Upper thermal limits of aquatic macroinvertebrates: Comparing critical thermal maxima with 96-LT50 values

Water temperature is an important abiotic driver of aquatic ecosystems. It influences many aspects of an organism's existence including its metabolic, growth and feeding rates; fecundity; emergence; behaviour and ultimately survival. This study determined the upper thermal limits of a range of aquatic macroinvertebrates from rivers in the south-western Cape, South Africa, using two methods: the critical thermal method (CTM) and the LT50 method, which is equivalent to the incipient lethal temperature (ILT) technique. The relationship between the two methods was examined with the intention of establishing if the simpler CTM could be used for future testing of thermal sensitivity thereby allowing for extrapolation of longer duration thermal stress. Of the ten species examined, Aphanicerca capensis, Paramelita nigroculus, Chimarra ambulans, Castanophlebia sp. and Afronurus barnardi were all thermally sensitive and had distinct thermal endpoints making them excellent test organisms. There was a significant positive linear relationship between the estimated incipient lethal upper temperature (ILUT) and critical thermal maximum (CTmax), which facilitates future experimental work based on CTM. Future evaluation of hourly in situ stream temperatures to identify periods of exceedance of these 96 h LT50s, in addition to experimental 10 day LT50s, will enable comparison of laboratory data with field conditions, and ultimately the derivation of target water temperature thresholds for management purposes.

► Upper thermal limit of ten species of aquatic macroinvertebrates was determined. ► Estimates of lethality based on critical thermal method (CTM) and LT50 method were compared. ► Comparison of upper limits with in situ stream temperatures will contribute to the derivation of thermal thresholds.