Thursday, March 16, 2017

A Matter of Millimeters: Helping Our Salt Marshes Survive Sea Level Rise


Some seven years ago, Save The Bay began to notice signs of deterioration of Narragansett Bay’s salt marshes. That’s not surprising, since the rate of sea level rise in Rhode Island has nearly doubled since 1999, from 2.78 to 5.22 millimeters per year, and the rate in the northeast is considerably higher than the global average. To document these changing conditions, Save The Bay and partners from the Narragansett Bay Research Reserve conducted a rapid salt marsh assessment of 44 marshes throughout Rhode Island and southeastern Massachusetts, recording widespread marsh degradation in the form of stunted or dead marsh plants, stagnant ponded water and unstable marsh soil.

At Round Marsh in Jamestown, areas of dead salt marsh 
grasses show where salt water gets trapped on the marsh 
and eventually causes the plants to become stressed and die off. 
Even though marsh plants can tolerate the twice daily high tides, they can’t grow in standing water. As sea levels rise, water remains trapped on the marsh even during low tides, and the excess water causes marsh plants to drown in place and die off. Without plants to trap sediment and roots to bind the marsh soil, the surface of the marsh begins to sink and converts to open water. Then, these new shallow-water areas become perfectly suited for mosquito breeding, because they no longer support small fish that feed on mosquito larvae.

Historically, salt marshes were able to keep pace with sea level rise, increasing their elevation a few millimeters each year.

Marsh plants trap sediment suspended in the water, and when they decay, they add organic material to the marsh soil. But now, with sea level rise accelerating, many salt marshes are approaching a tipping point and are not able to keep up.

Save The Bay has shifted our focus from restoring marshes affected by past human activities to helping marshes adapt to accelerated sea level rise. Four years ago, from Westerly and the Narrow River to Potowomut and Warren, our restoration team began trying new techniques to help marshes survive rising sea levels—digging narrow, shallow creeks within the marsh to allow impounded water to drain so that plants can recolonize the marsh. Along with volunteers, we dig the creeks either by hand, if the marsh soil is very unstable, or with the use of the R.I. Department of Environmental Management Mosquito Abatement Program’s specially designed excavator. The excavated soil is used to fill old ditches acting as mosquito breeding habitat or to raise the marsh elevation where it has subsided. Our monitoring thus far shows that plants are able to recolonize areas once flooded and that mosquito production is reduced.

The most radical adaptation approach to date is to raise marsh elevation by placing sand on the marsh so that marsh grasses can survive the rising water levels.
Top of Page: At Sachuest salt marsh in Middletown, 
sand is placed on the marsh to raise its elevation, helping 
it keep pace with sea level rise. Above: Volunteers, Danielle Perry 
and Alicyn Murphy, plant Spartina alterniflora on the newly elevated 
Sachuest salt marsh. 
Last winter, the U.S. Fish and Wildlife Service (USFWS), in partnership with The Nature Conservancy and Save The Bay, adopted this approach at Sachuest Marsh in Middletown, where the small, elusive saltmarsh sparrow is losing habitat as marsh grasses die off. The saltmarsh sparrow only nests at higher elevations in the marsh that flood during moon tides. To ensure that the elevations are suitable for sparrow nesting, contractors used GPS-equipped bulldozers to carefully place sand. More than 100 volunteers and students helped us and USFWS staff plant 17,000 marsh grasses across the new marsh surface. By raising the elevation of the marsh and replanting, we hope to give both the plants and the birds a chance at survival.

This technique of raising or rebuilding the marsh is being implemented on two other marshes this fall — one on Ninigret Pond in Charlestown and one on the Narrow River in Narragansett. At both of these marshes, sand will be dredged from nearby waters and spread across the marsh to build their height.

The overarching goal of our adaptation activities is to increase the lifespan and resiliency of salt marshes as sea levels rise and coastal storms intensify due to climate change. Why go to such measures? Because salt marshes form the base of the Bay’s food web and are critical to the ecological health of Narragansett Bay. They serve as nurseries and safe havens for many fish, shellfish and bird species to breed and grow, from egrets and saltmarsh sparrows to mummichogs and blue crabs. They filter pollutants and absorb excess nutrients, and during storm events, they can lessen coastal erosion by reducing wave energy.

The reality is that many of our existing marshes will transition to open water in the coming decades as sea level continues to rise, but for other marshes and the species that depend on them, adaptation may buy some time — to move inland, that is. In certain low lying areas around the Bay, where development isn’t right up to water’s edge, we see that marsh grasses are growing under mature trees and that such coastal plants as bayberry bushes and cedar trees are dying off as tides creep further inland.

In areas with space, we’re working with partners to identify ways to prepare inland areas for salt marsh migration. In Tiverton, a low lying field bordering Seapowet Point is one such place. Working with the R.I. Department of Environmental Management, we’ve secured funding to limit vehicular access and to convert a potato field into a natural grassland that can tolerate occasional inundation of salt water.

We cannot stop sea level rise, but we can help protect areas for our precious salt marshes to migrate inland. From Tiverton to Ninigret Pond and the Narrow River, Save The Bay and our dedicated partners will persevere in raising and restoring the salt marshes that are so critical to the Bay.

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