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Longitudinal thermal heterogeneity in rivers and refugia for coldwater species: effects of scale and climate change
Fullerton, A., C. Torgersen, J. Lawler, E. Steel, J.L. Ebersole, AND S. Lee. Longitudinal thermal heterogeneity in rivers and refugia for coldwater species: effects of scale and climate change. AQUATIC SCIENCES. Birkhaeuser Verlag AG, Basel, Switzerland, 80:3, (2018).
Many rivers and streams in the Pacific Northwest are currently listed as impaired under the Clean Water Act as a result of high summer water temperatures. Warm stream and river temperatures can impede migration of fish like adult salmon, slowing migration and increasing exposures to warm temperatures, predation, and other effects. Where available, areas of colder water within warm streams and rivers can potentially serve as ‘stepping stones’ allowing salmon and other coldwater fish to migrate through otherwise inhospitable temperatures. The spatial configuration and availability of colder water areas that may function in this way remains poorly understood. In this paper, we describe methodologies that can be used to characterize spatial patterns of temperature with implications for data resolution and scale. We describe the size and distribution of cold water patches in rivers across the Pacific Northwest, and the implications of climate change; both of which vary across the region. Our results can inform long-term monitoring programs as well as near-term climate-adaptation strategies. This article contributes to ACE CIVA 2.8 and SSWR 3.01C.
Climate-change driven increases in water temperature pose multiple challenges for aquatic organisms. Predictions of climate change impacts to biota typically do not account for fine-grained spatiotemporal patterns of stream networks; yet patches of cooler water within rivers could serve as important refuges for anadromous species that migrate during the warm season. We used high-resolution, remotely sensed, water temperature data to characterize summer thermal heterogeneity patterns for over 12,000 km of 2nd- to 7th-order rivers throughout the Pacific Northwest and northern California (403 river segments). We evaluated (1) the ability to resolve patterns in water temperature data at different spatial resolutions, (2) the frequency, size, and spacing of cold-water patches suitable for salmon, and (3) the potential influences of climate change on the availability of cold-water refuges. Thermal heterogeneity patterns were nonlinearly related to the spatial resolution of water temperature data, and heterogeneity at fine resolution (<1 km) would have been difficult to quantify without spatially continuous remotely sensed data. Cool patches suitable for salmon were generally >2.7 and <13.0 km long, and spacing among these patches was generally >5.7 and <49.4 km. However, thermal heterogeneity patterns varied across rivers, with some having long uninterrupted stretches of warm water, and others having many cool patches. Using two approaches to predict potential future thermal heterogeneity patterns, we found little change in patchiness across rivers, but large change within individual rivers. Our results can inform long-term monitoring programs as well as near-term climate-adaptation strategies.