07 Apr 2021

07 Apr 2021

Review status: a revised version of this preprint is currently under review for the journal BG.

Partitioning carbon sources in a tropical watershed (Nyong River, Cameroon) between wetlands and terrestrial ecosystems – Do CO2 emissions from tropical rivers offset the terrestrial carbon sink?

Moussa Moustapha1, Loris Deirmendjian2,3, David Sebag4,5,6, Jean-Jacques Braun2,3,7,8, Stéphane Audry2, Henriette Ateba Bessa7, Thierry Adatte9, Carole Causserand2, Ibrahima Adamou1, Benjamin Ngounou Ngatcha1, and Frédéric Guérin2,3 Moussa Moustapha et al.
  • 1Université de Ngaoundéré, Faculté des Sciences, BP 454 Ngaoundéré, Cameroun
  • 2Géosciences Environnement Toulouse (GET-Université de Toulouse, CNRS, IRD), Université de Toulouse Paul Sabatier, 14 Avenue Edouard-Belin, 31400 Toulouse, France
  • 3IRD, UR 234, GET, 14 Avenue E. Belin, 31400, Toulouse, France
  • 4Normandie Univ, UNIROUEN, UNICAEN, CNRS, M2C, 76000 Rouen, France
  • 5HSM, IRD, CNRS, Université de Montpellier, Montpellier, France
  • 6IFPEN, Geosciences Dept, Rueil-Malmaison, France
  • 7Institut de Recherches Géologiques et Minières/Centre de Recherches Hydrologiques, BP 4110, Yaoundé, Cameroun
  • 8International Joint Laboratory DYCOFAC, IRGM-UY1-IRD, Rue Joseph Essono Balla, Quartier Elig Essono, BP 1857, Yaoundé, Cameroun
  • 9Institut des Sciences de la Terre (ISTE), Université de Lausanne, GEOPOLIS, CH-1015 Lausanne, Switzerland

Abstract. We characterized the spatio-temporal dynamics of carbon (C) in rivers of the tropical Nyong catchment (South Cameroon). In 2016, we measured fortnightly at 6 locations along an upstream-downstream gradient from groundwater to the main stream of order 6, total alkalinity, dissolved inorganic C (DIC) used together with pH to compute pCO2, dissolved and particulate organic C (DOC and POC) and total suspended matter. Forest, groundwater had low DOC content (< 1 mg L−1) as its leaching was probably prevented in the overlaying lateritic soils. Forest groundwater was supersaturated in CO2 (~50 times the atmospheric value) because of the solubilisation of the CO2 originating from soil respiration. Wetlands water exhibited higher DOC (> 14 mg L−1) and similar DIC concentrations than the forest groundwater. Surface runoff was considered negligible in the basin due to low slopes and high infiltration capacity of the soils, making wetlands and forest groundwater the two main sources of C for surface waters. The influence of wetlands on C dynamics in rivers was significant during periods of high waters when the hydrological connectivity between surface waters and wetlands was enhanced. On annual scale, wetlands exported 60 % (15.4 ± 7.2 t C km−2 yr−1) of the total amount of C transferred laterally to surface waters, the remaining 40 % (12.1 ± 5.8 t C km−2 yr−1) being transferred from forest groundwater. Heterotrophic respiration in rivers averaged 89 mmol m−2 d−1 whereas CO2 degassing was 1260 mmol m−2 d−1, which shows that it is unlikely that the river heterotrophic respiration was the main process sustaining CO2 emission. The comparison of the hydrological export of terrestrial C via forest groundwater with the net terrestrial C sink in the Nyong watershed shows that only ~4 % of the net terrestrial C sink reach the aquatic ecosystem. The carbon mass balance of the Nyong watershed highlights that attributing to a unique terrestrial source the whole amount of riverine carbon emitted to the atmosphere and exported to the ocean and ignoring the river–wetland connectivity can lead to the misrepresentation of C dynamics in tropical watersheds.

Moussa Moustapha et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-69', Anonymous Referee #1, 09 May 2021
  • RC2: 'Comment on bg-2021-69', Anonymous Referee #2, 10 May 2021
  • CC1: 'Downstream contribution to C mass balance', Kleiton Rabelo de Araújo, 15 May 2021
  • RC3: 'Comment on bg-2021-69', Scott Winton, 16 May 2021

Moussa Moustapha et al.

Moussa Moustapha et al.


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Short summary
We monitored the spatial and temporal variability of organic and inorganic carbon species in the tropical Nyong River in Cameroon from groundwaters to the river mainstem. We found that 60 % of the carbon spiralling in the river originated from wetlands connected to the river and the remaining 40 % originated from terrestrial ecosystems. Only ~4 % of the net terrestrial C sink reach the aquatic ecosystem. 80 % of the carbon entering the river is outgassed to the atmosphere as CO2.