Seasonal dynamics of carbon and nutrients from two contrasting tropical floodplain systems in the Zambezi River Basin
Authors: Zuijdgeest A. L. , Zurbrügg R. , Blank N. , Fulcri R. , Senn D. B. , Wehrli B. 
Published: July 2015

Abstract. Floodplains are important biogeochemical reactors during fluvial transport of carbon and nutrient species towards the oceans. In the tropics and subtropics pronounced rainfall seasonality results in highly dynamic floodplain biogeochemistry. Massive construction of hydropower dams, however, has significantly altered the hydrography and chemical characteristics of many (sub)tropical rivers. In this study, we compare organic matter and nutrient biogeochemistry of two large, contrasting floodplains in the Zambezi River Basin in Southern Africa, the Barotse Plains and the Kafue Flats. Both systems are of comparable size, but differ in anthropogenic influence: while the Barotse Plains are still relatively pristine, the Kafue Flats are bordered by two hydropower dams. While the Barotse Plains retain particles during the wet season, annual yields of particulate organic carbon and nitrogen are higher than previously reported for the Zambezi and other tropical rivers. Enhanced wet-season runoff adds soil-derived dissolved organic carbon and nitrogen to the Zambezi River, with a corresponding increase in the Barotse Plains. Soil-derived organic matter dominates the particulate phase year-round in the Barotse Plains, and a varying influence of C3- and C4-plant vegetation can be observed throughout the year. In contrast to the Barotse Plains, net export of particulate matter from the Kafue Flats has been observed during the wet season, but over an annual cycle, the Kafue Flats are effectively accumulating dissolved carbon and nutrients. In the Kafue Flats, the runoff-induced increase in dissolved organic carbon and nitrogen concentrations is delayed by the upstream dam operation. The dam reservoir also causes a shift in the source of the particulate organic matter – from soil-derived during the dry season to aquatically produced in the wet season – in the downstream Kafue Flats. Spatial zonation in vegetation and temporal flooding dynamics in the Kafue Flats result in mostly C3-derived particulate organic matter during wet season, and a dominance of C4-derived material during dry season. This pattern results from dam-induced changes in vegetation, as dam construction along the Kafue River has led to encroachment of woody plant species onto the Kafue Flats. The two systems exhibit different flooding dynamics, with a~larger contribution of floodplain-derived water in the Kafue Flats and a stronger peak flow in the Barotse Plains. Differences in the biogeochemistry of the two systems that can be linked to the dams are the timing of the runoff-driven dissolved organic carbon and nitrogen pulses in the wet season and the origin and inputs of particulate organic matter. This study reveals clear effects of dam construction on organic matter and nutrient dynamics on the downstream floodplain. Man-made reservoirs alter the origin of organic matter, and change the timing of precipitation-driven carbon and nitrogen pulses. Environmental assessments of dam impacts should therefore consider changes in water quality.