Abstract
In the Western United States, volcanic spring-fed rivers are anticipated to become increasingly more important for salmonids and other native fishes, as these rivers will retain coldwater habitats as the climate warms. Despite this, little is known about the hydro-biogeochemical interactions within these ecosystems. A review of existing literature on spring-fed rivers, coupled with a decade of research on volcanic spring-fed rivers of northern California, finds that these systems are exceptionally productive and exhibit stable environmental conditions. These unique conditions stem from hydrogeologic processes typical of young volcanic terrains. Aquatic macrophytes, common to some nutrient-rich spring-fed systems, play a disproportionate role in hydrologic and geomorphic processes by facilitating ecological interactions and velocity conditions that improve juvenile salmonid growth. We find that volcanic spring-fed rivers are also resilient to climate change, due not only to their ability to dampen water temperature changes through deep groundwater flow but also because of their nutrient-driven high ecosystem productivity, which may enable coldwater species to metabolically compensate for marginal increases in water temperature. Understanding the fundamental geomorphic and ecological differences between these rare ecosystems and their numerically dominant runoff rivers is essential for developing long-term conservation strategies for coldwater species under a rapidly changing climate.
Highlights
The loss of freshwater taxa has occurred at unprecedented rates [1,2]
Much of the literature regarding coldwater fishes, including salmonids, has focused on the physical, biological, and chemical processes associated with runoff-dominated rivers or those generating discharge from rain events and snowmelt
Volcanic spring-fed rivers are often dominated by smaller substrate classes, such as sands and gravels, exhibit poorly developed channel bar features, rectangular cross-sections, and large width-todepth ratios when compared with runoff rivers [10,37,40]
Summary
The loss of freshwater taxa has occurred at unprecedented rates [1,2]. In California alone, 74% of native salmonids are projected to be extirpated by 2100 [3] if present trends continue. Much of the literature regarding coldwater fishes, including salmonids, has focused on the physical, biological, and chemical processes associated with runoff-dominated rivers or those generating discharge from rain events and snowmelt (hereafter, runoff rivers or systems) This stems from the fact that runoff rivers are numerically dominant throughout western North America and elsewhere, are relatively common study systems, and are anticipated to be strongly affected by climate change. Volcanic spring-fed rivers, or those deriving discharge from groundwater and occurring in volcanic terrains, have received far less attention in the scientific literature [5,6] These ecosystems exhibit a distinctive interplay between abiotic and biotic processes and are important contributors to coldwater fish production [7]. We combined a literature review with over a decade of research on volcanic spring-fed rivers in Northern California, USA to understand the principal geomorphic and ecological differences between volcanic spring-fed and runoff rivers, the response of volcanic spring-fed rivers to climate change, and the implications for coldwater biota, and why these systems are critical and historically undervalued habitats for coldwater fishes
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