Carbon dynamics of two river systems in Zambia: A comparative study of the Zambezi and the Kafue Rivers

Abstract

This study focuses on the spatial and temporal variability of various carbon fractions and fluxes in relation to major biogeochemical parameters in the Zambezi River in Zambia and its major tributary, the Kafue River. Concentrations and composition of stable isotopes of dissolved organic carbon, particulate organic carbon and dissolved inorganic carbon were measured for a range of biogeochemical characteristics. Physico-chemical parameters, aquatic metabolism and partial pressure of CO2 (pCO2) were measured using multi-meters and headspace technique to determine the dynamics of carbon. To determine factors influencing the two river systems, stable isotope compositions were also used. Results indicate that pH decreased (7.4 to 6.9 on average) whereas dissolved organic carbon (DOC) increased (3.1 to 4.5 mg/L on average) over sections of the river that traverse extensive floodplains in both rivers. Weathering and dissolution of silicate elements predominated in the Zambezi whereas carbonate weathering and dissolution in the Kafue River, making dissolved inorganic carbon (DIC) twice as high (1.6 mmol/L on average) in the Kafue River compared to the Zambezi River (0.7 mmol/L on average). Heterotrophic respiration in the floodplains resulted in the depletion of dissolved oxygen (6.7 to 1.8 mg/L on average). The pCO2 averaged 2,880 and 4,426 ppm during the wet season and 944 and 2,790 ppm during the dry season on the Zambezi and Kafue rivers, respectively. The isotopic signature for DOC (δ13C-DOC) for both rivers averaged around -22.0 ‰ suggesting DOC is of mixed origin, C3 (65% ) and C4 (35%) vegetation. Isotopic signature for POC (δ13C-POC) in both rivers ranged between -29‰ and -24‰ suggesting more aquatic produced phytoplankton on reservoirs (C3 origin) and terrestrially derived C3 vegetation along the river channels. The variations in the isotopic signature for DIC (δ13C – DIC) values (-22 to -5‰) for both rivers could largely be explained by the different relative contributions of silicate and carbonate weathering as observed in the Si/Ca ratio. The isotopic signature on the Kariba Reservoir was enriched (-4‰ to -1‰) in the 13C isotope. The carbon dynamics of the Zambezi and the Kafue rivers are different. Differences in geology, land-use (agriculture and mining) and hydrological regimes of the two river systems account for the majority of the differences observed in the patterns of carbon on the two river systems.