Thermal tolerances of two stream invertebrates exposed to diurnally varying temperature

For two key New Zealand freshwater invertebrates (the mayfly Deleatidium autumnale and the snail Potamopyrgus antipodarum) the upper thermal tolerances were measured in the laboratory under both constant and diurnally varying temperatures. At constant temperature 50% mortality in 96 h (constLT50) occurred at 24.2 +/- 0.9 degrees C for mayflies and 31.0 +/- 0.6 degrees C for snails: values similar to previously published estimates (Quinn et al. 1994). For diurnally varying temperatures (daily amplitude 10 degrees C) 50% mortality occurred when the daily mean temperature was 21.9 +/- 0.7 degrees C (mayflies) and 28.6 +/- 0.4 degrees C (snails) which is c. 10% (2.5 +/- 1.3 degrees C) lower than the constLT50 Conversely, 50% mortality occurred when the daily maximum temperature was 26.9 +/- 0.7 degrees C (mayflies) and 33.6 +/- 0.4 degrees C (snails) which is c. 10% (2.5 +/- 1.3 degrees C) higher than the constLT50 Many published temperature limits for stream organisms are derived from constant temperature experiments. Our results indicate that such limits should be applied to a temperature midway between the daily average and the daily maximum of a diurnal profile.