The Environmental Science Graduate Students at Watlings Castle, San Salvador, Bahamas.(TME 98)
The Salt Marsh: A Valued and Protected, Marine Ecosystem
Within the marine environment, some of the Earth’s most productive ecosystems are formed when streams and rivers merge with ocean water in areas known as estuaries. Organisms found within these estuaries experience strong changes in temperature and salt concentration as the fresh water and salt-water mix. The nutrient rich soil that is carried into these areas by rivers, supports a vast diversity of life both in the water and on surrounding lands. Estuaries not only serve as breeding grounds for many invertebrate and fish species, but as nesting and feeding areas for a wide range of birds (Campbell et. al., 2004). The wetlands formed by these estuaries are typically classified into several environments depending on there physical characteristics. The different types of vegetation, which grow in these areas, flood frequency, and fluvial and tidal processes all give a wetland a unique characteristic. For example, shallow subaqueous flats are areas which are more frequently submerged than not. Mud and sand flats are slightly higher in elevation than subaqueous flats and can be characterized by having blue-green algae and moist or wet surfaces. Salt water marshes are more defined based on vegetation, soil moisture, and the proximity of the land area to bay, lagoon, or estuary waters. Brackish marshes experience more of an effect from flooding of estuary waters, and freshwater precipitation.
These wetlands serve as a transition between salt and fresh marshes. Finally, fresh water marshes are located more inland along river or fluvial systems and are typically found beyond the point of salt water flooding (University of Texas, 2003).
Not only do these coastal wetlands experience a variety of physical characteristics, but also offer significant ecological benefits. In addition to providing grounds for breeding and habitats to a wide variety of wildlife, they maintain coastal water quality by acting as a filter to sediments and excess nutrients and provide a region for the dilution of pollutants. Coastal wetlands also slow the flow of water and protect shorelines from erosion and damage during storms; this aids and maintains the integrity of surrounding property. Additionally, two-thirds of the U.S. fishing industry relies heavily on the characteristics of coastal estuaries and marshes for nursery and spawnin grounds (Miller, 1996).
Along most of the east coast of the U.S., the major ecosystems found in estuaries are salt marshes. Within these regions of low shore energy, there are usually distinct tidal fluctuations and the vegetation in these areas displays various tolerances to salt water concentration (EPA, 2003). Generally, salt marshes are recognized by their characteristic grassy vegetation: cordgrass in lower regions of the marsh and salt hay in the high marsh. These plant species are able to thrive in marsh soils, which are commonly anaerobic and concentrate salt through seawater evaporation (University of Georgia, 2003). Often times, these salt concentrations depend on the frequency of flooding. Flood frequency also plays a major role in the types of animals and plant species that may be found in a particular area. Low marsh zones, typically flood twice on a daily basis. This creates areas of highly saturated, anaerobic soil with very high salt concentrations producing very harsh conditions for plant and animal species in these lower zones. In contrast, high marsh zones are defined as areas, which only flood during storms or unusually high tides (SCDNR, 2003).
More specifically, salt marshes can be divided into seven different ecological zones; these zones are based on the time and depth of the tidal flood and include the following: tidal creek, levee, low marsh, high marsh, marsh border, transition communities, and maritime climax forest. Tidal creeks are units of flowing water through the marsh, and levees are the habitats located on the banks of these creeks. These two zones, in addition to the low marsh, are typified by constant sea water flooding and little change in salinity and temperature levels. At a slightly higher elevation are the high marsh zones, which are characterized by very little flooding throughout the day. Because of this longer exposure to air and continued evaporation, these zones experience very high concentrations of salt and low amounts of plant growth. Surrounding these high marsh zones are marsh borders. These regions experience tidal flooding solely on a seasonal scale, and because of this, runoff from fresh water is more prominent and there is strong differentiation in the number of plant species. Transitional communities allow for more woody vegetation to become established and lead to maritime forests and mature tree species (University of Georgia, 2003).
Despite the seemingly simple appearance of a salt marsh ecosystem, these areas are actually quite complex and play a very valuable role in the health and integrity of all coastal ecosystems. A major function of salt marshes is that they greatly aid in the control of flooding and improve coastal water quality. Because of their buffer-like qualities, marshes add nutrients and microorganisms, greatly contributing to coastal food webs. These areas also function as a safe habitat for a wide variety of birds, fish and other species of wildlife. Salt marshes also provide people with attractive natural areas, allowing for opportunities of education, recreation, and tourism (Ecology Action Centre– EAC, 2003).
Salt marshes are ordered among the top ecosystems in the world; in terms of production and the organisms that inhabit these areas, they play a large part in productivity. Beginning at the microscopic level, decaying marsh plants provide nutrients in the form of detritus for a large variety of bacteria, fungi, and small algae species. These organisms take part in an important role by breaking down certain portions of detritus, which cannot be digested by larger animals, at the same time providing valuable fertilizer for future seasonal marsh plants. These microorganisms also provide a food source for large numbers of primary and secondary consumers, mainly invertebrates. Although drastic changes in salinity and temperature may limit the number of invertebrates inhabiting these areas, those that can withstand these factors make a valuable link in the food chain. Marsh snails, fiddler crabs and marsh mussels are among the more typical invertebrates that may be found in these ecosystems, additionally, highly valued oysters inhabit the borders of salt marshes (SCDNR, 2003).
Other abundant types of invertebrates in the marsh food web are insects. Similar to all other organisms, insects rely heavily on plant detritus; however, insects largest role is the primary source of food they provide for the abundant number of bird species (including herons, egrets, and red-winged blackbirds) and fish for much of the year. Additionally, as mentioned, marshes provide safe dwellings for a variety of fish species and shrimp. Each type of organism within a salt marsh plays a fundamental role in maintaining the delicate food chain and overall integrity of these wetland ecosystems (SCDNR, 2003).
The component of salt marshes that is most important for productivity and function comes from various species of vegetation. One particular plant species, which dominates the heavily flooded low marsh zones, is cordgrass (Spartina alterniflora). Cordgrass is able to withstand high salt concentrations using special glands that secrete excess salt; additionally their strong root systems and narrow blades enable them to withstand frequent flooding and temperature fluctuations. In addition, Cordgrass protects marsh surfaces and soils as well as provide detritus and nutrients for organisms in the food web. Higher marsh zones display different types of grass species due to the differences in flood frequency and salt concentrations; in these areas Saltgrass (Distichlis spicata) and Black needle rush (Juncus roemerianus) are the dominant species of plants (EPA, 2003).
Although grasses are the most dominant forms of plant life inhabiting salt marshes, many other types of species are also present and provide unique characteristics to these wetlands. Glasswort (Salicornia virginica) and saltwort (Batis maritima) are two common succulent species that are capable of water storage and provide organisms with a valuable food source (EPA, 2003). These species are typically found in higher marsh zones and transitional communities where environmental factors such as salinity levels and temperatures are less extreme. However, despite their difference in appearance and locality from grass species, these plants play a similar role in providing organisms in these wetlands with an abundant food source, maintaining the health of salt marshes.
Although the benefits of a healthy salt marsh may be quite apparent, there are unfortunate and common instances of these valuable wetlands becoming degraded or “sick.” Many factors may affect the health of a marsh; these factors can be natural, but oftentimes are caused by human populations. A common occurrence is the blockage of tidal flow from structures such as roads, railroads, even bridges and culverts that may restrict the flow of tidal flooding. By restricting the flood frequency of an area, the environmental conditions created are no longer suitable for naturally occurring grass species and other organisms (NRCS, 2003).
Another common problem occurs when people attempt to reduce mosquito populations by digging ditches and draining these areas. Many times, this results in the increase in the number of mosquitoes due to standing water and greatly degrades the amount of vegetation within the marsh. This drainage, in turn, may lead to natural invasion of nonnative species such as common reed (Phragmites australis) and purple loosestrife (Lythrum salicaria). These plant species typically invade unhealthy salt marshes and crowd out native grasses and other beneficial species, while providing very little value to wildlife (NRCS, 2003). It is extremely important to maintain the integrity and health of these wetland ecosystems and the species within these areas. Accomplishing this ensures water quality safety for all species and protects valuable shoreline lands and habitats.
What is being done to secure salt marsh health? There are two areas which may be approached in order to achieve salt marsh health security, from both regulatory and restoration processes. Point and nonpoint source pollution are common factors in negatively affecting salt marsh health, however regulatory permits provided by state level EPA greatly diminish, if not alleviate point source pollutions from discharge pipes and single sources. Nonpoint source pollution is much more difficult to provide protection against due to often unknown origin and multiple sources. However, monitoring and regulation of local land use practices (i.e. installing buffer strips, detention basins, and porous pavement) can greatly reduce the affect of nonpoint pollutants on local salt marsh areas (SCDNR, 2003).
Regulations offer solutions in preventing damage to these valuable wetlands; however, if damage has already occurred, or the health of an area has been greatly diminished, then restoration projects provide an answer. One of the most frequent problems with the installation of structures such as culverts in salt marshes is improper placement, thereby reducing tidal flow. Correcting the placement of these culverts is one of the more common restoration projects in these types of areas. Simply lowering a culvert below creek bed elevation or replacing an existing culvert with a larger one can directly reinstate tidal flooding to a particular area (EAC, 2003). Other restoration projects may involve replacement of native plants and grasses or rerouting or redesigning roads and bridges. In any case, ensuring and protecting the safety of these valuable salt marsh ecosystems and the organisms found within, plays an important role in protecting all aquatic and coastal habitats and ecosystems.
Cambell, N., Williamson, B., Heyden, R. Biology: Exploring Life. Pub-Prentice Hall 2004. pp.759.
“How Healthy is Your Salt Marsh.” New Hampshire Natural Resources Conservation Service website. Online. Last Accessed May 2003. http://www.nh.nrcs.usda.gov/ecosystem_restoration/salt_marsh_restoration_in_NH/Saltmarsh.html
“Indian River Lagoon National Estuary Program: Salt Marsh Habitats.” Online. Last Accessed April 2003. http://www.epa.gov/owow/oceans/lagoon/saltmarsh.html
“Marine Education Center and Aquarium: The Salt Marsh.” The University of Georgia website. Online. Last Accessed April 2003. http://www.uga.edu/aquarium/NAT_HISTORY/marsh.html
“Nova Scotia’s Salt Marshes – Summary.” Ecology Action Centre website. Online. Last Accessed April 2003. http://www.ecologyaction.ca/EAC_WEB_1/PROJECTS/salt_marsh/tid_restrictions.shtml
Sea Science: Dynamics of the Salt Marsh.” South Carolina Department of Natural Resources website. Online. Last Accessed May 2003. http://www.dnr.state.sc.us/marine/pub/seascience/dynamic.html
“Wetland Ecosystems.” University of Texas website. Online. Last Accessed May 2003. http://www.tsgc.utexas.edu/trcp/wetland.html
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