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


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Recent studies on the pelagic ecosystem of Lake Kariba identified a number of changes in its thermal regime, planktonic communities and fishery production, concluding they were the result of climate change, particularly warming. This study re-examines these conclusions and suggests alternative explanations for these changes. Historical data suggest there was no warming of the lake until at least the 1990s. Furthermore, lack of recent data makes it difficult to conclude that the lake’s temperature has increased by 2 °C. It is also not clear that the pattern of thermal stratification has changed, or that the thermocline has risen and become more stable. Although one of the suggested effects of climate change was a decreased number of larger zooplankton, this change occurred in the 1970s, when there had been no change in the lake temperature. Rather, there is strong evidence that the zooplankton composition changed as a result of selective predation by the introduced clupeid, Limnothrissa miodon. Furthermore, the loss of large grazing zooplankton species could have affected the composition of the phytoplankton, although this phenomenon also has been attributed to climate change. Although the phytoplankton communities in the lake are not as well documented as the zooplankton communities, it is clear that many changes in the lake actually began in the 1970s. Finally, the decline in the pelagic fishery for Limnothrissa was also linked to temperature changes. However, because the fisheries on the Zambian and Zimbabwean sides exhibited very different behaviours, there is little evidence to support this conclusion. It is concluded that the impacts of climate change on Lake Kariba are likely to be complex and that possible over-simplification in identifying these impacts will not facilitate our understanding of these complexities.