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African river basins: catchment-scale carbon fluxes and transformations

Final Report Summary - AFRIVAL (African river basins: catchment-scale carbon fluxes and transformations.)

While the importance of the oceans and the land biosphere in the global carbon (C) budget is well acknowledged, freshwater systems have received much less attention so far. The amount of terrestrial C processed in rivers, lakes and reservoirs is, however, thought to be similar or even higher than the global net terrestrial ecosystem production. The interface between terrestrial and aquatic ecosystems often functions as a biogeochemical ‘hotspot’, where hydrological flowpaths converge with substrates or other flowpaths containing complementary or missing reactants, leading to increased biogeochemical process rates. Data on tropical rivers are relatively scarce, despite their disproportionate importance in terms of, for example, riverine C transport. AFRIVAL addresses a number of key research priorities within the framework of quantifying and understanding the biogeochemical functioning of tropical aquatic ecosystems, and their linkage with the terrestrial environment in the catchment. The final output of this proposal is expected to form an important contribution to our understanding of fundamental biogeochemical processes and terrestrial-aquatic interactions. The main activities within AFRIVAL deal with the following research questions: (i) what is the impact of catchment land-use and vegetation patterns on the inputs, origin and biogeochemical processing of organic matter in aquatic systems? How dynamic is the composition of dissolved and particulate organic C in river systems – to which extent are the pool sizes, origin and age modified by biogeochemical processing during their transit from the terrestrial biome towards the coastal zone? (ii) How much of the lateral (terrestrial) C inputs in river networks is mineralized and outgassed as CO2 before it reaches the coastal zone? What is the metabolic balance and CO2 source/sink strength of tropical rivers, and its relationship to lateral inputs from the terrestrial biome? Which particular sites (headwaters, reservoirs, lowland rivers) are the primary hotspots for CO2 evasion? (iii) Which factors drive the degree of transport versus processing in river networks? Can general patterns be found from a combination of proxies for particle origin and residence time on the one hand, and organic proxies on the other hand?
To address these questions, we have combined extensive fieldwork throughout specific river basins (from headwater streams to the freshwater part of the estuary where possible), with setting up monitoring sites in collaboration with local scientists/institutions whereby field measurements and samples are taken at regular intervals (e.g. twice per month). Extensive field sampling has been performed in the Congo basin (DRC, Rwanda, Central African Republic), Zambezi basin (Zambia, Mozambique), Tana and Sabaki basins (Kenya), and Rianila and Betsiboka basins (Madagascar). Monitoring sites have been operational on the Niger River (Niamey, Niger), Oubangui River (Bangui, Central African Republic), Tana River (Kenya, various sites), Sabaki River (Malindi, Kenya), Ogooué River (Lambaréné, Gabon), Congo River (Kinshasa and Kisangani, DRC), and the Zambezi River (close to Lusaka, Zambia) and its main tributary, the Kafue River.
A wide range of physico-chemical and biogeochemical parameters are employed. These include quantification and characterisation of different organic and inorganic C pools, nutrient and major element concentrations, greenhouse gas exchange, primary production and aquatic respiration measurements, and –for monitoring sites where hydrological data are available - riverine transport flux measurements. Our dataset comprises >2500 site/date combinations for this broad suite of parameters, with an additional dataset of ~23,000 continuous measurements of selected parameters (%O2, pCO2, …) during 2 cruises on the Congo River. This represents by far the greatest effort to date to use state-of-the-art techniques in describing and understanding the biogeochemical functioning of tropical river networks, and multiplies by a large factor the knowledge on African river systems. Key findings of the AFRIVAL project include (i) the critical importance of residence time/seasonality in the degree of inorganic nutrient processing in semi-arid rivers, (ii) the quantitatively significant emissions of GHGs, in particular CO2 and CH4 from African inland waters at the global scale, (iii) the critical role played by reservoirs and floodplains in regulating the downstream variations in riverine biogeochemical proxies, (iv) the importance of riparian zones as the dominant source of organic matter to rivers, at least in semi-arid catchments, (v) emissions of CO2 to the atmosphere appears to be sustained only to a limited degree by in situ mineralization, hence lateral CO2 inputs (groundwater and floodwater exchange) are primary pathways, (vi) the importance of various catchment characteristics as key factors explaining the variation in the continuum of biogeochemical characteristics of rivers, suggesting that coupling with spatial datasets on physical and biotic catchment properties may strongly contribute to efforts at upscaling process or flux data to a wider geographical scale, and allowing to predict that changes on the catchment proprieties (climate change, land use, etc…) will strongly impact river functioning in wide spectrum of variables including the emission of GHGs to the atmosphere.