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Scale Issue for Organic and Inorganic Carbon Exports to Oceans: Case Study in the Sub-Tropical Thukela River Basin, South Africa

Author

Listed:
  • Macdex Mutema

    (Agricultural Research Council-Agricultural Engineering, Private Bag X519, Silverton, Pretoria 0127, South Africa)

  • Sandiswa Figlan

    (Department of Agriculture and Animal Health, University of South Africa, Private Bag X6, Florida 1710, South Africa)

  • Vincent Chaplot

    (Laboratoire d’Océanographie et du Climat, Expérimentations et Approches Numériques, Institut de Recherche pour le Développement (IRD), UMR 7159, IRD-CNRS-UPMC-MNHN, 4 Place Jussieu, CEDEX 05, 75252 Paris, France
    School of Agricultural, Earth and Environmental Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209, South Africa)

Abstract

Despite carbon (C) exports from continents being crucial in the connection between terrestrial, atmospheric, and oceanic C, there is still limited understanding of the dynamics of C within river basins. The objective of this work was to assess the changes in particulate (POC) and dissolved organic (DOC) and inorganic C (PIC: particulate inorganic carbon, DIC: dissolved inorganic carbon) content, quality, and fluxes within a river basin from its headwaters to its exit at the ocean. A survey was designed in the Thukela basin (from 2012 to 2013 and at six nested catchments ranging from ~10 to ~30,000 km 2 ) in the east of South Africa to evaluate the content, fluxes, and quality (UV spectral slope, 13 C, CO 2 effluxes from runoff) of the transported C in conjunction with chemical elements (Si, Na) for discriminating between the water sources and estimating C dynamics during low flows. Total carbon exports decreased continuously from 9.75 km 2 in the headwater (31.9 kg C km −2 y −1 ) to ocean (4.7 kg C km −2 y −1 ) with the highest decrease occurring between the catchment (7614 km 2 ) and large catchment (14,478 km 2 ). About 80% of C exports from the headwaters were POC, followed by DIC (10%) and DOC (10%), while at the ocean, the proportions were 31% (POC), 45% (DIC), 23% (DOC), and 0.7% (PIC). Moreover, there was a sharp decrease in the dissolved organic matter aromaticity from the headwater to ocean and for both DOC and POC that did not correspond to changes in water sources along the river (as indicated by a relatively constant Si/Na ratio). This pointed to the decomposition in the river of the dissolved organic matter originating from soils and to the within-stream organic production. Further in situ investigations need to be performed to quantify the within-stream inputs.

Suggested Citation

  • Macdex Mutema & Sandiswa Figlan & Vincent Chaplot, 2023. "Scale Issue for Organic and Inorganic Carbon Exports to Oceans: Case Study in the Sub-Tropical Thukela River Basin, South Africa," Land, MDPI, vol. 12(4), pages 1-10, April.
    • Handle: RePEc:gam:jlands:v:12:y:2023:i:4:p:815-:d:1115057
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    References listed on IDEAS

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    1. Emilio Mayorga & Anthony K. Aufdenkampe & Caroline A. Masiello & Alex V. Krusche & John I. Hedges & Paul D. Quay & Jeffrey E. Richey & Thomas A. Brown, 2005. "Young organic matter as a source of carbon dioxide outgassing from Amazonian rivers," Nature, Nature, vol. 436(7050), pages 538-541, July.
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