Spatial variability and temporal dynamics of greenhouse gas (CO 2 , CH 4 , N 2 O) concentrations and fluxes along the Zambezi River mainstem and major tributaries

© Author(s) 2014. Spanning over 3000 km in length and with a catchment of approximately 1.4 million km 2 , the Zambezi River is the fourth largest river in Africa and the largest flowing into the Indian Ocean from the African continent. As part of a∼broader study on the riverine biogeochemistry in the Zambezi River basin, we present data on greenhouse gas (GHG, carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O)) concentrations and fluxes 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 GHGs concentrations showed high spatial variability (coefficient of variation: 1.01 for CO 2 , 2.65 for CH 4 and 0.21 for N 2 O). 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, and the presence of man-made structures (reservoirs) and natural barriers (waterfalls, rapids). Highest CO 2 and CH 4 concentrations in the mainstream river were found downstream of extensive floodplains/wetlands. Undersaturated CO 2 conditions, in contrast, were characteristic for the surface waters of the two large reservoirs along the Zambezi mainstem. N 2 O concentrations showed the opposite pattern, being lowest downstream of floodplains and highest in reservoirs. Among tributaries, highest concentrations of both CO 2 and CH 4 were measured in the Shire River whereas low values were characteristic for more turbid systems such as the Luangwa and Mazoe rivers. The interannual variability in the Zambezi River was relatively large for both CO 2 and CH 4 , and significantly higher concentrations (up to two fold) were measured during wet seasons compared to the dry season. Interannual variability of N 2 O was less pronounced but generally higher values were found during the dry season. Overall, both concentrations and fluxes of CO 2 and CH 4 were well below the median/average values reported for tropical rivers, streams and reservoirs. 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 DIC), while only 38% of total C yield is annually emitted into the atmosphere, mostly as CO 2 (98%), and 3% is removed by sedimentation in reservoirs.

Publication year:2014