Predicting the effects of time-varying temperatures on stream invertebratemortality

Stream managers often need to predict the impacts of high and time-varying temperature on key stream invertebrates. A simplified model has been develo ped and calibrated using mortality observations made at constant temperatur e for two important New Zealand stream invertebrates: the snail Potamopyrgu s antipodarum and the mayfly Deleatidium autumnale. A close fit was obtaine d for Deleatidium but Potamopyrgus showed evidence of acclimation during th e 96-h tests. The model successfully predicted the mortality observed when temperature varied diurnally by +/-5 degrees C in seven experiments but ove r-estimated mortality in the eighth. The experiments used to calibrate and test the model were performed on test animals collected in winter (April-Ju ne) and acclimated at 16-17 degrees C. The model was less successful at pre dicting mortality on Deleatidium collected in summer (December-January) and /or acclimated at 21-22 degrees C. The likely reason is that the model, cal ibrated using winter data, does not account for summer acclimation.