Stream Warming and Brook Trout in the Southeast

Since brook trout are adapted to thrive in cold water, they are susceptible to increases in stream temperature, which is occurring as a result of climate change and changes to the terrestrial landscape.

Air temperatures in the Southeast are projected to increase 1.4 to 1.9°C between 2036 and 2065 and an additional 2.8 to 4.8°C between 2071 and 2100 as a result of global climate warming.

Warmer air temperatures will raise stream temperatures and negatively affect organisms where stream temperatures increase above which animals can adapt to.

Forest clearing and runoff from urban and agricultural land futher increase water temperatures.

Riparian buffers surrounding streams are important for maintaining stable and suitable temperatures, but they are under threat from human development.

Thus, it is important to understand how stream temperatures are changing and will continue to change to predict how brook trout populations may change.

Forested streams like those in the Southern Appalachians play an important role in regulating the carbon dioxide balance in Earth’s ecosystems.

They are important habitats for organisms like crayfish, stoneflies, fungi, salamanders and fishes, including brook trout.

The research team studied how stream organisms and energy processes in rivers respond to temperature to better predict how these ecosystems will respond to future changes in temperature.

We scaled the results of these studies up to the stream network level to investigate the consequences of warming at the extent of a whole watershed.

Check back here in the future to learn more about this larger project. Continue on to learn more about stream temperature and brook trout.

Stream temperature is only monitored in select locations due to financial and logistical constraints. But we're often interested in temperatures in other locations.

We can use statistical models to estimate stream temperature in other locations, to make predictions, and inform management actions.

The area we studied is in the Southern Appalachian Mountains in western North Carolina, encompassing the cities of Franklin and Cullowhee, and the Nantahala National Forest.

In collaboration with Trout Unlimited and Mainspring Conservation Trust, we measured the temperature of many streams in the Upper Tennessee River watershed between 2010 and 2021. We then used the data to create a statistical model that predicts stream temperature across the entire watershed.

We calculated the mean July stream temperature, a time that brook trout are most likely to experience thermal stress, for each year and each site where temperature was measured.

To predict temperature where there are no temperature measurements, we built a model to quantify the relationship between stream temperature and variables such as elevation, latitude, air temperature, and percentage of the stream reach with urban development.

We found that higher elevations and cooler air temperature were associated with lower stream temperatures, and high percentages of urban development were associated with higher stream temperatures.

We applied our stream temperature model to the network to create a current stream temperature map based on July 2021 conditions, and future stream conditions based on potential climate scenarios with increased air temperatures.

We can now compare future stream temperature projections with the current stream temperatures.

This allows us to identify stream reaches which are the most at risk for stream temperature rise and those that could serve as climate refugia for thermally sensitive cold water species.

A main objective for our stream temperature model is to identify the habitats within our study area that could serve as climate refugia for brook trout, and areas of high concern where currently suitable habitat may become unsuitable.

Brook trout can not sustain populations at temperatures greater than approximately 20.2°C, and 13.0 -18.0 °C is their ideal thermal range.

We classified stream temperatures of <18.0 °C as ideal habitat for brook trout, 18.0 - 19.5°C as fair habitat, and above 19.5°C as unsuitable habitat.

Given our analysis, thermally* ideal (<18°C) brook trout habitat is currently found in 3,836 miles of streams in our study area, but that will decrease by 23% if temperatures rise 1°C (2,974 miles) or 91% (341 miles) at 4°C.

*We only analyzed brook trout habitat suitability based on temperature. This does not account for stream size and other considerations.

Fair (18-19.5°C) habitat is currently in 661 miles of streams in our study area, increasing by 94% if temperatures rise 1°C (1,282 miles) or 98% (1,315 miles) at 4°C.

Unsuitable (>19.5°C) habitat is currently in 117 miles of streams in our study area, increasing by 206% if temperatures rise 1°C (358 miles) or 2,527% (2,957 miles) at 4°C.

Warming stream temperatures are caused by many factors. Warmer air temperatures (climate change), deforestation, and runoff from urban environments all contribute to warming water. As we saw in our models, the amount of urban land use in a watershed can contribute signficanlty to elevated stream temperatures. The loss of riparian forest cover that shades streams can also greatly increase stream temperatures.

Thus, it is important to maintain streamside forest cover to help reduce the impact of warming air temperatures.

Groundwater also plays an important role in controlling stream temperature. Groundwater can keep streams cooler during the summer and provide a stabilizing effect year-round. Loss of groundwater inputs due to extraction or prolonged drought can decrease groundwater inputs, leaving streams more susceptible to riparian forest cover loss and warmer air temperatures.

What is the likelihood of these various warming scenarios?

The warming scenarios we used were based on current estimates of warming trends for streams in the USA. A warming scenario of + 1 C streamwater may be likely within a few decades while +4 C could be seen by the end of the century*. Many factors effect the rate of warming including the amount and source of groundwater as well as geographical location. However, since landuse change like loss of forest cover can also increase stream temperatures, realized rates of warming could be more extreme than our scenarios which only account for increased air temperature.

*Hare et al. 2021. Continental-scale analysis of shallow and deep groundwater contributions to streams. Nature Communications. 12: 1450.

How else are streams changing because of warming?

As you can imagine, Brook trout aren’t the only organisms that have limits to the temperatures they can tolerate.  In the southeast, we enjoy some of the highest numbers of native freshwater organisms found anywhere in the world.  Some of those organisms are known to be sensitive to high temperatures. Thus, mainting forest cover and reducing runoff from urban environments is an important management action to help mitigate the increase in air temperature.

As part of the larger project that the Brook trout analysis was conducted, we determined the effects of higher temperatures on the main food resources that support the base of the food web that these fish depend on.  The food web base is the leaves and wood that fall into streams, that together with algae, feed the small bugs that fish feed on.  We found that because the leaves and wood decay faster at increased temperatures, that there might be less food at the base of the food web in a warming world. Check back here in the future for a link to another StoryMap describing these findings!