Seasonal and Longitudinal Changes in Invertebrate Functional Groups in the Dolores River, Colorado

Abstract

A 46 km section of the Dolores River, in southwest Colorado, was studied to determine the relative abundance of invertebrate functional groups over an altitudinal gradient of 500 m during three seasons. The Dolores River is a third order stream with an average width of 11 m in the upper 8 km of the study area. In the lower 38 km of the study area, it is a fourth order stream with an average width of 15 m. Benthic invertebrates were collected with a modified Hess sampler in October 1980, and March and August 1981, from 11 stations on the Dolores River. Despite little change in either stream order, width or apparent food resources in the study area, there were noticeable differences in the relative abundance of functional groups, with shredders most abundant upstream and collectors most abundant in the mid-reaches. The observed trends were highly dependent upon season with shredders abundant at most stations only in spring. This was a result of life history patterns of winter stoneflies, the primary shredders. Collector-gatherers were most abundant at the upper-middle stations in summer, but were less abundant in the other two seasons. In general, the pattern appeared to conform more to the altitudinal shifts in benthic species composition than to stream order or width. This led to shifts in the assigned functional groups without noticeable changes in food resources.