Stream invertebrates and water temperature : evaluating thermal tolerances in the Cape floristic region (South Africa) - implications of climate change

Master Thesis


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University of Cape Town

Temperature is an important environmental variable for aquatic invertebrates as it affects their development, reproduction and survival. Temperature also affects the abundance and distribution of individuals in a stream or river. Stream temperatures are affected by human impacts which include not only impacts such as pollution, abstraction of water and the removal of riparian vegetation, but also the affects of climate change. In the Western Cape Province, South Africa, it is predicted that air temperatures will increase and winter rainfall will decrease. This in turn will affect water availability and water temperatures in streams and rivers. Thermally sensitive species are threatened by increases in water temperature, but currently very little information exists on the thermal tolerances of aquatic invertebrates in South Africa. In order to rectify this problem baseline data on thermal tolerances of aquatic species needs to be collected, together with stream temperature and associated invertebrate community data. An attempt was made to collect some of these much needed data in the Western Cape Province. Two methods, namely Critical Thermal Maxima (CTM) and LT₅₀ experiments, were used to determine the thermal tolerances for a range of aquatic insect nymph species from the southwest fynbos bioregion and the south coast fynbos bioregion. The results from the experiments provided information on the relative thermal sensitivities of the species. Overall Aphanicerca capensis (form C and undescribed form), Notonemouridae ranked as the most thermally sensitive for both experiments. The A. capensis species complex (and possible other notonemourid stoneflies) may potentially be used as an indicator of changing stream temperatures in the Western Cape Province. To rapidly determine thermal sensitivities the CTM experiments are recommended rather than the more time consuming LT₅₀ experiments as the relative thermal tolerance for the species tested ranked the same for both experiments. It is suggested that LT₅₀ experiments of longer duration be investigated in order to compare the experiments to naturally-occurring thermal stress. Stream temperature and community composition data were collected from Window Gorge Stream, on Table Mountain, to provide baseline data for future monitoring and understanding of potential changes in thermal profiles. Temperature loggers were placed at six sites along the stream. Stream temperatures were fairly low but the stream ran dry during the late summer months (February through to April/May). From the community composition data collected the highest diversity was found during the winter months, as expected. The community composition did change down the length of the stream, with the species composition found near the source being quite different to that of the species composition found lower down the mountain. Temperature and associated oxygen saturation were two important variables related to the community composition down the length of the stream. It is important that experimental data be combined with field data, enabling field sampling to focus on the collection of information on the abundance of the thermally sensitive species (e.g. A. capensis species complex). Recorded stream temperatures also provide reference conditions for the species tested in the laboratory. Climate change is likely to have an affect not only on stream temperatures but also on water availability, which will both influence stream communities and ecosystems and it is important to understand what these potential effects might be.

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