A decline in the depth of the mixed layer and changes in other physical properties of Lake Kariba’s water over the past two decades

A meta-analysis of multiple experimental data collected monthly over two time periods, 1986–1987 and 2007–2008 demonstrated significant changes in the physical properties of the water of Lake Kariba’s Sanyati Basin. Increased lake water temperatures were noted in 6 months of the year, with a smaller decline in water temperature with increasing depth in 2007–2008 than 1986–1987; resulting in a significantly higher (by 1.5°C) mean annual value of 24.7°C. Plots of water temperature gradients against depth revealed a decline in thermocline from 10- to 15-m depth in 1986–1987 to a consistent 5-m depth in 2007–2008. Lake water pH declined significantly from a mean annual value of 7.89 in 1986–1987 to 6.96 in 2007–2008; this decrease restricted to 6 months of the year. Lake water dissolved oxygen contents in 8 months of the year, increased significantly from a mean annual value of 5.45 mg l−1 in 1986–1987 to 6.94 mg l−1 in 2007–2008, with a substantially smaller decline in dissolved oxygen content with increasing depth evident in 2007–2008 than 1986–1987. Significant higher lake water conductivities in 2007–2008 than 1986–1987 were confined to 2 autumn months. These findings are compared with those in other tropical African lakes and potential links to climate warming are discussed.