Watershed Controls on Stream Thermal Regimes: Effects on Salmon Spawn Timing and Species Interactions

Global surface temperatures are warming at unprecedented rates and are expected to continue to warm for the rest of the century. Yet, we know very little about how freshwater ecosystems will respond to a warmer climate. Regardless of uncertainties in global climate projections and unknown thermal responses in freshwater ecosystems, recent responses of freshwater environments to climate variation can provide important insight to future conditions. In this dissertation, I describe the role of geomorphic variation within river basins as a driver of thermal heterogeneity among streams in southwest Alaska. Further, I describe the importance of this thermal heterogeneity in supporting variation in salmon life histories and the consequences of this life-history diversity for species that directly and indirectly depend on salmon.;In Chapter 2, I explore how geomorphic variation produces intraspecific diversity in salmon spawn timing throughout a river system due to large thermal differences among tributaries within a river basin. Here, salmon spawn later in warmer tributaries draining flatter watersheds with lakes and earlier in cooler tributaries draining steeper, high elevation watersheds. Research in Chapter 3 further explores the mechanisms for heterogeneity in water temperature among streams across the geomorphic gradient. Here, I show how snowmelt buffers streams from warmer air temperatures in steeper watersheds. In Chapter 4, I show how lakes, well-known geomorphic features in boreal river systems, affect the thermal regimes of outlet-rivers through wind driven upwelling events. Rivers draining lakes are traditionally thought to be thermally stable environments, but I show how wind action on large upstream lakes can create temperature excursions in the river by 10°C in few hours. Chapter 5 explores how the variation in salmon spawn timing among streams, supported by differences in water temperature, is associated with the phenology in a riparian plant. Specifically, I test the hypothesis that a pollinator mutualism indirectly ties riparian plant flower timing to the seasonal influx of salmon resources to streams.;Taken together, this research highlights how geomorphic variation within river basins filters climate responses in freshwater streams and rivers. Thus, river drainages may express a range of reactions to environmental change due to their geomorphic complexity. Aquatic species that have adapted specific life-history strategies to features within river basins may experience and exploit local filtering of climate forcing rather than responding to regionally coordinated climate responses.