Submerged wood inhabiting macoinvertebrates of the Elm Fork of the Trinity River, Texas: Assemblage analyses, comparisons to the benthic fauna, and a study of Cyrnellus fraternus (Trichoptera: Polycentropodidae)

Taxonomic composition and spatio-temporal variability of macroinvertebrates inhabiting snags were examined from an 8-km segment of the Elm Fork of the Trinity River. Seventy-seven macroinvertebrate taxa were collected from submerged wood during the study. Assemblages were highly variable spatially and temporally, especially among seasons. An ordination of species and environment did not find snag taxa to have striking correlations with many of the selected environmental variables, although there were some trends in flow-mediated species distributions. There were significant seasonal differences in the abundances of some taxa, particularly among the Chironomidae (Diptera). The Tribe Orthocladiini was numerically abundant during the winter, while the Tribes Chironomini and Tanytarsini dominated the summer and autumn. Additionally, this study included a secondary production and microdistribution analysis of a population of the caddisfly Cyrnellus fraternus. Distributions of C. fraternus were restricted to snags, the most stable substratum for retreat building and for resisting periodic high waters. Larvae were not found in the soft sand and mud benthic habitats near submerged wood. The insects were multivoltine with a minimum of three generations per year. Pupae were collected July through September and adults emerged from May through October. The population overwintered as larvae. Estimated annual production was 1,015.4 mg/m2/yr. Mean standing stock biomass was 61.24 mg/m2, the cohort production/biomass ratio was 4.59, and the annual production/biomass rate was 16.58/yr for this population. Standing stock biomass ranged from 1.6 to 349.1 mg/m2 during the year and was significantly higher in autumn and summer than winter. Larvae were more abundant in the lower portions of the study area where flow was reduced for much of the year, than in the upper reaches, characterized by higher current velocities. C. fraternus larvae were found in their highest numbers on snags with dense deposits of silt and sand