SREL Reprint #1878
Differential Effects of a Brief Thermal Disturbance on Caddisffies
(Trichoptera) in a Regulated River
NEAL J. VOELZ, N. LEROY POFF AND J. V. WARD
Biology Department, Colorado State University, Fort Collins 80523
ABSTRACT.-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 dani.
SREL Reprint #1878
Voelz, N.J., N.L. Poff, and J.V. Ward. 1994. Differential effects of a brief thermal disturbance on caddisflies (Trichoptera) in a regulated river. The American Midland Naturalist 132:173-182.
