Wet-season observations of dissolved carbon and CO2–CH4 emissions in inland waters of the Lower Ogooué floodplain (Gabon)

Floodplain wetlands in tropical river basins play a disproportionate role in carbon cycling, yet their contribution to greenhouse-gas emissions in Central Africa remains poorly constrained. We quantified dissolved carbon, CO 2 and CH 4 concentrations, together with floating-chamber fluxes, across small streams, flooded forests, lakes, and the main channel of the Lower Ogooué River floodplain (Gabon) during two representative wet-season campaigns. Dissolved organic carbon (5–50 mg L −1 ), pCO 2 (1500–16,000 µatm), and dissolved CH 4 showed their highest observed values in small tributaries and flooded forests, whereas the main stem generally exhibited lower values. The highest observed chamber-measured CO 2 fluxes occurred in tributaries, although differences among water body types were not statistically significant. Despite 5–10-fold spatial variability in concentrations, CO 2 fluxes varied by only ∼2-fold among water body types and showed little diel or between-campaign variability. Dissolved oxygen was strongly associated with CO 2 distributions, whereas its relationship with CH 4 was weaker and season-dependent. First-order area-based upscaling suggests that flooded forests and floodplain lakes were the principal contributors to wet-season CO 2 evasion, whereas the main river contributed a smaller share. Limited chamber-based CH 4 flux observations ( n = 5, one campaign) suggest that flooded forests may be methane-emission hotspots, but do not support quantitative floodplain-scale partitioning. Scenario calculations applying wet-season CO 2 emission factors to assumed regional water coverage suggest that aquatic evasion could exceed downstream dissolved-carbon export by several-fold; however, these values represent order-of-magnitude illustrations rather than measured annual budgets. These results highlight the need to explicitly account for floodplain hydrology, wetland extent, and aquatic-landscape heterogeneity when constraining carbon budgets and projecting responses of humid tropical river basins to climate-driven changes in flooding dynamics.