Drifting Invertebrate Composition, Densities, and Biomass from Fall Through Winter in Three Wyoming Tail Waters

Abstract

We studied three regulated rivers downstream from reservoirs (tailwaters) in Wyoming from October 1997 through February 1998 to assess the temporal variation in composition, density, and biomass of drifting invertebrates. Macroinvcrtebrates were numerically dominated by Diptera (primanly Chironomidae) and Ephemeroptera in the North Platte and Big Horn rivers with Trichoptera, Oligochaeta, and Arachnida (Hydracarina) also common. In the Shoshone River, the predominant macroinvertebrate taxon was Oligochaeta followed by Diptera and Ephemeroptera. Densities of drifting macroinvertebrates were lowest in, November and highest in January in the North Platte and Big Horn rivers. However, densities or drifting macroinvertebrates were lowest in October and highest in November in the Shoshone River. The composition of drifting invertebrates changed from exclusively macroinvertebrates in October and November to predominantly zooplankton in December through February in all three rivers. However, drifting macroinvertebrates were relatively abundant throughout the winter in all three tailwaters compared to other river systems.