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)

Abstract

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.

Stream respiration exceeds CO2 evasion in a low‐energy, oligotrophic tropical stream

“…Potential contribution of NEP to CO 2 evasion in different streams and rivers, extracted from Abril et al (2014), Bernal et al (2022), Borges et al (2019), Borges et al (2015), Carter et al (2022), Cole and Caraco (2001), Demars (2019), Duvert et al (2019); Ellis et al (2012), Gómez‐Gener et al (2016), Hotchkiss et al (2015), Lupon et al (2019), Lynch et al (2010), Marzolf et al (2022), Moustapha et al (2022), Rasilo et al (2017), Rocher‐Ros et al (2020), Taillardat et al (2022), Teodoru et al (2015), Wang et al (2021), and Winterdahl et al (2016). 1 Estimates for 2005.…”

“…5). External CO 2 sources in this case may have comprised soil water and groundwater inputs of terrestrially derived CO 2 (Johnson et al 2008; Duvert et al 2018), a pathway that can be facilitated by the high connectivity between wetland soils, riparian areas and the stream network at high flow (Birkel et al 2020; Moustapha et al 2022). Given the concomitantly high internal CO 2 production via NEP during the early storms and wet season (Fig.…”

“…Marzolf et al (2022) found that 16% of the CO 2 evasion from a small, steep stream in Costa Rica can be sourced from stream metabolism, similar in magnitude to groundwater and upstream inputs of CO 2 . In tropical Africa, estimations by Borges et al (2015), Borges et al (2019), Moustapha et al (2022), andTeodoru et al (2015) indicate that stream metabolism contributes little CO 2 to the evasion flux, with contributions ranging from 0% to 33% in individual rivers, and 14% at a continental scale. Overall, these studies illustrate the huge variability in processes across river orders and morphologies and suggest that more work is needed to better understand and quantify the key controls on these fluxes for different tropical climates and landscapes.…”

Basin‐Scale CO2 Emissions From the East River in South China: Importance of Small Rivers, Human Impacts and Monsoons

“…In addition, wetland, known as a reliable C source for rivers (Abril & Borges, 2019), accounts for less than 1% of the total area of the ERB (Figure S5 in Supporting Information ). Therefore, a considerable proportion of terrestrial C could not be converted into a more labile form in wetlands before entering the river network (Algesten et al., 2004; Moustapha et al., 2021), restricting the in‐stream decomposition of terrestrial C.…”

Carbon Dioxide and Methane Dynamics in a Peatland Headwater Stream: Origins, Processes and Implications

“…This suggests that wetlands, when hydrologically connected to streams, act as steady sources of GHGs. The paramount importance of lateral carbon inputs from wetlands to explain GHGs emissions from streams was also reported in Africa, such as the Congo River network (Borges et al, 2019) and the Nyong watershed (Moustapha et al, 2022), high altitude tropical wetlands (Schneider et al, 2020), but also in temperate and boreal peatlands (Billett & Harvey, 2013;Billett & Moore, 2008;Dinsmore et al, 2010). Considering the relatively flat topographies draining wetlands and the large carbon stocks they hold, particularly in peatlands, it is worth exploring their carbon GHG dynamics and identifying their biogeochemical specificities, in comparison to other headwater streams.…”