Aquatic insects in sediment traps

Abstract

Sediment traps were set in three tributaries and in three areas of the upper Gallatin River. The kind, sizes and amount of sedimentation was analyzed and related to numbers and orders of aquatic insects found in gravel in the traps. From August, 1972, to October, 1973, samples were taken at monthly intervals in a monthly renewed sample and in a cumulative sample undisturbed since the beginning of the project. These sampling methods show that living space is of prime importance to aquatic insects and sculpins, and that it is a limiting factor for insect communities presently in the upper Gallatin. Measurable current speeds (up to 2.5 fps) did not seem to affect insect numbers. High percentages of organic matter in sediment coincided with high insect numbers, but actual weight had no apparent relationship. High numbers of insects were present in the spring, but were drastically reduced after runoff. They recovered much more slowly and to lower levels in the cumulative traps, which remained filled with sediment. The low sedimentation rates through fall and winter resulted in higher insect numbers in cumulative traps, presumably because they were undisturbed. Diptera was the dominant order in all areas.