many bays, estuaries, and coastal tidal flats are bordered inland by a vegetated intertidal area containing grasses or shrublike mangrove swamps. mangroves do not tolerate freezing conditions so are found only at low latitudes, whereas salt marshes are found at all latitudes. These coastal wetlands or salt marshes host a range of water salinities, from salty and brackish to nearly fresh. As estuaries age or mature, they tend to become progressively filled in first by tidal flats, then by salt marshes or coastal wetlands. Thus the degree to which estuaries are filled in can indicate the state of their maturity.
salt marshes form on the upper part of the intertidal zone where organic rich sediments are rarely disturbed by tides, providing a stable environment for grasses to take root. The low marsh area is defined as the part of the marsh that ranges from the beginning of vegetation to the least mean high tide. The high marsh extends from the mean high tide up to the limit of tidal influence. Different genera and species of grass form at different latitudes and on different continents, but in North America high parts of salt marshes are dominated by Juncus grasses, known also as needle- or black-rush, which can be 5 to 6 feet (2 m) tall, with sharp, pointed ends. Low parts of salt marshes tend to be dominated by dense growths of knee-high spartina grasses.
salt marshes must grow upward to keep up with rising sea levels. To do this they accumulate sediments derived from storm floods moving sediment inland from the beach environment, from river floods bringing in sediment from the mainland, and from the accumulation of organic material that grew and lived in the salt marshes. When plants in salt marshes are suddenly covered by sediment from storms or floods, they quickly recover by growing up through the new sediment, thereby allowing the marsh to survive and grow upward. some salt marshes grow upward so efficiently that they raise themselves above tidal influence and eventually become a freshwater woodland environment. With the increasing rate of sea-level rise predicted for the next century, however, many scientists are concerned that sea level will start to rise faster than marsh sedimentation can keep pace with it. This problem is particularly exacerbated in places where the normal supply of river and flood sediments is cut off, for instance, by levees along rivers. If this happens, many of the fragile and environmentally unique coastal marsh settings will disappear. marshes are among the most productive of all environments on Earth; they serve as nurseries for many organisms and are large producers of oxygen through photosynthesis. The disappearance of coastal marshes is already happening at an alarming rate in places such as the mississippi River delta, where coastal subsidence, loss of delta replenishment, together with sea-level rise leads to more than 0.39 inches (1 cm) of relative sea-level rise each year. salt marshes are disappearing at an alarming rate along the mississippi River delta, as discussed in a later chapter.
many coastal marshes in low latitudes are covered with dense mangrove tidal forest ecosystems, known also as mangals. These have fresh to brackish water and are under tidal influence. mangrove stands have proven to be extremely effective protective barriers against invaders from the sea, including hostile armies, storm surges, and tsunamis. The destruction of many coastal mangrove forests in recent years has proven catastrophic to some regions, such as areas inundated by the 2004 Indian ocean tsunami that were once protected by mangroves. Many local governments removed the mangroves to facilitate development and shrimp farming, but when the tsunami hit, it swept far inland in areas without mangroves, and was effectively stopped in places where the mangroves were still undisturbed. There are many examples of places where mangrove-dominated coasts have withstood direct hits from hurricanes and storm surges, yet protected the coastline to the extent that there was little detectable change after the storm.
several dozen or more types of mangroves are known, occurring on many coasts of North America, Africa, south America, India, southeast Asia, and elsewhere around the Pacific. mangroves prefer protected, low-energy coasts such as estuaries, lagoons, and back-barrier areas. mangroves develop extensive root systems and propagate by dropping seeds into the water, where they take root and spread. mangrove stands have also been known to be uprooted by storms, float to another location, and take root in the new setting.
The extensive root network of mangrove stands slows many tidal currents and reduces wave energy by a factor of 10, forming lower-energy conditions inside the mangrove forest. These lower-energy conditions favor the deposition of sediment, enhancing seaward growth of the mangrove forest.
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