The riverine source of tropospheric CH 4 and N 2 O from the Republic of Congo , Western Congo Basin

We report concentrations of dissolved CH4, N2O, O2, NO3 and NH4, and corresponding CH4 and N2O emissions for river sites in savanna, swamp forest and tropical forest, along the Congo main stem and in several of its tributary systems of the Western Congo Basin, Republic of Congo, during November 2010 (41 samples; “wet season”) 15 and August 2011 (25 samples; “dry season”; CH4 and N2O only). Dissolved inorganic nitrogen (DIN: wet season; NH4 + NO3) was dominated by NO3 (63 ± 19% of DIN), total DIN concentrations (1.5-45.3 mol L) being consistent with small agricultural, domestic and industrial sources. Dissolved O2 (wet season) was mostly undersaturated in swamp forest (36 ± 29%) and tropical forest (77 ± 36%) rivers but predominantly super-saturated in savannah rivers (100 ± 17%). Dissolved CH4 and N2O were within previously reported ranges for sub-Saharan 20 African rivers. While CH4 was always super-saturated (11.2 9553 nmol L; 440-354400%), N2O ranged from strong under-saturation to strong super-saturation (3.2-20.6 nmol L; 47-205%). Evidently, rivers of the ROC are persistent local sources of tropospheric CH4 but can be small sources or sinks for N2O. Dry season concentration means and ranges of CH4 and N2O were indistinguishable for all three land types and seasonal differences in means and ranges were not significant for N2O for any land type or for CH4 in savannah rivers; the latter is consistent with seasonal 25 buffering of river discharge by an underlying sandy-sandstone aquifer. By contrast, swamp and forest river CH4 was significantly higher in the wet season, possibly reflecting CH4 derived from floating macrophytes during flooding and/or enhanced methanogenesis in adjacent flooded soils. Swamp rivers exhibited both low (47%) and high (205%) N2O saturations but wet season values were overall significantly lower than in either tropical forest or savannah rivers, which were always super-saturated (103-266%) and for which the overall means and ranges of N2O were not 30 significantly different. In swamp and forest rivers % O2 co-varied negatively with log % CH4 and positively with % N2O. The strong positive N2O O2 correlation in swamp rivers was coincident with strong N2O and O2 undersaturation, indicating N2O consumption by sediment denitrification. In savannah rivers persistent N2O supersaturation and a negative N2O O2 correlation may indicate N2O production mainly by nitrification, consistent with a stronger correlation between N2O and NH4 than between N2O and NO3. Our range in CH4 and N2O emissions fluxes 35 (33-48705 mol CH4 m d; 1-67 mol N2O m d), is wider than previously estimated for sub-Saharan African rivers but it includes uncertainties deriving from our use of “basin-wide” values for CH4 and N2O gas transfer velocities. Even so, because we did not account for any contribution from ebullition, which for CH4 is likely to be at least 20%, our emissions estimates for CH4 are probably conservative.


Study site and sample locations
The ~4700 km long Congo River (Fig. 1) has an equatorial location that affords it a bimodal hydrological regime, with maximal flows in December and May and minimal flows in August and March (Coynel et al., 2005).The Congo Basin (9°N -14°S; 11° -31°E) is the largest hydrological system in Central Africa, covering ~3.8 × 10 6 km 2 (~ 12% of the total African land mass; Fig. 1) and incorporating the world's fourth largest wetland area ~3.6 x 10 5 85 km 2 (Laporte et al., 1998).The Congo's annual freshwater discharge is the world's second largest at ~1300 km 3 (Borges et al., 2015b), 50% of all freshwater flow from Africa to the Atlantic Ocean.Rivers and streams in the Congo Basin have a total open water surface area ~2.7 x 10 4 km 2 (Raymond et al., 2013).The climate is warm (mean annual temperature 24.8 ± 0.8 o C) and humid with an annual rainfall ~1800 mm (Laraque et al., 2001).
We sampled the Congo main stem, several of its tributary rivers and some of their sub-tributaries, at sites within the 90 Republic of Congo (ROC: area 3.4 x 10 5 km²), in the western Congo Basin (Figure 1).Individual catchment areas, freshwater discharge rates and rainfall are listed in Table 1.Around 50% of the ROC land area is classified as tropical forest, with the remainder classified as either swamp or savannah in approximately equal proportion (Clark and Decalo, 2012).Sampling sites were selected to represent each of these three land cover types (Fig. 1), which were georeferenced to the World Geodetic System 1984 (WGS84) and intersected with the highest level sub-95 watershed polygons defined by the HYDRO1K global hydrological dataset (U.S. Geological Survey, 2000).This enabled assigning the fractional cover for each land cover type, and hence the dominant land cover type, to the areas immediately surrounding each sampling location.Swamp includes both temporally and permanently inundated areas of "forest", with vegetation adapted to poorly drained, anaerobic soils (Mayaux et al., 2002).For all three land cover types the mean annual temperature range (period 1990-2012) is ~1-3 o C, temperatures being lowest (~22-24 o C) in 100 July-August and highest (~25-26 o C) in March-April (http://sdwebx.worldbank.org/climateportal/).For savannah the average monthly rainfall during July- May (1990May ( -2012) ) is ~120-260 mm, typically being maximal in October-November, but < 40 mm falls during June-August (http://sdwebx.worldbank.org/climateportal/).For forest and swamp the annual range in monthly rainfall is less pronounced.Both have two discernable rainfall maxima, during April-May and October-November (~150-240 mm month -1 ), and a minimum in June-August (~40-120 mm month -1 ) 105 (http://sdwebx.worldbank.org/climateportal/). ROC swamp and forest (Fig. 1) broadly correspond to the westernmost part of the "Cuvette Centrale" (Central Basin).This is a large shallow depression composed mainly of dense, humid forest and extending from approximately 15 o W to 25 o W and 5 o N to 4 o S, the central western part of which remains flooded throughout the rainy seasons.Rivers 110 sampled in this region (Sangha, Likouala-aux-Herbes, Likouala, Lengoue, Mambili: Fig. 1, Table 1) drain predominantly sandy or clayey quaternary deposits.The Kouyou basin (Fig. 1) borders the 'Batéké Plateaux', a 600-700m relief sandstone formation to the south, intersected by dry valleys and covering much of the southern ROC.
Here, bushy savannah is intersected by the Alima, Nkéni and Léfini rivers.Due to water storage in an underlying sandy-sandstone aquifer the hydrological regimes of these three rivers are largely independent of rainfall; they all 115 show only weak seasonality in discharge despite the relatively large variation in monthly precipitation (Laraque et al., 2001).

Sample collection and analytical techniques
We collected 66 surface water samples (~0.2 m) from central river channels for dissolved CH4, N2O, O2, NO3 monthly rainfall distribution, for convenience we hereinafter refer to these as "wet season" and "dry season" respectively.Samples were slowly decanted into a series of 125 ml glass screw top septum bottles (Sigma-Aldrich, UK) via a silicon rubber tube, over filling each by at least one sample volume to avoid bubble entrainment.Samples were inoculated with 25 l 0.1 M HgCl2 to arrest microbial activity, sealed to leave no headspace and subsequently returned to Newcastle for dissolved gas analysis within several weeks of collection.Dissolved gas samples treated in 125 this way can be successfully stored for several months (Elkins, 1980).
Samples for dissolved NH4 + and NO3 -were filtered on collection (Whatman 0.7 m GF/F; precombusted at 550 °C for 8 h), directly into clean glass vials and stored acidified (pH 2) at 4 °C in the dark for several weeks prior to analysis by segmented flow (Astoria Analyzer; Astoria-Pacific, USA) at Woods Hole, using established methods (U.S.
Emission fluxes, F (mol m -2 d -1 ), of CH4 and N2O were estimated using F = k w LΔp, where k w is the transfer velocity 145 of CH4 or N2O (cm hr -1 ), L is the solubility of CH4 or N2O (mol cm -3 atm -1 ) (Wiesenburg and Guinasso, 1979;Weiss and Price, 1980) and Δp is the corresponding water-to-air partial pressure difference.kw values were derived from two corresponding estimates for CO2 in the Congo.Raymond et al. (2013) estimated a basin-wide kw of 5.2 m d -1 for CO2, using hydraulic equations involving basin slope and flow velocity.The uncertainty in this estimate is ~ ± 10% (Raymond et al., 2013).In contrast Aufdenkampe et al. (2011) applied constant kw values for CO2 in streams (< 100 150 m wide: 3.0 m d -1 ) and in rivers (>100m wide: 4.2 m d -1 ).Adjusting for the relative areas of these in the Congo basin (Borges et al., 2015b) gives a basin-wide mean kw ~3.9 m d -1 for CO2.We converted these estimates to kw for CH4 and N2O by multiplying by (Sc/470.7) -0.5 , where 470.7 is the Schmidt number of CO2 in freshwater, and Sc is the Schmidt number of CH4 or N2O (ScCH4=486.8;ScN2O=476.9),assuming an ambient temperature of 25 o C (Wanninkhof, 1992).The resulting kw estimates are 5.1 and 3.9 m d -1 for CH4 and 5.2 and 4.0 m d -1 for N2O.Resulting emissions 155 estimates are consequently ~30% higher based on Raymond et al. (2013).Using both sets of kw estimates facilitates a direct comparison with the largest study of CH4 and N2O fluxes for African rivers that also used this approach (Borges et al., 2015b).While other relevant work used wind based kw estimates (Koné et al., 2010;Bouillon et al., 2012) the unavailability of wind speeds precludes their use here.We applied the global mean tropospheric mixing ratios of CH4 (1797 ppbv) and N2O (323 ppbv) for the year 2010 (http://www.eea.europa.eu/data-and-maps/).NH4 + and wet season DIN (NO3 -+ NH4 + ) is only reported for samples for which both NO3 -and NH4 + are available.

Dissolved CH4 and N2O
Table 2 summarises ranges, means and medians of riverine CH4 and N2O concentrations and percent saturations for the three land cover types.All samples were highly CH4 super-saturated, concentrations spanning two orders of magnitude (11.2 -9553 nmol L -1 ; 440-354400% saturation).N2O spanned a much narrower concentration range and varied from strong under-saturation to strong super-saturation (3.2-20.6 nmol L -1 ; 47-205%).Evidently, while rivers 185 of the ROC are strong local sources of tropospheric CH4 they can act as both small sources and sinks for N2O.
Swamp rivers exhibited both the lowest and among the highest N2O saturations (Table 2) but during the wet season had overall significantly lower N2O than either forest or savannah rivers, which were both always super-saturated (103-266%; Table 2) and for which the overall means and ranges of N2O were not significantly different (Mann-Whitney, one-tailed: swamp vs forest and swamp vs savannah, P = 0:004).For CH4, concentration means and 190 ranges during the wet season did not differ significantly between swamp and forest rivers but they were significantly higher in both than in savannah rivers (Mann-Whitney, one-tailed: swamp vs savannah, P = 0:004; forest vs savannah, P = 0.03).In contrast, during the dry season concentration means and ranges of both CH4 and N2O were indistinguishable for all three land cover types.Seasonal differences in concentration means and ranges were not significant for N2O for any of the three land cover types or for CH4 in savannah rivers, but in both swamp and forest 195 rivers CH4 was significantly higher during the wet season (Mann-Whitney, one-tailed: swamp P= 0.01; forest P = 0.003).

CH4 and N2O emission fluxes
Table 3 summarises ranges, means and medians of CH4 and N2O emission fluxes using kw derived from Raymond et al. (2013) and Aufdenkampe et al. (2011).Fluxes broadly followed the distribution of concentrations, for CH4 being lowest overall in savannah rivers and highest in swamp and forest rivers and for N2O being lowest in swamp rivers and highest in savannah and forest rivers.Fluxes were always to air at all sites for CH4 and at all savannah and forest 230 sites for N2O.However, swamp rivers were predominantly a N2O sink during the wet season (11 of 16 individual flux estimates) and predominantly a N2O source during the dry season (10 of 16 individual flux estimates).As far as we are aware the wet season sink for N2O in swamp rivers is the first such reported for African rivers.

235
The concentrations of dissolved CH4 and N2O at any specified river location reflect a dynamic and complex balance of in situ production and consumption impacted by import and export mechanisms that include upstream and downstream advection, groundwater inputs, local surface runoff and water-air exchange.
Notwithstanding this complexity, the coexistence of CH4 with dissolved O2 in rivers of the ROC (Fig. 2a which CH4 saturations ranged from ~4000-10000 % (Fig. 2a).These observations seem counterintuitive because the classical view of methanogenesis is that it is exclusively anoxic, carried out by severely O2-limited archaea (Bridgham et al., 2013).However, recent evidence is for a greater complexity of CH4 production in river catchments.For 245 example, methanogenesis in "anoxic microsites" within oxic soils is widely acknowledged (e.g.Teh et al., 2005;von Fisher & Hedin, 2007).Methanogens are now considered to be widespread in oxic soils and they are activated during flooding (Bridgham et al., 2013), their activity relating to soil carbon age and composition (Bridgham et al., 1998;Chanton et al., 2008) and likely involving substrate competition and other interactions.Production by soil macrofauna (Kammann et al., 2009), archeal production related to plant productivity (Updegraff et al., 2001;Dorodnikov et al., 250 2011) and non-microbial, direct aerobic production, both by living plant tissue (Keppler et al., 2006;2009) and in soils (Hurkuck et al., 2012, have all also been observed.Further, methanogenesis by photoautotroph-attached archaea has been detected in oxic lake water (Grossart et al., 2011), analogous to the "anoxic micro-niches" associated with dead and living particles in oxic sea water (de Angelis and Lee, 1994;Oremland, 1979;Ditchfield et al., 2012).
Additional production in oxic seawater may involve biological uptake of organic PO4 3- (Karl et al., 2008) and 255 methylotrophic methanogenesis (Damm et al., 2010), both mechanisms being associated with nutrient stress, but neither has yet been identified in freshwaters.Additional to this variability in production mechanisms and rates, CH4 is subject to variable and rapid aerobic and anaerobic microbial oxidation (Megonigal et al., 2004); CH4 loss rates have been variously estimated at between a few percent and >100% of the rate of methanogenesis (Bussmann, 2013;Shelley et al., 2015).Despite such potentially high losses, water to air exchange by ebullition and by turbulent 260 diffusion driven by wind stress, water depth and flow velocity (Raymond and Cole, 2001) is usually considered the major CH4 loss term, with ebullition frequently considered the dominant of these two mechanisms (Stanley et al., 2016).Despite this complexity of dissolved CH4 cycling in rivers, it is nevertheless informative to speculate on our principal observations in the context of potential CH4 sources and sinks.

265
The first notable feature of our results is the contrasting relationship between CH4 and O2 in swamp and forest rivers (negative) and in savannah rivers (positive) (Fig. 2a).Dissolved O2 in rivers is primarily driven by the balance between photosynthesis and respiration (Houser et al., 2015) but may also be impacted by varying contributions from water-air exchange that under conditions of extreme turbulence may lead to supersaturations as high as 150% (Li et al., 2010).The overall positive relationship between CH4 and O2 in savannah rivers (Fig 2a .)could, at least in part, 270 reflect high macrophyte-related productivity, which can give rise to positive relationships by direct CH4 production (Stanley et al., 2016) and by indirect production via trapping fine-grained organic sediments that support methanogenesis (Sanders et al., 2007).Similar relationships were observed in Amazon floodplain lakes (Devol et al., 1990).Offsetting this, stems and roots respire O2 (Caraco et al., 2006).Further inspection of the data shows that the highest dissolved O2 saturation found in savannah rivers (134%) deviates from the general CH4 vs O2 trend (Fig. 2a).

275
This sample was collected close to an area of rapids in the Congo main stem, in the vicinity of Stanley Pool (Fig. 1) where other samples were also O2 super-saturated.Intense water-air exchange in this region via increased turbulence would tend to enhance dissolved O2 (Li et al., 2010) while depleting dissolved CH4.To summarise, notwithstanding possible additional CH4 losses via oxidation, the CH4 vs O2 relationship in savannah rivers (Fig. 2a) could be explained by net macrophyte production imprinted by water-air gas exchange.The inverse of this relationship for 280 swamp and forest rivers (Fig. 2a), was similarly reported for the Zambezi and Amazon Basins, for the latter in fast flowing waters (Teodoru et al., 2015;Richey et al., 1988;Devol et al., 1990).Again, high gas exchange rates are plausible, especially for the small number of tropical forest samples for which O2 was close to or in excess of 100% (Fig. 2a).For the majority of samples that were O2 under-saturated however, additional mechanisms must be invoked.high CH4, low O2 groundwater but another possibility is that this relationship is the aggregate of this and several of the other processes previously discussed.
A second important aspect of the overall CH4 distributions is that swamp and forest river CH4 was highest during the wet season, whereas savannah samples revealed no such inter-seasonal contrast (Table 2).The constancy of CH4 in 290 savannah rivers might well reflect the buffering of seasonal river discharge by the sandy-sandstone aquifer that underlies this region (Laraque et al., 2001).For swamp and forest rivers a number of alternative but not mutually exclusive possibilities might be invoked.In addition to direct and indirect macrophyte production (Stanley et al., 2016;Sanders et al., 2007), as discussed for savannah rivers, methanogenesis following the activation of archaea during the flooding of adjacent soils (Bridgham et al., 2013) is also plausible, especially given that swamp and forest 295 soils are comparatively poorly drained (Mayaux et al., 2002).In contrast, an opposing behaviour was reported for three rivers of the Ivory Coast (Comoé, Bia, Tanoé).In these, overall decreases in CH4 during the dry to wet season transition (Koné et al., 2010) were similar to trends recorded in some temperate (European) rivers (Middelburg et al. 2002).Koné et al. (2010) ascribed the CH4 seasonality in Ivory Coast rivers to a combination of the dilution of high CH4 baseflow by low CH4 surface runoff (e.g.Jones and Mulholland 1998a, b), higher degassing rates during flooding 300 (Hope et al. 2001) and/or decreased in-stream methanogenesis towards high discharge (de Angelis and Scranton, 1993).Conversely, Bouillon et al. (2012) attributed relatively stable high discharge CH4 concentrations (~100 nmol l -1 ) in the Oubangui, a major tributary of the Congo, to terrestrial soil production in conjunction with baseflow transport.
The largest fractional CH4 contribution from baseflow often occurs in high elevation headwaters with high soil organic content, while progressive downstream increases in CH4 in lowland rivers have been linked to increasing in-305 stream methanogenesis (Jones and Mulholland 1998a).Assuming such processes are also operative in ROC swamp and tropical forest, interpreting or predicting the direction of any seasonal CH4 trend in a specified river system is evidently complex.
Although we found no statistically significant differences in the means and ranges of wet or dry season N2O 320 concentrations for any land cover type, higher N2O concentrations and emissions are considered likely where soilwater filled pore spaces exceed 60 % due to enhanced microbial production (Davidson, 1993), as has been observed in African savanna during the rainy season (Castaldi et al., 2006) and throughout much of the year in humid tropical forests (Castaldi et al., 2013).The discrepancy between these and our observations to some extent likely reflects a complex balance between the principal sites (groundwater and in-stream) and mechanisms of N2O cycling, as 325 evidenced by the variable relationships between N2O, O2 and DIN we observed.For example, we found both positive and negative relationships between N2O and O2 (Fig. 2b).Sediment processes and water concentrations are evidently closely coupled in tropical catchments (Harrison and Matson, 2003)  between N2O and O2 in swamp rivers coincident with strong under-saturation of both N2O and O2 (Fig. 2b) is consistent with N2O consumption by sediment denitrification.Although positive relationships between N2O and 330 NO3 − have been variously interpreted to reflect nitrification (Silvennoinen et al., 2008;Beaulieu et al., 2010), or both denitrification and nitrification (Baulch et al., 2011), for swamp rivers the stronger correlation between N2O and NO3 - than between N2O and NH4 + , which has previously been taken to indicate a sediment N2O source from denitrification (Dong et al., 2004), supports our conclusion of a swamp river denitrification sink for N2O.Similar N2O vs O2 relationships were identified in the Amazon and Zambezi river basins (Richey et al., 1988;Teodoru et al., 2015) and 335 in the Adyar river-estuary, S.E.India (Nirmal Rajkumar et al., 2008).In both the Amazon and the Adyar, N2O was undetectable in fully anoxic waters (Richey et al., 1998;Nirmal Rajkumar et al., 2008).N2O and NO3 -were also correlated in the Oubangui (Bouillon et al., 2012) and a similar, persistent correlation in a temperate river was ascribed to denitrification in hypoxic/anoxic sediment, favoured by the ambient low river flow and high temperatures leading to high community respiration and low O2 solubility (Rosamond et al., 2012).Even though denitrification in 340 rivers may be limited by low levels of NO3 -(Garcia-Ruiz et al., 1998) a temperate creek nevertheless was a N2O sink for combined NO2 -and NO3 -concentrations < 2.7 μmol l -1 (Baulch et al., 2011), broadly similar to the majority of NO3 -concentrations we observed (Fig. 3a).By contrast, N2O and NO3 -were uncorrelated in the Zambezi, for which there was also no correlation of N2O with NH4 + (Teodoru et al., 2015).For savannah rivers, in which N2O was always super-saturated (Fig. 2b), a negative correlation between N2O and O2 may indicate N2O production mainly by 345 nitrification, a conclusion supported by the corresponding stronger correlation between N2O and NH4 + than between N2O and NO3 -, the opposite to what we found for swamp rivers.Although published measurements of N2O production via in-stream nitrification are lacking, nitrification rates may frequently exceed denitrification rates in streams and rivers (Richardson et al., 2004;Arango et al., 2008) and nitrification rates are estimated to exceed denitrification rates two-fold globally (Mosier et al., 1998).In addition to O2 and DIN amount and speciation, pH and dissolved organic 350 carbon are important in controlling net N2O production via nitrification and denitrification (Baulch et al., 2011) and it has been suggested that due to variable N2O yields from these processes, simple diagnostic relationships for N2O production in rivers may prove elusive (Beaulieu et al., 2008).
To conclude, while our data have allowed us to draw some conclusions regarding the production and cycling of CH4 355 and N2O in contrasting rivers of the ROC, we consider these to be more robust for N2O given that its aquatic sources are the least diverse.However, for both gases an unequivocal identification of the primary controls of their riverine distributions would require additional detailed measurements.
The overall ranges of CH4 and N2O emissions from rivers of the ROC (33-48705 mol CH4 m -2 d -1 ; 1-67 mol N2O m -2 d -1 ) are somewhat wider than these earlier estimates for African and temperate rivers, the maximum values (Table 375 3) being around twice as high as previously reported.Nevertheless, it should be acknowledged that the use of "basinwide" values for kw is a necessity that takes no account of spatial and temporal kw variability, that our emissions based on kw derived from Raymond et al. (2013) are 30% higher than those derived from Aufdenkampe et al. (2011) and that other available kw parameterizations show five-fold variability (Barnes and Upstill-Goddard, 2011).Additionally, we did not measure CH4 ebullition fluxes.Borges et al (2015b) report an average 20% ebullition contribution to total 380 CH4 emissions from the Congo and Zambezi, although their maximum estimates are considerably higher than this, and for some other tropical rivers and lakes ebullition is thought to account for 30-98% of total CH4 emissions (Melack et al., 2004;Bastkviken et al., 2010;Sawakuchi et al., 2014).The uncertainties related to kw notwithstanding, our emissions estimates for CH4 at least, are therefore probably conservative.

385
Our data from the ROC support the growing consensus that river systems in Africa may be disproportionately large contributors to the global freshwater sources of tropospheric CH4 and N2O, as they are for CO2, although the potential for significant sinks lends a note of caution for N2O.Nevertheless, the wide ranges of emissions estimates for CH4 and N2O now available for African rivers clearly illustrate the difficulty in deriving representative total emissions given both the comparatively small size of the available data set and the various approaches that are typically used to 390 derive these emissions.This applies, not only to African rivers but to tropical rivers in general and indeed to freshwaters globally.At least equally important is an insufficiently mature understanding of the processes that link emissions to the environmental controls of process rates and their temporal variability, and to river catchment characteristics that include sources and seasonality of organic inputs and variability in the balance between baseflow and surface runoff.Our understanding of these interactions must improve if the system responses to future climate 395 and land use changes are to be predicted and planned for.Lastly, the measurement of CH4 and N2O, data calibration and the emissions estimates deriving would all benefit from agreed, standardized protocols.This is an issue that is yet to be adequately addressed, not only for freshwaters but for aquatic systems more generally.Table 1.Relevant physical characteristics of the rivers studied in this work.
) initially 240 seems enigmatic.While dissolved O2 was under-saturated in the majority of samples, being as low as 4% in one wet season swamp sample, it was always detectable and indeed was super-saturated in several savannah river samples in Biogeosciences Discuss., doi:10.5194/bg-2016-404,2016 Manuscript under review for journal Biogeosciences Published: 11 November 2016 c Author(s) 2016.CC-BY 3.0 License.
One possibility is that these distributions largely reflect the mixing of relatively well-oxygenated river waters with 285 Biogeosciences Discuss., doi:10.5194/bg-2016-404,2016 Manuscript under review for journal Biogeosciences Published: 11 November 2016 c Author(s) 2016.CC-BY 3.0 License.

Figure 1 .
Figure 1.Locations of river sampling stations in the Republic of Congo.

Figure 2 .
Figure 2. (a) Log percent dissolved methane saturation vs percent dissolved O2 saturation and (b) percent dissolved 655

Figure 3 .
Figure 3. (a) Dissolved nitrate vs dissolved ammonium and (b) percent nitrous oxide saturation vs DIN (NO3 -+ NH4 + ) for rivers of the Republic of Congo during the wet season: open circles, savannah rivers; filled black circles, swamp forest rivers; filled grey circles, tropical forest rivers.660 Figure 1.

Table 3 .
Emissions of CH4 and N2O to air from rivers in the Republic of Congo.Negative values in parentheses indicate uptake from the atmosphere.Emissions of CH4 and N2O published for African rivers: (A) refers to emissions estimated using the relationship of Aufdenkampe et al (2011) and (B) refers to emissions estimated using the relationship ofRaymond et al. (2013).

Table 2 .
Dissolved CH4 and N2O in rivers of the Republic of Congo.

Table 3 .
Emissions of CH4 and N2O to air from rivers of the Republic of Congo.Negative values in parentheses indicate uptake from the atmosphere.

Table 4 .
Emissions of CH4 and N2O published for African rivers:

Table 1 .
Relevant physical characteristics of rivers studied in this work.All rainfall data are from Laraque et al. (2001), Djoue catchment area and discharge data are from Laraque et al. (1994) and all other data are from Laraque et al. (2009).Biogeosciences Discuss., doi:10.5194/bg-2016-404,2016 Manuscript under review for journal Biogeosciences Published: 11 November 2016 c Author(s) 2016.CC-BY 3.0 License.