Productivity and Distribution of Trichoptera Larvae in a Colorado Mountain Stream

Summary

From June 1968 to late May, 1969 collections of more than 20,600 immature Trichoptera were made from the St. Vrain River of Colorado. Agapetus sp., Arctopsyche grandis, Brachycentrus americanus, Ecclisomyia maculosa, Hydropsyche sp., Hydropsyche occidentalis, Helicopsyche borealis, Leptocella sp., Neothremma alicia, Rhyacophila acropedes, and Sortosa sp. were taken at eight sites ranging from 1565 to 3200 m in altitude. The Trichoptera of the St. Vrain exhibited a distinct altitudinal zonation with Zone I (1565 m) dominated by Leptocella, Hydropsyche sp., and Helicopsyche borealis; Zone II (1750 to 2350 m) was dominated by Brachycentrus americanus and Hydropsyche occidentalis; Zone III (2350 m) was considered a transitional area. In Zone IV (2650 m) and Zone V (2950 to 3250 m ) Neothremma alicia and Arctopsyche grandis were the most successful genera, respectively. Temperature is proposed as the major variable controlling attitudinal zonation.

Standing crop productivity of Trichoptera was determined from timed collections and expressed as population density (number of organisms collected per 30 minutes) and organic biomass (dry organic matter represented by organisms collected per 30 minutes). Intermediate altitudes (2300 m) exhibited the lowest productivity but greatest species diversity. Highest productivity occurred below 1600 m but organic biomass was surprisingly high above 2900 m. Changes in productivity in relation to altitude are discussed in relation to temperature, food availability, severe flooding, and other variables. Productivity on a yearly basis was found to be greatest in spring and at minimum levels during autumn and early winter. The holometabolous life cycle of Trichoptera, temperature, and current seem to be the major governing factors of seasonal productivity.