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Amazon River carbon dioxide outgassing fuelled by wetlands

Nature volume 505, pages 395–398 (2014)Cite this article

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Abstract

River systems connect the terrestrial biosphere, the atmosphere and the ocean in the global carbon cycle1. A recent estimate suggests that up to 3 petagrams of carbon per year could be emitted as carbon dioxide (CO2) from global inland waters, offsetting the carbon uptake by terrestrial ecosystems2. It is generally assumed that inland waters emit carbon that has been previously fixed upstream by land plant photosynthesis, then transferred to soils, and subsequently transported downstream in run-off. But at the scale of entire drainage basins, the lateral carbon fluxes carried by small rivers upstream do not account for all of the CO2 emitted from inundated areas downstream3,4. Three-quarters of the world’s flooded land consists of temporary wetlands5, but the contribution of these productive ecosystems6 to the inland water carbon budget has been largely overlooked. Here we show that wetlands pump large amounts of atmospheric CO2 into river waters in the floodplains of the central Amazon. Flooded forests and floating vegetation export large amounts of carbon to river waters and the dissolved CO2 can be transported dozens to hundreds of kilometres downstream before being emitted. We estimate that Amazonian wetlands export half of their gross primary production to river waters as dissolved CO2 and organic carbon, compared with only a few per cent of gross primary production exported in upland (not flooded) ecosystems1,7. Moreover, we suggest that wetland carbon export is potentially large enough to account for at least the 0.21 petagrams of carbon emitted per year as CO2 from the central Amazon River and its floodplains8. Global carbon budgets should explicitly address temporary or vegetated flooded areas, because these ecosystems combine high aerial primary production with large, fast carbon export, potentially supporting a substantial fraction of CO2 evasion from inland waters.

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Figure 1: Study area.
Figure 2: CO2 advection from flooded forest to open waters.
Figure 3: Vegetation-mediated control of CO2 outgassing in floodplain lakes.

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Acknowledgements

This research is a contribution to the CARBAMA project, funded by the French National Agency for Research (grant number 08-BLAN-0221), the French INSU national programme EC2CO, and the National Council of Research and Development (CNPq), Brazil (Universal Program number 477655/2010-6). It was conducted under the auspices of the Environmental Research Observatory Hydrology and Geochemistry of the Amazon Basin (HYBAM), supported by the INSU and the IRD (Institute for Research and Development, France). F.R. was supported by CNPq and a Brazilian ‘Excellent Researcher’ fellowship. We thank all the participants of the CARBAMA cruises.

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Authors and Affiliations

  1. Laboratoire Environnements et Paléoenvironnements Océaniques et Continentaux (EPOC), CNRS, Université Bordeaux 1, Avenue des Facultés, 33405 Talence, France,

    Gwenaël Abril, Nicolas Savoye & Jonathan Deborde

  2. Laboratoire Géosciences et Environnement de Toulouse, Institut de Recherche pour le Développement, Université Paul Sabatier, 14 avenue Edouard Belin, 31400 Toulouse, France,

    Gwenaël Abril, Jean-Michel Martinez & Patricia Moreira-Turcq

  3. Laboratoire d’Océanologie et Géosciences, CNRS, Université du Littoral Côte d’Opale, 32 avenue Foch, 62930 Wimereux, France,

    L. Felipe Artigas

  4. Equipe Géochimie des Eaux, Institut de Physique du Globe de Paris, Université Paris Diderot, Sorbonne Paris Cité, 35 rue Hélène Brion, 75205 Paris Cedex 13, France,

    Marc F. Benedetti

  5. Departamento de Biologia, Laboratório de Ecologia Aquática, Universidade Federal de Juiz de Fora, Rua José Lourenço Kelmer, MG 36036-900 Juiz de Fora, Brazil,

    Luciana Vidal & Fabio Roland

  6. Laboratoire Biologie des Organismes et Ecosystèmes Aquatiques (BOREA), Muséum National d’Histoire Naturelle, CNRS, IRD, UPMC, 61 rue Buffon, 75005, Paris, France,

    Tarik Meziane

  7. Department of Marine Organic Biogeochemistry, NIOZ (Royal Netherlands Institute for Sea Research), Texel, 1790 AB Den Burg, The Netherlands,

    Jung-Hyun Kim

  8. Programa de Geoquímica, Universidade Federal Fluminense, Outeiro São João Batista, RJ 24020015 Niterói, Brazil,

    Marcelo C. Bernardes

  9. Instituto de Geociências, Universidade de Brasília, Campus Universitário Darcy Ribeiro, DF 70.910-900 Brasília, Brazil,

    Edivaldo Lima Souza

  10. Institut des Sciences de la Terre d’Orléans, 1A rue de la Férollerie, 45071 Orléans Cedex 2, France,

    Patrick Albéric

  11. Laboratório de Oceanografia Química, Universidade Estadual de Santa Cruz, Rodovia Ilhéus-Itabuna, 45662-900 Ilhéus, Bahia, Brazil,

    Marcelo F. Landim de Souza

Authors
  1. Gwenaël Abril
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Contributions

G.A., J.-M.M., P.M.-T., L.F.A., T.M. and M.F.B. conceived and designed the study. G.A. coordinated project and fieldwork. G.A., J.D., M.F.L.d.S. and N.S. performed the p co 2 measurements. J.-M.M. and E.L.S. analysed the remote sensing data. L.F.A. measured Chl a and fluorescence. L.V. and F.R. measured respiration. All authors contributed to the interpretation of the data. G.A. wrote the manuscript, J.-M.M., L.F.A. and F.R. contributed to manuscript writing and P.M.-T., L.V., T.M., J.-H.K., M.C.B., N.S. and M.F.B. commented on the manuscript.

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Correspondence to Gwenaël Abril.

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Extended data figures and tables

Extended Data Figure 1 Instrumental set-up for continuous measurement of p co 2 and ancillary parameters while underway in the Amazon River and floodplain lakes.

Side-view diagrams of both boats are illustrated with photos of the equipment. See detailed description in the Methods.

Extended Data Figure 2 Continuous record of water p co 2 in the Amazon River, tributaries, and floodplain lakes during the high water (June 2009).

a, Track of the ship in the main stem (brown), floodplain lakes (green), and major tributaries (blue). Land occupation derived from SAR data is shown as flooded forest (light grey), temporary open waters (dark grey) and permanent open waters (black). b, Conductivity values show that water in the floodplain lakes primarily originates from the flooding of the Solimões and Amazon rivers with modest contribution from local drainage. c, The distribution of water at maximum flooding shows the predominance of supersaturation with a net decrease downstream, in parallel with the extent of vegetation in the floodplains (percentage of total floodplain area is given in parentheses for each lake).

Extended Data Figure 3 Continuous record of water p co 2 in the Amazon River, its tributaries and floodplain lakes during the low water of October 2009.

a, Track of the ship in the main stem (brown), floodplain lakes (green), and major tributaries (blue). Land occupation derived from SAR data is shown as flooded forest (light grey), temporary open waters (dark grey) and permanent open waters (black). b, Conductivity values show that water in the floodplain lakes primarily originates from the flooding of the Solimões and Amazon rivers with modest contribution from local drainage. c, The distribution of water shows large contrasts between channel and floodplains, with a significant decrease downstream (as during the high water), in parallel with the extent of vegetation in the floodplains (percentage of total floodplain area is given in parentheses for each lake). Undersaturation in occurs at low water in dense phytoplankton blooms in almost isolated lakes.

Extended Data Figure 4 Conceptual diagram for carbon dioxide outgassing fuelled by Amazonian wetlands.

In flooded forests, aerial gross primary production absorbs CO2 from the atmosphere and sequesters part of this carbon in wood. Most of the sequestration in wood occurs during the terrestrial phase and is supposed to be balanced by natural tree mortality associated with channel migration. Leaves and wood also respire CO2 back to the atmosphere. Litter falls from flooded trees primarily during flooding and constitutes a significant organic carbon input to the water. Floating plants in the Amazon grow above the water level, where they perform aerial photosynthesis, and as the water level progressively rises, their biomass is recycled and decomposes underwater. Because no significant burial of macrophyte material is observed in sediment, it is assumed that all their annual net primary production (NPP) is transferred to water as organic carbon (litter fall). Below water, the respiration of roots of flooded trees and floating macrophytes releases CO2 to the water. With the establishment of anoxic conditions in forest soils, tree metabolism deviates to an anaerobic pathway that generates fermentation products, which are exuded from the roots into the surrounding water. Carbon flux between the Amazonian wetlands and rivers thus occurs through two distinct pathways. CO2 export from the wetlands is derived from root and sediment respiration within the wetlands, whereas organic carbon export from the wetlands is derived from litter fall and from fermentative products released by roots. Quantitative information is missing for the latter exudation flux. In rivers and floodplains, water movement is fast enough relative to gas exchange to generate a lateral CO2 flux with the water mass, and this flux should be taken into account in the interpretation of the spatial and temporal patterns of CO2 outgassing. In water and sediments of the entire aquatic system, microbial heterotrophic respiration continuously converts organic carbon to CO2. In open lakes, phytoplankton uses CO2 dissolved in water (that is, primarily derived from the surrounding wetland vegetation) and infrequently uses atmospheric CO2 because the lakes were rarely net CO2 sinks on a daily basis. The phytoplankton biomass produced in open lakes constitutes an additional source of biodegradable organic carbon. Both C3 and C4 plants are well represented in the wetland. Isotopic and molecular tracers may distinguish woody from non-woody material. However, it is difficult to differentiate woody material from the flooded forest and woody material from the non-inundated forest, particularly as many species are common to both. More detailed discussion and references can be found in the Supplementary Information.

Extended Data Figure 5 Modelling how far dissolved CO2 is transported before being outgassed.

a, We assessed the potential for lateral CO2 transport in rivers and floodplains using a simple one-dimensional model that simultaneously calculates the CO2 lost by outgassing and the CO2 that remains dissolved in water and is transported downstream by the currents. The model starts from a point source in the wetland (set here at 12,000 p.p.m.v., which is a typical value observed in the vicinity of a flooded forest; Fig. 2b). The iteration time was 1 min. In the model, F(CO2) is calculated from , using representative values of k600. The quantity of CO2 lost to the atmosphere during one iteration is subtracted from the initial CO2 quantity present in a column of water of a determined depth H. Note that this procedure is adequate only for acidic, non-buffered waters, such as those in the Amazon. b, c, When integrated (Supplementary Information), the equation gives a one-phase exponential decay function of versus the distance x, the water current velocity w, the normalized gas transfer velocity k600, and the water depth H. The curves give the potential extent of saturation that can be maintained without the necessity of aquatic respiration (Fig. 2b). D½ is the half-evasion distance, which is the theoretical distance the water mass travels before outgassing half of its initial excess CO2. T½ is the associated half-evasion time. d, Typical half-evasion distances of wetland CO2 in river–floodplain systems vary from less than 1 km in a shallow, stagnant, wind- and heat-protected lake to more than 300 km in a deep and fast-flowing river. This illustrates, on the one hand, how far wetland CO2 can be exported downstream, and on the other hand, the large heterogeneity of the transport and outgassing processes in the river–floodplain complex.

Extended Data Table 1 Surface area of flooded land and aquatic biomes in the Central Amazon and study sites
Full size table
Extended Data Table 2 Water p co 2 , conductivity and Chl a in the river and floodplain lakes of quadrant c
Full size table
Extended Data Table 3 Tentative revised carbon budget for the central Amazon River and wetlands
Full size table

Supplementary information

Supplementary Information

This file contains Supplementary Text 1-4 and additional references. (PDF 409 kb)

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Abril, G., Martinez, JM., Artigas, L. et al. Amazon River carbon dioxide outgassing fuelled by wetlands. Nature 505, 395–398 (2014). https://doi.org/10.1038/nature12797

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