DIFFERENTIAL-EFFECTS OF A BRIEF THERMAL DISTURBANCE ON CADDISFLIES (TRICHOPTERA) IN A REGULATED RIVER

During studies on filter-feeding and grazing caddisflies in the regula ted upper Colorado River, a problem developed with the deep-release me chanism of Granby Dam and water had to be released from the surface fo r 16 days in August 1986. Maximum water temperatures exceeded the norm al summer maxima by at least 4-5 C for up to 12 km below the reservoir . Samples taken along the longitudinal profile below the reservoir sho wed that populations of several caddisflies species, which had been nu merically abundant in previous years, were virtually eliminated after this period of increased water temperature. Some species experienced i mmediate reductions, presumably due to larval/pupal mortality (e.g., B rachycentrus americanus, Glossosoma ventrale, G. parvulum), while othe rs exhibited lagged responses over the next several months, presumably due to reduced hatching success and extensive winter mortality (e.g., Glossosoma verdona). Interestingly, at some sites, B. americanus and other caddisfly species were largely unaffected by the elevated temper atures. Species showing the greatest resilience to the thermal disturb ance were either those having abundant terrestrial adults present at t he time (eg., Agapetus boulderensis) or those potentially having broad thermal tolerances (e.g., Brachycentrus occidentalis). Most studies c oncerning the effects of extreme temperature change on lotic organisms have dealt with heated effluents or the general thermal impacts induc ed by river regulation. This is the first report, that we are aware of , detailing the potential effect of short-term elevated temperatures o n lotic macroinvertebrates in a river regulated by a deep-release dam.