SS1 Biology Lesson Note on Terrestrial Habitat-Marsh

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TERRESTRIAL HABITAT-MARSH

Marshland is a treeless land in which the water table is at, above, or just below the surface of the ground; it is dominated by grasses, reeds, sedges, and cattails. These plants typify emergent vegetation, which has its roots in soil covered or saturated with water and its leaves held above water.

The salt marsh channels on Llanrhidian Marsh in South Wales slowly fill with water on the incoming tide, watched by a flock of oystercatchers and gulls. The diurnal cycle of the tides flooding and exposing the flats generates a unique ecosystem rich in marine life. Salt marshes are found in the intertidal zone along low-energy coastlines, forming along the margins of estuaries, where freshwater from the land mixes with sea water. The extensive root systems of salt marsh plants enable them to withstand strong winds, waves, and flooding from storms, and act as natural buffers against storm damage to upland development.

Marshes may be freshwater or salt. Freshwater marshes develop along the shallow margins of lakes and slow-moving rivers, forming when ponds and lakes become filled with sediment. Salt marshes occur on coastal tidal flats. Inland salt marshes occupy the edges of saline lakes. The nature of a marsh—its plant composition, species richness, and productivity—is strongly influenced by its relationship to surrounding ecosystems. They affect the supply of nutrients, the movement of water, and the type and deposition of sediment.

In the prairie pothole country of glaciated central North America, freshwater marshes undergo a cyclic renewal that is induced by periodic drought and dependent on the feeding habits of muskrats. The cycle begins with a nearly dry marsh in which seeds of aquatic plants germinate in the mud. When the marsh fills, the aquatic plants grow densely. Muskrats eat large areas of the emergent vegetation, creating patches of open water. This causes the shallow-water emergent to decline, but the submerged and floating species persist. When the next drought comes, the cycle begins again.

Salt marshes are best developed on the Atlantic coasts of North America and Europe. In eastern North America the low marsh is dominated by a single species, salt-marsh cord grass. The high marsh consists of a short cord grass called hay, spike grass, and glasswort. Glasswort is the dominant plant of Pacific Coast salt marshes.

WATER AND VEGETATION PATTERN
In some marshes, such as the saw-grass wetlands of the Everglades or in salt marshes that are swept twice daily by tidal floods, water flows like a sheet across the surface, and the terrain is typically dominated by one or two species of emergent vegetation. In other marshes the water flows in channels rather than in sheets, flooding only at times of snowmelt and heavy precipitation and bringing in nutrients and sediment. Such irregular deposition of sediments provides variations in water depth, thus creating conditions favourable for a variety of wetland species. Deep marsh water is colonized by aquatic submerged plants (pond weeds) and floating plants (pond lilies). Shallower water supports reeds and wild rice. Very shallow water supports sedges, bulrushes, and cattails.

As sediments and organic deposits raise the bottom of a marsh above the water table, aquatic vegetation is gradually replaced by shrubs and eventually by a terrestrial ecosystem of upland grasses or forest trees.

IMPORTANCE

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