Seasonal variation of CO2, CH4 and N2O fluxes from tropical streams and rivers under forest and cropland landuses: A case study of the Mara river basin in Kenya

Greenhouse gas (GHG) emission estimates from tropical African rivers are underrepresented in global datasets, resulting in uncertainties in their contributions to global emissions. To better constrain the contribution of rivers and streams to GHG emissions from tropical landscapes and to determine possible underlying controlling processes, we implemented a monthly synoptic sampling program from January 2019 – December 2019, in which CO₂, CH₄ and N₂O concentrations and fluxes, along with water quality and sediment parameters were measured from 60 river sites in the upper and middle catchments of the Mara River in Kenya (~8450 km²).

Consistent with previous studies, Mara basin streams and rivers were mostly sources of GHGs, and were comparable to previous studies in tropical and temperate regions. Based on CO₂ equivalents, CO₂ accounted for >60% of the emissions, while CH₄ and N₂O (<35%) were minor contributors. There were higher mean values of CO₂ and N₂O fluxes in streams draining croplands (92±9 CO₂ mmol m^-2 d^-1 and 14±2 N₂O µmol m^-2 d^-1) compared to those draining forested areas (45±5 CO₂ mmol m^-2 d^-1 and 3±0.6 N₂O µmol m^-2 d^-1). CH₄ fluxes showed no significant variation with land use. CO₂ and CH₄ concentrations had a negative correlation with dissolved oxygen (DO) and a positive correlation with dissolved organic carbon (DOC) and fine benthic organic matter (FBOM), while N₂O was positively correlated to nitrate (NO₃-N) and negatively correlated to DO. Based on the significant relationships of all three gases with DO and their substrates, we inferred that GHG concentrations were mainly controlled by in-stream biogeochemical processes - i.e. methanogenesis for CH_4, net heterotrophy for CO₂ and coupled nitrification-denitrification for N₂O. Changes in discharge, driven by precipitation events, significantly accounted for the seasonal variation in GHGs concentration and flux, with clear differences between the driest months (March and April) and the wettest (October-December). During low-discharge periods, streams were characterized by lower DO, lower nitrate NO₃-N, higher DOC, and higher FBOM concentrations compared to the wet season. This resulted in significantly higher CH₄ and CO₂ concentrations, which could be attributed to increased in-stream production through the aforementioned processes as a result of increased water residence times. In contrast, N₂O concentrations in the dry season were lower than in the wet season, indicating that due to low DO and NO₃-N concentrations, produced N₂O may have been further reduced to N₂ during denitrification. However, as fluxes are a function of both concentration and the discharge-related gas transfer velocity (k), all GHG’s exhibited higher fluxes in the wet season compared to the dry season. Mean monthly CO₂ and N₂O concentrations also responded positively to discharge, suggesting that terrestrial inputs could also account for higher fluxes during the wet season.

In future studies, we therefore plan to incorporate process measurements (e.g. nitrification, denitrification and ecosystem metabolism) across seasons in conjunction with measurements of GHG fluxes and environmental parameters. This will allow to a) elucidate the importance of in-stream production versus terrestrial inputs as controls of fluxes of GHGs and to b) attribute observed fluxes to specific biogeochemical processes.