The impact of human activities on the concentrations and composition of dissolved organic matter (DOM) and particulate organic matter (POM) was investigated in the Walloon Region of the Meuse River basin (Belgium). Water samples were collected at different hydrological periods along a gradient of human disturbance (50 sampling sites ranging from 8.0 to 20,407 km²) and during a 1.5 year monitoring of the Meuse River at the city of Liège. This dataset was completed by the characterization of the DOM pool in groundwaters. The composition of DOM and POM was investigated through elemental (C:N ratios), isotopic (δ¹³C) and optical measurements including excitation emission matrix fluorescence with parallel factor analysis (EEM–PARAFAC). Land use was a major driver on fluvial OM composition at the regional scale of the Meuse Basin, the composition of both fluvial DOM and POM pools showing a shift toward a more microbial/algal and less plant/soil-derived character as human disturbance increased. The comparison of DOM composition between surface and groundwaters demonstrated that this pattern can be attributed in part to the transformation of terrestrial sources by agricultural practices that promote the decomposition of soil organic matter in agricultural lands and subsequent microbial inputs in terrestrial sources. In parallel, human land had contrasting effects on the autochthonous production of DOM and POM. While the in-stream generation of fresh DOM through biological activity was promoted in urban areas, summer autochthonous POM production was not influenced by land use. Finally, soil erosion by agricultural management practices favored the transfer of terrestrial organic matter via the particulate phase. Stable isotope data suggest that the hydrological transfer of terrestrial DOM and POM in human-impacted catchment are not subject to the same controls, and that physical exchange between these two pools of organic matter is limited.

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 ${\mathrm{NH}}_{\mathrm{4}}^{+}$ (2.3±1.3 µmol L⁻¹) and ${\mathrm{NO}}_{\mathrm{3}}^{-}$ (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, ${\mathrm{NH}}_{\mathrm{4}}^{+}$ , ${\mathrm{NO}}_{\mathrm{3}}^{-}$ and dissolved oxygen saturation level (%O₂) indicate N₂O removal by soil or sedimentary denitrification in low O₂, high ${\mathrm{NH}}_{\mathrm{4}}^{+}$ and low ${\mathrm{NO}}_{\mathrm{3}}^{-}$ environments (typically small and organic matter rich streams) and N₂O production by nitrification in high O₂, low ${\mathrm{NH}}_{\mathrm{4}}^{+}$ and high ${\mathrm{NO}}_{\mathrm{3}}^{-}$ (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).

The processing of terrestrially derived dissolved organic matter (DOM) during downstream transport in fluvial networks is poorly understood. Here, we report a dataset of dissolved organic carbon (DOC) concentrations and DOM composition (stable carbon isotope ratios, absorption and fluorescence properties) acquired along a 1700 km transect in the middle reach of the Congo River basin. Samples were collected in the mainstem and its tributaries during high-water (HW) and falling-water (FW) periods. DOC concentrations and DOM composition along the mainstem were found to differ between the two periods because of a reduced lateral mixing between the central water masses of the Congo River and DOM-rich waters from tributaries and also likely because of a greater photodegradation during FW as water residence time (WRT) increased. Although the Cuvette Centrale wetland (one of the world's largest flooded forests) continuously releases highly aromatic DOM in streams and rivers of the Congo Basin, the downstream transport of DOM was found to result in an along-stream gradient from aromatic to aliphatic compounds. The characterization of DOM through parallel factor analysis (PARAFAC) suggests that this transition results from (1) the losses of aromatic compounds by photodegradation and (2) the production of aliphatic compounds by biological reworking of terrestrial DOM. Finally, this study highlights the critical importance of the river–floodplain connectivity in tropical rivers in controlling DOM biogeochemistry at a large spatial scale and suggests that the degree of DOM processing during downstream transport is a function of landscape characteristics and WRT.

Natural lakes are thought to be globally important sources of greenhouse gases (CO ₂ , CH ₄ , and N ₂ O) to the atmosphere although nearly no data have been previously reported from Africa. We collected CO ₂ , CH ₄ , and N ₂ O 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 CO ₂ 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 CH ₄ concentrations were generally higher than in boreal systems. The lowest CO ₂ and the highest CH ₄ concentrations were observed in the more shallow and productive lakes. Emissions of CO ₂ 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.