Partitioning carbon sources between wetland and well-drained ecosystems to a tropical first-order stream – implications for carbon cycling at the watershed scale (Nyong, Cameroon)

• Published in: Biogeosciences 2022-01, Vol.19 (1), p.137-163
• Main Authors: Moustapha, Moussa, Deirmendjian, Loris, Sebag, David, Braun, Jean-Jacques, Audry, Stéphane, Ateba Bessa, Henriette, Adatte, Thierry, Causserand, Carole, Adamou, Ibrahima, Ngounou Ngatcha, Benjamin, Guérin, Frédéric
• Format: Article
• Language: English
• Subjects: Applied geology; Aquatic ecosystems; Aquatic plants; Carbon; Carbon cycle; Carbon dioxide; Carbon sources; Catchment area; Catchments; Chemical partition; Creeks & streams; Degassing; Downstream; Dry season; Earth Sciences; Ecosystems; Environmental aspects; Exports; Floating plants; Forests; Geomorphology; Groundwater; Headwaters; Hydrologic cycle; Hydrology; Land use; Macrophytes; Metabolism; Organic matter; Parameters; Particulate organic carbon; Partitioning; Rainy season; Respiration; River networks; Rivers; Sciences of the Universe; Seasonal variations; Seasonality; Seasons; Stream order; Streams; Surface area; Tropical climate; Watersheds; Wet season; Wetlands
• ISSN: 1726-4189; 1726-4170; 1726-4189
• DOI: 10.5194/bg-19-137-2022

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 2