Researcher finds nature’s hidden threshold for saltiness in the space where forests meet streams

Riparian zones, the space where forests and streams meet, are intimately connected. Yet a wholistic understanding about these zones is lacking because such work requires a combination of aquatic and terrestrial sciences.

That’s soon to change with the help of a nearly $1 million National Science Foundation grant led by scientists at Virginia Tech and the University of Arkansas. Their work will explore how salt impacts the movement of resources like carbon in riparian zones, along with its impact on processes and organisms.

Sally Entrekin, professor of aquatic entomology in Virginia Tech’s College of Agriculture and Life Sciences, has joined forces with Natalie Clay, associate professor of entomology and plant pathology for the Arkansas Agricultural Experiment Station, and Michelle Evans-White, professor of biological sciences at the University of Arkansas, for the study.

Salts are minerals that can either be essential nutrients or contaminants, depending on the amount and location. For forests far from the coast or saline rivers, salt is lacking and welcomed in moderation by lifeforms that need it to live. In freshwater aquatic systems, however, scientists have documented unwelcome salinization. So, what about that place where they meet — the riparian zone?

“Aquatic scientists have been approaching salt from a totally different perspective than terrestrial systems, and yet these two systems are connected,” Clay said. “They’re usually studied in silos. Terrestrial people often don’t talk to aquatic people and aquatic people often don’t talk to terrestrial people, and the result is a lack of understanding of these systems as a whole.”

“Understanding these terrestrial-aquatic linkages is key in moving forward to understand the pathway and consequences for biodiversity and food webs that are being impacted by global salinization,” Entrekin said.

The National Science Foundation is providing $948,291 to the three researchers for the project. Not only are they merging two fields of science, but they’re bringing the next generation of scientists with them.

Crowdsourced data, some of which will be collected by K-12 students and teachers across the nation, will bolster the study’s data points and expose youth to a major science project quantifying riparian zone chemistry.

The crowdsourcing will be done with Pennsylvania-based Stroud Water Research Center. Tara Muenz, Stroud Center assistant director of education and Leaf Pack Network coordinator, is leading the recruitment of individuals from volunteer stream monitoring partners like Virginia Stream Team, Izaak Walton League’s Save-Our-Streams program, SciStarter.org, Stroud Center's Leaf Pack International Network, and other professional networks.

A changing world

Globally, salinization is an issue that scientists are keeping close tabs on in both terrestrial soil and freshwater systems because it can change water quality and plant productivity.

Freshwater salinization is often preceded by terrestrial salinization from human activities like urbanization, agricultural practices, and road salting. However, predicting how salinization alters carbon cycling across riparian-stream boundaries is not yet possible. The National Science Foundation-backed research will help quantify how salinization alters carbon cycling across terrestrial-aquatic boundaries.

Carbon cycling is the process in which carbon atoms, the building blocks of all life on Earth, continually travel from the atmosphere into the ground and then back into the atmosphere. While the amount of carbon on Earth does not change, the location of those carbon atoms is constantly in flux.

“Salt is rare in terrestrial systems, unless you live on the coast, so adding a little bit causes this flurry of activity in all kinds of organisms,” Clay said. “We see animals showing sodium-seeking behavior, so in terrestrial systems it’s usually in shortfall.”

Entrekin and her fellow researchers predict that natural processes like decomposition and ecosystem respiration in the riparian zone will react positively to salinization to a certain point, since sodium is a biologically essential nutrient. After that optimal threshold is met, however, the ecosystem is expected to decline as salt becomes a stressor at sub-lethal levels and a toxicant at higher levels.

Finding that threshold will define the fundamental principles of how aquatic and terrestrial systems respond to increased salinization across a salinization gradient.

“By identifying these thresholds, we can also help predict changes to aquatic insect diversity and their contribution to both the aquatic and terrestrial food webs,” Entrekin said.

Written John Lovett, University of Arkansas System Division of Agriculture, Arkansas Agricultural Experiment Station, with Marya Barlow