An assessment of climate change and stratification in Lake Kariba (Zambia–Zimbabwe)

Abstract

Lake Kariba is a monomictic reservoir, isothermal in winter (June–August), and stratified for the rest of the year. Its temperature has increased since the 1990s, with some researchers suggesting it now has a stronger and more stable thermocline as a result. It was also claimed that the depth of the epilimnion had decreased, and nutrient depletion has led to a decrease in the depth of the euphotic zone, algal biomass and primary productivity. A re‐analysis of the data suggests this view is incorrect. The epilimnion is more homogenous now than in the 1980s, with less vertical variation in temperature and dissolved oxygen in the upper 20 m of the lake. The depth of the euphotic zone has not changed since the 1960s, and the algal biomass and primary productivity have increased. The thermocline appears to have weakened with temperature and density gradients being much smaller than in the 1980s because of a more rapid rate of warming in deeper water. The reasons for this phenomenon are not clear, although evaporative cooling at the surface may contribute to it. There was a significant increase in Birgean heat budgets over time, although there was none for stability. This may be explained by the fact that the lake is monomictic, cooling to 22°C in winter, with a new cycle of stratification beginning each year, and it may be affected by variable factors such as evaporation, rainfall, river flow and internal wave movements.

Sub-Saharan Africa Freshwater Fisheries under Climate Change: A Review of Impacts, Adaptation, and Mitigation Measures

“…A weak thermocline has been detected in studies by Mahere et al [85] and Marshall [86] in Lake Kariba, which has been acknowledged to be experiencing warming temperatures. On the other hand, Mahere et al [85] and Marshall [86] claimed that the perceived warming temperatures of Lake Kariba led to an increase in algal biomass and primary productivity. Therefore, the thermal stratification of inland freshwater bodies has and will continue to affect fish production, either by increasing or decreasing fish productivity.…”

“…Contrary to Vincent [82] and Ndebele-Murisa et al [18], who state that warming temperatures promote stratification, Mahere et al [85] and Marshall [86] claimed that warming temperature weakens the thermocline (less pronounced stratification). A weak thermocline has been detected in studies by Mahere et al [85] and Marshall [86] in Lake Kariba, which has been acknowledged to be experiencing warming temperatures. On the other hand, Mahere et al [85] and Marshall [86] claimed that the perceived warming temperatures of Lake Kariba led to an increase in algal biomass and primary productivity.…”

The Relationship between Benthic Macroinvertebrate Assemblages and Water Quality Parameters in the Sanyati Basin, Lake Kariba, Zimbabwe

“…The lake has a surface area of 4 364 km 2 at the normal operation level of 484 m a.s.l, a length of 276 km, an average width of 19 km, and an average depth of 29 m [ 24 ]. The lake is warm, nutrient-poor, and monomictic [ 25 , 26 ]. It is divided into five major hydrological basins: Mlibizi (Basin 1), Binga (Basin 2), Sengwa (Basin 3), Bumi (Basin 4), and Sanyati (Basin 5).…”

Impacts of climate change on streamflow and reservoir inflows in the Upper Manyame sub-catchment of Zimbabwe

“…They concluded that climatic variability was contributing to the declines in fish catches and changes in species composition. Marshall (2017) noted that the thermocline in the Lake has weakened, with much smaller density and thermal gradients compared to the 1980s. Mhlanga et al(2018) found that under climate change scenarios RCP 8.5 and 6.0 areas suitable for dairy farming will decrease by 25.4% and 2.7%, respectively.…”