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We discuss concentrations of dissolved CH4, N2O,O2, NO3− and NH4+, and emission fluxes of CH4 andN2O for river sites in the western Congo Basin, Republic of Congo (ROC).Savannah, swamp forest and tropical forest samples were collected from theCongo main stem and seven of its tributaries during November 2010 (41samples; <q>wet season</q>) and August 2011 (25 samples; <q>dry season</q>;CH4 and N2O only). Dissolved inorganic nitrogen (DIN: NH4++NO3−; wet season) was dominated by NO3− (63 ± 19 %of DIN). Total DIN concentrations (1.5–45.3 µmol L−1) wereconsistent with the near absence of agricultural, domestic and industrialsources for all three land types. Dissolved O2 (wet season) was mostlyundersaturated in swamp forest (36 ± 29 %) and tropical forest(77 ± 36 %) rivers but predominantly supersaturated in savannahrivers (100 ± 17 %). The dissolved concentrations of CH4 andN2O were within the range of values reported earlier for sub-SaharanAfrican rivers. Dissolved CH4 was found to be supersaturated(11.2–9553 nmol L−1; 440–354 444 %), whereas N2O rangedfrom strong undersaturation to supersaturation (3.2–20.6 nmol L−1;47–205 %). Evidently, rivers of the ROC are persistent local sources ofCH4 and can be minor sources or sinks for N2O. During the dryseason the mean and range of CH4 and N2O concentrations were quitesimilar for the three land types. Wet and dry season mean concentrations andranges were not significant for N2O for any land type or for CH4 insavannah rivers. The latter observation is consistent with seasonal bufferingof river discharge by an underlying sandstone aquifer. Significantly higherwet season CH4 concentrations in swamp and forest rivers suggest thatCH4 can be derived from floating macrophytes during flooding and/orenhanced methanogenesis in adjacent flooded soils. Swamp rivers alsoexhibited both low (47 %) and high (205 %) N2O saturation butwet season values were overall significantly lower than in either tropicalforest or savannah rivers, which were always supersaturated (103–266 %)and for which the overall means and ranges of N2O were not significantlydifferent. In swamp and forest rivers O2 saturation co-varied inverselywith CH4 saturation (log %) and positively with % N2O. Asignificant positive correlation between N2O and O2 saturation inswamp rivers was coincident with strong N2O and O2undersaturation, indicating N2O consumption during denitrification inthe sediments. In savannah rivers persistent N2O supersaturation and anegative correlation between N2O and O2 suggest N2O productionmainly by nitrification. This is consistent with a stronger correlationbetween N2O and NH4+ than between N2O and NO3−. Ourranges of values for CH4 and N2O emission fluxes(33–48 705 µmol CH4 m−2 d−1;1–67 µmol N2O m−2 d−1) are within the rangespreviously estimated for sub-Saharan African rivers but they includeuncertainties deriving from our use of <q>basin-wide</q> values for CH4 andN2O gas transfer velocities. Even so, because we did not account for anycontribution from ebullition, which is quite likely for CH4 (at least20 %), we consider our emission fluxes for CH4 to be conservative

biogeosciences

The riverine source of CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O from the Republic of Congo, western Congo Basin