Carbon dioxide emissions to the atmosphere from inland waters-streams, rivers, lakes and reservoirs-are nearly equivalent to ocean and land sinks globally. Inland waters can be an important source of methane and nitrous oxide emissions as well, but emissions are poorly quantified, especially in Africa. Here we report dissolved carbon dioxide, methane and nitrous oxide concentrations from 12 rivers in sub-Saharan Africa, including seasonally resolved sampling at 39 sites, acquired between 2006 and 2014. Fluxes were calculated from published gas transfer velocities, and upscaled to the area of all sub-Saharan African rivers using available spatial data sets. Carbon dioxide-equivalent emissions from river channels alone were about 0.4 Pg carbon per year, equivalent to two-thirds of the overall net carbon land sink previously reported for Africa. Including emissions from wetlands of the Congo river increases the total carbon dioxide-equivalent greenhouse-gas emissions to about 0.9 Pg carbon per year, equivalent to about one quarter of the global ocean and terrestrial combined carbon sink. Riverine carbon dioxide and methane emissions increase with wetland extent and upland biomass. We therefore suggest that future changes in wetland and upland cover could strongly a ect greenhouse-gas emissions from African inland waters.C limate predictions necessitate a full and robust account of natural and anthropogenic greenhouse-gas (GHG) fluxes, especially for CO 2 (refs 1-3), CH 4 (ref. 4) and N 2 O (ref. 5), which together accounted for 94% of the anthropogenic global radiative forcing by well-mixed GHGs in 2011 relative to 1750 (ref. 6). Inland waters (streams, rivers, lakes and reservoirs) are increasingly recognized as important sources of GHGs to the atmosphere, with global CO 2 and CH 4 emissions estimated at 2.1 PgC yr −1 (ref.3) and 0.7 PgC yr −1 (CO 2 -equivalents; CO 2 e) (ref. 4) (1 Pg = 10 15 g), respectively. Considering that the oceanic and land carbon (C) sinks correspond to ∼1.5 and ∼2.0 PgC yr −1 (ref. 7), respectively, the GHG flux from inland waters is significant in the global C budget.In a recent global compilation of inland CO 2 data 3 , <20 data points (out of 6,708, that is, <0.3%) represented African inland waters (with the exception of South Africa, which has been densely sampled), even though they account for ∼12% of both global freshwater discharge 8 and riverine surface area 3 , and include some of the largest rivers and lakes in the world. Equally for the global CH 4 database, there is a strong under-representation of tropical inland waters, whereby a recent synthesis 4 resorted to extrapolating CH 4 fluxes from temperate rivers.The prevailing large uncertainty involved in GHG flux estimates for inland waters, essentially due to the paucity of available data, is coupled to a poor understanding of underlying processes, both of which preclude gauging of future fluxes in response to human pressures. In particular, there is a need to further understand the link between inland water GHG fluxes and ...

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River damming reduces the ecological heterogeneity of ecosystems, with a resultant shift of communities adapted to lotic conditions to those better adapted to the newly created lentic environment. Whereas the response of phytoplankton and zooplankton community composition and structure to river damming has been extensively studied in individual reservoirs, this study assessed three cascading reservoirs on the Tana River, Kenya. A total of five sampling campaigns were carried out in 2011, 2012 and 2013 for different sampling sites within each of the reservoirs, as well as upstream and downstream of the reservoirs. Plankton communities within each of the three reservoirs, and upstream of the reservoirs, were compared, indicating (i) the disappearance of 13 phytoplankton and 2 zooplankton taxa; (ii) the appearance (invasion) by 26 phytoplankton and 33 zooplankton taxa; and (iii) an opportunistic presence of 7 phytoplankton and 6 zooplankton taxa. Thirty-two genera of potentially harmful algae were also encountered, suggesting a potential future threat of harmful algal blooms. A significant reduction in phytoplankton diversity and taxa evenness was also observed, as well as increases in overall abundance in the reservoirs, compared to the river upstream of the study reservoirs. However, these ecological indices were restored to the original levels observed upstream of the study reservoirs at sites hundreds of kilometres downstream of the reservoirs.

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The existing reservoirs on the River Tana (Kenya) were mainly constructed for hydropower generation, with inadequate consideration of the long-term impacts on downstream livelihoods. We investigated the impacts of the reservoirs on people's livelihoods downstream. The results showed a few positive impacts in the vicinity of the reservoirs and numerous negative impacts downstream (i.e. reduced flood-recess agriculture and floodplain pastoralism, and escalating resource-use conflicts). Inadequate stakeholders' consultation during reservoir development was also observed. We recommend a detailed basin-wide socioeconomic assessment for future reservoir developments and controlled flood release to simulate the natural flow regime, thereby restoring indigenous flood-based livelihoods while retaining sufficient reserves for power generation.

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