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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.

Inland waters have been recognized as a significant source of carbon dioxide (CO2) to the atmosphere at the global scale. Fluxes of CO2 between aquatic systems and the atmosphere are calculated from the gas transfer velocity and the water–air gradient of the partial pressure of CO2 (pCO2). Currently, direct measurements of water pCO2 remain scarce in freshwaters, and most published pCO2 data are calculated from temperature, pH and total alkalinity (TA). Here, we compare calculated (pH and TA) and measured (equilibrator and headspace) water pCO2 in a large array of temperate and tropical freshwaters. The 761 data points cover a wide range of values for TA (0 to 14 200 μmol L−1), pH (3.94 to 9.17), measured pCO2 (36 to 23 000 ppmv), and dissolved organic carbon (DOC) (29 to 3970 μmol L−1). Calculated pCO2 were >10% higher than measured pCO2 in 60% of the samples (with a median overestimation of calculated pCO2 compared to measured pCO2 of 2560 ppmv) and were >100% higher in the 25% most organic-rich and acidic samples (with a median overestimation of 9080 ppmv). We suggest these large overestimations of calculated pCO2 with respect to measured pCO2 are due to the combination of two cumulative effects: (1) a more significant contribution of organic acids anions to TA in waters with low carbonate alkalinity and high DOC concentrations; (2) a lower buffering capacity of the carbonate system at low pH, which increases the sensitivity of calculated pCO2 to TA in acidic and organic-rich waters. No empirical relationship could be derived from our data set in order to correct calculated pCO2 for this bias. Owing to the widespread distribution of acidic, organic-rich freshwaters, we conclude that regional and global estimates of CO2 outgassing from freshwaters based on pH and TA data only are most likely overestimated, although the magnitude of the overestimation needs further quantitative analysis. Direct measurements of pCO2 are recommended in inland waters in general, and in particular in acidic, poorly buffered freshwaters.

Export of soil carbon to superficial water through the drainage of groundwater is a significant but poorly documented component of the continental carbon budget. We monitored the concentrations of dissolved organic and inorganic carbon (DOC and DIC) in groundwaters and first order streams of a small temperate, forested and sandy watershed where hydrology occurs exclusively through drainage (no surface runoff). The studied watershed was also implemented for continuous measurements of groundwater table, precipitation, evapotranspiration, river discharge, and net ecosystem exchanges of sensible and latent heat fluxes as well as CO2. On a monthly basis, we found a good consistency between precipitation and the sum of evapotranspiration, drainage and groundwater storage. DOC and DIC temporary storage in groundwater and export to streams varied drastically during the hydrological cycle, the residence times of these two carbon forms varying from one month to several years. DOC concentrations in groundwater and streams were maximal at high water table and high stream discharge, when the water table reached the superficial organic rich layer of the soil. A large fraction of this winter DOC maximum was temporarily stored and further mineralized to DIC in the groundwater and only about 15 % was exported to streams during winter periods. In contrast, DIC, which was present in majority in the form of dissolved CO2 in groundwater and streams, was apparently diluted at high water table: DIC concentrations were maximum at low water table and low discharge in late summer and maximum pCO2 in groundwater corresponded to the late summer period of heterotrophic conditions (i.e., Reco > GPP). Groundwater DIC peaked in late summer and was followed by a rapid loss of excess CO2 from stream surface to the atmosphere. Overall, mean carbon export was 7.5 g C m−2 yr−1 (50 % as DOC and 50 % as DIC) and represented only 1.5 % of the NEE. About 65 % of the DIC exported from groundwaters returned to the atmosphere in the form of CO2 in first order streams.