For years, scientists have observed a slow decay of salt marshes all along the Atlantic coast without understanding why. Now a newly released study conducted in the local Great Marsh is shedding light on the cause of salt marsh decay and the impact it could have on the environment.
Scientists from the Marine Biology Laboratory, a nonprofit biological research group based out of Woods Hole, have determined that higher levels of nutrients such as nitrogen and phosphorous are contributing to the widespread decay of salt marshes along the East Coast.
The group, led by MBL scientist Linda Deegan, has been introducing nitrogen and phosphorous into a portion of Sweeney Creek in the Plum Island Estuary since 2003. On Wednesday, the group announced that significant decay in the experimental area has occurred as a result. The creek is near the Rowley/Ipswich line in a remote section of the marsh.
“We have known that we’ve been losing marshes in places like Connecticut and Long Island and Delaware, but we haven’t really understood why,” Deegan said in a video interview that her team posted on YouTube. “On the basis of this experiment, we’re beginning to understand that at least some of that marsh loss is probably due to this widespread coastal nutrient enrichment from upland development.”
Over the course of nine years, the scientists added nitrogen and phosphorous to the tidal water flushing through the marsh’s creeks at levels typical of nutrient enrichment in densely developed areas, such as Cape Cod and Long Island, they said.
The scientists found that the higher level of nutrients caused the grass on the banks of the tidal creeks to grow taller while their roots grew shorter. As the grass grew taller, it eventually drooped over into the water, where the currents started pulling on it.
“Since they didn’t have as many roots, they got tugged on and pulled on, and it’s breaking up the integrity of the entire creek bank edge,” Deegan said. “That’s why we’re getting these fissures, these fractures, in the edge of the creek, and the plants are falling into the edge of the creek themselves.”
Photos of the test area of the marsh show great clumps of marsh sod that has collapsed into creeks.
The long-term effect of elevated nutrient levels was a conversion of the vegetated marsh into a mud flat, which Deegan called a much less productive ecosystem that doesn’t provide the same benefits to humans or as suitable a habitat for fish and wildlife.
The Great Marsh stretches from Gloucester to Hampton, N.H., a stretch of some 25 miles. Plum Island and the estuaries and marshes on the landward side of it compose the center of the marsh. It is the largest salt marsh in New England and the largest north of Long Island, N.Y.
Deegan called salt marshes a critical interface between land and sea, saying they provide a habitat for fish, birds and shellfish while taking nutrients out of the water coming from upland areas, protecting coastal bays from over-pollution. One of the reasons for conducting the experiment, she said, was to see what level of nutrients the marshes could handle, if there was a limit at all.
“The thinking at the time was that marshes could absorb nutrients almost without end,” Deegan said. “It would simply grow more grass, and it does indeed do that, but it turns out that there is a limit to the amount of nutrients that marshes can take out.”
That knowledge is critical for conservationists who have been looking for ways to reduce the ongoing trend and save the salt marshes, especially in highly developed urban areas that produce a lot of nutrients as a result of septic systems, sewerage or fertilizer runoff from people’s lawns.
“If every individual could make the decision not to add nitrogen to their lawn, that would help the coastal areas a great deal,” Deegan said.
The next step for Deegan and her team will be to see if they can reverse the damage done to the Sweeney Creek marsh system.
The scientists will stop adding more nutrients and will then observe how long it takes for the marsh system to recover.
“After we stop adding the nitrogen, how long does it take the system to rebound to its natural state?” Deegan said. “This information will be important in reclaiming the health of salt marshes that are currently suffering from nutrient enrichment.”