THERMAL-SHOCK TOLERANCE OF 3 SPECIES OF AQUATIC INSECTS IN A NORTHERNCALIFORNIA, GEOTHERMALLY INFLUENCED STREAM

This study assessed the response of aquatic insects to short-term ther mal shocks, such as those encountered upon drifting into heated areas of geothermally influenced streams. The upper incipient lethal tempera ture (UILT) thresholds were determined for three common species in Big Sulphur Creek, Sonoma County, California, and the effect of acclimati on temperature on the UILT was determined for two caddisflies, both of which overwinter in the stream as larvae. When acclimated at 28 degre es C, the LT(50) of Centroptilum convexum (Ide) ranged from 36.3 degre es to 38.5 degrees, that of Gumaga nigricula (McLachlan) from 37.8 deg rees to 41.0 degrees, and that of Helicopsyche borealis (Hagen) from 3 8.5 degrees to 41.4 degrees. When acclimated at 5.5 degrees, the LT(50 ) for G. nigricula ranged from 35.7 degrees to 38.9 degrees and that o f H. borealis from 36.6 degrees to 41.1 degrees. Helicopsyche borealis has the highest UILT of any caddisfly, 39.0 degrees for 30 min. The m ost important factors determining the UILTs in these insects are accli mation temperature, shock duration, and the maximum temperature of exp osure.