Troubled waters: VCU researchers are studying the impact of saltwater intrusion on tidal wetlands

Monday, July 27, 2015

Along the muddy banks of the Pamunkey River in Virginia’s New Kent County, Virginia Commonwealth University researchers have built an irrigation system that is allowing them to simulate the potential effects of climate change on tidal wetlands.

The system, designed and built by Dong-Yoon Lee, Ph.D., a postdoctoral researcher in the Department of Biology, pumps saltwater into several plots of land in Cumberland Marsh, mimicking what will happen as the sea level rises due to climate change and intrudes increasingly into freshwater ecosystems.

“We decided to run a real-time simulation in which we built a saltwater pumping system in a natural wetland further upstream, where saltwater rarely flowed before,” Lee said.

Lee’s experiment, which is underway this summer, is part of a multiyear National Science Foundation grant that is enabling VCU biologists to study the impact of climate change on tidal wetlands.

While previous research has examined the effects of saltwater on freshwater marshes in the lab, this project is among the first studies of its kind to take place in the natural environment.

As part of the project, researchers are measuring the effects of saltwater on the soil’s microbial community, marsh plants and greenhouse gas emissions, as well as attempting to discover how these changes in tidal wetlands might affect the larger ecosystem.

Rima Franklin, Ph.D., associate professor of biology in the College of Humanities and Sciences, is principal investigator of the $780,000 grant, titled “Climate Change Effects on Coastal Wetlands — Linking Microbial Community Composition and Ecosystem Responses.” She said tidal freshwater wetlands are serving as the researchers’ model system.

“One of the main ways that saltwater intrusion is expected to affect the microbial community is by shifting the pathways involved in carbon cycling,” she said. “With our field manipulation, we will be able to track how this change in microbial activity scales up to affect the overall carbon cycling of the wetland. In particular, we will learn whether these microbial community changes affect the carbon storage capacity of the wetland and determine rates of greenhouse gases emissions — carbon dioxide and methane.”

A natural resource at risk

Wetlands provide a variety of important services to the environment. They filter nutrients and improve water quality for the rest of the estuary. They serve as the habitat of bald eagles, ospreys, great blue herons and egrets. They provide migratory and wintering habitats for waterfowl. They support a diverse array of plant species. And they play an important role in carbon sequestration, the process of capturing and storing carbon dioxide, which helps slow global warming by keeping carbon dioxide out of the atmosphere.

Yet as the sea level rises, the researchers say, all of those services are at risk.

With sea level rise and saltwater intrusion, we may end up losing these wetlands.

“With sea level rise and saltwater intrusion, we may end up losing these wetlands,” said Scott Neubauer, Ph.D., assistant professor of biology and co-investigator on the study.

The sea level is rising at a rate of 3 or 4 millimeters per year, Neubauer said, though that rate is accelerating and expected to reach 6 millimeters or more by the end of the century.

“There’s kind of a hotspot here in the mid-Atlantic in terms of sea level rise,” he said. “It’s increasing over time.”

Neubauer is interested in studying what happens to the carbon stored in the soil once saltwater intrudes.

“If we’re doing something that disturbs the carbon cycle — like bringing in saltwater and [consequently] reducing carbon sequestration — that could mean the wetland is growing vertically more slowly, which means as the sea level rises, the marsh is going to fall behind,” he said. “And once the marsh gets flooded too much, the plants can’t survive and you lose all the important functions of the wetlands.

“If you convert your tidal wetland to a mudflat, you’ve lost a lot of those services provided by the wetland.”

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