Advances in Understanding Landscape Influences on Freshwater Habitats and Biological Assemblages

<i>Abstract.</i>—Climatic variation is a key driver of freshwater physical processes that in turn control stream fish growth and population dynamics at fine spatial scales and species distributions across broad landscapes. A recent downturn in Chinook Salmon <i>Oncorhynchus tshawytscha </i>returns across the Yukon River basin, Alaska, USA and Yukon Territories, Canada has led to hardship among user groups and increased interest in understanding how freshwater processes affect population persistence within this important commercial, recreational, and subsistence fishery. Here, we present results for the Chena River basin, interior Alaska, where we used field observations and riverscape-scale spatially explicit models to assess the influence of stream temperature on juvenile Chinook Salmon growth potential among years (2003–2015) and across 438 stream kilometers. We ran bioenergetic simulations for warm and cool year scenarios and contrasted temperature model precision and growth among different habitat types (small and large tributaries, main stem, and side channels) based on field estimates of growth, size, diet, and measured stream temperatures. Stream temperature regimes predicted from remotely sensed land surface temperatures were precise during the open water season (<I>R</I>² > 0.87; root-mean-squared error < 1.1°C), although the relationship was weakest in groundwater-mediated tributary habitats. Field observations revealed salmon were 67% larger by mass (g) in September during a warm year versus a cool year from main-stem sites. Bioenergetic simulations predicted that, on average, growth potential was 42% higher in warm years, although growth potential varied across the riverscape as much as 60% between cool upstream and warm downstream habitats. Climate variability is clearly an important driver of freshwater habitat conditions and has a large role in controlling freshwater growth of juvenile salmon. A better understanding of how climate influences growth conditions in different habitat types and across broad landscapes will be critical for conservation and management of Alaskan Chinook Salmon stocks under an expected warmer and more variable climate.