“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.