Abstract
Abstract. Watershed-scale stream temperature models are often one-dimensional because they require fewer data and are more computationally efficient than two- or three-dimensional models. However, one-dimensional models assume completely mixed reaches and ignore small-scale spatial temperature variability, which may create temperature barriers or refugia for cold-water aquatic species. Fine spatial- and temporal-resolution stream temperature monitoring provides information to identify river features with increased thermal variability. We used distributed temperature sensing (DTS) to observe small-scale stream temperature variability, measured as a temperature range through space and time, within two 400 m reaches in summer 2015 in Nevada's East Walker and main stem Walker rivers. Thermal infrared (TIR) aerial imagery collected in summer 2012 quantified the spatial temperature variability throughout the Walker Basin. We coupled both types of high-resolution measured data with simulated stream temperatures to corroborate model results and estimate the spatial distribution of thermal refugia for Lahontan cutthroat trout and other cold-water species. Temperature model estimates were within the DTS-measured temperature ranges 21 % and 70 % of the time for the East Walker River and main stem Walker River, respectively, and within TIR-measured temperatures 17 %, 5 %, and 5 % of the time for the East Walker, West Walker, and main stem Walker rivers, respectively. DTS, TIR, and modeled stream temperatures in the main stem Walker River nearly always exceeded the 21 ∘C optimal temperature threshold for adult trout, usually exceeded the 24 ∘C stress threshold, and could exceed the 28 ∘C lethal threshold for Lahontan cutthroat trout. Measured stream temperature ranges bracketed ambient river temperatures by −10.1 to +2.3 ∘C in agricultural return flows, −1.2 to +4 ∘C at diversions, −5.1 to +2 ∘C in beaver dams, and −4.2 to 0 ∘C at seeps. To better understand the role of these river features on thermal refugia during warm time periods, the respective temperature ranges were added to simulated stream temperatures at each of the identified river features. Based on this analysis, the average distance between thermal refugia in this system was 2.8 km. While simulated stream temperatures are often too warm to support Lahontan cutthroat trout and other cold-water species, thermal refugia may exist to improve habitat connectivity and facilitate trout movement between spawning and summer habitats. Overall, high-resolution DTS and TIR measurements quantify temperature ranges of refugia and augment process-based modeling.
Highlights
Trout and salmon avoid heat stress by sheltering in thermal refugia, or pockets of cooler water, when stream temperatures are near upper thermal tolerances (Dunham et al, 2003; Sutton et al, 2007)
We measured small-scale stream temperature variability that was unquantified in an existing one-dimensional, basin-scale model
The warmest temperatures were measured by thermal infrared (TIR) in the East Walker River (26.5 ◦C), but by Distributed temperature sensing (DTS) in the main stem Walker River (32.9 ◦C), indicating that these methods complement each other, and suggesting that different years may result in alternate temperature distributions along the river (Tables 2 and 3)
Summary
Trout and salmon avoid heat stress by sheltering in thermal refugia, or pockets of cooler water, when stream temperatures are near upper thermal tolerances (Dunham et al, 2003; Sutton et al, 2007). Where stream temperatures are warming or where cold-water fish species are near the margins of their ranges, measuring stream temperatures at small temporal and spatial scales is important to quantify thermal refugia and stream temperature heterogeneity (Vatland et al, 2015). Dzara et al.: Quantifying thermal refugia connectivity tudinal stream temperature changes at the watershed scale but are poor predictors of thermal micro-habitats. High-resolution temperature monitoring provides micro-habitat information but is typically conducted over small spatial extents and difficult to extrapolate to the watershed scale for management and restoration decisions
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