Basin‐scale estimates of greenhouse gas emissions from the Mara River, Kenya: Importance of discharge, stream size, and land use/land cover

Greenhouse gas fluxes (CO$_2$, CH$_4$, and N$_2$O) from African streams and rivers are under-represented in global datasets, resulting in uncertainties in their contributions to regional and global budgets. We conducted year-long sampling of 59 sites in a nested-catchment design in the Mara River, Kenya in which fluxes were quantified and their underlying controls assessed. We estimated annual basin-scale greenhouse gas emissions from measured in-stream gas concentrations, modeled gas transfer velocities, and determined the sensitivity of up-scaling to discharge. Based on the total annual CO$_2$-equivalent emissions calculated from global warming potentials (GWP), the Mara basin was a net greenhouse gas source (294 ± 35 Gg CO$_2$ eq yr$^{-1}$). Lower-order streams (1–3) contributed 81% of the total fluxes, and higher stream orders (4–8) contributed 19%. Cropland-draining streams also exhibited higher fluxes compared to forested streams. Seasonality in stream discharge affected stream widths (and stream area) and gas exchange rates, strongly influencing the basin-wide annual flux, which was 10 times higher during the high and medium discharge periods than the low discharge period. The basin-wide estimate was underestimated by up to 36% if discharge was ignored, and up to 37% for lower stream orders. Future research should therefore include seasonality in stream surface areas in upscaling procedures to better constrain basin-wide fluxes. Given that agricultural activities are a major factor increasing riverine greenhouse gas fluxes in the study region, increased conversion of forests and agricultural intensification has the possibility of increasing the contribution of the African continent to global greenhouse gas sources

Basin‐scale estimates of greenhouse gas emissions from the Mara River, Kenya: Importance of discharge, stream size, and land use/land cover

Greenhouse gas fluxes (CO2, CH4, and N2O) from African streams and rivers are under‐represented in global datasets, resulting in uncertainties in their contributions to regional and global budgets. We conducted year‐long sampling of 59 sites in a nested‐catchment design in the Mara River, Kenya in which fluxes were quantified and their underlying controls assessed. We estimated annual basin‐scale greenhouse gas emissions from measured in‐stream gas concentrations, modeled gas transfer velocities, and determined the sensitivity of up‐scaling to discharge. Based on the total annual CO2‐equivalent emissions calculated from global warming potentials (GWP), the Mara basin was a net greenhouse gas source (294 ± 35 Gg CO2 eq yr−1). Lower‐order streams (1–3) contributed 81% of the total fluxes, and higher stream orders (4–8) contributed 19%. Cropland‐draining streams also exhibited higher fluxes compared to forested streams. Seasonality in stream discharge affected stream widths (and stream area) and gas exchange rates, strongly influencing the basin‐wide annual flux, which was 10 times higher during the high and medium discharge periods than the low discharge period. The basin‐wide estimate was underestimated by up to 36% if discharge was ignored, and up to 37% for lower stream orders. Future research should therefore include seasonality in stream surface areas in upscaling procedures to better constrain basin‐wide fluxes. Given that agricultural activities are a major factor increasing riverine greenhouse gas fluxes in the study region, increased conversion of forests and agricultural intensification has the possibility of increasing the contribution of the African continent to global greenhouse gas sources.Deutscher Akademischer Austauschdienst http://dx.doi.org/10.13039/501100001655IHE Delft Institute for Water EducationFederal Ministry of Education and Research http://dx.doi.org/10.13039/501100002347Helmholtz Association http://dx.doi.org/10.13039/501100009318TERENO Bavarian Alps/ Pre‐Alps Observator

Land Use, Not Stream Order, Controls N2O Concentration and Flux in the Upper Mara River Basin, Kenya

Anthropogenic activities have led to increases in nitrous oxide (N2O) emissions from river systems, but there are large uncertainties in estimates due to lack of data in tropical rivers and rapid increase in human activity. We assessed the effects of land use and river size on N2O flux and concentration in 46 stream sites in the Mara River, Kenya, during the transition from the wet (short rains) to dry season, November 2017 to January 2018. Flux estimates were similar to other studies in tropical and temperate systems, but in contrast to other studies, land use was more related to N2O concentration and flux than stream size. Agricultural stream sites had the highest fluxes (26.38 ± 5.37 N2O-N μg·m–2·hr–1) compared to both forest and livestock sites (5.66 ± 1.38 N2O-N μg·m–2·hr–1 and 6.95 ± 2.96 N2O-N μg·m–2·hr–1, respectively). N2O concentrations in forest and agriculture streams were positively correlated to stream carbon dioxide (CO2-C(aq)) but showed a negative correlation with dissolved organic carbon, and the dissolved organic carbon:dissolved inorganic nitrogen ratio. N2O concentration in the livestock sites had a negative relationship with CO2-C(aq) and a higher number of negative fluxes. We concluded that in-stream chemoautotrophic nitrification was likely the main biogeochemical process driving N2O production in agricultural and forest streams, whereas complete denitrification led to the consumption of N2O in the livestock stream sites. These results point to the need to better understand the relative importance of nitrification and denitrification in different habitats in producing N2O and for process-based studies