salt marshes

salt marshes Salt marshes are areas of land covered mainly by salt-tolerant (halophytic) plant communities which are regularly flooded by the rise and fall of tides. Salt marshes occur in mid and high latitudes along intertidal shores worldwide, being largely replaced by mangrove swamps in tropical and sub-tropical regions. Salt-marsh development requires a net accumulation of mainly fine-grained sediment to provide a stable substrate; so some degree of shelter from wave action is required. Such shelter may be found in estuaries, in embayments, and on sections of open coast which are protected by wide intertidal flats and barrier complexes. Large expanses of salt marsh have often developed in these areas and have an important role in protecting the coastline from erosion by extreme storm events. Salt marshes are also valuable wildlife habitats, providing roosting and feeding sites for birds and acting as a nursery site for many economically important fish and shellfish. The halophyte communities which cover the marshes are biologically interesting in their own right and are extremely productive, exporting large amounts of organic material to adjacent waters.

The dominant organisms found in salt marshes are rooted plants, originally of terrestrial origin, which have adapted to the stresses imposed by the intertidal environment, such as high salinity and periodic tidal inundation. Colonization of sand or mud flats by vegetation can begin only when sediment has accumulated to a sufficiently high level in the tidal cycle for halophyte plants to survive and reproduce. Initial colonization is carried out by pioneer species such as Salicornia (glasswort) and Spartina (cord-grass). Once vegetation is established, the salt marsh may rapidly grow vertically as it traps and intercepts more incoming sediment, and organic matter (produced by the vegetation itself) is added to the marsh surface. This raising of the salt-marsh surface leads to an increase in drainage and a decreased submergence in salt water. Conditions are thus made suitable for the arrival of less hardy species which may displace the initial pioneers. This results in a zonation of the vegetation, pioneer species giving way to a more terrestrial-type vegetation near the landward margin of the marsh. An individual marsh may therefore develop a series of morphological zones as it matures, open mud flats with scattered clumps of pioneer vegetation giving way to a zone of continuous vegetation dominated by pioneer species (the low-marsh), which is subject to adjacent estuarine or marine conditions. This grades into a zone which is less frequently flooded and more species-rich (the high-marsh), dominated by plants that are more terrestrial in nature. A generalized transect across a typical North European salt marsh is shown in Fig. 1.

Salt marshes exhibit a number of characteristic geomorphological features. Cliffs may occur at the seaward edge of the marsh because of wave erosion. Creeks or marsh drainage systems are often well developed, and provide important pathways for energy and material transfer between the marsh and surrounding areas. To a certain extent the initial pattern of marsh drainage is inherited from the original tidal flat on which the marsh formed. A dendritic (or branched) creek pattern may develop which is the most efficient in terms of dissipating incoming wave and tidal energy by friction. Pans are natural depressions in the marsh which retain water even at low tide. They may form when the outlet of a tidal creek or depression is blocked by vegetation or accumulating sediment.

The behaviour of relative sea level is a major driving force behind the evolution of salt marshes in the medium to long term. A falling sea level may allow salt marsh progradation, whereas a rising sea level may cause landward migration. Studies of salt marshes on the western Atlantic coast have shown that, where sediment supply or organic production is sufficiently high, salt marshes are able to keep pace with rising sea levels. In such areas the vertical growth of salt marshes provides a measure of the rate of sea-level rise. In other areas, such as the Mississippi delta and parts of southern England, marshes are not successfully maintaining their elevation and are eroding and dying off as sea level rises. Other salt marsh areas are threatened by land reclamation and pollution. In recent decades, however, the ecological importance of salt marsh areas has been recognized and many are now designated conservation areas.

Andrew B. Cundy

Bibliography

Allen, J. R. L. and and Pye, K. (1992) Saltmarshes. Morphodynamics, conservation and engineering significance. Cambridge University Press.
Boaden, P. J. S. and and Seed, R. (1985) An introduction to coastal ecology. Blackie, Glasgow.
Frey, R. W. and and Basan, P. B. (1985) Coastal salt marshes. In Davis, R. A., Jnr (ed.) Coastal sedimentary environments (2nd edn), pp. 225–301. Springer-Verlag, New York.