Critical Thermal Maxima of aquatic macroinvertebrates: towards identifying bioindicators of thermal alteration

Water temperature is an important abiotic driver of aquatic ecosystems. It influences many aspects of an organism's existence including its growth, feeding and metabolic rates; emergence; fecundity; behaviour and ultimately survival. All organisms have an optimum temperature range within which they survive and are able to thrive. Determining upper thermal limits provides insight into the relative sensitivity of organisms to elevated temperatures. Thermally sensitive taxa may be useful as bioindicators of thermal alteration and used in the generation of target thermal thresholds for aquatic systems. This study determined the upper thermal limit (CTmax) of a range of aquatic macroinvertebrates from rivers in the south-western Cape, South Africa, using the dynamic Critical Thermal Method. The study focused on the taxonomic level of family as an initial screening tool for ranking thermal sensitivity. Of the 27 families examined, four were both thermally sensitive and highly suitable as test organisms, including Paramelitidae, Notonemouridae, Teloganodidae and Philopotamidae. Five families were moderately sensitive and highly suitable, including Palaemonidae, Heptageniidae, Leptophlebiidae, Corydalidae and Aeshnidae. Preliminary experiments to determine potential sources of variation in CTmax revealed that thermal sensitivity was relatively uniform within families, but that acclimation temperature influenced CTmax. Further investigation of the influence of thermal history, acclimation temperature and rate of temperature change on CTmax is necessary. Target water temperatures for river management will be derived using CTmax data, in addition to longer duration experimental data, which will be linked to in situ temperature data.