Historical and future contributions of inland waters to the Congo Basin carbon balance

Abstract

Abstract. As the second largest area of contiguous tropical rainforest and second largest river basin in the world, the Congo basin has a significant role to play in the global carbon (C) cycle. Inventories suggest that terrestrial net primary productivity (NPP) and C storage in tree biomass has increased in recent decades in intact forests of tropical Africa, due in large part to a combination of increasing atmospheric CO2 concentrations and climate change, while rotational agriculture and logging have caused C losses. For the present day, it has been shown that a significant proportion of global terrestrial NPP is transferred laterally to the land-ocean aquatic continuum (LOAC) as dissolved CO2, dissolved organic carbon (DOC) and particulate organic carbon (POC). Whilst the importance of LOAC fluxes in the Congo basin has been demonstrated for the present day, it is not known to what extent these fluxes have been perturbed historically, how they are likely to change under future climate change and land use scenarios, and in turn what impact these changes might have on the overall C cycle of the basin. Here we apply the ORCHILEAK model to the Congo basin and show that 4% of terrestrial NPP (NPP\u2009=\u20095,800\u2009±\u2009166\u2009Tg\u2009C\u2009yr−1) is currently exported from soils to inland waters. Further, we found that aquatic C fluxes have undergone considerable perturbation since 1861 to the present day, with aquatic CO2 evasion and C export to the coast increasing by 26\u2009% (186\u2009±\u200941\u2009Tg\u2009C\u2009yr−1 to 235\u2009±\u200954\u2009Tg\u2009C\u2009yr−1) and 25\u2009% (12\u2009±\u20093\u2009Tg\u2009C\u2009yr−1 to 15\u2009±\u20094\u2009Tg\u2009C\u2009yr−1) respectively, largely because of rising atmospheric CO2 concentrations. Moreover, under climate scenario RCP 6.0 we predict that this perturbation will continue; over the full simulation period (1861–2099), we estimate that aquatic CO2 evasion and C export to the coast will increase by 79\u2009% and 67\u2009% respectively. Finally, we show that the proportion of terrestrial NPP lost to the LOAC also increases from approximately 3\u2009% to 5\u2009% from 1861–2099 as a result of increasing atmospheric CO2 concentrations and climate change.