Cutting Down On The Salt: Researchers Determine That Freshwater Table Increases As Temps Rise In Salt Marsh Grasses

By Jeffrey M. Brodeur, Woods Hole Sea Grant

Frodo was tasked with a crucial mission recently and although it had nothing to do with a ring or battling evil, it was aimed at helping save the planet.

"Frodo" is the nickname given by Woods Hole Sea Grant-funded researchers to a machine that measures the change in carbon dioxide concentration during photosynthesis. And it was helping them try to determine why large chunks of salt marshes are suddenly disappearing across a large swath of the eastern United States.

Led by principal investigator Ivan Valiela, a professor with The Ecosystems Center at the Marine Biological Lab in Woods Hole, the scientists wanted to see whether global warming and the effect of sea level rise were impacting the Spartina alterniflora grass that is predominant in salt marshes.

Sea Grant-funded researchers are looking into increased dieback of salt marsh grasses, such as in Falmouth's Great Sippewissett Marsh.

Photo by Ylva Olsen, The Ecosystems Center

"The question is how this is happening," Valiela says. "The concern is that this is a very valuable habitat that provides lots of services for adjoining coastal waters. So if it starts dying off, there are going to be consequences."

Valiela's team opted to focus initially on atmospheric warming, because other colleagues were working on other possible factors such as grazing by crabs or fungal infections. The group set out experimental chambers in the Spartina of Great Sippewissett Marsh on Cape Cod, with the goal of driving up the temperature without affecting other variables. Their goal, in essence, was to turn Falmouth into a more tropical spot.

Sophia Fox, a postdoctoral fellow working with Valiela, says they compared controls at an ambient temperature of approximately 30o Celsius, or 90o Fahrenheit, with other chambers that allowed some air exchange to bump the temperature up to about 35o C, and with nearly closed chambers, with a temperature of 40o C.

Valiela says one of the assumptions was that as the temperature increased, and freshwater was released, the water left behind would be saltier. He knew that 20 parts per thousand of salinity within the sediment pore water was detrimental to the plant, and expected to see numbers approaching that tipping point.

Sophia Fox adjusts a chamber attached to "Frodo", which measures carbon dioxide concentration during photosynthesis.

Photo by Ylva Olsen, The Ecosystems Center

Instead, "it turns out, amazingly enough, that there wasn't any increase, and there was some evidence that salinity might actually have decreased," says Ylva Olsen, also a postdoctoral fellow at The Ecosystems Center.

Valiela says that by raising the temperature, they "created enough of an upward negative pressure on the fresh groundwater below, so that the freshwater table was moved up."

"While the finding of the freshening does not tell us much about dieback, it does say much about the close coupling of salt marshes and the groundwater below," Valiela says. "If the groundwater were actually to go down because of larger human use, that means the roots would be in saltier water.

"The tight connection between marsh and groundwater also says that whatever comes in the groundwater — perhaps toxics or nutrients—is going to be impacting the roots."

Valiela's team continues its effort into root causes of the roots' failing, including another round of data collection next spring and looking into the possibility of involvement of fungal pathogens in marsh dieback.

"There's going to be lots of interesting surprises," he says.