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.
- Borges AV, Darchambeau F, Teodoru CR, Marwick T, Tamooh F, Geeraert N, Omengo FO, Guérin F, Lambert T, Morana C, Okuku E, Bouillon S. 2015. Globally significant greenhouse gas emissions from African inland waters. Nature Geoscience (in press).Google Scholar
- Dewitte O, Jones A, Spaargaren O, Breuning-Madsen H, Brossard M, Dampha A, Deckers J, Gallali T, Hallett S, Jones R, Kilasara M, Le Roux P, Michéli E, Montanarella L, Thiombiano L, Van Ranst E, Yemefack M, Zougmore R. 2013. Harmonisation of the soil map of Africa at the continental scale. Geoderma 211–212:138–53.CrossRefGoogle Scholar
- Downing BD, Boss E, Bergamaschi BA, Fleck JA, Lionberger MA, Ganju NK, Schoelhamer DH, Fujii R. 2009. Quantifying fluxes and characterizing compositional changes of dissolved organic matter in aquatic systems in situ using combined acoustic and optical measurements. Limnology and Oceanography Methods 7:119–31.CrossRefGoogle Scholar
- Jaffé R, Yamashita Y, Maie N, Cooper WT, Dittmar T, Dodds WK, Jones JB, Myoshi T, Ortiz-Zayas JR, Podgorski DC, Watanabe A. 2012. Dissolved organic matter in headwater streams: compositional variability across climatic regions of North America. Geochimica Et Cosmochimica Acta 94:95–108.CrossRefGoogle Scholar
- O’Donnell JA, Aiken GR, Walvoord MA, Butler KD. 2012. Dissolved organic matter composition of winter flow in the Yukon River basin: implications of permafrost thaw and increased groundwater discharge. Global Biogeochemical Cycles 26(4):GB0E06.Google Scholar
- Pérez MAP, Moreira-Turcq P, Gallard H, Allard T, Benedetti MF. 2011. Dissolved organic matter dynamic in the Amazon basin: sorption by mineral surfaces. Chemical Geology 286:158–68.Google Scholar
- Prairie Y, Del Giorgio PA, Roehm C, Tremblay A. 2010. Insights on riverine metabolism from continuous measurements of CDOM fluorescence in Eastmain-1 Reservoir. Quebec Verhandlungen des Internationalen Verein Limnologie 30:1545–8.Google Scholar
- Raymond PA, McClelland JW, Holmes RM, Zhulidov AV, Mull K, Perterson BJ, Striegl RG, Aiken GR, Gurtovaya TY. 2007. Flux and age of dissolved organic carbon export to the Artic Ocean: a carbon isotopic study of the five lalarge rivers. Global Biogeochemical Cycles 21:GB4011.Google Scholar
- Salisbury J, Vandemark D, Campbell J, Hunt C, Wisser D, Reul N, Chapron B. 2011. Spatial and temporal coherence between Amazon River discharge, salinity, and light absorption by colored organic carbon in western tropical Atlantic surface waters. Journal of Geophysical Research Oceans 116:C00H02.Google Scholar
- Seyler P, Coynel A, Moreira-Turcq P, Etcheber H, Colas C, Orange D, Bricquet JP, Laraque A, Guyot JL, Olivry JC, Meybeck M. 2004. Organic carbon transported by the Equatorial rivers: example of Congo-Zaire and Amazon basins. In: Roose ELR, Feller C, Barthès B, Stewart BA, Eds. Soil erosion and carbon dynamics. Boca Raton (FL): Taylor et Francis, pp. 255–274.Google Scholar
- Spencer RGM, Stubbins A, Hernes PJ, Baker A, Mopper K, Aufdenkampe AK, Dyda RY, Mwamba VL, Mangangu AM, Wabakanghanzi JN, Six J. 2009. Photochemical degradation of dissolved organic matter and dissolved lignin phenols from the Congo River. Journal of Geophysical Research 114:G03010.Google Scholar
- USGS. 2000. HYDRO1K elevation derivative database, Center for Earth Resources Observation and Science, Sioux Falls, S.D, http://edc.usgs.gov/products/elevation/gtopo30/hydro.
- Wynn JG, Bird MI. 2007. C4-derived soil organic carbon decomposes faster than its C3 counterpart in mixed C3/C4 soils. Global Biogeochemical Cycles 13:1–12.Google Scholar
- Yamashita Y, Maie N, Briceno H, Jaffé R. 2010. Optical characterization of dissolved organic matter in tropical rivers of the Guayana Shield, Venezuela. Journal of Geophysical Research 115:G00F10.Google Scholar