Variations in dissolved greenhouse gases (CO₂, CH₄, N₂O) in the Congo River network overwhelmingly driven by fluvial-wetland connectivity

• Main Authors: A. V. Borges, F. Darchambeau, T. Lambert, C. Morana, G. H. Allen, E. Tambwe, A. Toengaho Sembaito, T. Mambo, J. Nlandu Wabakhangazi, J.-P. Descy, C. R. Teodoru, S. Bouillon
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-10-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/16/3801/2019/bg-16-3801-2019.pdf

<p><span id="page3802"/>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 (<span class="inline-formula">CO₂</span>), methane (<span class="inline-formula">CH₄</span>) and nitrous oxide (<span class="inline-formula">N₂O</span>) concentrations. We investigate the possible drivers of the spatial variations in dissolved <span class="inline-formula">CO₂</span>, <span class="inline-formula">CH₄</span> and <span class="inline-formula">N₂O</span> 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 <span class="inline-formula">CO₂</span>, <span class="inline-formula">CH₄</span> and <span class="inline-formula">N₂O</span> 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&thinsp;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&thinsp;% of the total length of the main stem. Additionally, three time series of <span class="inline-formula">CH₄</span> and <span class="inline-formula">N₂O</span> 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 <span class="inline-formula">N₂O</span> concentrations were modest, with values oscillating around the concentration corresponding to saturation with the atmosphere, with <span class="inline-formula">N₂O</span> saturation level (%<span class="inline-formula">N₂O</span>, where atmospheric equilibrium corresponds to 100&thinsp;%) ranging between 0&thinsp;% and 561&thinsp;% (average 142&thinsp;%). The relatively narrow range of %<span class="inline-formula">N₂O</span> variations was consistent with low <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M19" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="b054521cc8a5d2267742c16e315b1d01"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-3801-2019-ie00001.svg" width="24pt" height="15pt" src="bg-16-3801-2019-ie00001.png"/></svg:svg></span></span> (<span class="inline-formula">2.3±1.3</span>&thinsp;<span class="inline-formula">µ</span>mol&thinsp;L<span class="inline-formula">⁻¹</span>) and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M23" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="9d4fcb6817fb360ca45e4c6c5f1d67f7"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-3801-2019-ie00002.svg" width="25pt" height="16pt" src="bg-16-3801-2019-ie00002.png"/></svg:svg></span></span> (<span class="inline-formula">5.6±5.1</span>&thinsp;<span class="inline-formula">µ</span>mol&thinsp;L<span class="inline-formula">⁻¹</span>) levels in these near pristine rivers and streams, with low agriculture pressure on the catchment (croplands correspond to 0.1&thinsp;% of catchment land cover of sampled rivers), dominated by forests (<span class="inline-formula">∼70</span>&thinsp;% of land cover). The covariations in %<span class="inline-formula">N₂O</span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M29" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="8926cb25afd85746c56ef60531d71c8b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-3801-2019-ie00003.svg" width="24pt" height="15pt" src="bg-16-3801-2019-ie00003.png"/></svg:svg></span></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M30" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="cbd0aa2bda73584a7a23dafea6b5761c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-3801-2019-ie00004.svg" width="25pt" height="16pt" src="bg-16-3801-2019-ie00004.png"/></svg:svg></span></span> and dissolved oxygen saturation level (%<span class="inline-formula">O₂</span>) indicate <span class="inline-formula">N₂O</span> removal by soil or sedimentary denitrification in low <span class="inline-formula">O₂</span>, high <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M34" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="5acffb7624bc47d59d396460984849a8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-3801-2019-ie00005.svg" width="24pt" height="15pt" src="bg-16-3801-2019-ie00005.png"/></svg:svg></span></span> and low <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M35" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="3408ea7bd52ee01c6f48215a5b5364fb"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-3801-2019-ie00006.svg" width="25pt" height="16pt" src="bg-16-3801-2019-ie00006.png"/></svg:svg></span></span> environments (typically small and organic matter rich streams) and <span class="inline-formula">N₂O</span> production by nitrification in high <span class="inline-formula">O₂</span>, low <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M38" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="dcfda7f7e9e6d10ee21653c7e18f979d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-3801-2019-ie00007.svg" width="24pt" height="15pt" src="bg-16-3801-2019-ie00007.png"/></svg:svg></span></span> and high <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M39" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="0ed2522f87147883b53f48851745fd62"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-3801-2019-ie00008.svg" width="25pt" height="16pt" src="bg-16-3801-2019-ie00008.png"/></svg:svg></span></span> (typical of larger rivers that are poor in organic matter). Surface waters were very strongly oversaturated in <span class="inline-formula">CO₂</span> and <span class="inline-formula">CH₄</span> with respect to atmospheric equilibrium, with values of the partial pressure of <span class="inline-formula">CO₂</span> (<span class="inline-formula"><i>p</i>CO₂</span>) ranging between 1087 and 22&thinsp;899&thinsp;ppm (equilibrium <span class="inline-formula">∼400</span>&thinsp;ppm) and dissolved <span class="inline-formula">CH₄</span> concentrations ranging between 22 and 71&thinsp;428&thinsp;nmol&thinsp;L<span class="inline-formula">⁻¹</span> (equilibrium <span class="inline-formula">∼2</span>&thinsp;nmol&thinsp;L<span class="inline-formula">⁻¹</span>). Spatial variations were overwhelmingly more important than seasonal variations for <span class="inline-formula"><i>p</i>CO₂</span>, <span class="inline-formula">CH₄</span> and %<span class="inline-formula">N₂O</span> as well as day–night variations for <span class="inline-formula"><i>p</i>CO₂</span>. The wide range of <span class="inline-formula"><i>p</i>CO₂</span> and <span class="inline-formula">CH₄</span> variations was consistent with the equally wide range of %<span class="inline-formula">O₂</span> (0.3&thinsp;%–122.8&thinsp;%) and of dissolved organic carbon (DOC) (1.8–67.8&thinsp;mg&thinsp;L<span class="inline-formula">⁻¹</span>), 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 <span class="inline-formula">CO₂</span> to the atmosphere from riverine surface waters was on average about 10 times higher than the flux of <span class="inline-formula">CO₂</span> produced by aquatic net heterotrophy (as evaluated from measurements of pelagic respiration and primary production). This indicates that the <span class="inline-formula">CO₂</span> emissions from the river network were sustained by lateral inputs of <span class="inline-formula">CO₂</span> (either from terra firme or from wetlands). The <span class="inline-formula"><i>p</i>CO₂</span> and <span class="inline-formula">CH₄</span> values decreased and %<span class="inline-formula">O₂</span> 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 <span class="inline-formula">CO₂</span> and <span class="inline-formula">CH₄</span> concentrations in the Congo river network, as well as driving spatial variations: the rivers draining the CCC were characterized by significantly higher <span class="inline-formula"><i>p</i>CO₂</span> and <span class="inline-formula">CH₄</span> and significantly lower %<span class="inline-formula">O₂</span> and %<span class="inline-formula">N₂O</span> values than those not draining the CCC; <span class="inline-formula"><i>p</i>CO₂</span> and %<span class="inline-formula">O₂</span> 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&thinsp;mmol&thinsp;m<span class="inline-formula">⁻²</span>&thinsp;d<span class="inline-formula">⁻¹</span> for <span class="inline-formula">CO₂</span> (range 86 and 7110&thinsp;mmol&thinsp;m<span class="inline-formula">⁻²</span>&thinsp;d<span class="inline-formula">⁻¹</span>), 12&thinsp;553&thinsp;<span class="inline-formula">µ</span>mol&thinsp;m<span class="inline-formula">⁻²</span>&thinsp;d<span class="inline-formula">⁻¹</span> for <span class="inline-formula">CH₄</span> (range 65 and 597&thinsp;260&thinsp;<span class="inline-formula">µ</span>mol&thinsp;m<span class="inline-formula">⁻²</span>&thinsp;d<span class="inline-formula">⁻¹</span>) and 22&thinsp;<span class="inline-formula">µ</span>mol&thinsp;m<span class="inline-formula">⁻²</span>&thinsp;d<span class="inline-formula">⁻¹</span> for <span class="inline-formula">N₂O</span> (range <span class="inline-formula">−</span>52 and 319&thinsp;<span class="inline-formula">µ</span>mol&thinsp;m<span class="inline-formula">⁻²</span>&thinsp;d<span class="inline-formula">⁻¹</span>). The estimate of integrated <span class="inline-formula">CO₂</span> emission from the Congo River network (<span class="inline-formula">251±46</span>&thinsp;TgC (10<span class="inline-formula">¹²</span>&thinsp;gC)&thinsp;yr<span class="inline-formula">⁻¹</span>), corresponding to nearly half the <span class="inline-formula">CO₂</span> emissions from tropical oceans globally (565&thinsp;TgC&thinsp;yr<span class="inline-formula">⁻¹</span>) and was nearly 2 times the <span class="inline-formula">CO₂</span> emissions from the tropical Atlantic Ocean (137&thinsp;TgC&thinsp;yr<span class="inline-formula">⁻¹</span>). Moreover, the integrated <span class="inline-formula">CO₂</span> 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&thinsp;TgC&thinsp;yr<span class="inline-formula">⁻¹</span>). This shows that it is unlikely that the <span class="inline-formula">CO₂</span> 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).</p>