Big Hole River Macroinvertebrate Community Structure: Baseline 2024 (Wisdom, Montana)

A year ago, we completed a summary of the first four years of a long-term ecological monitoring program examining spatial and longitudinal changes in benthic macroinvertebrate community structure. It is an extensive report (178 pp.) summarizing many quantitative aspects of the benthic community of the Big Hole River. Still, probably the most critical finding was an apparent ecological impairment of the River near the town of Wisdom, MT. The dominant species of riffle areas were midges and worms. Other insects occurred at the site, but midges dominating the site were collector-gatherers and collector-filterers, presumably using excessive algae growth as refugia from predation and flow. Furthermore, the site stood out by having the highest Hilsenhoff Biotic Index (HBI) scores, and would have been classified by Hilsenhoff (1987) as impaired. The average (n=3) HBI scores ranged >6.0 to 7.0.

Collectively, chironomid midges comprised an average of 58-75% of the benthic community between 2019-2022 (inclusive). Although dominance by midges can indicate impairment, midges remain ecologically important contributors to most lotic ecosystems in Montana. Furthermore, the most-tolerant midges--species tolerant to periodic anaerobic conditions that would aphixiate most insects--were not observed in the Big Hole River during the monitoring period covered by the report. Thus, although the very high abundance of midges does suggest ecological impairment, it is clear that the site and the river overall can still get worse. There are still some mayflies and caddisflies at the location. There are even a few stoneflies. However, the taxonomic composition of the site's benthic fauna indicates this region of the Big Hole River has an impaired community--likely due to excessive algae growth. For more details, download and read the entire report!

Enjoy. Feel free to reach out with questions.

BigHoleBugs 10-10-24 RCC.pdf

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Keywords: Big Hole River, benthic macroinvertebrate taxonomy, aquatic insects, aquatic entomology, midges, bioassessment