Location: Barn Island, Connecticut
Oceanic Region: Western Atlantic Coast

Restoration of Tidal Salt Marshes

Tidal salt marshes occur in sheltered areas along the U.S. East Coast in most temperate regions. Such wetlands, which are typically dominated by cordgrass (Spartina), are regularly flooded by the tides. Tidal flooding and the salinity of the water are major factors influencing the presence and distribution of organisms. Salt marshes are highly productive ecosystems, generating large amounts of plant material each year. Very little of this plant material is grazed directly by animals; most of it dies and is converted to detritus by microorganisms. The detritus, along with algae, is consumed by a wide variety of animals including snails, amphipods, isopods, fiddler crabs, shrimp, and minnows. Many of these detritus/algae feeders are in turn eaten by carnivores such as mummichogs, eels, bluefish, striped bass, blue crabs, herons, and egrets. The tides, together with nekton (fishes and larger crustaceans), link these productive salt marshes with the adjacent estuarine and nearshore coastal waters.

As a result of human intrusion, thousands of hectares of tidal salt marshes along the Atlantic coast of North America were destroyed or degraded. Tidal exchange with many coastal marshes was reduced or completely blocked by the construction of highways, railroads, and impoundments. Elimination of the normal pattern of tidal flooding resulted in the loss of typical salt marsh organisms within the affected areas. In many cases, tall reedy vegetation, cattail (Typha angustifolia), or common reedgrass (Phragmites australis) replaced the marsh grasses. In other cases, the marshes were converted into sparsely vegetated areas with standing water. Changes in the physical environment, habitat structure, and food resources contributed to shifts in the animal populations.

Figure 1. Aerial photograph of Impoundment 1. Arrowhead indicates the dike. Figure 2. Impoundment 1 dominated by cordgrass (Spartina) 21 years following the re-establishment of tidal flooding. Figure 3. The marsh snail (Melampus bidentatus) in Impoundment 1. Figure 4. Relative abundance of the marsh snail (Melampus) in restoring and reference regions of four marshes at Barn Island, Connecticut in relation to the number of years in restoration.

As a result of increased awareness of the ecological importance of tidal salt marshes, legislation has been enacted to protect remaining marshes. Restoration of many degraded marshes has been undertaken. In addition, some new marshes have been created. Re-opening impounded marshes to tidal flooding alone is often sufficient to bring about restoration of a thriving salt marsh community. The saltwater kills salt-intolerant plants such as cattail and reedgrass, which outcompete the salt marsh plants under low salt conditions. Tidal transport of seeds and larval and adult animals from nearby healthy marshes also affects recolonization of the degraded marsh area. However, full recovery of the salt marsh community may require many years to achieve. Furthermore, various marsh attributes may return at different rates and, to some degree, are independent of one another.

A good example of successful marsh restoration is provided by the Barn Island marshes situated in southeastern Connecticut near the mouth of Long Island Sound. Four valley marshes within this system were impounded during the late 1940s to increase waterfowl habitat. Impoundment 1 (figure 1), which was most extensively studied, was converted from a cordgrass (Spartina) meadow marsh to a brackish cattail marsh inhabited by frogs, turtles, and water snakes. In 1978, a 5-ft diameter culvert was placed in the impoundment dike. A 7-ft diameter culvert was added 4 years later. By 1988, after 10 years of renewed tidal flooding, cattail cover had dramatically declined and smooth cordgrass (Spartina alterniflora) cover had increased from less than 1% to nearly 50%. Other salt marsh plants were also established. Since then, these plants have continued to spread (figure 2).

By the early 1990s, a characteristic assemblage of salt marsh invertebrates, including snails, ribbed mussels, amphipods, isopods, and fiddler crabs, was present in the restoring marsh. One of the more abundant and widely distributed of these marsh invertebrates, and one that can be quantitatively sampled with relative ease, is the marsh snail (Melampus bidentatus) (figure 3). Twelve years after the re-establishment of tidal flooding to Impoundment 1, the density of the marsh snail (Melampus) there was about half that in the reference marsh below the dike and in a nearby unimpounded valley marsh. However, after 21 years in restoration, Impoundment 1 supported a Melampus population comparable to that of the reference marsh (figure 4).

Similarly, by 1991 a typical assemblage of tidal salt marsh fishes was present in Impoundment 1. Essentially the same species were found above and below the impoundment dike. These included the common mummichog, sheepshead minnow, striped killifish, and Atlantic silverside. However, the numerically dominant mummichog (Fundulus heteroclitus) appeared to be less abundant in Impoundment 1 than in the reference marshes; but 8 years later, the numbers of mummichogs trapped in the restored marsh were comparable to those caught below the dike.

The studies at Barn Island and elsewhere suggest that salt marsh plants and animals may become re-established on a restoring marsh relatively soon following the return of tidal flooding, but a much longer period of time may be required for some species to achieve normal abundances.

Key Principles

1. During the past century, large numbers of tidal salt marshes around the world have been degraded or completely destroyed.
2. Many degraded marshes can be reclaimed. For impounded marshes, the re-introduction of tidal flooding, adjusted to current marsh elevations, is often sufficient to set in motion a natural process of recovery. Restoration can therefore be carried out at modest cost.
3. Marsh restoration is frequently a slow process, and full recovery may require decades. This fact must be kept in mind when assessing the success of restoration projects.

Ethical Considerations

1. Tidal salt marshes are ecologically valuable as integral components of many coastal ecosystems, may provide habitat for endangered species, and have considerable aesthetic value.
2. As marine biologists and informed citizens, we should be advocates for protecting and restoring our coastal tidal marshes.

Author
Paul E. Fell
Department of Zoology
Connecticut College
New London, CT 06320