Environmental state of Lake Kariba and Zambezi River Valley: Lessons learned and not learned

Abstract

Lake Kariba, still the largest reservoir in the world by volume, is 60 years old. It has undergone changes in its thermal properties, associated with global warming, which reflect in turn on its limnology. These changes include a shallower eipilimnion, higher heat content and increased tropicality to near equatorial status. The role of Lake Kariba with regard to its energy characteristics is discussed in light of global warming findings. The lake's water residence time has increased from 3.7 years to %5.7 years, attributable to a reduced inflow from the Zambezi River. The phytoplankton communities have changed towards a cyanophyceae-dominated community, leading to a decline in entomostracan zooplankton, and a near collapse of the planktivorous Limnothrissa miodon fishery. Prolonged use of pesticides to control Glossina has led to measurable ecosystem level impacts on both terrestrial and aquatic biota. The impacts of the forced relocation of the Tonga people were still evident during this study. Siltation from resettlement areas has led to the loss of habitat and biodiversity in the inflowing streams to the lake. Unplanned shoreline development in the early history of the lake poses health problems. It is projected that global warming will cause the lake temperature to rise by %4°C by the end of the century. Higher temperatures will be accompanied by windier conditions, thereby enhancing the risks from storms on the lake. The appropriateness of administrative structures intended to manage the Zambezi River Basin environment also is discussed herein. It is concluded that the management protocol is institutionally a non-inclusive process lacking the capacity to involve other stakeholders in managing the lake's resources, and even less so in the integrated management of the basin.

Phytoplankton biomass and primary production dynamics in Lake Kariba

“…Analyses of climatic data for the middle Zambezi Valley indicate that warming around Lake Kariba is proceeding at a faster rate than predicted with regional models (Magadza 2010, 2011; Ndebele‐Murisa et al. 2011a,b).…”

Indications of the effects of climate change on the pelagic fishery of Lake Kariba, Zambia–Zimbabwe

“…Magadza (2010) previously discussed the limnochemistry and physical properties of Lake Kariba. In regard to the lake’s thermal properties, he indicated the lake has significantly warmed since the mid‐1960s.…”

Climate change does not explain historical changes in the pelagic ecosystem of Lake Kariba (Zambia–Zimbabwe)

“…4 in Magadza (2010)), raising some uncertainty about their dates. The estimation of temperature is further complicated by the fact that there is extensive variation between years, being well illustrated in the profiles provided by Magadza (2010). During the 1988–1992 period, the surface (25.0–28.2 °C) and mean epilimnetic temperatures (25.1–27.9 °C) in March varied from year to year by about 3 °C, while the temperature at 40 m depth varied by about 2 °C.…”

“… Historical temperature profiles from Lake Kariba during periods of maximum thermal stratification (△ = February 1961 (from Harding 1964); • = February 1968 (from Coche 1974); ○ = March 1991 (from Magadza 2010); grey line = profile labelled ‘July 2007’ (from Magadza 2010)). …”