Microdistributions, food resources and feeding habits of filter-feeding Trichoptera in the Upper Colorado River

A year-round study was conducted in the Colorado River, USA, to examine microdistributions and feeding habits of the filter-feeding Trichoptera (caddisflies) Arctopsyche grandis, Brachycentrus occidentalis and Hydropsyche cockerelli. Only one significant relationship was found between individual rock surface area and the density of larvae during the sampling periods. Contrary to our predictions that current-exposed microhabitats would be selected, all species occurred primarily on the bottom and lower lateral sides of rocks at all sites. Clumped distributions in the absence of spatial segregation by size class suggest common microhabitat preferenda for species and instars. Organic seston in the study reach consisted primarily of detritus, diatoms and Closterium sp. (a green alga). The diets of B. occidentalis and H. cockerelli generally corresponded with the observed proportions of food resources in the seston. Based on HORN'S index, we found high intraspecific dietary overlap for A. grandis and H. cockerelli. Dietary overlap was usually lower between A. grandis and other species. Dietary overlap between B. occidentalis and H. cockerelli larvae was high for most months. Ontogenetic niche shifts, especially in microhabitat use, were demonstrated in this study. The similar use of microhabitat, and the species-specific feeding habits by the filter-feeding Trichoptera suggest that examining relationships at a finer scale is important for a more thorough understanding of species' habitat requirements and overall distribution patterns.