Long-term trends of stream temperature in western North American streams
Ivan Arismendi, Sherri Johnson, Jason Dunham, Roy Haggerty
Recent warming of terrestrial climate in most parts of the world has motivated concern about corresponding increases in water temperature. Cold water obligate organisms found in western North America are especially sensitive to changes in the temperature of streams. The link between terrestrial climate and stream temperature is complicated in part by uncertainties regarding the role of climate change versus other factors, such as changes in land or water use, land cover, and complexities of the heat budget of streams. Based on observed trends in air temperature and stream flow, we expected to find warming of streams as well as increasing temperature variability over time. Contrary to this expectation, we found both cooling and warming trends in 18 minimally human-influenced and 45 more human-influenced streams across western North America (See Figure) based on data available from 1951-2009. The most recent two decades (1987-2009) produced mostly cooling trends in minimally human-influenced sites whereas more human-influenced systems showed mixed responses. Lack of coherence between air and stream temperature in recent decades may be related to complex and lagged interactions among non-climatic and climatic variables. Regardless of the processes driving these patterns, our results suggest that climate impacts on air and stream water cannot be simply correlated to infer future responses of stream temperature. Addressing gaps in our understanding of climate impacts on stream temperatures can provide an informed approach to sustaining key freshwater ecosystem services.
Acknowledgements. Financial support was provided by US Geological Survey, the US Forest Service Pacific Northwest Research Station and Oregon State University. Data were provided by the HJ Andrews Experimental Forest and LTER research program, funded by the National Science Foundation's Long-Term Ecological Research Program (DEB 08-23380), US Forest Service Pacific Northwest Research Station, and Oregon State University.
Figure legend - Study area and location of long-term stream gauge stations of temperature records for minimally human-influenced (dotted circle; n = 18) and more human-influenced watersheds (yellow square; n = 45) in western North America.