A new approach by U.S. Geological Survey scientists to modeling water temperatures resulted in more realistic predictions of how climate change will affect fish habitat by taking into account effects of cold groundwater sources.

The study, recently published in the journal Ecological Applications, showed that groundwater is highly influential but also highly variable among streams and will lead to a patchy distribution of suitable fish habitat under climate change.  This new modeling approach used brook trout, but can be applied to other species that require coldwater streams for survival.

"One thing that has been missing from other models is the recognition that groundwater moderates the temperature of headwater streams," said Nathaniel Hitt, a fish biologist and study coauthor. "Our paper helps to bring the effects of groundwater into climate change forecasts for fish habitat."

Climate change models predict that summer air temperatures will increase between 2.7 and 9 degrees Fahrenheit in the eastern United States over the next 50 to 100 years. Such increases in air temperatures will increase water temperatures of streams and rivers and pose a significant threat to fish like brook trout that have low resistance to warming water temperatures. 

Brook trout are an important cultural and recreational species with specific restoration outcomes identified in the new Chesapeake Bay Agreement.

However, how these global and regional predictions regarding a changing climate translate to water temperatures in specific streams or stream reaches, a process called “downscaling”, remains an important and challenging question for scientists and natural resource managers.

Previous efforts to downscale global and regional estimates of air temperature change down to water temperatures in individual streams and river networks have relied on the assumption that the exchange of heat between the water and the surrounding air is the primary driver of water temperature within an individual section of a stream. However, the exchange of heat between cold groundwater and warmer surface water can also be very important, especially in headwater streams where the volume of water is relatively small.

"Our models help improve the spatial resolution of climate change forecasts in headwater streams," said Craig Snyder, a USGS research ecologist and lead author of the study. "This work will assist conservation and restoration efforts by connecting climate change models to places that matter for stream fishes."

The study is available online:  Snyder, C.D., N.P. Hitt, and J.A. Young. 2015. Accounting for groundwater in stream fish thermal habitat responses to climate change. Ecological Applications 25:1397-1419.