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)
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 
Published: January 2022

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.