Yesterday's Island Today's Nantucket
Volume 38 Issue 14 • July 31-Aug. 6, 2008
now in our 37th season

Restoring Our Marshes

by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station

Decades of human manipulation have transformed our salt marshes.  Formerly considered as worthless water sodden land that bred diseases and mosquitoes, many marshes and swamps were filled in or drained.  Entire neighborhoods were built on filled tidelands along the east coast, with Boston’s Back Bay as a prime example. 

In the mid 1930s, during the Depression, the government put many people to work in programs designed to increase employment through agencies such as the short-lived Civil Works Administration and the much more successful, WPA (Works Progress Administration, later renamed the Works Projects Administration).  One of the jobs created was ditch-digging in salt marshes.  More than 90% of all east coast salt marshes were ditched, with close to 3000 miles of ditches dug in Massachusetts alone.  This ditching was originally designed to eliminate mosquito-breeding habitat on the marsh by draining the water off the marsh quickly after high tides.

In the short term, this worked while the ditches were clear of infill.  One of the unforeseen side effects, though, was that once the marshes were drained, they were no longer used by wildlife. Another problem was that the ditching was not always maintained and in some situations the ditches that were designed to prevent mosquito breeding, became prime mosquito breeding habitat as the water in the ditches stagnated. This can be easily observed on Nantucket in areas around the island near functioning marshes like Folgers’ marsh at the Nantucket Field station versus other ditched and restricted marshes, such as Medouie Creek, located on the northeastern edge of Polpis Harbor. Ditching was also done to facilitate salt marsh haying and in some areas fish weirs were established by the native Wampanoags that diverted natural tidal inlets.

Today, federal, state, local, and private groups are using techniques like Open Marsh Water Management (OMWM) to restore our marshes to their former glory.  OMWM (gotta love those acronyms and initialisms) involves filling in some ditches, reconnecting other ditches to deeper pools, and recreating salt pannes in a marsh system to allow normal tidal flow and mosquito-larvae-eating fish such as Fundulus (killifish) to reach all areas of a marsh where mosquitoes might breed.  OMWM also allows native marsh vegetation such as Spartina alterniflora (smooth cordgrass or saltmarsh cordgrass) and Spartina patens (salt meadow cordgrass) to fill in naturally.  All these efforts are an attempt to restore normal tidal flow.  The tide is the dominating characteristic of a salt marsh. The salinity (salt content) of the incoming tide defines the plants and animals that can survive in various sections of a marsh. The vertical range of the tide determines flooding depths and thus the height of the vegetation, and the tidal cycle controls how often and how long vegetation is submerged.  Two areas are delineated by the tide: the low marsh and the high marsh. The low marsh generally floods and drains twice daily with the rise and fall of the tide; the high marsh, which is at a slightly higher elevation, floods less frequently.

The salt marsh is one of the most productive ecosystems in nature, with the total biomass created daily rivaling a typical rainforest.  In addition to the solar energy that drives the photosynthetic process of higher rooted plants and the algae growing on the surface muds, tidal energy repeatedly spreads nutrient-enriched waters over the marsh surface.  Some of this enormous supply of live plant material may be consumed by marsh animals, but most of the vegetation dies and is decomposed by microorganisms to form detritus. Dissolved organic materials are released, providing an essential energy source for bacteria that mediate wetland biogeochemical cycles (carbon, nitrogen, and sulfur cycles). A healthy marsh typically increases in height gradually (by millimeters) as this sediment accumulates; this provides a fragile balance between subsiding coasts and increasing sea levels.  The salt marsh serves as a sediment sink, a nursery habitat for fishes and crustaceans, a feeding and nesting site for waterfowl and shorebirds, a habitat for numerous unique plants and animals, a nutrient source, a reservoir for storm water, an erosion control mechanism, and a site for aesthetic pleasure. Appreciation of the importance of salt marshes has led to federal and state legislation aimed at their protection.

While we are on this topic, it has always bothered me when someone calls a marsh a “swamp.”  The basic distinction between swamps and marshes depends on whether or not the wetland contains trees.  Swamps are forested wetlands, containing trees and large shrubs.  Marshes, on the other hand, are primarily filled with grasses and various soft-stemmed plants. Swamps also often have more open water and tend to be deeper than marshes.  I enjoy both swamps and marshes, but I have found that when someone uses the term “swamp,” it’s not meant as an endearment.

One of the many avoidable tragedies of Hurricane Katrina’s assault on Mississippi and Louisiana was in areas where salt marshes were drained to make canals or filled in and subsequently built up.  America's Wetland, a Baton Rouge organization, estimates that more than 1,900 square miles of the Louisiana wetlands (approximately 25% or 1.2 million acres) have disappeared since the 1930s due to development and the construction of levees and canals. These areas could have helped to absorb the storm surge that came as far as 50 miles inland in some areas by providing a frictional force that slows down the flooding water.  Scientists have calculated that for every square mile (640 acres) of salt marsh filled in, another foot of storm surge occurs.  Now, as sea level rise is overcoming sinking or dredged wetlands, we are starting to value, both scientifically and economically, the worth of this land.

Here on Nantucket, conservation groups are proactively working to the natural functions of our salt marshes.  The Nantucket Conservation Foundation (NCF) in collaboration with the Massachusetts Office of Coastal Zone Management Wetlands Restoration Program (MWRP) and the University of Massachusetts at Boston Nantucket Field Station has been conducting research on the Medouie Creek marsh system since 2004 to determine what type of restoration is needed to counteract or slow the establishment and spread of the invasive common reed (Phragmites australis) within the marsh. The spread of Phragmites in our nation’s wetlands has pushed out native plants and the animals that depend on them through the establishment of a monoculture which supplants the natural biodiversity of the wetlands.  The next time you drive along coastal areas in New Jersey, Rhode Island, Connecticut, or Massachusetts, look along the roadsides for this 10-20 foot tall reed in order to understand the magnitude of the problem.

Some severely impacted areas can never be reclaimed, but it is worth the effort for areas such as Medouie Creek marsh.  Medouie’s tidal exchange with the harbor has been severely restricted by the presence of dike roads and the partial blockage of the only remaining connecting channel.  The NCF project proposes to restore substantial tidal exchange by installing a three-foot by three-foot box culvert at a low point in the dike road and by opening up channels that originally led to the southern end of the marsh before the dike road was built. If successful, this project will allow salt water to reach the back portions of the marsh and improve water quality while holding the insidious Phragmites at bay. In 2003, the MWRP designated Medouie Creek as a high priority restoration site due to the value of its habitat and the potential for restoration.  Over the past five years, the NCF, with some help from the Nantucket Field Station, has done an extensive amount of research work including installing pressure transducers through the marsh system to measure tidal height, measuring soil salinities at various depths, obtaining exact elevation measurements, and doing vegetative profiles to document the “before restoration” conditions.  NCF also hired hydrologists (study of water movement, properties, and effects) and worked with the CZM to determine the best plan of action based on their data and all alternatives.

The proposed restoration of a tidal connection through the eastern dike road at Medouie Creek will likely result in significant improvement in drainage of fresh water from the area that is currently impounded, and will also allow  for a corresponding increase in salt water inundation to this area.  The combined influence of increased freshwater drainage and salt water inundation are predicted to improve habitat conditions for native salt marsh flora and decrease habitat suitability for non-native P. australis.  There is little doubt that the current tidally-restricted marsh was once naturally connected to Polpis Harbor in several places. If this initial project is successful, it may be possible to re-establish other tidal connections.  The Nantucket Conservation Foundation received a $9100.00 grant from the CZM for scientific monitoring of the success of this project. This is another example of excellent collaboration between island researchers and conservationists and state agencies to achieve much more than could be accomplished by any one agency alone. Thankfully, on Nantucket, we know our marshes are more than just useless land.

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