Salt Marsh

by Geoff Wilson
February 2005

Salt Marshes are one of our nation's most important natural resources. These seas of grass are inundated by salt water twice daily, and provide nursery habitat for almost 90% of our coastal fish species.

The area comprising a salt marsh is neither ocean nor land. The tidal currents that dictate all of the natural biorhythms flood the salt marsh 730 times each year. As the waters recede, the enriched waters provide as much as 54% of the soluble and particulate nutrients to the surrounding coastal waters.

Salt Marsh Profile

Salt marshes are the most productive natural habitats on the face of the earth. The nutrients exchanged between the salt marshes and the open ocean nourishes the coastal waters for a distance of nearly three miles out to sea. It is in this way that these two separate ecosystems complement each other. The effect of the nourishing waters starts with the nutrients available for the production of phyto-plankton, and is transferred up the food web to the top predators. In a sense the entire natural food web starts in the estuaries. As the mud flat invertebrates feed on the particulate nutrients and phyto-plankton, the invertebrates excrete a high nitrogen waste that flows back into the salt marsh feeding the grasses. In a well balanced estuary as much as 85% of the water exiting the estuary returns on the next tide cycle.

An additional benefit of salt marshes is as Nursery Ground for coastal fishes. Nearly all of the finfish found in coastal waters depend on salt marshes for at least one of its life stages. All shellfish rely on the stability of salt marshes to harbor their larva.

Zones of the Salt Marsh

The salt marsh can be divided into two distinctly different zones. The lowest zone is inundated by the tide twice each day and is called the Regularly Flooded Zone. The second receives tidal inundations only periodically, and is called the Irregularly Flooded Zone.

Within these two zones there are three classes of habitats. Each of these habitats provides distinct and very diverse wildlife values with none more productive than the other. The most noticeably different habitat is where water pools on the surface of the marsh. These areas are called Salt Pannes. Another type of habitat is the High Marsh and is comprised of the fine textured grass that grows in the seemingly flat surface of the Irregularly Flooded Zone. The third is the Low Marsh, a narrow band of tall grasses that line the lower elevations of the Regularly Flooded Zone.

Salt Pannes

Salt Pannes are in most cases where people experience their first taste of marshland wildlife. During different tidal cycles these pool areas are attractive to multitudes of marsh birds. Salt Pannes can appear in both the Regularly Flooded Zone and the Irregularly Flooded Zone. High Marsh Pannes are cyclical in nature. As Hydrogen Sulfide accumulates in the soils, the grass around the panne dies, causing the water surface to migrate to a creek or ditch. After the water drains, the Hydrogen Sulfide is flushed from the soils and the plants reestablish, plugging the opening. This cycle is called Rejuvenation and is important to the evolution process of salt marsh pannes. Without rejuvenating, the Sulfides found in the panne soils bond with the available Nitrogen reducing the invertebrate production.

There are four different classes of Pannes. Each Panne type is important to many different wildlife species. Deep Perennial pannes provide deep-water refuges for baitfish stranded on the marsh surface at low tide. Shallow Perennial pannes provide feeding and resting areas for marshland wading birds during periods of high water. Temporal pannes provide the most important invertebrate and insect larval habitats that are safe from feeding fish. Vegetated pannes are important as feeding areas for migrating waterfowl. Each of these panne types link together like the links of a chain, by removing one link the chain is broken and not nearly as productive.

High Marsh

The grasses of the high marsh comprise the real production factory of the salt marsh. A well tuned high marsh can out produce the best cornfield four times over. The nutrients carried in on each passing tide permeates the marsh peat and, fueled by intense competition, the grasses grow thick and lush.

Each season as the new growth dies, the plant parts break down into particulate matter or Detritus. Detritus is the hub of the coastal food web.

Low Marsh

The low marsh is easily defined as the tall grasses that line the sides of creeks and ditches or extends from the high marsh down to the mud flats. The primary function of the low marsh is as a nursery area for 90% of the coastal fishery species.

These grass-lined banks provide safe haven for small fish as they proceed in and out of the marsh to feed. Without the proper ratio of creeks and ditches, inaccessible areas of the marsh would only be available for feeding fish for a handful of hours each month. As the tide receded from these areas untold millions of fish would be stranded on the marsh surface or in warm water pannes to perish.

Salt Marsh

The coastal food web is dependant on the productivity of salt marshes. The effects of a large estuary can be evident two miles out to sea. The baitfish and gamefish that utilized the salt marshes as a nursery ground migrate the entire eastern seaboard and out to the edges of the continental shelf.

Salt marshes can be beneficial to more areas than just the surrounding coastal waters. The system of marshes lining the Atlantic Coast is extremely important to migratory birds. Many bird species, most notably the shore birds, make planned stops at Areas of Ecological Importance as they migrate both north and south each season. These areas are instinctual stops that reliably provide an abundant high-energy food source, such as Horseshoe Crab eggs or Sea Worms.

Because of their filtering nature, salt marshes also readily remove many of the pollutants found in surface and ground water, such as heavy metals. The removed pollutants are then stored within the accumulating peat of the marsh.

In the recent past, salt marshes were considered to be wastelands and breeding places for Mosquitoes that have little value to society. As a result they could not be filled fast enough. Many coastal cities have been built on the marshes, including the whole Back Bay area of Boston Massachusetts. Only during the last two decades has the importance of salt marshes been discovered. These new lines of thought have made salt marshes the most protected habitats in the United States.

Salt Marsh

A picture is worth a thousand words. This little oasis in Danvers, MA has the spectrum of coastal values in a microcosm. The salt marsh blends with coastal woodlands, open shrub, dense shrub, and wet meadow. This area is small and as such has limited year round values; however it becomes very important during migration periods and to winter resident species.


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