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

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.