In aquatic animals, temperature can affect metabolism, growth, and reproduction, as well as response to pathogens and sensitivity to pollutants. Thus temperature is a key driver of the ecological processes that control population and community structure in aquatic ecosystems.
Across large watersheds, temperature partly defines species distributions. At the reach or stream level, temperature can define the extent and connectivity of suitable habitats, including those used during critical life stages such as foraging and breeding.
Human activities that have altered the thermal regime of streams and rivers include operation of hydropower dams, development of land adjacent to streams, and water withdrawal for irrigation.
Climate change is predicted to further alter thermal and hydrologic conditions in streams and rivers across the Pacific Northwest. For coldwater species such as Pacific salmon, real challenges to conservation can result from increases in temperature, altered variability in thermal regimes, and changes to the configuration of thermal refuges.
To ensure success, many restoration plans seek to incorporate climate adaption strategies, which consider threatened stocks in terms of their vulnerability to expected changes.
Recent advances in sampling technology enable us to compare patterns in temperature across hundreds of rivers throughout the Pacific Northwest. With these data, we can characterize patterns in stream temperature that are important to salmon within and across reaches, rivers, or watersheds.
Below we briefly describe completed, ongoing, and planned analyses that will enable us to conserve thermal habitat for Pacific salmon. Our collaborative partners include the University of Washington, U.S. Geological Survey, and U.S. Forest Service, as well as specialists from the private sector.
Our challenge is to characterize present and future spatial patterns of thermal habitat in order to map locations where Pacific salmon may be vulnerable in responding to expected changes in temperature. By learning about features of thermal habitat that are important to fish, managers can better understand how to conserve and restore key habitats. Research questions that guide our analysis include:
To address these questions, we are using remotely–sensed summertime stream temperature from hundreds of watersheds across the Pacific Northwest and California.
We compared whole–river thermal profiles (stream temperature vs. distance) of over 60 rivers. At right is an example of such a profile for the Molalla River, Oregon. Both the map and graph at right illustrate thermal heterogeneity at multiple spatial scales.
We also investigated spatial patterns of water temperature within rivers, looking at metrics such as the length and spacing of thermal "patches " of river below a biological threshold. For example, we quantified what proportion of the river fell below 15°C, the length of each cold patch, and how far apart cold patches were spaced. We looked at how these metrics differed among salmon species and with landscape characteristics such as stream size and elevation.
To identify the potential implications for salmon of alterations to natural thermal regimes, we addressed research questions that would assess the temporal variability of water temperature at hourly, daily, seasonal, and annual scales.
To address the first of these questions, we used year–round temperature data collected from over 30 monitoring stations throughout a small watershed (pictured here), along with continuous spatial data from one summer.
Focusing on cold–water patches identified with the spatial data, we used this dataset to consider whether fluctuations in temperature differ among sites, and to determine the duration of each cold–water patch.
These evaluations may tell us how thermal refuges change over time, and whether such changes are consistent throughout the watershed.
To explore how salmon respond to altered thermal regimes, we used two separate approaches. First, we conducted a laboratory experiment to determine whether different levels of thermal variation would influence the size, condition, development, or emergence timing of Chinook salmon.
For this experiment, we exposed Chinook salmon to eight different thermal regimes during egg and alevin incubation. Each temperature regime differed in frequency, magnitude, and duration, but all had the same mean temperature.
Our second approach has been to begin development of a spatially explicit, individual–based model to help us evaluate the potential effects of altered thermal regimes on survival and smolt migration timing.
Funding for this work was provided by the Northwest Fisheries Science Center Internal Grants Program (2009), the NOAA Fisheries Advanced Studies Program (2010–2012), a Eugene Maughan Scholarship from the Western Division of the American Fisheries Society (2013), and the North Pacific Landscape Conservation Cooperative (2014).
Fullerton, A.H., C.E. Torgersen, J.J. Lawler, R.N. Faux, E.A. Steel, T.J. Beechie, J.L. Ebersole, and S.G. Leibowitz. 2015. Rethinking the longitudinal stream temperature paradigm: region–wide comparison of thermal infrared imagery reveals unexpected complexity of river temperatures. Hydrological Processes 10.1002/hyp.10506.
Steel, E.A., A. Tillotson, D.A. Larsen, A.H. Fullerton, K.P. Denton, and B.R. Beckman. 2012. Beyond the mean: The role of variability in predicting ecological impacts of stream temperature. Ecosphere 3(11). Article 104.