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dc.contributor.author
Borges, Alberto V.
dc.contributor.author
Darchambeau, François
dc.contributor.author
Lambert, Thibault
dc.contributor.author
Morana, Cédric
dc.contributor.author
Allen, George H.
dc.contributor.author
Tambwe, Ernest
dc.contributor.author
Toengaho Sembaito, Alfred
dc.contributor.author
Mambo, Taylor
dc.contributor.author
Nlandu Wabakhangazi, José
dc.contributor.author
Descy, Jean-Pierre
dc.contributor.author
Teodoru, Cristian R.
dc.contributor.author
Bouillon, Steven
dc.date.accessioned
2019-10-21T10:09:11Z
dc.date.available
2019-10-20T03:02:39Z
dc.date.available
2019-10-21T10:09:11Z
dc.date.issued
2019
dc.identifier.other
10.5194/bg-16-3801-2019
en_US
dc.identifier.uri
http://hdl.handle.net/20.500.11850/371750
dc.identifier.doi
10.3929/ethz-b-000371750
dc.description.abstract
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).
en_US
dc.format
application/pdf
en_US
dc.language.iso
en
en_US
dc.publisher
Copernicus Publications
en_US
dc.rights.uri
http://creativecommons.org/licenses/by/4.0/
dc.title
Variations in dissolved greenhouse gases (CO2, CH4, N2O) in the Congo River network overwhelmingly driven by fluvial-wetland connectivity
en_US
dc.type
Journal Article
dc.rights.license
Creative Commons Attribution 4.0 International
dc.date.published
2019-10-07
ethz.journal.title
Biogeosciences
ethz.journal.volume
16
en_US
ethz.journal.issue
19
en_US
ethz.pages.start
3801
en_US
ethz.pages.end
3834
en_US
ethz.version.deposit
publishedVersion
en_US
ethz.identifier.scopus
ethz.publication.place
Göttingen
en_US
ethz.publication.status
published
en_US
ethz.date.deposited
2019-10-20T03:02:46Z
ethz.source
SCOPUS
ethz.eth
yes
en_US
ethz.availability
Open access
en_US
ethz.rosetta.installDate
2019-10-21T10:09:33Z
ethz.rosetta.lastUpdated
2019-10-21T10:09:33Z
ethz.rosetta.versionExported
true
ethz.COinS
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