Title: Impact of the Itezhi-Tezhi reservoir on dissolved oxygen and greenhouse gas dynamics in the Kafue River, Zambia
Author: Namakau Muyumbana
Supervising Institution: IHE Delft - Institute for Water Education
Many aquatic ecosystems have been altered by anthropogenic activities. To meet human needs, impoundments and dams have been built but often have significant impacts on aquatic ecosystems. Dams have been understood to act as traps that disrupt the continuity of sediment, nutrients and organic matter. They also change the flow of water by increasing the residence time of water thus causing the system to have lake-like characteristics. This causes the accumulation of organic matter, and stratification of the water column, thus disrupting the geochemical processes of the system. The accumulation of the organic matter leads to microbial degradation, which consumes oxygen, hence causing anoxic conditions in the system. The release of this oxygen-depleted water has adverse effects on the downstream ecosystem. In this study, we investigated the impact of a reservoir on dissolved oxygen (DO) and greenhouse gas (GHG) dynamics in and below the Itezhi-Tezhi (ITT) dam in the Kafue River, Zambia. The aim was to understand the factors controlling DO and GHG concentration in the reservoir and the dynamics of recovery downstream of the river after the release of hypolimnetic water from the reservoir. We explored this topic by measuring in situ physical and chemical variables and collecting water samples in the reservoir and in the river for GHG analysis. The results revealed that the build-up of thermal stratification in the reservoir over the sampling period led to the development of anoxic condition in the hypolimnion, and an accumulation of GHGs. CH4 concentration increased from 0.0001 mg l-1 in the epilimnion to 0.2 mg l-1 in the hypolimnion. N2O was not detected in the uppermost 10 m of the reservoir and reached concentrations up to 0.02 mg l-1 in the hypolimnion. CO2 concentration ranged from 0.4 mg l-1 at the surface to 18 mg l-1 in the hypolimnion. The depletion of DO and accumulation of GHGs in the reservoir through the campaigns manifested in the outflow downstream of the dam. We observed high concentration of GHGs at the outlet which decreased with distance downstream. DO concentration was low (≤2 mg l-1) at the outlet and increased with distance. The quantity of CH4 emitted through the lake surface ranged between 1 and 2 mg m-2 d-1. CO2 ranged between 362 and 532 mg m2 d-1. In the river, CH4 fluxes were highest at the outlet, ranging between 2 and 68 mg m-2 d-1 and decreasing with distance to ≤2 mg m-2 d-1 at 45 km, CO2 fluxes in the river ranged between 2868 and 15800 mg m-2 d-1. Observed DO recovery in the river initially followed those estimated with a simple exchange model with high piston velocity. Further downstream, recovery was slower than projected by the model, suggesting other physical or biological processes could play a role in DO recovery downstream of the dam.