A salt marsh is an environment in the upper coastal intertidal zone between land and salt water or brackish water, it is dominated by dense stands of halophytic (salt-tolerant) plants such as herbs, grasses, or low shrubs.[1][2] These plants are terrestrial in origin and are essential to the stability of the salt marsh in trapping and binding sediments. Salt marshes play a large role in the aquatic food web and the exporting of nutrients to coastal waters. They also provide support to terrestrial animals such as migrating birds as well as providing coastal protection.[2]
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Salt marshes occur on low-energy coasts in temperate and high-latitudes.[3] These typically include sheltered environments such as embankments, estuaries and the leeward side of barrier islands and spits. In the tropics and sub-tropics they are replaced by mangroves; an area that differs to a salt marsh in that instead of herbaceous plants, they are dominated by salt-tolerant trees.[1] Most salt marshes have a low topography with low elevations but a vast wide area, making them hugely popular for human populations.[4]
Salt marshes are located among different landforms based on their physical and geomorphological settings. Such marsh landforms include deltaic marshes, estuarine, back-barrier, open coast, embayments and drowned-valley marshes. Deltaic marshes are associated with large rivers where many occur in Southern Europe such as the Camargue in the Rhone delta or the Ebro delta. They are also highly extensive within the rivers of the Mississippi Delta.[2] In New Zealand, most salt marshes occur at the head of estuaries[5] in areas where there is little wave action and high sedimentation. Such marshes are located in Awhitu Regional Park in Auckland, the Manawatu Estuary and the Avon-Heathcote Estuary in Christchurch. Back-barrier marshes are sensitive to the reshaping of barriers in the landward side of which they have been formed.[2] They are common along much of the eastern coast of the United States and the Frisian Islands. Large, shallow coastal embayments can hold salt marshes with examples including Morecambe Bay and Portsmouth in Britain and the Bay of Fundy in North America.[2] Salt marshes are sometime included in lagoons, and the difference is not very marked: e.g. a major part of the Venetian Lagoon is made up of these sorts of animals and or living organisms belonging to this ecosystem.
Coastal salt marshes can be distinguished from terrestrial habitats by the daily tidal flow that occurs and continuously floods the area.[1] It is an important process in delivering sediments, nutrients and plant water supply to the marsh.[4] At higher elevations in the upper marsh zone, there is much less tidal inflow, resulting in lower salinity levels.[1] Soil salinity in the lower marsh zone is fairly constant due to everyday annual tidal flow. However, in the upper marsh, variability in salinity is shown as a result of less frequent flooding and climate variations. Rainfall can reduce salinity and evapotranspiration can increase levels during dry periods.[1] As a result, there are microhabitats populated by different species of flora and fauna dependant on their physiological abilities. The flora of a salt marsh is differentiated into levels according to the plants' individual tolerance of salinity and water table levels. Vegetation found at the water must be able to survive high salt concentrations, periodical submersion, and a certain amount of water movement, while plants further inland in the marsh can sometimes experience dry, low-nutrient conditions. It has been found that the upper marsh zones limit species through competition and the lack of habitat protection, while lower marsh zones are determined through the ability of plants to tolerate physiological stresses such as salinity, water submergence and low oxygen levels.[6][7]
The New England salt marsh is subject to strong tidal influences and shows distinct patterns of zonation.[7] In low marsh areas with high tidal flooding, a monoculture of the smooth cordgrass, Spartina alterniflora dominate, then heading landwards, zones of the salt hay, Spartina patens, black rush, Juncus gerardii and the shrub Iva frutescens are seen respectively.[6] These species all have different tolerances that make the different zones along the marsh best suited for each individual.
Plant species diversity is relatively low, since the flora must be tolerant of salt, complete or partial submersion, and anoxic mud substrate. The most common salt marsh plants are glassworts (Salicornia spp.) and the cordgrass (Spartina spp.), which have worldwide distribution. They are often the first plants to take hold in a mudflat and begin its ecological succession into a salt marsh. Their shoots lift the main flow of the tide above the mud surface while their roots spread into the substrate and stabilize the sticky mud and carry oxygen into it so that other plants can establish themselves as well. Plants such as sea lavenders (Limonium spp.), plantains (Plantago spp.), and varied sedges and rushes grow once the mud has been vegetated by the pioneer species.
Salt marshes are quite photosynthetically active and are extremely productive habitats. They serve as depositories for a large amount of organic matter and are full of decomposition, which feeds a broad food chain of organisms from bacteria to mammals. Many of the halophytic plants such as cordgrass are not grazed at all by higher animals but die off and decompose to become food for micro-organisms, which in turn become food for fish and birds.
The coast is a highly attractive natural feature to humans through its beauty, resources, and accessibility. As of 2002, over half of the world’s population was estimated to being living within 60 km of the coastal shoreline,[2] making our coastlines highly vulnerable to human impacts from daily activities that put pressure on these surrounding natural environments. In the past, salt marshes were perceived as coastal ‘wastelands,’ causing considerable loss and change of these ecosystems through land reclamation for agriculture, urban development, salt production and recreation.[4][8][9] The indirect effects of human activities such as nitrogen loading also play a major role in the salt marsh area.
Reclamation of land for agriculture by converting marshland to upland was historically a common practice.[4] Dikes were often built to allow for this shift in land change and to provide flood protection further inland. For centuries, livestock such as sheep and cattle grazed on the highly fertile salt marsh land.[1][10] Land reclamation for agriculture has resulted in many changes such as shifts in vegetation structure, sedimentation, salinity, water flow, biodiversity loss and high nutrient inputs. There have been many attempts made to eradicate these problems for example, in New Zealand, the cordgrass Spartina anglica was introduced from England into the Manawatu River mouth in 1913 to try and reclaim the estuary land for farming.[5] A shift in structure from bare tidal flat to pastureland resulted from increased sedimentation and the cordgrass extended out into other estuaries around New Zealand. Native plants and animals struggled to survive as non-natives out competed them. Efforts are now being made to remove these cordgrass species, as the damages are slowly being recognised.
Following the period of marshland conversion for agriculture, these areas were later transformed into urban, residential and industrial land.[4] Major cities such as Boston, San Francisco, Amsterdam, Rotterdam, Venice and Tokyo have all expanded out into areas of salt marsh.[4] The remaining marshes surrounding these urban areas are under immense pressure from the human population as human-induced nitrogen enrichment enters these habitats.
Nitrogen loading through human-use indirectly affects salt marshes causing shifts in vegetation structure and the invasion of non-native species.[6] Human impacts such as sewage, urban run-off, agricultural and industrial wastes are running into the marshes from nearby sources. Salt marshes are nitrogen limited[6][11] and with an increasing level of nutrients entering the system from anthropogenic effects, the plant species associated with salt marshes are being restructured through change in competition.[4] For example, the New England salt marsh is experiencing a shift in vegetation structure where S. alterniflora is spreading from the lower marsh where it predominately resides up into the upper marsh zone.[6] Additionally, in the same marshes, the reed Phragmites australis has been invading the area expanding to lower marshes and becoming a dominant species. P. australis is an aggressive halophyte that can invade disturbed areas in large numbers outcompeting native plants.[4][12][13] This loss in biodiversity is not only seen in flora assemblages but also in many animals such as insects and birds as their habitat and food resources are altered.
Earlier in the 20th century, it was believed that draining the salt marshes would help mitigate mosquito populations. In many locations, particularly in the northeastern United States, residents and local and state agencies dug straight-lined ditches deep into the marsh flats. The end result, however, was a depletion of killifish habitat. The killifish is a mosquito predator, so the loss of habitat actually led to higher mosquito populations, and adversely affected wading birds that preyed on the killifish. These ditches can still be seen, though with the realization of the impacts on the marshes' health, some places have begun efforts to refill the ditches.[14]
The perception of bay salt marshes as a coastal ‘wasteland’ has since changed, acknowledging that they are one of the most biologically productive habitats on earth, rivalling tropical rainforests. Salt marshes are ecologically important providing habitats for native migratory fish and acting as sheltered feeding and nursery grounds.[9] They are now protected by legislation in many countries to look after these ecologically important habitats.[15] In the United States and Europe, they are now accorded to a high level of protection by the Clean Water Act and the Habitats Directive respectively. With the impacts of this habitat and its importance now realised, a growing interest in restoring salt marshes, through managed retreat or the reclamation of land has been established. However, many Asian countries such as China are still to recognise the value of marshlands. With their ever-growing populations and intense development along the coast, the value of salt marshes tends to be ignored and the land continues to be reclaimed.[4]
Bakker et al. (1997)[16] suggests two options available for restoring salt marshes. The first is to abandon all human interference and leave the salt marsh to complete its natural development. These types of restoration projects are often unsuccessful as vegetation tends to struggle to revert back to its original structure and the natural tidal cycles are shifted due to land changes. The second option suggested by Bakker et al. (1997)[16] is to restore the destroyed habitat into its natural state either at the original site or as a replacement at a different site. Under natural conditions, recovery can take 2–10 years or even longer depending on the nature and degree of the disturbance and the relative maturity of the marsh involved.[15] Marshes in their pioneer stages of development will recover more rapidly than mature marshes[15] as they are often first to colonize the land. It is important to note, that restoration can often be sped up through the replanting of native vegetation.
This last approach is often the most practised and generally more successful than allowing the area to naturally recover on its own. The salt marshes in the state of Connecticut in the United States have long been an area lost to fill and dredging. As of 1969, the Tidal Wetland Act was introduced that seized this practise,[13] but despite the introduction of the act, the system was still degrading due to alterations in tidal flow. One area in Connecticut is the marshes on Barn Island. These marshes were diked then impounded with salt and brackish marsh during 1946-1966.[13] As a result the marsh shifted to a freshwater state and became dominated by the invasive species P. australis, Typha angustifolia and T. latifolia that have little ecological connection to the area.[13] By 1980, a restoration programme was put in place that has now been running for over 20 years.[13] This programme has aimed to reconnect the marshes by returning tidal flow along with the ecological functions and characteristics of the marshes back to their original state. In the case of Barn Island, declines in the invasive species have initiated, re-establishing the tidal-marsh vegetation along with animal species such as fish and insects. This example highlights that considerable time and effort is needed to effectively restore salt marsh systems. Times in marsh recovery can depend on the development stage of the marsh; type and extent of the disturbance; geographical location; and the environmental and physiological stress factors to the marsh-associated flora and fauna.
Although much effort has gone into restoring salt marshes worldwide, further research is needed. There are many setbacks and problems associated with marsh restoration that requires careful long-term monitoring. Information on all components of the salt marsh ecosystem should be understood and monitored from sedimentation, nutrient, and tidal influences, to behaviour patterns and tolerances of both flora and fauna species.[15] Once we have a better understanding of these processes and not just locally, but over a global scale, we can then suggest more sound and practical management and restoration efforts that can be used to preserve our valuable marshes and put them back to their original state.
While humans are situated along coastlines, there will always be the possibility of human-induced disturbances despite the number of restoration efforts we plan to implement. Dredging, pipelines for offshore petroleum resources, highway construction, accidental toxic spills or just plain carelessness are examples that will for some time now and into the future be the major influences of salt marsh degradation.[15]
In addition to restoring and managing salt marsh systems based on scientific principles, the opportunity should be taken to educate public audiences of their importance biologically and their purpose as serving as a natural buffer for flood protection.[9] Because salt marshes are often located next to urban areas, they are likely to receive more visitors than remote wetlands. By physically seeing the marsh, people are more likely to take notice and be more aware of the environment around them. An example of public involvement occurred at the Famosa Slough wetland in San Diego, where a “friends” group worked for over a decade in trying to prevent the area from being developed.[17] Eventually, the 5 hectare site was bought by the City and the group worked together to restore the area. The project involved removing invasive species and replanting with natives, along with public talks to other locals, frequent bird walks and clean-up events.[17]
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