Sea Level Rise and Salt Marsh Migration
DIGGING TRENCHES AIDS RESTORATION
A dome of blue-gray sky cups the open expanse, ensuring the day will be a chilly one. Low tide on Round Marsh in Jamestown exposes last year’s growth of grasses, including the omnipresent Phragmites, still only in its dead-beige phase. Early April means that green growth is still a few weeks of warmth away, but the whistling and calling of the red-winged blackbirds hold the promise of spring.
There’s a farm on the far side of the marsh, and cows are grazing in their enclosures. It was a similar scene in centuries past, though farmers in those days were able to graze their cattle much farther into the marsh. They dug trenches to keep the salt water from standing in place on the marsh, and the ebb and flow of salt water would allow grasses to grow. The trenches provided the added benefit of keeping the mosquito population down by draining off any pools of standing water. It was a simple way to maintain the health of the marsh, and it had worked well for farmers around Rhode Island for the past 200 years. By the start of the 20th century, though, things had begun to change, as much of the land surrounding the marsh ceased being farmland. Some of the trenches dug years before had begun to fill in, leaving portions of the marsh to drown under the trapped water left behind. The contours and vegetated composition of the marsh began to shift. Then, in the 1960s, the Newport Bridge was built, and some of the detritus from the construction was dumped at the edge of the marsh. Housing developments were popping up, altering the way the water could rise and recede. Measures of carbon emissions and sea level rise revealed that changes were taking place at a faster, more alarming rate than they had in the past. Native grasses, less adaptable to changing conditions, gave way to the invasive Phragmites. Without intervention, the marsh would eventually be lost. But what could be done? A partial answer to that lies with the excavator, a large piece of construction machinery, now trundling across the marsh.
Marshes Lost to Development, Rising Seas
“In the past 200 years, we have lost half of our marshes to development,” said James Boyd, coastal policy analyst for the state Coastal Resources Management Council (CRMC). “Until the 1900s, salt marshes have been able to accrete (gain sediment that raises elevation) in order to keep up with sea level rise; however, this began to change in the 20th century.”
As the sea level rises, salt marshes migrate, moving to higher ground in order to survive. With housing developments, sea walls, bridges, and other manmade structures blocking this natural movement, agencies across the state are working to develop strategies that will accommodate the migration. In some cases, this means taking a very hands-on approach, such as the one taking place in Round Marsh.
Excavators, Runnels, and Rejuvenation
Wenley Ferguson, director of habitat restoration for Save The Bay, looks out from an observation platform at Round Marsh. She points out the complex web of water, Phragmites, and high and low water points. Her maps and photographs show evidence of new standing water that has materialized just since 2007.
According to Ferguson, where impounded water (water that isn’t allowed to drain off with the tidal cycle) occurs, vegetation has died off. She hypothesizes that this may be a result of sea level rise. Part of the work from this project includes the digging of runnels to drain off the standing water. Recently, the Phragmites has been cut down to allow for better viewing of water flow patterns and to aid in restoration work. The eight-foot-tall reed is highly invasive, and its spread can choke off other vital plants, resulting in a reduction in the biodiversity of plant life. Phragmites is, in essence, the crabgrass of the salt marsh world. That is part of the reason there is an excavator on the marsh at this moment. It is a low-impact machine, designed to be lighter and more maneuverable on softer ground conditions as it re-digs trenches that have long since filled in. “In the 1930s, farmers dug trenches to drain off the pools of standing water in an attempt at mosquito abatement. Unfortunately, this also sometimes drained the natural pools. What was sometimes left was water that was too shallow to allow mummichogs [fish] (which would feed on mosquito eggs) to survive,” Ferguson said. The restoration work now underway will attempt to address some of the water flow issues. It is not an easy task: even digging 18 inches or more reveals the persistent root system of Phragmites,the stubborn plants so adept at taking over an area. As the excavator digs, there’s a pungent, sickly sweet smell that increases as the day rolls on, until it seems to stay inside the nostrils for hours after leaving the area. Still, there is a steady flow of water sliding into the new channel, an indicator that the project is on the right track. It is just one attempt to address sea level rise impacts on marshes.
Mapping the Coast and Predicting the Future
In 2010, Rhode Island’s CRMC received a $30,000 grant to update sea level rise information and improve elevation maps. The result was a more accurate model of the land elevations of coastal Rhode Island, particularly salt marshes. The first was developed for North Kingstown; later, the decision was made to do similar studies of all 21 coastal communities. The result of the studies will enable towns to make better- informed decisions about their future.
“What are the issues in permitting development that will come from this?” Pamela Rubinoff, coastal management extension specialist with Rhode Island Sea Grant and URI’s Coastal Resources Center (CRC), is discussing a new set of maps that forecast impacts of sea level rise. “Should we make the buffers between the shore and development bigger? We might identify an area that’s really vital to fisheries or to bird watching. How do we address those concerns? That’s what this project is about. It’s about management, protection, and restoration of salt marshes and the coastal areas.”
Rubinoff teamed with Boyd to present the results of these assessments to groups around the state. By early February 2014, the two had reviewed their findings with all 21 coastal communities. The meetings with town councils, planners, and other interested parties were productive, according to Boyd. “We bolstered our presentations with information about what’s being seen (trends) and what’s already been seen. I can’t think of a single time when we were met with skepticism. What we’re trying to do is to build a consensus of all the parties in a community (about how to address the implications of sea level rise).” Boyd continued: “Coastal marshes are some of the most productive resources in the world. If the model we’ve created is correct and we’ll have so much loss to our marshes, what does that mean?” It’s still an open question. If it appears some loss of marshes is inevitable, what then? “We may have to let some fringe marshes go in favor of saving larger marshes,” said Rubinoff.
Drowning in Place
Ferguson explained that Save The Bay’s role in the marsh assessments began in 1996, when over 90 volunteers conducted salt marsh evaluations throughout the bay. She identified a variety of restoration projects that they have been working on ever since. One of the ways restoration needs are assessed is through an examination of a surface elevation table, such as the one Save The Bay did at Gooseneck Beach in Newport. This is accomplished by driving steel rods into the marsh until they hit bedrock. The elevation reading is the distance from the bedrock to the surface. “A marsh may be accreting (building up) or decaying,” Ferguson said. “A combination of those readings and our photography shows the progress of open water from 2006 to 2010, when much of the marsh became covered with standing water.” When water becomes impounded, important marsh grass like Spartinaalterniflora cannot grow.
“Spartina is a main marsh building material,” Ferguson explained. “It causes marshes to accrete quite well. But if there’s water on the marshes, it can’t grow. It needs that rising and falling where the water drops periodically. Spartina alterniflora can stand being flooded twice a day, but it will die if it’s constantly underwater.”
In Gooseneck Cove, elevation tables showed a band of Spartina holding water in. The warming and shallowness of the water created algal blooms, and studies revealed that the source was the decaying peat at the base of the Spartina. “Because of changes in the salinity brought on by constant immersion in salt water, the peat began to decay, releasing excessive nitrogen, which in turn produced the algae,” Ferguson said. “(Former coastal ecologist for Save The Bay) Marci Cole Ekberg did a study that showed a huge algae bloom in the middle of an open water section of the cove. At first, we thought it could have been caused by broken sewer lines or other manmade encroachments, but we eventually determined that decay of the peat was the cause. We’ve estimated that Gooseneck has suffered 16 acres of loss since the 1930s.” The hypothesis from Ferguson is that with sea level rise, there is less marsh growth and increasing degradation to the existing marshland. Scientists are seeing that marshes are essentially drowning in place.
The maps that Rubinoff and Boyd helped develop and promote have been adopted by the state, in order to help make better-informed decisions regarding development and conservation. “The important thing is that decisions are made on sound science,” says Rubinoff. Meanwhile, Ferguson directs the excavator operator, selecting where to dig the drainage ditch and where the excavated material should be placed so that it will do the most good. The work being done on this day must be complete before the songbirds return to nest. One species is of a particular concern. “The salt marsh sparrows need high marsh vegetation for their nests. And right now the high marsh vegetation has standing water on it. These guys actually plan their egg laying around the moon tide, and they create a kind of dome over the eggs so that the eggs can float around a bit (at moon high tide), and the birds will go right back on the nest for incubation. But they cannot withstand the constant flooding that would occur in an area that lacks high marsh vegetation. U.S. Fish and Wildlife Service is very concerned about the salt marsh sparrows, and there has been a population viability assessment done through UConn (the University of Connecticut). It does not look good for them through sea level rise,” Ferguson said.
“The salt marsh sparrow is the polar bear of the salt marsh.”
The current project to carve passages on either side of the marsh is designed to allow passage of fresh water in and out of the marsh areas. Once the stand- ing water is eliminated and the salinity levels return to normal, the marsh may begin to heal itself. “Getting water to drain off is the key,” Ferguson says peering out over the acres. “Millimeters matter in a marsh.”
— By Hugh Markey Photographs by Monica Allard Cox