Abstract
Abstract. Any regular interaction of land and river during flooding affects carbon pools within the terrestrial system, riverine carbon and carbon exported from the system. In the Amazon basin carbon fluxes are considerably influenced by annual flooding, during which terrigenous organic material is imported to the river. The Amazon basin therefore represents an excellent example of a tightly coupled terrestrial–riverine system. The processes of generation, conversion and transport of organic carbon in such a coupled terrigenous–riverine system strongly interact and are climate-sensitive, yet their functioning is rarely considered in Earth system models and their response to climate change is still largely unknown. To quantify regional and global carbon budgets and climate change effects on carbon pools and carbon fluxes, it is important to account for the coupling between the land, the river, the ocean and the atmosphere. We developed the RIVerine Carbon Model (RivCM), which is directly coupled to the well-established dynamic vegetation and hydrology model LPJmL, in order to account for this large-scale coupling. We evaluate RivCM with observational data and show that some of the values are reproduced quite well by the model, while we see large deviations for other variables. This is mainly caused by some simplifications we assumed. Our evaluation shows that it is possible to reproduce large-scale carbon transport across a river system but that this involves large uncertainties. Acknowledging these uncertainties, we estimate the potential changes in riverine carbon by applying RivCM for climate forcing from five climate models and three CO2 emission scenarios (Special Report on Emissions Scenarios, SRES). We find that climate change causes a doubling of riverine organic carbon in the southern and western basin while reducing it by 20 % in the eastern and northern parts. In contrast, the amount of riverine inorganic carbon shows a 2- to 3-fold increase in the entire basin, independent of the SRES scenario. The export of carbon to the atmosphere increases as well, with an average of about 30 %. In contrast, changes in future export of organic carbon to the Atlantic Ocean depend on the SRES scenario and are projected to either decrease by about 8.9 % (SRES A1B) or increase by about 9.1 % (SRES A2). Such changes in the terrigenous–riverine system could have local and regional impacts on the carbon budget of the whole Amazon basin and parts of the Atlantic Ocean. Changes in riverine carbon could lead to a shift in the riverine nutrient supply and pH, while changes in the exported carbon to the ocean lead to changes in the supply of organic material that acts as a food source in the Atlantic. On larger scales the increased outgassing of CO2 could turn the Amazon basin from a sink of carbon to a considerable source. Therefore, we propose that the coupling of terrestrial and riverine carbon budgets should be included in subsequent analysis of the future regional carbon budget.
Highlights
Research on the effects of climate and land use change on terrestrial and riverine systems has been extensively conducted
We want to deepen the understanding of consequences of climate change on riverine carbon fluxes, which are fuelled by vegetation, and on the export of carbon from the terrestrial part to the atmosphere and the ocean
We aimed to develop a coupled terrestrial–riverine model to understand the effects of climate change on carbon fluxes in such a coupled system
Summary
Research on the effects of climate and land use change on terrestrial and riverine systems has been extensively conducted. In this study we focus on the coupled terrestrial–riverine system in the Amazon basin In this region the Amazon River and, in particular, the annually recurring flooding of parts of the forests shape the manifold Amazonian ecosystems. The input of terrigenous organic material affects the riverine system enormously on a local scale (Melack and Forsberg, 2001; Waterloo et al, 2006) It acts, for instance, as fertilizer and food source (Anderson et al, 2011; Horn et al, 2011) and is a modifier of habitats and interacting local carbon cycles (Hedges et al, 2000; Irmler, 1982; Johnson et al, 2006; McClain and Elsenbeer, 2001). The low aquatic productivity in the river system is caused by a high sediment load and high turbidity in white-water rivers and a low nutrient supply in the blackwater rivers (Benner et al, 1995; Richey et al, 1990; Sioli, 1957)
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