Walking through the Parker River Wildlife Refuge on Plum Island with wildlife biologist Nancy Pau is to take a tour through the various ways humans have disrupted the saltwater marsh for the last 300-plus years.
The area is filled with what Pau calls “legacy infrastructure” — colonial-era irrigation canals and 20th century mosquito control ditches.
These drainage canals were good for 18th century hay farmers and 21st century residents concerned about mosquitoes, but Pau says they weren’t good for many animals and the marsh’s overall health. So beginning in the 1990s, scientists tried to fix the problem by blocking some ditches with small “ditch plugs” made from soil.
“It worked and we got water back onto the marsh,” Pau says.
But it may have worked too well, leading to an effort today to reverse decades of well-intentioned meddling in the marsh and help protect it against sea level rise by mimicking nature.
"Our concern about too much draining has shifted, and the concern now is that the marsh is getting too much flooding," she says. “It’s important for the marsh to get flooded, but also for the water to come back off. Anything that interrupts either of those two processes can negatively impact the marsh."
The Parker River Refuge is part of The Great Marsh, the largest continuous salt marsh in New England. Saltwater marshes are important ecosystems. They protect inland areas from flooding, provide habitat for many coastal animals and store carbon underground.
Healthy saltwater marshes are also natural buffers against sea level rise. As the tide comes in and out, grasses trap sediment, allowing the peat underneath the grass to actually grow in elevation.
But if something happens to that grass, there’s nothing to trap sediment — let alone keep it in place. The prospect of that erosion is what Pau began worrying about a decade ago as she noticed an important marsh grass was disappearing.
There are two dominant types of grass in a saltwater marsh: smooth cordgrass and salt marsh hay. The former grows tall and thrives in wetter, lower-elevation areas; the latter is shorter, softer and thrives in higher-elevation areas where its roots get more exposure to air. Both grasses play an important ecological role, but the salt marsh hay, in particular, provides critical nesting and feeding habitat for a lot of wildlife — including the endangered saltmarsh sparrow.
After installing the ditch plugs, big areas of the marsh were inundated with water and the salt marsh hay drowned. Sometimes the cordgrass grew in its place, but sometimes the land just turned to open mud-flats.
Equally alarming, a few years ago, a lacy covering of pink, green and yellow algae started spreading over pools of standing water. Pau and her colleagues think the flooding is causing the soil to decompose, leading to a release of nutrients that the algae is gobbling up.
She calls the algae "a signal or a flag that says, ‘Hey, something is going on here.' " In other words, something she and her colleagues couldn’t ignore.
By 2010, she was so worried about wildlife and marsh erosion — especially because climate change models show sea levels will continue to rise — she devised an experiment. What would happen if she removed some ditch plugs in one area of the marsh?
Pau’s pilot project began in 2015, and already, the results are visible. Compared to other areas of the refuge, this plot is less soggy and more densely vegetated.
“Visually, I’m seeing cues that the restoration is doing what it should be doing,” she says, surveying the land. “You have smooth cordgrass coming in, [and] I think we’re beginning to see a little bit of the salt marsh hay grass.”
The pilot’s success inspired her to remove more ditch plugs on the refuge. It took a while to get the permitting squared away, but a few weeks ago, Geoff Wilson of Northeast Wetland Restoration began removing the first of 23 plugs.
Using a small excavator, Wilson scoops out small chunks of grass-covered dirt, eventually clearing an area about the size of a large refrigerator. He carefully creates a meandering channel, reconnecting two parts of a previously blocked ditch. The water starts draining immediately, and a thin trickle of water heads toward a small river that leads to the ocean.
Though removing the ditch plugs isn’t the only project planned for the refuge, what makes it exciting is how quickly the change occurs. With the excess water gone, and the tidal flow restored, grass should start growing in this area before the end of the summer.
And more grass means more trapped sediment, which for the marsh, means improved resiliency.
Saltwater marshes throughout New England are threatened by development, pollution and climate change. In the grand scheme of things, ditch plugs aren’t the biggest danger to the marsh. But by removing them, Pau hopes her team can at least help give this area a fighting chance against sea level rise.
This segment aired on July 2, 2019.