Variations in dissolved greenhouse gases (CO2, CH4, N2O) in the Congo River network overwhelmingly driven by fluvial-wetland connectivity
Authors: Borges Alberto V. , Darchambeau François , Lambert Thibault , Morana Cédric , Allen George H. , Tambwe Ernest , Toengaho Sembaito Alfred , Mambo Taylor , Nlandu Wabakhangazi José , Descy Jean-Pierre , Teodoru Cristian R. , Bouillon Steven 
Published: October 2019

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 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).