Harnessing the NEON data revolution to advance open environmental science with a diverse and data-capable community. Ecosphere 12(12):e03833.

Abstract. Tropical rivers emit large amounts of carbon dioxide (CO2) to the atmosphere, in particular due to large wetland-to-river carbon (C) inputs. Yet, tropical African rivers remain largely understudied, and little is known about the partitioning of C sources between wetland and well-drained ecosystems to rivers. In a first-order sub-catchment (0.6 km2) of the Nyong watershed (Cameroon 27 800 km2), we fortnightly measured C in all forms and ancillary parameters in groundwater in a well-drained forest (hereafter referred to as non-flooded forest groundwater) and in the stream. In the first-order catchment, the simple land use shared between wetland and well-drained forest, together with drainage data, allowed the partitioning of C sources between wetland and well-drained ecosystems to the stream. Also, we fortnightly measured dissolved and particulate C downstream of the first-order stream to the main stem of order 6, and we supplemented C measurements with measures of heterotrophic respiration in stream orders 1 and 5. In the first-order stream, dissolved organic and inorganic C and particulate organic C (POC) concentrations increased during rainy seasons when the hydrological connectivity with the riparian wetland increased, whereas the concentrations of the same parameters decreased during dry seasons when the wetland was shrinking. In larger streams (order > 1), the same seasonality was observed, showing that wetlands in headwaters were significant sources of organic and inorganic C for downstream rivers, even though higher POC concentration evidenced an additional source of POC in larger streams during rainy seasons that was most likely POC originating from floating macrophytes. During rainy seasons, the seasonal flush of organic matter from the wetland in the first-order catchment and from the macrophytes in higher-order rivers significantly affected downstream metabolism, as evidenced by higher respiration rates in stream order 5 (756 ± 333 gC-CO2 m−2 yr−1) compared to stream 1 (286 ± 228 gC-CO2 m−2 yr−1). In the first-order catchment, the sum of the C hydrologically exported from non-flooded forest groundwater (6.2 ± 3.0 MgC yr−1) and wetland (4.0 ± 1.5 MgC yr−1) to the stream represented 3 %–5 % of the local catchment net C sink. In the first-order catchment, non-flooded forest groundwater exported 1.6 times more C than wetland; however, when weighed by surface area, C inputs from non-flooded forest groundwater and wetland to the stream contributed to 27 % (13.0 ± 6.2 MgC yr−1) and 73 % (33.0 ± 12.4 MgC yr−1) of the total hydrological C inputs, respectively. At the Nyong watershed scale, the yearly integrated CO2 degassing from the entire river network was 652 ± 161 GgC-CO2 yr−1 (23.4 ± 5.8 MgC CO2 km−2 yr−1 when weighed by the Nyong watershed surface area), whereas average heterotrophic respiration in the river and CO2 degassing rates was 521 ± 403 and 5085 ± 2544 gC-CO2 m−2 yr−1, which implied that only ∼ 10 % of the CO2 degassing at the water–air interface was supported by heterotrophic respiration in the river. In addition, the total fluvial C export to the ocean of 191 ± 108 GgC yr−1 (10.3 ± 5.8 MgC km−2 yr−1 when weighed by the Nyong watershed surface area) plus the yearly integrated CO2 degassing from the entire river network represented ∼ 11 % of the net C sink estimated for the whole Nyong watershed. In tropical watersheds, we show that wetlands largely influence riverine C variations and budget. Thus, ignoring the river–wetland connectivity might lead to the misrepresentation of C dynamics in tropical watersheds.

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.