Date of Award:


Document Type:


Degree Name:

Master of Science (MS)


Watershed Sciences

Committee Chair(s)

Janice Brahney


Janice Brahney


James Dunnigan


Phaedra Budy


Charles Hawkins


Didymosphenia geminata (Didymo) is a nuisance algae that can cover entire streambeds under certain environmental conditions. Numerous studies have shown that it changes the composition of stream invertebrates. Fishes in many headwaters are known to feed almost exclusively on invertebrates. Thus, there is concern changes to the amount or type of invertebrates caused by Didymo blooms will impact fishes such as trout, charr, and sculpin. In the Kootenai River basin of Montana and British Columbia, we examined stream invertebrates and fish diets, condition, and growth across 25 streams during the summers of 2018 and 2019. The severity of Didymo blooms in these streams ranged from 0 – 80% coverage of the entire streambed. In 2018, we observed significant shifts in the types of stream invertebrates available to trout in Didymo-affected streams. However, trout diets and growth rate were not affected. In 2019, trout, charr, and sculpin diets in streams with severe Didymo blooms were similar to streams with little to no Didymo. Condition of all three types of fish were unaffected. We therefore conclude that summer Didymo blooms have no obvious impacts on the diet, condition, or growth of these fishes. We suggest further studies document potential impacts during winter months and on sensitive invertebrates such as freshwater mussels.