The Drivers and Ecological Importance of Streamflow Permanence for Native Salmonids: Evaluated at Multiple Spatio-Temporal Scales

In freshwater ecosystems, aquatic resources are distributed patchily across both space and time. Non-permanent streams, which go through periodic episodes of drying and rewetting, account for more than half of all streams in North America and help create much of the diversity that defines aquatic ecosystem form and function. While non-permanent streams have become increasingly common under climate change, we have a poor understanding of both their distribution across the landscape as well as how native stream fishes make use of them when they are flowing. I analyzed a large data set that spanned multiple decades across the northern Rocky Mountains to describe the dominant climate and landscape conditions that make a stream and its watershed vulnerable to summer drying. Additionally, I examined the sensitivity of habitat supporting native salmonids to changes in climate and landcover. Accelerated climate warming will increase the amount of habitat lost to drying, particularly in small, arid catchments that support cutthroat trout. While summer habitat may decline under anticipated climate futures, it is important to understand if non-permanent streams are useful when they are flowing. As such, I described the spatio-temporal variability in annual spawning suitability for a native salmonid species that occupies streams with different flow classes in an Intermountain West river basin. I then characterized how suitability is impacted by watershed physical and annual climatic conditions. In the low snowpack year, suitable spawning habitat was concentrated in permanent tributaries, as non-permanent streams did not support sufficient surface flows. In a high snowpack year, non-permanent tributaries provided extensive suitable spawning habitat, while permanent tributaries demonstrated reduced suitability due to cold temperatures. Importantly, aggregate, basin- wide spawning opportunities remained relatively constant between years, as suitability gains in non-permanent tributaries balanced the losses in permanent ones. ...