In Iceland stream, possible glimpse of warming future

"As we warm up the planet, the consequences for a community's natural ecosystem will depend on dispersal," said Dr. Jon Benstead, a freshwater ecologist and UA professor of biological sciences. "Communities will change and receive new species that can disperse that not only do well under those warm conditions, but can get there.

"If you don't do well in warmer temperatures, and you can't disperse, you're toast."

Benstead's former doctoral student, Dr. Daniel Nelson, is the lead author on two papers published in the journals Ecology and Global Change Biology. Nelson is now a post-doctoral researcher at the University of Oklahoma.

Nelson said the ability of a species to disperse and optimally perform in warmer temperatures is often overlooked in theoretical studies.

"They are important biological traits when it comes to be able to predict how organisms will reassemble and survive in a warm future," Nelson said.

Co-authors at UA include Benstead and Dr. Alex Huryn, an ecologist who directs UA's Center for Freshwater Studies and professor of biological sciences. Dr. Philip Johnson, UA professor emeritus of civil engineering, is also a co-author.

Other co-authors on the papers included researchers from Montana State and Ohio State universities as well as University of Iceland and the Institute of Freshwater Fisheries in Reykjavık, Iceland.

As part of a larger project in Iceland, the research team used the mix of the island's cold climate and geothermal energy to heat part of a normally cold stream. A heat exchanger custom-built by Johnson let the team carefully control water temperatures for experiments.

The team dammed a cold stream to create a pool for a pipe to take on water. Using gravity, the water ran down into a pool created by damming an adjacent, warmer stream. The cool water zigzagged through pipes immersed in a warmer stream. This heated it to the desired temperatures before it flowed down an exit pipe back into the cool stream to heat it.

The result was the cool stream warmed about 6.8 degrees Fahrenheit.

The team set out to see what would happen to life once the stream warmed, specifically the invertebrate species since no known vertebrates such as fish lived in the stream. A scientific concept called metabolic theory predicted that, as the stream warmed, fewer organisms would survive, but those who did would grow at a faster rate.

In fact, the surviving organisms should grow at such a rate as to balance out the loss of life. That would mean secondary production, or the turnover in sum biomass of all non-plant life living in a fixed area over a certain time, should stay the same.

The team confirmed the predictions of metabolic theory, but not in the way expected. Biomass production did stay level, and the number of individual organisms decreased. What changed was the distribution of the size of the organisms, according to the findings.

"We got fewer relatively small organisms that grow quickly, and more large-bodied individuals that grow slower," Benstead said. "That's what gave rise to this lack of change in total production."

In fact, two species that did not live in the stream before the temperature changes colonized the warmed stream, Nelson said.

The reason why confirms the usefulness of conducting the experiment in nature, as opposed to a lab, Benstead said.

The experiment took place on the slopes of the dormant volcano Hengill. Its geothermal activity underneath warms many of the streams flowing along its surface, but not all. The result is that smaller species with fast growth rates adapted to the cold of the north Atlantic Ocean thrive in the cooler streams while more warm-adapted, larger and fast-growing species live in the hotter streams next door.

The larger, warm-adapted species such as snails and black flies found a way over to the warmed stream while the smaller species such as midges began declining.

"When we warmed up a naturally cold stream, there's plenty of dispersal especially of animals with aerial adults," Benstead said.

In a controlled experiment in a lab, this result would not have happened, he said.

"We would have been dealing with entirely closed populations and communities that couldn't receive anything from the outside," Benstead said of a lab experiment. "We may have seen certain species struggle or do well, but we never would have gotten this amazing result that's driven by dispersal."