[en] 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 NH+4 (2.3±1.3 µmol L−1) and NO−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, NH+4, NO−3 and dissolved oxygen saturation level (%O2) indicate N2O removal by soil or sedimentary denitrification in low O2, high NH+4 and low NO−3 environments (typically small and organic matter rich streams) and N2O production by nitrification in high O2, low NH+4 and high NO−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).
Research Center/Unit :
FOCUS - Freshwater and OCeanic science Unit of reSearch - ULiège
Disciplines :
Aquatic sciences & oceanology
Author, co-author :
Borges, Alberto ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Chemical Oceanography Unit (AGO)
Darchambeau, François ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Chemical Oceanography Unit (AGO)
Lambert, Thibault ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Chemical Oceanography Unit (AGO)
Morana, Cédric ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Chemical Oceanography Unit (AGO)
Allen, G. H.
Tambwe, Ernest
Toengaho Sembaito, A.
Mambo, T.
Nlandu Wabakhangazi, J.
Descy, Jean-Pierre ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Chemical Oceanography Unit (AGO)
Teodoru, C. R.
Bouillon, S.
Language :
English
Title :
Variations in dissolved greenhouse gases (CO2, CH4, N2O) in the Congo River network overwhelmingly driven by fluvial-wetland connectivity
Abril, G. and Borges, A. V.: Carbon leaks from flooded land: do we need to re-plumb the inland water active pipe?, Biogeosciences, 16, 769-784, https://doi.org/10.5194/bg-16-769-2019, 2019.
Abril, G., Martinez, J.-M., Artigas, L. F., Moreira-Turcq, P., Benedetti, M. F., Vidal, L., Meziane, T., Kim, J.-H., Bernardes, M. C., Savoye, N., Deborde, J., Alberic, P., Souza, M. F. L., Souza, E. L., and Roland, F.: Amazon river carbon dioxide outgassing fuelled by wetlands, Nature, 505, 395-398, https://doi.org/10.1038/nature12797, 2014.
Abril, G., Bouillon, S., Darchambeau, F., Teodoru, C. R., Marwick, T. R., Tamooh, F., Omengo, F. O., Geeraert, N., Deirmendjian, L., Polsenaere, P., and Borges A. V.: Technical note: Large overestimation of pCO2 calculated from pH and alkalinity in acidic, organic-rich freshwaters, Biogeosciences, 12, 67-78, https://doi.org/10.5194/bg-12-67-2015, 2015.
Aho, K. S. and Raymond, P. A.: Differential response of greenhouse gas evasion to storms in forested and wetland streams, J. Geophys. Res., 124, 649-662, https://doi.org/10.1029/2018JG004750, 2019.
Allen, G. H. and Pavelsky, T. M.: Global extent of rivers and streams, Science, 28, eaat0636, https://doi.org/10.1126/science.aat0636, 2018.
Almeida, R. M., Pacheco, F. S., Barros, N., Rosi, E., and Roland, F.: Extreme floods increase CO2 outgassing from a large Amazonian river, Limnol. Oceanogr., 62, 989-999, https://doi.org/10.1002/lno.10480, 2017.
Alsdorf, D., Beighley, E., Laraque, A., Lee, H., Tshimanga, R., O'Loughlin, F., Mahe, G., Dinga, B., Moukandi, G., and Spencer, R. G. M.: Opportunities for hydrologic research in the Congo Basin, Rev. Geophys., 54, 378-409, https://doi.org/10.1002/2016RG000517, 2016.
Amaral, J. H. F., Borges, A. V., Melack, J. M., Sarmento, H., Barbosa, P. M., Kasper, D., Melo, M. L., de Fex Wolf, D., da Silva, J. S., and Forsberg, B. R.: Influence of plankton metabolism and mixing depth on CO2 dynamics in an Amazon floodplain lake, Sci. Total Environ., 630, 1381-1393, https://doi.org/10.1016/j.scitotenv.2018.02.331, 2018.
APHA: Standard methods for the examination of water and wastewater, American Public Health Association, 1325 pp., 1998.
Balagizi, C. M., Darchambeau, F., Bouillon, S., Yalire, M. M., Lambert, T., and Borges, A. V.: River geochemistry, chemical weathering and atmospheric CO2 consumption rates in the Virunga Volcanic Province (East Africa), Geochem. Geophy. Geosy., 16, 2637-2660, https://doi.org/10.1002/2015GC005999, 2015.
Barbosa, P. M., Melack, J. M., Farjalla, V. F., Amaral, J. H. F., Scofield, V., and Forsberg, B. R.: Diffusive methane fluxes from Negro, Solimoes and Madeira rivers and fringing lakes in the Amazon basin, Limnol. Oceanogr., 61, S221-S237, https://doi.org/10.1002/lno.10358, 2016.
Bastviken, D., Ejlertsson, J., and Tranvik, L.: Measurement of methane oxidation in lakes: A comparison of methods, Environ. Sci. Technol., 36, 3354-3361, https://doi.org/10.1021/es010311p, 2002.
Bastviken, D., Tranvik, L. J., Downing, J. A., Crill, P. M., and Enrich-Prast, A. :, Freshwater methane emissions offset the continental carbon sink, Science, 331, p. 50, https://doi.org/10.1126/science.1196808, 2011.
Battin, T. J., Kaplan, L. A., Findlay, S., Hopkinson, C. S., Marti, E., Packman, A. I., Newbold, J. D., and Sabater, F.: Biophysical controls on organic carbon fluxes in fluvial networks, Nat. Geosci., 1, 95-100, https://doi.org/10.1038/ngeo101, 2008.
Baulch, H. M., Schiff, S. L., Maranger, R., and Dillon, P. J.: Nitrogen enrichment and the emission of nitrous oxide from streams, Global Biogeochem. Cy., 25, GB4013, https://doi.org/10.1029/2011GB004047, 2011.
Benstead, J. P. and Leigh, D. S.: An expanded role for river networks, Nat. Geosci., 5, 678-679, https://doi.org/10.1038/ngeo1593, 2012.
Billett, M. F., Palmer, S. M., Hope, D., Deacon, C., Storeton-West, R., Hargreaves, K. J., Flechard, C., and Fowler, D.: Linking land-atmosphere-stream carbon fluxes in a lowland peatland system, Global Biogeochem. Cy., 18, GB1024, https://doi.org/10.1029/2003GB002058, 2004.
Bird, M. I. and Pousai, P.: Variations of δ13C in the surface soil organic carbon pool, Global Biogeoch. Cy., 11, 313-322, https://doi.org/10.1029/97GB01197, 1997.
Bird, M. I., Giresse, P., and Chivas, A. R.: Effect of forest and savanna vegetation on the carbon-isotope composition from the Sanaga River, Cameroon, Limnol. Oceanogr., 39, 1845-1854, https://doi.org/10.4319/lo.1994.39.8.1845, 1994.
Bowen, G. J., Wassenaar, L. I., and Hobson, K. A.: Global application of stable hydrogen and oxygen isotopes to wildlife forensics, Oecologia, 143, 337-348, https://doi.org/10.1007/s00442-004-1813-y, 2005.
Bloom, A. A., Palmer, P. I., Fraser, A., Reay, D. S., and Frankenberg, C.: Large-scale controls of methanogenesis inferred from methane and gravity spaceborne data, Science, 327, 322-325, https://doi.org/10.1126/science.1175176, 2010.
Borges, A. V., Darchambeau, F., Teodoru, C. R., Marwick, T. R., Tamooh, F., Geeraert, N., Omengo, F. O., Guerin, F., Lambert, T., Morana, C., Okuku, E., and Bouillon, S.: Globally significant greenhouse gas emissions from African inland waters, Nat. Geosci., 8, 637-642, https://doi.org/10.1038/NGEO2486, 2015a.
Borges, A. V., Abril, G., Darchambeau, F., Teodoru, C. R., Deborde, J., Vidal, L. O., Lambert, T., and Bouillon, S.: Divergent biophysical controls of aquatic CO2 and CH4 in the World's two largest rivers, Sci. Rep., 5, 15614, https://doi.org/10.1038/srep15614, 2015b.
Borges, A. V., Darchambeau, F., Lambert, T., Bouillon, S., Morana, C., Brouyere, S., Hakoun, V., Jurado, A., Tseng, H.-C., Descy, J.-P., and Roland, F. A. E.: Effects of agricultural land use on fluvial carbon dioxide, methane and nitrous oxide concentrations in a large European river, the Meuse (Belgium), Sci. Total Environ., 610/611, 342-355, https://doi.org/10.1016/j.scitotenv.2017.08.047, 2018.
Borges, A. V. and Bouillon, S.: Data-base of CO2, CH4, N2O and ancillary data in the Congo River, available at: https://zenodo. org/record/3413449#.XYm2eUYzaUk, last access: 24 September 2019.
Bouillon, S., Abril, G., Borges, A. V., Dehairs, F., Govers, G., Hughes, H. J., Merckx, R., Meysman, F. J. R., Nyunja, J., Osburn, C., and Middelburg, J. J.: Distribution, origin and cycling of carbon in the Tana River (Kenya): a dry season basin-scale survey from headwaters to the delta, Biogeosciences, 6, 2475-2493, https://doi.org/10.5194/bg-6-2475-2009, 2009.
Bouillon, S., Yambele, A., Spencer, R. G. M., Gillikin, D. P., Hernes, P. J., Six, J., Merckx, R., and Borges, A. V.: Organic matter sources, fluxes and greenhouse gas exchange in the Oubangui River (Congo River basin), Biogeosciences, 9, 2045-2062, https://doi.org/10.5194/bg-9-2045-2012, 2012.
Bouillon, S., Yambele, A., Gillikin, D. P., Teodoru, C., Darchambeau, F., Lambert, T., and Borges, A. V.: Contrasting biogeochemical characteristics of right-bank tributaries and a comparison with the mainstem Oubangui River, Central African Republic (Congo River basin), Sci. Rep., 4, 5402, https://doi.org/10.1038/srep05402, 2014.
Bultot, F.: Atlas Climatique du Bassin Congolais Publications de L'Institut National pour L'Etude Agronomique du Congo (I.N.E.A.C.), Troisieme Partie, Temperature et Humidite de L'Air, Rosee, Temperature du Sol, 253 pp., 1972.
Butman, D. and Raymond, P. A.: Significant efflux of carbon dioxide from streams and rivers in the United States, Nat. Geosci., 4, 839-842, https://doi.org/10.1038/NGEO1294, 2011.
Bwangoy, J.-R. B., Hansen, M. C., Roy, D. P., De Grandi, G., and Justice, C. O.: Wetland mapping in the Congo Basin using optical and radar remotely sensed data and derived topographical indices, Remote Sens. Environ., 114, 73-86, https://doi.org/10.1016/j.rse.2009.08.004, 2010.
Canion, A., Overholt, W. A., Kostka, J. E., Huettel, M., Lavik, G., and Kuypers, M. M. M.: Temperature response of denitrification and anaerobic ammonium oxidation rates and microbial community structure in Arctic fjord sediments, Environ. Microbiol., 16, 3331-3344, https://doi.org/10.1111/1462-2920.12593, 2014.
Cardoso, S. J., Enrich-Prast, A., Pace, M. L., and Roland, F.: Do models of organic carbon mineralization extrapolate to warmer tropical sediments? Limnol. Oceanogr., 59, 48-54, https://doi.org/10.4319/lo.2014.59.1.0048, 2014.
Ciais, P., Bombelli, A., Williams, M., Piao, S. L., Chave, J., Ryan, C. M., Henry, M., Brender, P., and Valentini, R.: The carbon balance of Africa: synthesis of recent research studies, Philos. T. R. Soc. A, 369, 2038-2057, https://doi.org/10.1098/rsta.2010.0328, 2013.
Coplen, T. B. and Wassenaar, L. I.: LIMS for Lasers 2015 for achieving long-term accuracy and precision of δ2H, δ17O, and δ18O of waters using laser absorption spectrometry, Rapid Commun. Mass Spectr., 29, 2122-2130, https://doi.org/10.1002/rcm.7372,2015.
Cole, B. E. and Cloern, J. E.: An empirical model for estimating phytoplankton productivity in estuaries, Mar. Ecol. Prog. Ser., 36, 299-305, https://doi.org/10.3354/meps036299, 1987.
Cole, J. J. and Caraco, N. F.: Carbon in catchments: connecting terrestrial carbon losses with aquatic metabolism, Mar. Fresh. Res., 52, 101-110, https://doi.org/10.1071/MF00084, 2001.
Cole, J. J., Caraco, N. F., Kling, G. W., and Kratz, T. K.: Carbon dioxide supersaturation in the surface waters of lakes, Science, 265, 1568-1570, https://doi.org/10.1126/science.265.5178.1568, 1994.
Cole, J. J., Prairie, Y. T., Caraco, N. F., McDowell, W. H., Tranvik, L. J., Striegl, R. G. , Duarte, C. M., Kortelainen, P., Downing, J. A., Middelburg, J. J., and Melack, J.: Plumbing the global carbon cycle: Integrating inland waters into the terrestrial carbon budget, Ecosystems, 10, 171-184, https://doi.org/10.1007/s10021-006-9013-8, 2007.
Coynel, A., Seyler, P., Etcheber, H., Meybeck, M., and Orange, D.: Spatial and seasonal dynamics of total suspended sediment and organic carbon species in the Congo River, Global Biogeochem. Cy., 19, GB4019, https://doi.org/10.1029/2004GB002335, 2005.
Craine J. M., Elmore, A. J., Wang, L., Aranibar, J., Bauters, M., Boeckx, P., Crowley, B. E., Dawes, M. A., Delzon, S., Fajardo, A., Fang, Y., Fujiyoshi, L., Gray, A., Guerrieri, R., Gundale, M. J., Hawke, D.J., Hietz, P., Jonard, M., Kearsley, E., Kenzo, T., Makarov, M., Maranon-Jimenez, S., McGlynn, T. P., Mc-Neil, B. E., Mosher, S. G., Nelson, D. M., Peri, P. L., Roggy, J. C., Sanders-DeMott, R., Song, M., Szpak, P., Templer, P. H., Van der Colff, D., Werner, C., Xu, X., Yang, Y., Yu, G., and Zmudczynska-Skarbek, K.: Isotopic evidence for oligotrophication of terrestrial ecosystems, Nat. Ecol. Evol., 2, 1735-1744, https://doi.org/10.1038/s41559-018-0694-0, 2018.
Crawford J. T., Stanley, E. H., Dornblaser, M., and Striegl, R. G.: CO2 time series patterns in contrasting headwater streams of North America, Aquat. Sci., 79, 473-486, https://doi.org/10.1007/s00027-016-0511-2, 2017.
Dargie, G. C., Lewis, S. L., Lawson, I. T., Mitchard, E. T. A., Page, S. E., Bocko, Y. E., and Ifo, S. A.: Age, extent and carbon storage of the central Congo Basin peatland complex, Nature, 542, 86-90, https://doi.org/10.1038/nature21048, 2017.
Deirmendjian, L. and Abril, G.: Carbon dioxide degassing at the groundwater-stream-atmosphere interface: isotopic equilibration and hydrological mass balance in a sandy watershed, J. Hydrol., 558, 129-143, 2018.
Deirmendjian, L., Loustau, D., Augusto, L., Lafont, S., Chipeaux, C., Poirier, D., and Abril, G.: Hydro-ecological controls on dissolved carbon dynamics in groundwater and export to streams in a temperate pine forest, Biogeosciences, 15, 669-691, https://doi.org/10.5194/bg-15-669-2018, 2018.
Dinsmore, K. J., Wallin, M. B., Johnson, M. S., Billett, M. F., Bishop, K., Pumpanen, J., and Ojala, A.: Contrasting CO2 concentration discharge dynamics in headwater streams: a multi-catchment comparison, J. Geophys. Res., 118, 445-461, https://doi.org/10.1002/jgrg.20047, 2013.
Del Giorgio, P. A., Cole, J. J., Caraco, N. F., and Peters, R. H.: Linking planktonic biomass and metabolism to net gas fluxes in northern temperate lakes, Ecology, 80, 1422-1431, https://doi.org/10.1890/0012-9658(1999)080[1422:LPBAMT]2.0.CO;2, 1999.
Descy, J.-P., Hardy, M.-A., Stenuite, S., Pirlot, S., Leporcq, B., Kimirei, I., Sekadende, B., Mwaitega, S. R., and Sinyenza, D.: Phytoplankton pigments and community composition in Lake Tanganyika, Freshwater Biol., 50, 668-684, https://doi.org/10.1111/j.1365-2427.2005.01358.x, 2005.
Descy, J.-P., Darchambeau, F., Lambert, T., Stoyneva, M. P., Bouillon, S., and Borges, A. V.: Phytoplankton dynamics in the Congo River, Freshwater Biol., 62, 87-101, https://doi.org/10.1111/fwb.12851, 2017.
Doctor, D. H., Kendall, C., Sebestyen, S. D., Shanley, J. B., Ohte, N., and Boyer, E. W.: Carbon isotope fractionation of dissolved inorganic carbon (DIC) due to outgassing of carbon dioxide from a headwater stream, Hydrol. Process., 22, 2410-2423, https://doi.org/10.1002/hyp.6833, 2008.
Downing, J. A., Cole, J. J., Duarte, C. M., Middelburg, J. J., Melack, J. M., Prairie, Y. T., Kortelainen, P., Striegl, R. G., Mc-Dowell, W. H., and Tranvik, L. J.: Global abundance and size distribution of streams and rivers, Inland Waters, 2, 229-236, https://doi.org/10.5268/IW-2.4.502, 2012.
Duvert, C., Butman, D. E., Marx, A., Ribolzi, O., and Hutley, L. B.: CO2 evasion along streams driven by groundwater inputs and geomorphic controls, Nat. Geosci., 11, 813-818, https://doi.org/10.1038/s41561-018-0245-y, 2018.
Fisher, J. B., Sikka, M., Sitch, S., Ciais, P., Poulter, B., Galbraith, D., Lee, J.-E., Huntingford, C., Viovy, N., Zeng, N., Ahlstrom, A., Lomas, M. R., Levy, P. E., Frankenberg, C., Saatchi, S., and Malhi, Y.: African tropical rainforest net carbon dioxide fluxes in the twentieth century, Philos. T. R. Soc. B, 368, 20120376, https://doi.org/10.1098/rstb.2012.0376, 2013.
Fluet-Chouinard, E., Lehner, B., Rebelo, L.-M., Papa, F., and Hamilton, S. K.: Development of a global inundation map at high spatial resolution from topographic downscaling of coarsescale remote sensing data, Remote Sens. Environ., 158, 348-361, https://doi.org/10.1016/j.rse.2014.10.015, 2015.
Frankignoulle, M., Borges, A., and Biondo R.: A new design of equilibrator to monitor carbon dioxide in highly dynamic and turbid environments, Water Res., 35, 1344-1347, https://doi.org/10.1016/S0043-1354(00)00369-9, 2001.
Gaillardet, J., Dupre, B., Louvat, P., and Allegre C. J.: Global silicate weathering and CO2 consumption rates deduced from the chemistry of large rivers, Chem. Geol., 159, 3-30, https://doi.org/10.1016/S0009-2541(99)00031-5, 1999.
Gillikin, D. P. and Bouillon, S.: Determination of δ18O of water and δ13C of dissolved inorganic carbon using a simple modification of an elemental analyzer-isotope ratio mass spectrometer (EAIRMS): an evaluation, Rapid Commun. Mass Spectr., 21, 1475-1478, https://doi.org/10.1002/rcm.2968, 2007.
Gran, G.: Determination of the equivalence point in potentiometric titrations Part II, The Analyst, 77, 661-671, https://doi.org/10.1039/AN9527700661, 1952.
Hamilton, S. K., Sippel, S. J., and Melack J. M.: Comparison of inundation patterns among major South American floodplains, J. Geophys. Res., 107, LBA 5-1-LBA 5-14, https://doi.org/10.1029/2000JD000306, 2002.
Happell, J., Chanton, J. P., and Showers, W.: The influence of methane oxidation on the stable isotopic composition of methane emitted from Florida Swamp forests, Geochim. Cosmochim. Ac., 58, 4377-4388, https://doi.org/10.1016/0016-7037(94)90341-7, 1994.
Hedges, J. I., Clark, W. A., Quay, P. D., Richey, J. E., Devol, A. H., and de M. Santos, U.: Compositions and fluxes of particulate organic material in the Amazon River, Limnol Oceanogr., 31, 717-738, https://doi.org/10.4319/lo.1986.31.4.0717, 1986.
Hotchkiss, E. R., Hall Jr, R. O., Sponseller, R. A., Butman, D., Klaminder, J., Laudon, H., Rosvall, M., and Karlsson, J.: Sources of and processes controlling CO2 emissions change with the size of streams and rivers, Nat. Geosci., 8, 696-699, https://doi.org/10.1038/ngeo2507, 2015.
Hu, M., Chen, D., and Dahlgren, R. A.: Modeling nitrous oxide emission from rivers: a global Assessment, Glob. Change Biol., 22, 3566-3582, https://doi.org/10.1111/gcb.13351, 2016.
Hughes, R. H. and Hughes, J. S.: A directory of African wetlands, IUCN, ISBN 2-88032-949-3, 820 pp., 1992.
Huotari, J., Haapanala, S., Pumpanen, J., Vesala, T., and Ojala, A.: Efficient gas exchange between a boreal river and the atmosphere, Geophys. Res. Lett., 40, 5683-5686, https://doi.org/10.1002/2013GL057705, 2013.
Kindler, R., Siemens, J., Kaiser, K., Walmsley, D. C., Bernhofer, C., Buchmann, N., Cellier, P., Eugster, W., Gleixner, G., Grunwald, T., Heim, A., Ibrom, A., Jones, S. K., Jones, M., Klumpp, K., Kutsch, W., Steenberg Larsen, K., Lehuger, S., Loubet, B., McKenzie, R., Moors, E., Osborne, B., Pilegaard, K., Rebmann, C., Saunders, M., Schmidt, M. W. I., Schrumpf, M., Seyfferth, J., Skiba, U., Soussana, J.-F., Sutton, M. A.; Tefs, C., Vowinckel, B., Zeeman, M. J., and Kaupenjohann, M.: Dissolved carbon leaching from soil is a crucial component of net ecosystem carbon balance, Glob. Change Biol., 17, 1167-1185, https://doi.org/10.1111/j.1365-2486.2010.02282.x, 2011.
Klaus, M., Geibrink, E., Jonsson, A., Bergstrom, A.-K., Bastviken, D., Laudon, H., Klaminder, J., and Karlsson, J.: Greenhouse gas emissions from boreal inland waters unchanged after forest harvesting, Biogeosciences, 15, 5575-5594, https://doi.org/10.5194/bg-15-5575-2018, 2018.
Kokic, J., Sahlee, E., Sobek, S., Vachon, D., and Wallin, M. B.: High spatial variability of gas transfer velocity in streams revealed by turbulence measurements, Inland Waters, 8, 461-473, https://doi.org/10.1080/20442041.2018.1500228, 2018.
Kone, Y. J. M., Abril, G., Kouadio, K. N., Delille, B., and Borges, A. V.: Seasonal variability of carbon dioxide in the rivers and lagoons of Ivory Coast (West Africa), Estuar. Coast., 32, 246-260, https://doi.org/10.1007/s12237-008-9121-0, 2009.
Kone, Y. J. M., Abril, G., Delille, B., and Borges, A. V.: Seasonal variability of methane in the rivers and lagoons of Ivory Coast (West Africa), Biogeochemistry, 100, 21-37, https://doi.org/10.1007/s10533-009-9402-0, 2010.
Kosten, S., Pineiro, M., de Goede, E., de Klein, J., Lamers, L. P. M., and Ettwig, K.: Fate of methane in aquatic systems dominated by free-floating plants, Water Res., 104, 200-207, 2016.
Kroeze, C., Dumont, E., and Seitzinger, S. P.: Future trends in emissions of N2O from rivers and estuaries, J. Integr. Environ. Sc., 7, 71-78, https://doi.org/10.1080/1943815X.2010.496789, 2010.
Lambert, T., Bouillon, S., Darchambeau, F., Massicotte, P., and Borges, A.V.: Shift in the chemical composition of dissolved organic matter in the Congo River network, Biogeosciences, 13, 5405-5420, https://doi.org/10.5194/bg-13-5405-2016, 2016.
Laraque, A., Mietton, M. Olivry, J. C., and Pandi, A.: Impact of lithological and vegetal covers on flow discharge and water quality of Congolese tributaries from the Congo river, Rev. Sci. Eau., 11, 209-224, 1998.
Laraque, A., Bricquet, J. P., Pandi, A., and Olivry, J. C.: A review of material transport by the Congo River and its tributaries, Hydrol. Process., 23, 3216-3224, https://doi.org/10.1002/hyp.7395, 2009.
Lauerwald, R., Laruelle, G. G., Hartmann, J., Ciais, P., and Regnier, P. A. G.: Spatial patterns in CO2 evasion from the global river network, Global Biogeochem. Cy., 29, 534-554, https://doi.org/10.1002/2014GB004941, 2015.
Lauerwald, R., Regnier, P., Camino-Serrano, M., Guenet, B., Guimberteau, M., Ducharne, A., Polcher, J., and Ciais, P.: ORCHILEAK (revision 3875): a new model branch to simulate carbon transfers along the terrestrial-aquatic continuum of the Amazon basin, Geosci. Model Dev., 10, 3821-3859, https://doi.org/10.5194/gmd-10-3821-2017, 2017.
Le, T. T. H., Fettig, J., and Meon, G.: Kinetics and simulation of nitrification at various pH values of a polluted river in the tropics, Ecohydrol. Hydrobiol., 19, 54-65, 2019.
Liptay, K., Chanton, J., Czepiel, P., and Mosher, B.: Use of stable isotopes to determine methane oxidation in landfill cover soils, J. Geophys. Res., 103, 8243-8250, https://doi.org/10.1029/97JD02630, 1998.
Liss, P. S. and Slater, P. G.: Flux of gases across the air sea interface, Nature, 247, 181-184, https://doi.org/10.1038/247181a0, 1974.
Liu, S. and Raymond, P. A.: Hydrologic controls on pCO2 and CO2 efflux in US streams and rivers, Limnol. Oceanogr. Lett., 3, 428-435, https://doi.org/10.1002/lol2.10095, 2018.
Lynch, J. K., Beatty, C. M., Seidel, M. P., Jungst, L. J., and DeGrandpre, M. D.: Controls of riverine CO2 over an annual cycle determined using direct, high temporal resolution pCO2 measurements, J. Geophys. Res., 115, G03016, https://doi.org/10.1029/2009JG001132, 2010.
Maavara, T., Lauerwald, R., Laruelle, G.G., Akbarzadeh, Z., Bouskill, N. J., Van Cappellen, P., and Regnier, P.: Nitrous oxide emissions from inland waters: Are IPCC estimates too high?, Glob. Change Biol., 25, 473-488, https://doi.org/10.1111/gcb.14504, 2018.
Malhi, Y., Adu-Bredu, S., Asare, R. A., Lewis, S. L., and Mayaux, P.: African rainforests: past, present and future, Philos. T. R. Soc. B, 368, 20120312, https://doi.org/10.1098/rstb.2012.0312, 2013.
Mann, P. J., Spencer, R. G. M., Dinga, B. J., Poulsen, J. R., Hernes, P. J., Fiske, G., Salter, M. E., Wang, Z. A., Hoering, K. A., Six, J., and Holmes R. M.: The biogeochemistry of carbon across a gradient of streams and rivers within the Congo Basin, J. Geophys. Res.-Biogeo., 119, 687-702, https://doi.org/10.1002/2013JG002442, 2014.
Maurice, L., Rawlins, B. G., Farr, G., Bell, R., and Gooddy, D. C.: The influence of flow and bed slope on gas transfer in steep streams and their implications for evasion of CO2, J. Geophys. Res.-Biogeo., 122, 2862-2875, https://doi.org/10.1002/2017JG004045, 2017.
Marwick, T. R., Tamooh, F., Ogwoka, B., Teodoru, C., Borges, A. V., Darchambeau, F., and Bouillon, S.: Dynamic seasonal nitrogen cycling in response to anthropogenic N loading in a tropical catchment, Athi-Galana-Sabaki River, Kenya, Biogeosciences, 11, 1-18, https://doi.org/10.5194/bg-11-1-2014, 2014.
Marx, A., Dusek, J., Jankovec, J., Sanda, M., Vogel, T., van Geldern, R., Hartmann, J., and Barth, J. A. C.: A review of CO2 and associated carbon dynamics in headwater streams: A global perspective, Rev. Geophys., 55, 560-585, https://doi.org/10.1002/2016RG000547, 2017.
McDowell, M. J. and Johnson, M. S.: Gas transfer velocities evaluated using carbon dioxide as a tracer show high streamflow to be a major driver of total CO2 evasion flux for a headwater stream, J. Geophys. Res.-Biogeo., 123, 2183-2197, https://doi.org/10.1029/2018JG004388, 2018.
Melack, J. M., Hess, L. L., Gastil, M., Forsberg, B. R., Hamilton, S. K., Lima, I. B. T., and Novo, E. M. L. M.: Regionalization of methane emissions in the Amazon Basin with microwave remote sensing, Glob. Change Biol., 10, 530-544, https://doi.org/10.1111/j.1365-2486.2004.00763.x, 2004.
Meybeck, M.: Global chemical weathering of surficial rocks estimated from river dissolved loads, Am. J. Sci., 287, 401-428, https://doi.org/10.2475/ajs.287.5.401, 1987.
Millero, F. J.: The thermodynamics of the carbonate system in seawater, Geochem. Cosmochem. Ac., 43, 1651-1661, https://doi.org/10.1016/0016-7037(79)90184-4,1979.
Morana, C., Borges, A. V., Roland, F. A. E., Darchambeau, F., Descy, J.-P., and Bouillon, S.: Methanotrophy within the water column of a large meromictic, tropical lake (Lake Kivu, East Africa), Biogeosciences, 12, 2077-2088, https://doi.org/10.5194/bg-12-2077-2015, 2015.
Nkounkou, R. R. and Probst, J. L.: Hydrology and geochemistry of the Congo river system, Mitt. Geol-Palaont. Inst. Univ. Hamburg, SCOPE/UNEP, 64, 483-508, 1987.
O'Loughlin, F., Trigg, M. A., Schumann, G. J.-P., and Bates, P. D.: Hydraulic characterization of the middle reach of the Congo River, Water Resour. Res., 49, 5059-5070, https://doi.org/10.1002/wrcr.20398, 2013.
Peter, H., Singer, G. A., Preiler, C., Chifflard, P., Steniczka, G., and Battin, T. J.: Scales and drivers of temporal pCO2 dynamics in an Alpine stream, J. Geophys. Res.-Biogeo., 119, 1078-1091, https://doi.org/10.1002/2013JG002552, 2014.
Powell, R. L., Yoo, E.-H., and Still, C. J.: Vegetation and soil carbon-13 isoscapes for South America: integrating remote sensing and ecosystem isotope measurements, Ecosphere, 3, 1-25, https://doi.org/10.1890/ES12-00162.1, 2012.
Prairie, Y. T., Bird, D. F., and Cole, J. J.: The summer metabolic balance in the epilimnion of southeastern Quebec lakes, Limnol. Oceanogr., 47, 316-321, https://doi.org/10.4319/lo.2002.47.1.0316, 2002.
Raymond, P. A., Zappa, C. J., Butman, D., Bott, T. L., Potter, C., Mulholland, P., Laursen, A. E., McDowell, W. H., and Newbold, D.: Scaling the gas transfer velocity and hydraulic geometry in streams and small rivers, Limnol. Oceanogr. Fluids Environ., 2, 41-53, https://doi.org/10.1215/21573689-1597669, 2012.
Raymond, P. A., Hartmann, J., Lauerwald, R., Sobek, S., Mc-Donald, C., Hoover, M., Butman, D., Striegl, R., Mayorga, E., Humborg, C., Kortelainen, P., Durr, H., Meybeck, M., Ciais, P., and Guth, P.: Global carbon dioxide emissions from inland waters, Nature, 503, 355-359, https://doi.org/10.1038/nature12760, 2013.
Reiman, J. H. and Xu, J. Y.: Diel variability of pCO2 and CO2 outgassing from the Lower Mississippi River: Implications for riverine CO2 outgassing estimation, Water, 11, 1-15, https://doi.org/10.3390/w11010043, 2019.
Richardson, D. C., Newbold, J. D., Aufdenkampe, A. K., Taylor, P. G., and Kaplan, L. A.: Measuring heterotrophic respiration rates of suspended particulate organic carbon from stream ecosystems, Limnol. Oceanogr.-Method., 11, 247-261, https://doi.org/10.4319/lom.2013.11.247, 2013.
Richey, J. E., Devol, A. H., Wofy, S. C., Victoria, R., and Riberio, M. N. G.: Biogenic gases and the oxidation and reduction of carbon in Amazon River and floodplain waters, Limnol. Oceanogr., 33, 551-561, https://doi.org/10.4319/lo.1988.33.4.0551, 1988.
Richey, J. E., Melack, J. M., Aufdenkampe, A. K., Ballester, V. M., and Hess, L.: Outgassing from Amazonian rivers and wetlands as a large tropical source of atmospheric CO2, Nature, 416, 617-620, https://doi.org/10.1038/416617a, 2002.
Runge, J.: Large Rivers: Geomorpholgy and Management, edited by: Gupta, A., John Wiley & Sons., 293-309, 2008.
Santos, I. R., Maher, D. T., and, Eyre B. D.: Coupling automated radon and carbon dioxide measurements in coastal waters, Environ. Sci. Technol., 46, 7685-7691, https://doi.org/10.1021/es301961b, 2012.
Saunois, M., Bousquet, P., Poulter, B., Peregon, A., Ciais, P., Canadell, J. G., Dlugokencky, E. J., Etiope, G., Bastviken, D., Houweling, S., Janssens-Maenhout, G., Tubiello, F. N., Castaldi, S., Jackson, R. B., Alexe, M., Arora, V. K., Beerling, D. J., Bergamaschi, P., Blake, D. R., Brailsford, G., Brovkin, V., Bruhwiler, L., Crevoisier, C., Crill, P., Kovey, K., Curry, C., Frankenberg, C., Gedney, N., Hoglund-Isaksson, L., Ishizawa, M., Ito, A., Joos, F., Kim, H.-S., Kleinen, T., Krummel, P., Lamarque, J.-F., Langenfelds, R., Locatelli, R., Machida, T., Maksyutov, S., McDonald, K. C., Marshall, J., Melton, J. R., Morino, I., Naik, V., O'Doherty, S., Parmentier, F.-J. W., Patra, P. K., Peng, C., Peng, S., Peters, G., Pison, I., Prigent, C., Prinn, R., Ramonet, M., Riley, W. J., Saito, M., Sanyini, M., Schroeder, R., Simpson, I. J., Spahni, R., Steele, P., Takizawa, A., Thornton, B. F., Tian, H., Tohjima, Y., Viovy, N., Voulgarakis, A., van Weele, M., van der Werf, G., Weiss, R., Wiedinmyer, C., Wilton, D. J., Wiltshire, A., Worthy, D., Wunch, D. B., Xu, X., Yoshida, Y., Zhang, B., Zhang, Z., and Zhu, Q.: The global methane budget, Earth Syst. Sci. Data, 8, 697-751, https://doi.org/10.5194/essd-8-697-2016, 2016.
Sawakuchi, H. O., Bastviken, D., Sawakuchi, A. O., Krusche, A. V., Ballester, M. V. R., and Richey, J. E.: Methane emissions from Amazonian Rivers and their contribution to the global methane budget, Glob. Change Biol., 20, 2829-2840, https://doi.org/10.1111/gcb.12646, 2014.
Sawakuchi, H. O., Bastviken, D., Sawakuchi, A. O., Ward, N. D., Borges, C. D., Tsai, S. M., Richey, J. E., Ballester, M. V. R. and Krusche, A. V.: Oxidative mitigation of aquatic methane emissions in large Amazonian rivers, Glob. Change Biol., 22, 1075-1085, https://doi.org/10.1111/gcb.13169, 2016.
Scofield, V., Melack, J. M., Barbosa, P. M., Amaral, J. H. F., Forsberg, B. R., and Farjalla, V. F.: Carbon dioxide outgassing from Amazonian aquatic ecosystems in the Negro River basin, Biogeochemistry, 129, 77-91, https://doi.org/10.1007/s10533-016-0220-x, 2016.
Seitzinger, S. P. and Kroeze, C.: Global distribution of nitrous oxide production and N inputs in freshwater and coastal marine ecosystems, Global Biogeochem. Cy., 12, 93-113, https://doi.org/10.1029/97GB03657, 1998.
Simpson, H. and Herczeg, A.: Stable isotopes as an indicator of evaporation in the River Murray, Australia, Water Resour. Res., 27, 1925-1935, https://doi.org/10.1029/91WR00941, 1991.
Spencer, R. G. M., Hernes, P. J., Aufdenkampe, A. K., Baker, A., Gulliver, P., Stubbins, A., Aiken, G. R., Dyda, R. Y., Butler, K. D., Mwamba, V. L., Mangangu, A. M., Wabakanghanzi, J. N., and Six, J.: An initial investigation into the organic matter biogeochemistry of the Congo River, Geochim. Cosmochim. Ac., 84, 614-627, https://doi.org/10.1016/j.gca.2012.01.013, 2012.
SCA (Standing committee of Analysts): Ammonia in waters. Methods for the examination of waters and associated materials, 16 pp., 1981.
Stanley, E. H., Casson, N. J., Christel, S. T., Crawford, J. T., Loken, L. C., and Oliver, S. K.: The ecology of methane in streams and rivers: patterns, controls, and global significance, Ecol. Monogr., 86, 146-171, https://doi.org/10.1890/15-1027, 2016.
Still, C. J. and Powell, R. L.: Continental-scale distributions of vegetation stable carbon isotope ratios, edited by: West, J. B., Bowen, G. J., Dawson, T. E., Tu, K. P., Isoscapes, the Netherlands, Springer Netherlands, 179-193, 2010.
Takahashi, T., Sutherland, S. C., Wanninkhof, R., Sweeney, C., Feely, R. A., Chipman, D. W., Hales, B., Friederich, G., Chavez, F., Sabine, C., Watson, A., Bakker, D. C. E., Schuster, U., Metzl, N., Yoshikawa-Inoue, H.I., Ishii, M., Midorikawa, T., Nojiri, Y., Kortzinger, A., Steinhoff, T., Hoppema, M., Olafsson, J., Arnarson, T. S., Tilbrook, B., Johannessen, T., Olsen, A., Bellerby, R., Wong, C. S., Delille, B., Bates, N. R., and de Baar, H. J. W.: Climatological mean and decadal change in surface ocean pCO2, and net sea-air CO2 flux over the global oceans, Deep-Sea Res. Pt. II, 56, 554-577, https://doi.org/10.1016/j.dsr2.2008.12.009, 2009.
Tamooh, F., Borges, A. V., Meysman, F. J. R., Van Den Meersche, K., Dehairs, F., Merckx, R., and Bouillon, S.:Dynamics of dissolved inorganic carbon and aquatic metabolism in the Tana River basin, Kenya, Biogeosciences, 10, 6911-6928, https://doi.org/10.5194/bg-10-6911-2013, 2013
Teodoru, C. R., Nyoni, F. C., Borges, A. V., Darachambeau, F., Nyambe, I., and Bouillon, S.: Spatial variability and temporal dynamics of greenhouse gas (CO2, CH4, N2O) concentrations and fluxes along the Zambezi River mainstem and major tributaries, Biogeosciences, 12, 2431-2453, https://doi.org/10.5194/bg-12-2431-2015, 2015.
Tyler, S. C., Bilek, R. S., Sass, R. L., and Fisher, F. M.: Methane oxidation and pathways of production in a Texas paddy field deduced from measurements of flux, δ13C, and δD of CH4, Global Biogeochem. Cy., 11, 323-348, https://doi.org/10.1029/97GB01624, 1997.
Ulseth, A. J., Hall Jr, R. O., Canadell, M. B., Madinger, H. L., Niayifar, A., and Battin, T. J.: Distinct air-water gas exchange regimes in low-and high-energy streams, Nat. Geosci., 12, 259-263, https://doi.org/10.1038/s41561-019-0324-8, 2019
Upstill-Goddard, R. C., Salter, M. E., Mann, P. J., Barnes, J., Poulsen, J., Dinga, B., Fiske, G. J., and Holmes, R. M.: The riverine source of CH4 and N2O from the Republic of Congo, western Congo Basin, Biogeosciences, 14, 2267-2281, https://doi.org/10.5194/bg-14-2267-2017, 2017.
Ward, N. D., Krusche, A. V., Sawakuchi, H. O., Brito, D. C., Cunha, A. C., Sousa Moura, J. M., da Silva, R., Yager, P. L., Keil, R. G., and Richey, J. E.: The compositional evolution of dissolved and particulate organic matter along the lower Amazon River-Obidos to the ocean, Mar. Chem., 177, 244-256, https://doi.org/10.1016/j.marchem.2015.06.013, 2015.
Ward N. D., Sawakuchi, H. O., Neu, V., Less, D. F. S., Valerio, A. M., Cunha, A. C., Kampel, M., Bianchi, T. S., Krusche, A. V., Richey, J. E., and Keil, R. G.: Velocity-amplified microbial respiration rates in the lower Amazon River, Limnol. Oceanogr. Lett., 3, 265-274, https://doi.org/10.1002/lol2.10062, 2018.
Wassenaar, L. I., Coplen, T. B., and Aggarwal, P. K.: Approaches for achieving long-term accuracy and precision of δ18O and δ2H for waters analyzed using laser absorption spectrometers, Environ. Sci. Technol., 48, 1123-1131, https://doi.org/10.1021/es403354n, 2014.
Weiss, R. F.: Determinations of carbon dioxide and methane by dual catalyst flame ionization chromatography and nitrous oxide by electron capture chromatography, J. Chromatogr. Sci., 19, 611-616, https://doi.org/10.1093/chromsci/19.12.611, 1981.
Weiss, R. F. and Price, B. A.: Nitrous oxide solubility in water and seawater, Mar. Chem., 8, 347-359, https://doi.org/10.1016/0304-4203(80)90024-9, 1980.
Wissmar, R. C., Richey, J. E., Stallard, R. F., and Edmond, J. M.: Plankton metabolism and carbon processes in the Amazon river, its tributaries, and floodplain waters, Peru-Brazil, May-June 1977, Ecology, 62, 1622-1633, https://doi.org/10.2307/1941517, 1981.
Yoshida, N., Iguchi, H., Yurimoto, H., Murakami, A., and Sakai, Y.: Aquatic plant surface as a niche for methanotrophs, Front. Microbiol., 30, 1-9, https://doi.org/10.3389/fmicb.2014.00030, 2014.
Zhou, L., Tian, Y., Myneni, R. B., Ciais, P., Saatchi, S. L., Yi Y., Shilong, P., Chen, H., Vermote, E. F., Song, C., and Hwang, T.: Widespread decline of Congo rainforest greenness in the past decade, Nature, 509, 86-90, https://doi.org/10.1038/nature13265, 2014.