Marine Reserves: A Guide to Science,Design, and Use

There is evidence that global warming is changing the thermal regimes of tropical lakes and this paper examines the impact of warming on the limnology of Lake Kariba. Between 1986 and 2011 the mean temperature of the lake rose by 0.7 °C, a rate equivalent to 0.03 °C y^?1, but this increase was not uniform, being most pronounced in March and May (1.5 and 1.4 °C, respectively) and least in August and October (0.1 °C in both). These changes appear to have disrupted the thermal regime which now seems to be less predictable than before. Thermal gradients have weakened and, as a result, oxyclines are now less pronounced and persistent than in the past. Previous workers suggested that warming in Lake Kariba has strengthened stratification and has caused the thermocline to rise, reducing the volume of the epilimnion and thus reducing the productivity of the pelagic fishery. But this does not appear to be the case. Warming was not accompanied by more pronounced stratification or upward shift in the thermocline, but instead there was a downward transfer of heat that caused the thermocline to descend and weaken, thus reducing thermal stability. The dynamic nature of the thermocline highlights the need to exercise caution in making simple predictions and conclusions about biotic responses in a large complex ecosystem.