CARBon biogeochemistry and atmospheric exchanges in the AMAzon river system – CARBAMA

The Amazon basin is a central player in the global carbon balance because it stores large amounts of carbon in biomass above ground. This stock is currently being modified by a complex combination of processes that affect the carbon turnover and storage in forests and, in turns, the atmospheric CO2. There are still large uncertainties on the exact size of the contemporary net carbon fluxes within the Amazon basin and how deforestation could be compensated by forest regrowth and enhanced productivity associated with global change. Estimates of changes in carbon biomass for the entire Amazon basin could vary from a net loss of 0.5 PgC.y-1 (1015 g.y-1), to a net gain of 0.6 PgC.y-1. Because they connect the atmosphere, the land and the ocean, river systems are a vital link in the processes controlling the storage of atmospheric CO2. Rivers are continuously fed with terrestrial carbon; they act as a carbon source for the ocean and for the atmosphere by the degassing process. It was recently estimated that CO2 degassing by the Amazon River system was about 0.5 PgC.y-1, a number one order of magnitude higher than what was previously thought, when considering the export to the ocean alone. Such a large loss of terrestrial carbon from the Amazonian forest through the aquatic system is difficult to reconcile with studies upland, with atmospheric observations in the region, as well as with other studies elsewhere. In addition, the origin of the carbon that degas in the river is still unknown, and another recent work revealed that it was young carbon, much younger than the old terrestrial organic matter transported in the waters. Indentifying and quantifying this source is a key point in order to reconcile the Amazon inland and aquatic carbon budgets. One major weakness of the different works carried so far in the Amazon River system, is that they are based almost exclusively on sampling in river channels, which are not totally representative of the water surface areas. More than 70% of the water mirror in the central Amazon is constituted of floodplains, which form a network of interconnected shallow lakes that exchange water and suspended material with the deep main stem following the hydrological cycle of rising and falling waters Floodplains are known as very productive ecosystems, were primary producers are phytoplankton, aquatic macrophytes and herbaceous plants in drawdown areas. We recently acquire original data that suggest that primary production in floodplains, first, significantly reduces the CO2 degassing in the floodplain areas, and, second, constitutes an important source of young and biodegradable organic matter that fuels respiration and CO2 degassing in the channel. This hypothesis could also reconcile the difference in the age of organic carbon and CO2 in river channels. The CARBAMA project consists in multidisciplinary work devoted to a fair and precise estimation of the carbon budget of the Amazon River system, with peculiar emphasis on the role of floodplains. We propose first to re-evaluate CO2 degassing at the whole watershed level by combining extensive field observation using original automatic techniques with remote sensing of water color and surface areas; with this strategy, seasonal and spatial variations of CO2 degassing will be adequately integrated. Second, we will characterize and quantify the different sources of organic matter in the system, with a peculiar focus on the significance of material produced by aquatic primary production in the floodplains; a large scale hydrological modelling will allow a quantification of fluxes of young aquatic organic carbon from the floodplain to the river channels. Finally, we will investigate the biogeochemical processes that drive the metabolic status (Net Ecosystem Production) of floodplains, as well as their controlling factors at the ecosystem level; understanding these processes are fundamental for predicting future changes in the carbon budget of the River system; three contrasting study sites that cover the large range of environmental conditions of Amazonian floodplains have been selected for specific and combined hydrological, biological, biogeochemical and modelling studies. The combination of these different approaches will improve our understanding of the function of River systems in the continental carbon cycle and will allow a better estimate of the carbon balance of Amazonia.