Landscape Control on the Spatial and Temporal Variability of Chromophoric Dissolved Organic Matter and Dissolved Organic Carbon in Large African Rivers
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The characteristics of colored dissolved organic matter (CDOM) as well as the concentrations and stable isotope composition (δ13C) of dissolved organic carbon (DOC) were characterized in several large rivers of Africa including the Congo, Niger, Zambezi, and Ogooué basins. We compared the spatial and temporal patterns of dissolved organic matter (DOM) quantity and quality along with various environmental gradients, including hydrology, river size, catchment vegetation, and connectivity to land. The optical proxies used include the absorption coefficient at 350 nm, the specific ultra-violet absorbance, and the spectral slope ratio (S R = 275–295-nm slope divided by 350–400-nm slope). Our results show that land cover plays a primary role in controlling both DOC concentration and optical properties of DOM in tropical freshwaters. A higher cover of dense forest in the catchment leads to a higher quantity of highly aromatic DOM in the river network, whereas an increasing savannah cover results in lower DOC concentrations and less absorptive DOM. In addition to land cover, the watershed morphology (expressed by the average slope) exerts a strong control on DOC and CDOM in tropical rivers. Our results also show that the percentage of C3 and C4 vegetation cover is not an accurate predictor for DOM and CDOM quality in rivers due to the importance of the spatial distribution of land cover within the drainage network. The comparison of our results with previously published CDOM data in temperate and high-latitude rivers highlights that DOM in tropical freshwaters is generally more aromatic, and shows a higher capacity for absorbing sunlight irradiance.
KeywordsCarbon cycle Colored dissolved organic matter Tropical rivers Land cover Landscape Carbon isotopes
The dataset used in this study was collected in the framework of projects funded by the European Research Council (ERC-StG 240002, AFRIVAL—African river basins: Catchment-scale carbon fluxes and transformations), the Fonds National de la Recherche Scientifique (FRNS, Transcongo, 14711103), the Research Foundation Flanders (FWO-Vlaanderen), and the Belgian Federal Science Policy (BELSPO-SSD project COBAFISH). We thank C. Lancelot for access to the Perkin-Elmer UV/Vis 650S, T. Mambo Baba, and E. Tambwe Lukosha (Université de Kisangani, DRC) for collection of Congo at Kinsagani and Tshopo time-series data collection, Y. Yamashita and P. Mann for providing their datasets from the Guayana Shield and Congo Basin, respectively, subject-matter editor (Michael Pace) and two anonymous reviewers for constructive comments on the previous version of the ms. Data from Arctic Rivers were collected in the framework of projects funded by the Arctic Great Rivers Observatory (NSF-0732522 and NSF-1107774). TL is a postdoctoral researcher at the FNRS. AVB is a senior research associate at the FNRS.
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