How Do Fine Sediments and Hangman Creek Discharge Affect Benthic Macroinvertebrates in the Spokane River?

Abstract

One of the most common causes of stream degradation globally is fine sediment deposition. Most fine sediment deposition in stream beds is generated by land use practices such as timber harvest, agriculture and urbanization. Hangman Creek is a major tributary and source of fine sediment to the Spokane River. Benthic macroinvertebrates play a crucial function in food webs as an important source of food for many stream predators, including fish. Also, benthic invertebrates are effective indicators of the ecological condition of rivers, including those that are adversely affected by fine sediments. This study investigated the effects of fine sediments on Spokane River macroinvertebrates. I sampled macroinvertebrate communities from 3 substrate types with different levels of fine sediment (sand, 0 - 25% embedded cobbles, and 75-100 % embedded cobbles) in the Spokane River upstream and downstream of its confluence and in Hangman Creek. Water temperature, flow velocity, and depth were also measured. Hangman Creek tended to be warmer and shallower than the Spokane River. The three most common invertebrates in the Spokane River were Chironomidae, Baetis, and Hydropsyche. The three most common invertebrates in Hangman Creek were Chironomidae, Leptohyphidae and Hydropsyche. In early summer 2018, the substrates significantly affected the density of macroinvertebrates; family richness for Ephemeroptera, Plecoptera, and Trichoptera (EPT); proportion EPT; proportion clinger functional group; and the proportion of the dominant taxon. Family richness for all taxa; Evenness; Shannon-Weaver diversity; Hilsenhoff Family Biotic Index (HFBI); and Biological Sediment Tolerance Index (BSTI) were not affected. In late summer 2018, substrate significantly affected only the density of macroinvertebrates. Overall, the results suggest that areas with fine sediment were ecologically impaired compared to unembedded cobbles. However, not all indicators responded as expected. There was no evidence for the effects of fine sediment to increase through the summer, or of significant ecological degradation in the Spokane River directly below the confluence with Hangman Creek. This work will contribute to our understanding of the biotic effects of fine sediment in the Spokane River.