Differential effects of a brief thermal disturbance on caddisflies (Trichoptera) in a regulated river

During studies on filter-feeding and grazing caddisflies in the regulated upper Colorado River, a problem developed with the deep-release mechanism of Granby Dam and water had to be released from the surface for 16 days in August 1986. Maximum water temperatures exceeded the normal 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 showed that populations of several caddisflies species, which had been numerically abundant in previous years, were virtually eliminated after this period of increased water temperature. Some species experienced immediate reductions, presumably due to larval/pupal mortality (e.g., Brachycentrus americanus, Glossosoma ventrale, G. parvulum), while others 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 temperatures. Species showing the greatest resilience to the thermal disturbance were either those having abundant terrestrial adults present at the time (e.g., Agapetus boulderensis) or those potentially having broad thermal tolerances (e.g., Brachycentrus occidentalis). Most studies concerning the effects of extreme temperature change on lotic organisms have dealt with heated effluents or the general thermal impacts induced by river regulation. This is the first report, that we are aware of, detailing the potential effect of short-term elevated temperatures on lotic macroinvertebrates in a river regulated by a deep-release dam.