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
Authors: Mwanake Ricky , Gettel Gretchen , Butterbach-Bahl Klaus , Kiese Ralf 
Published: March 2020

&lt;p&gt;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 &amp;#8211; December 2019, in which CO&lt;sub&gt;2&lt;/sub&gt;, CH&lt;sub&gt;4 &lt;/sub&gt;and N&lt;sub&gt;2&lt;/sub&gt;O 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 km&lt;sup&gt;2&lt;/sup&gt;).&lt;/p&gt;&lt;p&gt;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 CO&lt;sub&gt;2&lt;/sub&gt; equivalents, CO&lt;sub&gt;2 &lt;/sub&gt;accounted for &gt;60% of the emissions, while CH&lt;sub&gt;4&lt;/sub&gt; and N&lt;sub&gt;2&lt;/sub&gt;O (&lt;35%) were minor contributors. There were higher mean values of CO&lt;sub&gt;2&lt;/sub&gt; and N&lt;sub&gt;2&lt;/sub&gt;O fluxes in streams draining croplands (92&amp;#177;9 CO&lt;sub&gt;2&lt;/sub&gt; mmol m&lt;sup&gt;-2&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt; and 14&amp;#177;2 N&lt;sub&gt;2&lt;/sub&gt;O &amp;#181;mol m&lt;sup&gt;-2&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt;) compared to those draining forested areas (45&amp;#177;5 CO&lt;sub&gt;2&lt;/sub&gt; mmol m&lt;sup&gt;-2&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt; and 3&amp;#177;0.6 N&lt;sub&gt;2&lt;/sub&gt;O &amp;#181;mol m&lt;sup&gt;-2&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt;). CH&lt;sub&gt;4&lt;/sub&gt; fluxes showed no significant variation with land use. CO&lt;sub&gt;2&lt;/sub&gt; and CH&lt;sub&gt;4&lt;/sub&gt; 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 N&lt;sub&gt;2&lt;/sub&gt;O was positively correlated to nitrate (NO&lt;sub&gt;3&lt;/sub&gt;-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&lt;sub&gt;4, &lt;/sub&gt;net heterotrophy for CO&lt;sub&gt;2&lt;/sub&gt; and coupled nitrification-denitrification for N&lt;sub&gt;2&lt;/sub&gt;O. 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 NO&lt;sub&gt;3&lt;/sub&gt;-N, higher DOC, and higher FBOM concentrations compared to the wet season. This resulted in significantly higher CH&lt;sub&gt;4&lt;/sub&gt; and CO&lt;sub&gt;2&lt;/sub&gt; concentrations, which could be attributed to increased in-stream production through the aforementioned processes as a result of increased water residence times. In contrast, N&lt;sub&gt;2&lt;/sub&gt;O concentrations in the dry season were lower than in the wet season, indicating that due to low DO and NO&lt;sub&gt;3&lt;/sub&gt;-N concentrations, produced N&lt;sub&gt;2&lt;/sub&gt;O may have been further reduced to N&lt;sub&gt;2&lt;/sub&gt; during denitrification. However, as fluxes are a function of both concentration and the discharge-related gas transfer velocity (k), all GHG&amp;#8217;s exhibited higher fluxes in the wet season compared to the dry season. Mean monthly CO&lt;sub&gt;2 &lt;/sub&gt;and N&lt;sub&gt;2&lt;/sub&gt;O concentrations also responded positively to discharge, suggesting that terrestrial inputs could also account for higher fluxes during the wet season.&lt;/p&gt;&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;