Background

Geography and geomorphology

Coastal marshes are a very sensitive and dynamic ecosystem. They form on low-lying coastal areas that are washed by tides, in the intertidal regions. The plants which colonize a coastal marsh trap the sediment washed in by the tides, and their own dead biomass produces a rich organic substrate that they grow in. As long as they can maintain their leaves above the lowest tide and as long as a supply of sediment and nutrient comes in on the tide, they can expand and cover many acres of coastal area.

The area modeled by the Coastal Marsh Project is the eastern coast of the United States. The study area follows the trailing edge of the North American plate where the slope of the coastline is very low and gentle, which facilitates the development of coastal marshes. The Project did not cover, for instance, the rocky shores of Maine, where high tides crash against cliffs, precluding the long, slow building of sediment that is necessary for coastal marshes to grow. Nor was most of Atlantic Florida included in the study, where the salt coastal ecosystem is mostly mangrove swamps rather than grassy marshland.

Beneficial aspects of coastal marshes

Coastal marshes have been impacted by many Western cultural activities. As the European settlement of North America developed, coastal marshes were utilized for haying and hunting. As these sustainable activities continued, however, many other activities have degraded coastal marsh areas. The marshes were filled in to expand settled areas, drained for additional cropland, ditched for mosquito control, and were used for dumping wastes. Anglo-american culture has preserved a certain distaste in the language for most wetlands: "bogged down," "swamped," and even the "bogeyman." They are, in fact, buggy, humid, changeable places that are difficult for humans to travel through. However, scientists and ecologists have found that coastal marshes are extremely valuable in providing many services to human settlements or other ecosystems.

Floods are a major hazard in many coastal, low-lying, humid regions. Big storms often generate high waves which crash into the shore, ripping away seawalls or sand dunes which protect inland areas. Coastal marsh hydrology, with its quick drainage and system of creeks that disperse water quickly and efficiently over a great area, is very effective for blunting the force of such storms and floods.

Many coastal and oceanic species spend part of their life cycle on or close to land, and many of these need coastal marshes as nurseries for the young. Commercially important species that spend part of their lives in coastal wetlands include shrimp, crabs, and fish such as menhaden and flounder. The National Marine Fisheries Service's 1990 report (Zimmerman, 1990) indicates that 90% of the commercially important fish and shellfish of the southeast Atlantic and Gulf coasts use these ecosystems as migratory protection.

Certainly both birders and hunters are very familiar with coastal marshes as a primary habitat for migratory bird species along the Atlantic flyway. Many birds also nest in coastal marshes, following the dozens of species of insects and spiders that have helped to give the ecosystem its reputation. Wrens, sparrows, and the clapper rail are all permanent residents, as well as wading birds like egrets, herons, and rails. Great populations of northern waterfowl overwinter in the relatively mild climate of the southern coastal marshes.

Coastal marsh habitat is considered an ecotone, a transition zone between two different plant communities. Suspended between dry land and open water, the coastal marsh is one of the most productive areas on earth. Mitsch and Gosselink (1993) have analyzed coastal wetlands and determined that such areas produce up to 80 metric tons of plant material per hectare per year.

Coastal marshes have been called the "kidneys of the coast", referring to their ability to transform pollution-laden water into fresh, clear water. Dense plant communities function as filters to capture many pollutants and remove them from the water. Some pollutants can then be used by the plants for growth. Other chemicals may be deposited into the soil or atmosphere.

Historical loss of American marshland

Studies differ in their analyses of the acreage extant at the beginning of the colonial period in North America and in the acreage currently considered coastal marsh. A convenient and customary rule of thumb is that half of all the coastal marsh along the east coast has been destroyed in the last three hundred years.

In the mid-century, more attention focused on this rich and significant ecosystem in the light of the new awareness of environmental protection. Many communities passed legislation to protect the remaining wetlands in their locale, and the federal government began its campaign to preserve all wetland types in the United States.

Any representative curve of marsh loss in the United States shows a definite tick mark in the 1970s, when much protective legislation went into effect. Two laws were probably the most significant for wetlands protection: the Clean Water Act (1972), whose Section 404 established the process by which developers had to get permits to change wetlands by dredging or filling; and the Coastal Zone Management Act (1972), which established the Office of Coastal Zone Management. Since then many wetlands ordinances have been passed at all levels.

Non-governmental organizations have also contributed to the "no net loss" of wetlands policy of the 80s and beyond: Ducks Unlimited and The Nature Conservancy have been very active in sequestering thousands of acres of wetlands for their value in waterfowl habitat enhancement, and many other groups with names like "Save the Bay" have concentrated on preserving wetlands for ecological reasons.

Mechanisms for Marsh Loss

The simplest answer to the question of what destroys a coastal marsh is that it loses its delicate balance between sea level and construction of its own substrate. A coastal marsh is built up slowly, the plants rooting into an organic ooze created from their own rotting bodies and the accumulated sediment that is brought in on the tide and not lost to the outgoing tide. Wherever sea level is rising or falling, the local coastal marshes have to adapt to the changing conditions, and historical records show that, to a remarkable extent, they do so. The marshes in Sandwich, Massachusetts are possibly 5000 years old (Redfield, 1972), and many areas in England can show similar dates.

But coastal marshes can decline in size even when human activities are halted, or where they never were implemented. Natural forces act on them as well. In order to set policy and protect the remaining marshes, it is important to know if they can even be saved; if they are declining even in the face of protective legislation, it might make sense to leave them alone and focus on other areas with more hope of salvation.

Coastal coastal marshes disappear for different reasons:

• In some cases, the liquid (groundwater or oil, or even natural gas) beneath the substrate has been extracted to an extent that the marsh surface collapses, reducing the elevation of the marsh to a point where it drowns in place.
• The sediment brought in by the daily tides carries both material for the continuing health of the substrate and nutrients for the plants that anchor the substrate. If a contributing river, for instance, is dammed or canalized, the supply of sediment can diminish to a point where the marsh cannot keep its elevation stable with the sea level.
• The tidal regime of the area might be changed, so that the outgoing tides become stronger than the incoming ones; again, the marsh becomes starved for sediment accretion.
• As an area becomes more urbanized, sediment that runs off from the paved surfaces can accumulate in the estuary or the tidal channels to an extent that the whole area is pressed down, raising the local sea level and flooding the marsh.
• Some areas of the globe are still reacting to ancient glacial activity; the surface of the earth might be rising or falling in response to the pressures put on it tens of thousands of years ago. In that case, local sea level will be changing. If it is changing faster than the marsh can migrate to follow its tidal heights, the marsh will die.

Dr. Michael Kearney of the University of Maryland spent many years studying the activities of the coastal marshes around Chesapeake Bay. After analyzing the different modes by which coastal marshes decline in natural and even protected conditions, he determined how to measure coastal marsh loss (Stevenson, 1988). Kearney found that marshes erode in a quantifiable pattern from the increasing areas of standing water within a marsh system. As a result of his research, he developed a model for estimating marsh loss as a function of the amount of water in a given area.

No matter what the original cause for the changing relationship between salt water and marsh surface, a pattern of decline can give a manager some idea of how much time elapses between different stages of deterioration, making possible timely intervention. The pattern of increased waterlogging, though not always exact, is sufficient to allow a model to be developed that categorizes the stages of deterioration:

Class 1:   0-20% water in the observed marsh: Healthy marsh, stable substrate

Class 2:   21-30% water as judged by spectral analysis: tidal streams widen

Class 3:   31-50% water: ponds enlarge and coalesce, streams widen further

Class 4:   >50% water: large ponds remain all year, streams cut off islands and further enlarge ponds, allowing greater wind fetch, creating a feedback that leads to more rapid deterioration.

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Combining Data with Theory

The Coastal Marsh Project was funded as part of NASA's Mission to Planet Earth in 1993. The intent was to provide a useful "back of the envelope" data base that would give coastal managers a quickly updated, small-scale GIS inventory of the condition of coastal coastal marshes.

The Landsat Thematic Mapper images, with a resolution of 28.5 meters per pixel, are coarse enough to miss small local marsh areas. The National Wetland Inventory maps are created using 7.5-minute United States Geological Survey topographic quadrangle maps; although they are large in scale, they take years to update and are dependent on hand work. The two data sets (TM and NWI) are combined in a Geographic Information System and processed to create a new data base. This new data base shows an approximation of the state of the marsh areas by defining the percentage of water in the substrate, and consequently, the degree of deterioration it is likely to be experiencing.

The model is produced by combining the two streams of data, running a spectral unmixing algorithm on them to define the spectral signal for water, and creating a GIS data set that shows the categories of stability of the marsh, from "healthy" marsh through the stages of deterioration to "completely deteriorated." (See Methodology for details.)

Since the model was developed in a microtidal area, some alterations had to be made to the model when it was applied in a mesotidal area like South Carolina, where acres of mud flat are left exposed at low tide; or a macrotidal area like Georgia, where tidal water rushes across acres of coastal marsh. The algorithm had to account for such tidal differences in the satellite images.

References

1. Mitsch, W.J., and Gosselink, J.G. 1993. Wetlands. (Second Edition) (New York: Van Nostrand Reinhold)

2. Redfield, A.C. 1972. Development of a New England salt marsh. Ecologic Monograph 42:201-237.

3. Stevenson, J. C., Ward, L. G., Kearney, M. S., 1988, Sediment transport and trapping in marsh systems: implications of tidal flux studies. Marine Geology 80, 37-59

4. Zimmerman, R.J., Minellos, T.J., Smith, D.L., and Kostera, J. 1990. The Use of Juncus and Spartina Marshes by Fisheries Species in Lavaca Bay, Texas, with reference to the Effects of Floods. (National Marine Fisheries Service, NOAA, Washington, D.C.: NOAA Technical Memo NMFS-SEFC-251)