Partitioning carbon sources in a tropical watershed (Nyong River, Cameroon) between wetlands and terrestrial ecosystems – Do CO 2 emissions from tropical rivers offset the terrestrial carbon sink?

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 ( 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.