Dynamics of greenhouse gases (CO2, CH4, N2O) along the Zambezi River and major tributaries, and their importance in the riverine carbon budget

[摘要] Spanning over 3000 km in length and with a catchment ofapproximately 1.4 million km², the Zambezi River is the fourth largestriver in Africa and the largest flowing into the Indian Ocean from theAfrican continent. We present data on greenhouse gas (GHG: carbon dioxide(CO₂), methane (CH₄), and nitrous oxide (N₂O)) concentrationsand fluxes, as well as data that allow for characterization of sources and dynamicsof carbon pools collected along the Zambezi River, reservoirs and several ofits tributaries during 2012 and 2013 and over two climatic seasons (dry andwet) to constrain the interannual variability, seasonality and spatialheterogeneity along the aquatic continuum. All GHG concentrations showedhigh spatial variability (coefficient of variation: 1.01 for CO₂, 2.65for CH₄ and 0.21 for N₂O). Overall, there was no unidirectionalpattern along the river stretch (i.e., decrease or increase towards theocean), as the spatial heterogeneity of GHGs appeared to be determinedmainly by the connectivity with floodplains and wetlands as well as the presenceof man-made structures (reservoirs) and natural barriers (waterfalls,rapids). Highest CO₂ and CH₄ concentrations in the main channelwere found downstream of extensive floodplains/wetlands. UndersaturatedCO₂ conditions, in contrast, were characteristic of the surface watersof the two large reservoirs along the Zambezi mainstem. N₂Oconcentrations showed the opposite pattern, being lowest downstream of thefloodplains and highest in reservoirs. Among tributaries, highestconcentrations of both CO₂ and CH₄ were measured in the ShireRiver, whereas low values were characteristic of more turbid systems such asthe Luangwa and Mazoe rivers. The interannual variability in the ZambeziRiver was relatively large for both CO₂ and CH₄, andsignificantly higher concentrations (up to 2-fold) were measured duringwet seasons compared to the dry season. Interannual variability of N₂Owas less pronounced, but higher values were generally found during the dryseason. Overall, both concentrations and fluxes of CO₂ and CH₄were well below the median/average values for tropical rivers, streams andreservoirs reported previously in the literature and used for globalextrapolations. 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% ofthe total C yield is annually emitted into the atmosphere, mostly asCO₂ (98%), and 3% is removed by sedimentation in reservoirs.

[学科分类] 地球化学与岩石