While most use the Second College Grant — a 27,000 acre area of land in Clarksville, New Hampshire — to canoe or fish, environmental studies professor Lauren Culler A&S’13 used its data to quantify the relationship between warming air temperatures and streams.
A recent Dartmouth-led study published in April found that an air temperature increase of one degree Celsius over five days corresponded to an increase in maximum daily stream temperature of 0.5 to 0.8 degrees Celsius, which could affect local ecosystems and brook trout populations. The study was published in April in science journal Freshwater Biology and was a collaboration between Dartmouth researchers and New Hampshire Fish and Game.
Culler, the study’s lead author and the science outreach coordinator at Dartmouth’s Institute of Arctic Studies, said she began working on the project in 2010 while she was a graduate student at Dartmouth working in the laboratory of biology professor Matt Ayres. Since 2001, Ayres had been collecting data on stream temperatures at the Second College Grant, which is home to the Dead Diamond River watershed, the largest watershed in New Hampshire and Maine.
At the time, Culler said she and Ayres began to discus potential research questions they could explore with “this really amazing dataset” and realized they could answer fundamental questions such as how stream temperature can vary with air temperature.
“In 2000 or so, we knew enough to understand the planet might be warming up but what we didn’t know is how changes in air temperature would relate to changes in water temperature,” Ayres said. “In order to predict what the consequences would be for any scenario of air warming, we needed to understand what the relationships are between air temperature and water temperature.”
Culler said she spent the next three years analyzing the data. Due to the dataset’s magnitude, Culler enlisted undergraduates to work with her and build the model that was published in the paper.
She added the study is important not because it shows that streams become warmer as air does, which is intuitive, but because her team was able to pin down to what extent air temperatures affect water temperatures.
“I think a lot of people could easily see the press release and think, ‘Well of course. If it’s warmer outside the water will be warmer,’” Culler said. “But we wanted to know by how much the water is warmed. That’s what we were able to quantify using this long-term data set of hourly temperatures.”
Using the Second College Grant as a natural laboratory, Ayres said that quantifying the change in water temperatures is important because of the ways in which increased water temperature can affect the organisms that live in streams.
“The Second College Grant is a jewel of Dartmouth, of New Hampshire, really of the whole region,” he said. “The cold water streams at the Grant are home to book trout, spring salamanders and hundreds of species of beautiful aquatic insects. But the attributes of those ecosystems depend on it remaining cool, on cold-water streams remaining cold-water streams.”
Culler found that stream temperatures were buffered so they did not change as much as air temperatures do. For every degree Celsius in air temperature increase over several days, streams generally warm by 0.5 degrees to 0.8 degrees.
Even a buffered warming is alarming for the ecosystem, according to ecology graduate student Keith Fritschie. He added that increases in summer temperatures affect species like trout in three major ways: their growth and survival, life cycles and population dispersion.
According to the study, the Dead Diamond River watershed is home to the last self-sustaining native population of brook trout in New Hampshire and Vermont.
“Above a certain temperature threshold, you start to see pretty strong effects on the survivability of fish,” Fritschie said. “You could have larger die-offs because fish can’t really process the food they’re eating into growth. They can barely sustain their normal bodily functions. There’s also a relationship between water temperature and the amount of oxygen it can hold. At some point, the water temperature gets so high that it doesn’t hold enough oxygen for fish to even breathe.”
He added that warmer waters will stunt the growth of fish, which could decrease the number of offspring in a population.
Second, increased stream temperatures affects their phenology, which is the timing of different life history events like spawning and egg hatching. Warmer temperatures could lead to fish spawning later in the season, which in turn delays when those eggs hatch in the spring.
Third, warmer streams can potentially affect the connectivity of different populations within a watershed.
“In the same way dams or road crossings might fragment a river, temperature has the potential to have a separate populations,” he said.
One of Culler’s most interesting findings was that not all streams warm equally. Some streams — particularly smaller headwater streams, which are streams that are close to the source — are more resistant to increases in air temperature.
Ayres noted that this finding is important for trout’s ability to adapt to warming waters.
“In the Diamond River Watershed, there are a few tributary streams that stay cool even on the warmest days,” Ayres said. “We see trout using those as refuge habitats. We now have the ability to look at a watershed and identify the most likely location of these streams that can be refuges. The population of trout within a 10 or 20 mile radius might depend on these few cold water streams.”
Preventing development in these areas could be a lesson that policy-makers take away from this study, he said.
“If development or something happened to take away these cold water refuges, even though they’re not a very big percentage of the habitat, it could lead to a loss of the trout population in the area,” Ayres said.
Culler said that New Hampshire Fish and Game has put trackers on fish so they can keep track of where fish are. This will help officials think about how to allow access to different parts of streams, which can show which areas are cold water refuges.
Ayres said that this study should be a warning for people in New England.
“In New York, Massachusetts and Pennsylvania, there’s a lot of places that used to have brook trout that don’t anymore,” Ayres said. “This is largely due to warmer temperatures.”
