Researchers study the impact of saltwater intrusion on tidal wetlands

July 28, 2015 by Brian Mcneill
Researchers study the impact of saltwater intrusion on tidal wetlands
Biology Professor Scott Neubauer, Ph.D works on setting up a chamber that will measure greenhouse gas emissions from a plot in Cumberland Marsh.

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 Daniel 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 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 .

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 , 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— 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," 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."

Effect on greenhouse gas emissions?

Clockwise from left: Biology major Allison Tillett, biology professor Scott Neubauer, Ph.D., and postdoctoral researcher Daniel Lee, Ph.D., set up a chamber that will measure greenhouse gas emissions from a plot in Cumberland Marsh.

On a recent afternoon, Neubauer and Lee installed several large transparent chambers over plots of marsh plants that will allow them to measure carbon dioxide and methane fluxes between the vegetation and the atmosphere.

"We can see all the vegetation here, but [the chamber will] show us an actual rate of photosynthesis and allow us to look at respiration by the plants and the microbes to get an idea of how much gas is going back to the atmosphere," Neubauer said.

Nearby sits Lee's irrigation system, which pumps water from the Pamunkey and distributes it to 15 plots of marsh. Some of the plots receive freshwater and some receive saltwater.

Within a few months, the researchers expect to see significant differences among the saltwater and freshwater plots.

"We're expecting to see that plant production will go down a lot because most of these plants out here can't tolerate much salt at all. So we should see big decreases in photosynthesis," Neubauer said. "What happens with respiration? I think at the scale we're looking at we'll see that over time respiration will decrease because of the saltwater. Over the long term, when the plants decline, they're adding less organic matter to the soil. So even if the microbes are more efficient, there's less organic matter there because the plants are growing more slowly because of the salt."

Allison Tillett, a biology major, is tasked with observing what happens to the plants—the majority of which are pickerelweed and arrow arum—within the 15 plots.

"I'm characterizing the plant community," she said. "I'm measuring the heights of the plants and determining what the percent cover [of plants] is within the plot. I'm going to monitor [the plots] and watch how the vegetation responds to the freshwater and the saltwater, as well as the control plots."

After just a few weeks, Tillett said, there is already a visible difference between the plants' growth levels in the saltwater and freshwater plots.

"The growth of some of them in the saltwater plots is slowing down," she said. "Some of the other ones are actually growing more than normal in response to the freshwater. They're really enjoying the freshwater that's coming in."

Moving an 'entire lab' out into a marsh

Lee designed and built the pumping system with the help of Olivia DeMeo, a technician in Neubauer's lab, and Richmond-area industrial designer Robert Thomas, whom Lee met at HackRVA, a local makerspace dedicated to providing opportunities for people to collaborate and create.

"I had never designed or built anything like this before, so I became a member of HackRVA as soon as I came to Richmond," Lee said. "I met Robert and he helped me figure out how to build this."

Thomas said he was intrigued by the project, as it required the design of a new kind of pumping system, with no precedent to copy.

"To do these experiments, you've got to basically move your entire lab out there onto a marsh," Thomas said. "When Daniel told me about the project, I said, 'Man, I just have to be involved in that.' That's the kind of stuff I love—taking something that's never been done before, [figuring out] the technology and putting it all together."

Daniel Lee, Ph.D., a postdoctoral researcher, shows off the system he built to irrigate 15 plots in Cumberland Marsh with saltwater, simulating the effects of saltwater intrusion brought about by the rising sea level.

Thomas helped Lee with the apparatus' microprocessor, wiring, solar panel support and more.

"I saw Daniel and what he was trying to do as an opportunity for me to contribute my 50-plus years of doing wacky crap," Thomas said.

Saltwater's impact on microbial communities in soil

Not far from the pumping system spot, a team of students and researchers led by Franklin has been collecting soil samples and analyzing them in the lab for microbial activity and diversity. Bonnie Brown, Ph.D., professor of biology and co-investigator on the grant, and George Giannopoulos, Ph.D., another postdoctoral researcher, are key contributors to this effort, which uses state-of-the-art DNA sequencing technology to track the growth and survival of tens of thousands of microbial species in response to the saltwater addition.

"We do this coincident with Scott's sampling so we can directly compare our measurements of the microbial community with their measurements of ecosystem processes," Franklin said.

One of Franklin's students, Chansotheary Dang, who is pursuing a master's degree in biology, is conducting an additional field experiment. She has taken soil samples from Cumberland Marsh and enclosed them in "microbial cages" that are now incubating at another site in the Taskinas Creek Marsh.

"This site is on the York River, downstream from Cumberland Marsh, and has a higher salinity," Franklin said. "The microbes are trapped in these cages, and Chansotheary samples every two weeks to study how these freshwater communities survive and adapt to the more saline conditions."

The researchers work at their experiment site alongside the Pamunkey River in New Kent County.

As part of Dang's experiment, she is analyzing soil and porewater chemistry, microbial enzyme activity—or looking at the various steps in the decomposition pathway—and carbon dioxide and methane production.

Saltwater intrusion likely impossible to stop

The researchers' work to understand how sea level rise will affect freshwater wetlands could apply to many rivers along the East Coast.

"It's possible that the research coming out of this experiment could be extrapolated out into similar freshwater ecosystems in the eastern United States," Lee said. "There are lots of wetlands—particularly in the Chesapeake Bay—where we can extrapolate these results and maybe predict how much carbon is sequestered when the saltwater intrudes, and how much methane might be produced."

These effects are important to understand, the researchers say, because the sea level is rising and it appears that little can be done to halt it.

"Saltwater intrusion is kind of hard to stop because it's being driven over decades by and ," Neubauer said. "Unfortunately, that's proven very difficult to get a handle on."

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