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Sea Level Rise and Shrinking Salt Marsh
By Peter Phippen, Eight Towns and the Bay, and Anne Donovan, CZM
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Situated on the North Shore of Massachusetts—covering more than 25,000 acres from Gloucester to Salisbury—sits the Great Marsh. This Commonwealth coastal jewel is the largest contiguous salt marsh north of Long Island, New York. More than marsh, however, the area includes barrier beaches, dunes, tidal rivers, estuary, mudflats, and islands.
But with sea levels rising, what is the likely future of the Great Marsh? This article looks at some of the basic science behind sea level rise and salt marsh dynamics, and discusses some of the likely impacts to this important region. They key message: All we do to think green/go blue can make a real difference in the Bay State’s own backyard, helping to ensure that shoreline ecosystems like the Great Marsh are here for future generations to enjoy.
Rising Seas/Sinking Shores
Global warming causes sea levels to rise by...
In addition to rising global sea levels, the tectonic plate that Massachusetts rests on is subsiding, resulting in relative sea level rise rates that are even more extreme. According to Climate’s Long-Term Impacts on Metro Boston (CLIMB)—a 4-year, million dollar research effort funded by the U.S. Environmental Protection Agency and conducted by experts from Tufts University, the University of Maryland, and Boston University—relative sea levels in Massachusetts rose by almost a foot over the last century. Their estimates indicate that by the end of the 21st century, relative sea levels in the Boston area will rise from two to three feet. (More than double the sea level rise from the last century—data taken at the Boston Tide gauge indicate that sea levels rose 0.87 feet from 1921 through 1999.)
At the lower end of the estimate, flooding would primarily occur only during storms. However, even moderate storms with 2-foot storm surges (which typically occur several times a year in Massachusetts) would flood Boston’s Financial District and parts of East Boston, South Boston, and Charlestown. For more on the CLIMB project, see www.tufts.edu/tie/climb and for a Boston Globe slideshow of potential flooding impacts in Boston, see www.boston.com/news/multimedia/interactive_bostonflood/.
The Birth and Growth of a Salt Marsh
Bacteria and fungi slowly decay the organic matter trapped by a growing stand of Spartina alterniflora. Over time, the accumulation of dead and decaying marsh plants and other material results in the formation of peat (organic matter that only partially decomposes because of the lack of oxygen in waterlogged environments). Through years and years of peat accumulation, the elevation of the young marsh increases enough to reduce flooding frequency. Once this occurs, high marsh plants can become established, allowing a greater diversity of salt marsh plants to grow.
Salt Marsh: From Stable to Shrinking
When the peat-formation process does not keep pace with increasing sea levels, parts of the marsh become submerged. Ultimately, this kills the plants and degrades the edges of the marsh, making it more vulnerable to continued rising sea levels.
Potential Impacts in the Great Marsh
As shown in Figure 2, as sea level rise increases, much of the area around Crane Beach (an important barrier beach in the Great Marsh system) will flood. As these areas are transformed to open water, many benefits of the system will be lost. Important habitats for fish, shellfish, and birds will be drowned; space for beach recreation will be reduced; and storm-damage prevention to inland areas will be compromised.
The rate of sea level rise is not fixed, however, and humans are having a big impact on global warming through release of greenhouse gases (see Global Warming for Dummies). To do what you can to help sustain the Great Marsh, see the feature section of this edition of Coastlines—and think green . . . go blue!
Mitsch, W.J. and J.G. Gosselink. 1993. Wetlands. Van Nostrand Reinhold, Inc. New York, NY.
Teal, J. and M. Teal. 1969. Life and Death of the Salt Marsh. Ballantine Books, New York, NY.
Tiner, R.W. 1987. Coastal Wetland Plants of the Northeastern United States. The University of Massachusetts Press, Amherst, MA.
U.S. Environmental Protection Agency. Coastal Zones and Sea Level Rise. www.epa.gov/climatechange/effects/coastal/index.html.
Peter Phippen provides technical assistance to the nine communities of the upper North Shore—promoting coastal stewardship, working to develop environmental initiatives and implement outreach and education strategies, and developing grant proposals to secure funding—on behalf of the Eight Towns and the Bay Local Governance Committee of the Massachusetts Bays Program.
Photos, etc.: Salt Marsh - Stephen Gersh, Salt Marsh Illustration - Ethan Nedeau, Sea Level Rise potential Figure 2 - Julia Knisel