Densities and Substrate Associations of Macroinvertebrates in Riffles of a Small, High Plains Stream

We described the densities of macroinvertebrates in riffles of Crow Creek, Laramie County, Wyoming, and the relations of substrate composition to densities. Dominant taxa were Hyallela azteca (Amphipoda), Tricorythodes minutus (Ephemeroptera), Naididae (Oligochaeta), Chironomidae (Diptera), and Dubiraphia sp. (Coleoptera), which collectively comprised 86.7% of the macroinvertebrates in the samples. The mean density of macroinvertebrates was high (770/0.1 m2) compared to montane streams. Riffles were primarily gravel, but the amount of fine material (particle diameters ≤ 0.42 mm) varied from 0.5 to 100.% of the dry weight of samples. As the amount of fine material increased, the densities of Dubiraphia sp., Hyallela azteca, and Helisoma spp. increased, whereas the densities of Tricorythodes minutus decreased. Our observations suggest that enhanced sediment deposition in small, high plains streams alters macroinvertebrate assemblages.