere from riverine surface waters was on average about 10 times higher than the flux of CO2 produced by aquatic net heterotrophy (as evaluated from measurements of pelagic respiration and primary production). This indicates that the CO2 emissions from the river network were sustained by lateral inputs of CO2 (either from terra firme or from wetlands). The pCO2 and CH4 values decreased and %O2 increased with increasing Strahler order, showing that stream size explains part of the spatial variability of these quantities. In addition, several lines of evidence indicate that lateral inputs of carbon from wetlands (flooded forest and aquatic macrophytes) were of paramount importance in sustaining high CO2 and CH4 concentrations in the Congo river network, as well as driving spatial variations: the rivers draining the CCC were characterized by significantly higher pCO2 and CH4 and significantly lower %O2 and %N2O values than those not draining the CCC; pCO2 and %O2 values were correlated to the coverage of flooded forest on the catchment. The flux of greenhouse gases (GHGs) between rivers and the atmosphere averaged 2469 mmol m−2 d−1 for CO2 (range 86 and 7110 mmol m−2 d−1), 12 553 µmol m−2 d−1 for CH4 (range 65 and 597 260 µmol m−2 d−1) and 22 µmol m−2 d−1 for N2O (range −52 and 319 µmol m−2 d−1). The estimate of integrated CO2 emission from the Congo River network (251±46 TgC (1012 gC) yr−1), corresponding to nearly half the CO2 emissions from tropical oceans globally (565 TgC yr−1) and was nearly 2 times the CO2 emissions from the tropical Atlantic Ocean (137 TgC yr−1). Moreover, the integrated CO2 emission from the Congo River network is more than 3 times higher than the estimate of terrestrial net ecosystem exchange (NEE) on the whole catchment (77 TgC yr−1). This shows that it is unlikely that the CO2 emissions from the river network were sustained by the hydrological carbon export from terra firme soils (typically very small compared to terrestrial NEE) but most likely, to a large extent, they were sustained by wetlands (with a much higher hydrological connectivity with rivers and streams).

Abstract. We report the spatial variations of dissolved carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) concentrations in the lowland part of the Congo River network obtained during ten field expeditions carried out between 2010 and 2015, in the eastern part of the basin (Democratic Republic of Congo). Two transects of 1,650 km were carried out from the city of Kisangani to the city of Kinshasa, along the longest possible navigable section of the river, and corresponding to 41 % of the total length of the mainstem. Additionally, three time series of CH4 and N2O were obtained at fixed points in the mainstem of the middle Congo (2013–2018, biweekly sampling), in the mainstem of the lower Kasaï (2015–2017, monthly sampling), and in the mainstem of the middle Oubangui (2010–2012, biweekly sampling). The variations of dissolved N2O concentrations were modest, with values oscillating around the concentration corresponding to saturation with the atmosphere, with N2O saturation level (%N2O) ranging between 0 % and 561 % (average 142 %). The relatively narrow range of %N2O variations was consistent with low NH4+ (2.3±1.3 µmol L−1) and NO3− (5.6±5.1 µmol L−1) levels in these near pristine rivers and streams with low agriculture pressure on the catchment (croplands correspond to 0.1 % of catchment land cover of sampled rivers), dominated by forests (~ 70 % of land cover). The co-variations of %N2O, NH4+, NO3−, and dissolved oxygen saturation level (%O2) indicate N2O removal by sedimentary denitrification in low O2, high NH4+ and low NO3− environments (typically small and organic matter rich streams) and N2O production by nitrification in high O2, low NH4+ and high NO3− (typical of larger rivers that are poor in organic matter). Surface waters were very strongly over-saturated in CO2 and CH4 with respect to atmospheric equilibrium, with values of the partial pressure of CO2 (pCO2) ranging between 1,087 and 22,899 ppm (equilibrium ~ 400 ppm), and dissolved CH4 concentrations ranging between 22 and 71,428 nmol L−1 (equilibrium ~ 2 nmol L−1). Spatial variations were overwhelmingly more important than seasonal variations for pCO2, CH4 and %N2O, and than diurnal (day-night) variations for pCO2. The wide range of pCO2 and CH4 variations was consistent with the equally wide range of %O2 (0.3–122.8 %) and of dissolved organic carbon (DOC) (1.8–67.8 mg L−1), indicative of intense processing of organic matter that generated these two greenhouse gases. However, the emission rate of CO2 to the atmosphere from riverine surface waters was on average about 10 times higher than the flux of CO2 produced by aquatic net heterotrophy (as evaluated from measurements of pelagic respiration and primary production). This indicates that the CO2 emissions from the river network were sustained by lateral inputs of CO2 (either from terra firme or from wetlands). The pCO2 and CH4 values decreased and %O2 increased with increasing Strahler order, showing that stream size explains part of the spatial variability of these quantities. In addition, several lines of evidence indicate that lateral inputs of carbon from wetlands (flooded forest and aquatic macrophytes) were of paramount importance in sustaining high CO2 and CH4 concentrations in the Congo river network, as well as driving spatial variations: the rivers draining the Cuvette Centrale Congolaise (CCC) (a giant wetland of flooded forest in the core of the Congo basin) were characterized by significantly higher pCO2 and CH4 and significantly lower %O2 and %N2O values than those not draining the CCC; pCO2 and %O2 values were correlated to the coverage of flooded forest on the catchment. The flux of GHGs between rivers and the atmosphere averaged 2,469 mmol m−2 d−1 for CO2 (range 86 and 7,110 mmol m−2 d−1), 12,553 µmol m−2 d−1 for CH4 (range 65 and 597,260 µmol m−2 d−1), 22 µmol m−2 d−1 for N2O (range −52 and 319 µmol m−2 d−1). The estimate of integrated CO2 emission from the Congo River network (251 TgC (1012 gC) yr−1) corresponded to nearly half the CO2 emissions from tropical oceans globally (565 TgC yr−1) and was nearly two times the CO2 emissions from the tropical Atlantic Ocean (137 TgC yr−1). Moreover, the integrated CO2 emission from the Congo River network is more than three times higher than the estimate of terrestrial net ecosystem exchange (NEE) on the whole catchment (77 TgC yr−1). This shows that it is unlikely that the CO2 emissions from the river network were sustained by the hydrological carbon export from terra firme soils (typically very small compared to terrestrial NEE), but most likely, to a large extent, they were sustained by wetlands (with a much higher hydrological connectivity with rivers and streams).

A process-based reservoir model of Flooded Forest Denitrification Decomposition (FF-DNDC) was developed to simulate carbon dioxide (CO2) exchange from flooded boreal landscapes. The reservoir model is based on Forest-DNDC, a terrestrial biogeochemistry model which supports detailed soil carbon (C) processes including redox chemistry, with modification to represent the disturbed soil and vegetation C dynamics due to the presence of an overlying water column on the ecosystems. Soil decomposition rates and temperature and oxygen profiles were changed, and sedimentation to the soil surface was added. FF-DNDC was evaluated using CO2 exchange measurements from the newly created Eastmain-1 reservoir in northern Quebec, Canada. For the first four years of the reservoir (2006 to 2009), simulated daily CO2 emissions averaged 1.42 g C m(-2) d(-1) (ranging from 0.75 to 3.24 g C m(-2) d(-1)) from the flooded forest and 0.74 g Cm-2 d(-1) (ranging from 0.51 to 1.09 gC m(-2) d(-1)) from the flooded peatland. The simulated emissions were smaller than the thin-filmed boundary layer exchanges based on measured partial pressure of carbon dioxide (pCO(2)) but were larger than the exchanges measured using an eddy covariance system. However, the temporal patterns of simulated and measured exchanges were similar. We simulated potential CO2 emissions over 100 years, the expected operating lifetime of the reservoir, with assuming no change in climate. Simulated CO2 emissions decreased with time since flooding especially for the first four decades. The 100-year cumulative emissions from the flooded peatland were larger than those from the flooded forest. Sensitivity analysis indicated that vegetation and soil inputs and parameters controlling the quality and/or quantity of decomposable soil C in flooded ecosystems (e.g. woody vegetation biomass, soil organic carbon in organic and mineral layers, and carbon:nitrogen ratio in woody vegetation and soil) were important to the reservoir CO2 emission. (C) 2016 Elsevier B.V. All rights reserved.

Large rivers transport considerable amounts of terrestrial dissolved organic matter (DOM) to the ocean. However, downstream gradients and temporal variability in DOM fluxes and characteristics are poorly studied at the scale of large river basins, especially in tropical areas. Here, we report longitudinal patterns in DOM content and composition based on absorbance and fluorescence measurements along the Zambezi River and its main tributary, the Kafue River, during two hydrological seasons. During high-flow periods, a greater proportion of aromatic and humic DOM was mobilized along rivers due to the hydrological connectivity with wetlands, while low-flow periods were characterized by lower DOM content of less aromaticity resulting from loss of connectivity with wetlands, more efficient degradation of terrestrial DOM and enhanced autochthonous productivity. Changes in water residence time due to contrasting water discharge were found to modulate the fate of DOM along the river continuum. Thus, high water discharge promotes the transport of terrestrial DOM downstream relative to its degradation, while low water discharge enhances the degradation of DOM during its transport. The longitudinal evolution of DOM was also strongly impacted by a hydrological buffering effect in large reservoirs in which the seasonal variability of DOM fluxes and composition was strongly reduced.