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Biogeosciences Discuss., 9, 7943-7981, 2012
www.biogeosciences-discuss.net/9/7943/2012/ doi:10.5194/bgd-9-7943-2012 © Author(s) 2012. This work is distributed under the Creative Commons Attribution 3.0 License. |
Organic carbon and nitrogen export from a tropical dam-impacted floodplain system
1Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Universitätstrasse 16, 8092 Zurich, Switzerland
2Eawag, Swiss Federal Institute of Aquatic Science and Technology, Surface Waters – Research and Management, Seestrasse 79, 6047 Kastanienbaum, Switzerland
3Institute of Environmental Geosciences, University of Basel, Bernoullistrasse 30, 4056 Basel, Switzerland
Abstract. Tropical floodplains play an important role in organic matter transport, storage, and transformation between headwaters and oceans. However, the fluxes and quality of organic carbon (OC) and organic nitrogen (ON) in tropical river-floodplain systems are not well constrained. We explored the quantity and characteristics of dissolved and particulate organic matter (DOM and POM) in the Kafue River flowing through the Kafue Flats (Zambia). The Kafue Flats are a tropical dam-impacted river-floodplain system in the Zambezi River basin. During the flooding season, >80% of the Kafue River water passed through the floodplain, mobilizing large quantities of OC and ON, which resulted in a net export of 75 kg OC km−2 d−1 and 2.9 kg ON km−2 d−1, 80% of which was in the dissolved form. Mass budget estimates showed that ON export, denitrification, and burial caused an annual deficit of ~21 000 t N yr−1 in the Kafue Flats. A N isotope balance and the δ15N of DON and PON suggest that N-fixation must level out the large N losses. The elemental C:N ratio of ~20, the δ13C values of higher than −24‰, and spectroscopic properties (excitation-emission matrices) showed that DOM in the river was mainly of terrestrial origin. Despite a threefold increase in OC loads due to inputs from the floodplain, the river DOM characteristics remained relatively constant along the sampled 400-km river reach. This suggested that floodplain DOM had similar properties than DOM from the upstream reservoir. In contrast, based on its low δ13C of −29‰ and the C:N ratio of ~8, POM originated from phytoplankton production in the upstream reservoir and in the floodplain. While the reservoir had little impact on DOM properties, terrestrial POM was efficiently trapped and, instead, phytoplankton-derived POM was discharged to the downstream Kafue Flats.
Citation: Zurbrügg, R., Suter, S., Lehmann, M. F., Wehrli, B., and Senn, D. B.: Organic carbon and nitrogen export from a tropical dam-impacted floodplain system, Biogeosciences Discuss., 9, 7943-7981, doi:10.5194/bgd-9-7943-2012, 2012.