Dynamics of greenhouse gases (CO<sub>2</sub>, CH<sub>4</sub>, N<sub>2</sub>O) along the Zambezi River and major tributaries, and their importance in the riverine carbon budget

Abstract

Abstract. Spanning over 3000 km in length and with a catchment of approximately 1.4 million km2, the Zambezi River is the fourth largest river in Africa and the largest flowing into the Indian Ocean from the African continent. We present data on greenhouse gas (GHG: carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)) concentrations and fluxes, as well as data that allow for characterization of sources and dynamics of carbon pools collected along the Zambezi River, reservoirs and several of its tributaries during 2012 and 2013 and over two climatic seasons (dry and wet) to constrain the interannual variability, seasonality and spatial heterogeneity along the aquatic continuum. All GHG concentrations showed high spatial variability (coefficient of variation: 1.01 for CO2, 2.65 for CH4 and 0.21 for N2O). Overall, there was no unidirectional pattern along the river stretch (i.e., decrease or increase towards the ocean), as the spatial heterogeneity of GHGs appeared to be determined mainly by the connectivity with floodplains and wetlands as well as the presence of man-made structures (reservoirs) and natural barriers (waterfalls, rapids). Highest CO2 and CH4 concentrations in the main channel were found downstream of extensive floodplains/wetlands. Undersaturated CO2 conditions, in contrast, were characteristic of the surface waters of the two large reservoirs along the Zambezi mainstem. N2O concentrations showed the opposite pattern, being lowest downstream of the floodplains and highest in reservoirs. Among tributaries, highest concentrations of both CO2 and CH4 were measured in the Shire River, whereas low values were characteristic of more turbid systems such as the Luangwa and Mazoe rivers. The interannual variability in the Zambezi River was relatively large for both CO2 and CH4, and significantly higher concentrations (up to 2-fold) were measured during wet seasons compared to the dry season. Interannual variability of N2O was less pronounced, but higher values were generally found during the dry season. Overall, both concentrations and fluxes of CO2 and CH4 were well below the median/average values for tropical rivers, streams and reservoirs reported previously in the literature and used for global extrapolations. A first-order mass balance suggests that carbon (C) transport to the ocean represents the major component (59%) of the budget (largely in the form of dissolved inorganic carbon, DIC), while 38% of the total C yield is annually emitted into the atmosphere, mostly as CO2 (98%), and 3% is removed by sedimentation in reservoirs.

Nitrous oxide and methane in two tropical estuaries in a peat-dominated region of northwestern Borneo

“…Another possible explanation is methane oxidation, which was shown to remove large fractions of CH 4 in some estuaries (de Angelis and Scranton, 1993;Abril et al, 2007). Although CH 4 concentrations tended to be higher during the wet season, a clear seasonal pattern of CH 4 concentrations was not apparent, which is in line with observations at other tropical Teodoru et al, 2014) and subtropical (Musenze et al, 2014) sites.…”

“…The generally positive relationship between CH 4 and pCO 2 has been observed in other tropical aquatic systems (Teodoru et al, 2014;Borges et al, 2015) and is indicative of organic matter decomposition as a source of both gases. The strong response of CH 4 to tidal variations indicates that CH 4 is produced in the sediments and released when the hydrostatic pressure drops during falling tide.…”

Effects of agricultural land use on fluvial carbon dioxide, methane and nitrous oxide concentrations in a large European river, the Meuse (Belgium)

“…Finally, in order to further characterize human-impacted river networks compared to relatively pristine ones, and also to compare temperate and tropical rivers we compared CH 4 and N 2 O in the Meuse with our own data-sets obtained in the Congo and Zambezi rivers (Borges et al, 2015a;Teodoru et al, 2015) (Fig. 12).…”

“…Enhanced nutrient inputs will fuel primary production leading to low CO 2 and high CH 4 concentrations, the latter related to enhanced organic matter delivery to sediments . Other human impacts that affect carbon and nitrogen cycling in river networks that can potentially influence cycling of GHGs are river bank stabilization and floodplain drainage that disrupt the river-wetland connectivity that is important for CO 2 and CH 4 dynamics in rivers (Abril et al, 2014;Teodoru et al, 2015;Borges et al, 2015aBorges et al, , 2015bSieczko et al, 2016). The introduction of invasive animal species such as the zebra mussel (Dreissena polymorpha) in US rivers and lakes (Caraco et al, 1997;Evans et al, 2011) (Hussner, 2012), some with high production and biomass (Hussner, 2009); invasive floating macrophytes such as the water hyacinth (Eichhornia crassipes) have been documented to increase CO 2 and CH 4 levels in tropical rivers (Koné et al, 2009(Koné et al, , 2010, but this remains undocumented in temperate rivers.…”

Nam Theun 2 Reservoir four years after commissioning: significance of drawdown methane emissions and other pathways

“…Degassing downstream the turbines is a wellknown phenomenon linked to pressure change and turbulence once the water has been released (Galy-Lacaux et al, 1997). Though it is not a new pathway, downstream degassing has been neglected in the past decade, even if few studies pointed out (Abril et al, 2005;Kemenes et al, 2007;Maeck et al, 2014;Teodoru et al, 2015) that it could be a significant one. Results from the Petit Saut reservoir study in French Guiana (Abril et al, 2005) have shown that degassing and emissions from the river downstream of the dam, if not the main pathway during the two first years after impoundment, becomes heavily dominant when accounted over the span of the first ten years after impoundment.…”