Characterizing and assessing seasonal variation of the benthic macroinvertebrate community in the Bighorn River in SE Montana

Abstract

Freshwater is a limited resource essential to the survival of humans and many other organisms and ecosystems on Earth, making its understanding and protection imperative. Benthic macroinvertebrates (BMI) are aquatic organisms found in these freshwater environments and they have long been used to characterize stream health as they demonstrate variable response to environmental conditions and play a large role as an intermediate trophic level within their food web, demonstrating both top-down and bottom-up effects. The Bighorn River Alliance (BHRA) is a nonprofit dedicated to the long-term health and viability of the Bighorn River in southeast Montana. They launched a research initiative in 2018 which includes biannual, seasonal collections of BMIs at several locations along the Bighorn River. Using data collected by the BHRA, I assessed BMI taxanomic diversity for differences in richness, alpha diversity, beta diversity, and rank abundance from eight collections sampled from fall 2020-fall 2024 across seven sites. I assessed this BMI community to set a baseline of expected seasonal variation in BMI assemblages, explore the impact of variable discharge and related factors on the BMI community, and as a tool to assess stream health. Seasonal differences are marked by increased species richness in fall, with abundant Naididae (sludge worms), Parakiefferiella spp. (midge), and Baetis tricaudatus (blue-winged olives) in spring and abundant Caecidotea sp. (aquatic sowbugs), Naididae, and Cricotopus spp. (midge) in fall. The Bighorn River in this region is a tailwater river with hydrology primarily regulated by the Yellowtail Dam. While dams typically reduce hydrological variation, the system saw record flows in the summer of 2023. Collections after this event demonstrated reduced evenness, dominance by Caecidotea sp., and more dissimilarity in community composition compared to collections prior to summer 2023. This shift appears to be concentrated to upper collection sites while lower collection sites demonstrated a shift towards more EPT (Ephemeroptera, Plecoptera, Trichoptera) taxa which are more sensitive to pollution than Caecidotea sp. This suggests that stream health may vary down the length of the river, but a more explicit site-wise analysis is needed.