Salt marsh

A salt marsh or saltmarsh, also known as a coastal salt marsh or a tidal marsh, is a coastal ecosystem in the upper coastal intertidal zone between land and open saltwater or brackish water that is regularly flooded by the tides. It is dominated by dense stands of 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 delivery of nutrients to coastal waters. They also support terrestrial animals and provide coastal protection.[2]

Salt pannes and pools high and low tide
Salt marsh during low tide, mean low tide, high tide and very high tide (spring tide).

Basic information

Salt marshes occur on low-energy shorelines in temperate and high-latitudes[3] which can be stable, emerging, or submerging depending if the sedimentation is greater, equal to, or lower than relative sea level rise (subsidence rate plus sea level change), respectively. Commonly these shorelines consist of mud or sand flats (known also as tidal flats or abbreviated to mudflats) which are nourished with sediment from inflowing rivers and streams.[4] 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 from 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.[5] 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, France in the Rhone delta or the Ebro delta in Spain. They are also extensive within the rivers of the Mississippi Delta in the United States.[2] In New Zealand, most salt marshes occur at the head of estuaries in areas where there is little wave action and high sedimentation.[6] 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 sometimes included in lagoons, and the difference is not very marked; the Venetian Lagoon in Italy, for example, is made up of these sorts of animals and or living organisms belonging to this ecosystem. They have a big impact on the biodiversity of the area. Salt marsh ecology involves complex food webs which include primary producers (vascular plants, macroalgae, diatoms, epiphytes, and phytoplankton), primary consumers (zooplankton, macrozoa, molluscs, insects), and secondary consumers.[7]

The low physical energy and high grasses provide a refuge for animals. Many marine fish use salt marshes as nursery grounds for their young before they move to open waters. Birds may raise their young among the high grasses, because the marsh provides both sanctuary from predators and abundant food sources which include fish trapped in pools, insects, shellfish, and worms.[8]

Worldwide occurrence

Saltmarshes across 99 countries (essentially worldwide) were mapped by Mcowen et al. 2017.[9] A total of 5,495,089 hectares of mapped saltmarsh across 43 countries and territories are represented in a Geographic Information Systems polygon shapefile. This estimate is at the relatively low end of previous estimates (2.2–40 Mha). The most extensive saltmarshes worldwide are found outside the tropics, notably including the low-lying, ice-free coasts, bays and estuaries of the North Atlantic which are well represented in their global polygon dataset.[9]

Formation

The formation begins as tidal flats gain elevation relative to sea level by sediment accretion, and subsequently the rate and duration of tidal flooding decreases so that vegetation can colonize on the exposed surface.[10] The arrival of propagules of pioneer species such as seeds or rhizome portions are combined with the development of suitable conditions for their germination and establishment in the process of colonisation.[11] When rivers and streams arrive at the low gradient of the tidal flats, the discharge rate reduces and suspended sediment settles onto the tidal flat surface, helped by the backwater effect of the rising tide.[4] Mats of filamentous blue-green algae can fix silt and clay sized sediment particles to their sticky sheaths on contact[12] which can also increase the erosion resistance of the sediments.[13] This assists the process of sediment accretion to allow colonising species (e.g., Salicornia spp.) to grow. These species retain sediment washed in from the rising tide around their stems and leaves and form low muddy mounds which eventually coalesce to form depositional terraces, whose upward growth is aided by a sub-surface root network which binds the sediment.[14] Once vegetation is established on depositional terraces further sediment trapping and accretion can allow rapid upward growth of the marsh surface such that there is an associated rapid decrease in the depth and duration of tidal flooding. As a result, competitive species that prefer higher elevations relative to sea level can inhabit the area and often a succession of plant communities develops.[10]

Tidal flooding and vegetation zonation

Bride-Brook-Salt-Marsh-s
An Atlantic coastal salt marsh in Connecticut.

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.[5] 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.[15][16]

Saltmarsh-Grass
High marsh in the Marine Park Salt Marsh Nature Center in Brooklyn, New York

The New England salt marsh is subject to strong tidal influences and shows distinct patterns of zonation.[16] 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.[15] 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.

Sediment trapping, accretion, and the role of tidal creeks

BloodyMarsh2008
Bloody Marsh in Georgia, USA

The factors and processes that influence the rate and spatial distribution of sediment accretion within the salt marsh are numerous. Sediment deposition can occur when marsh species provide a surface for the sediment to adhere to, followed by deposition onto the marsh surface when the sediment flakes off at low tide.[10] The amount of sediment adhering to salt marsh species is dependent on the type of marsh species, the proximity of the species to the sediment supply, the amount of plant biomass, and the elevation of the species.[17] For example, in a study of the Eastern Chongming Island and Jiuduansha Island tidal marshes at the mouth of the Yangtze River, China, the amount of sediment adhering to the species Spartina alterniflora, Phragmites australis, and Scirpus mariqueter decreased with distance from the highest levels of suspended sediment concentrations (found at the marsh edge bordering tidal creeks or the mudflats); decreased with those species at the highest elevations, which experienced the lowest frequency and depth of tidal inundations; and increased with increasing plant biomass. Spartina alterniflora, which had the most sediment adhering to it, may contribute >10% of the total marsh surface sediment accretion by this process.[17]

Salt marsh species also facilitate sediment accretion by decreasing current velocities and encouraging sediment to settle out of suspension.[10] Current velocities can be reduced as the stems of tall marsh species induce hydraulic drag, with the effect of minimising re-suspension of sediment and encouraging deposition.[18] Measured concentrations of suspended sediment in the water column have been shown to decrease from the open water or tidal creeks adjacent to the marsh edge, to the marsh interior,[17][18][19] probably as a result of direct settling to the marsh surface by the influence of the marsh canopy.[18][19]

Inundation and sediment deposition on the marsh surface is also assisted by tidal creeks[19] which are a common feature of salt marshes.[4][10][14][19][20] Their typically dendritic and meandering forms provide avenues for the tide to rise and flood the marsh surface, as well as to drain water,[14] and they may facilitate higher amounts of sediment deposition than salt marsh bordering open ocean.[20] Sediment deposition is correlated with sediment size: coarser sediments will deposit at higher elevations (closer to the creek) than finer sediments (further from the creek). Sediment size is also often correlated with particular trace metals, and can thus tidal creeks can affect metal distributions and concentrations in salt marshes, in turn affecting the biota.[21] Salt marshes do not however require tidal creeks to facilitate sediment flux over their surface[18] although salt marshes with this morphology seem to be rarely studied.

The elevation of marsh species is important; those species at lower elevations experience longer and more frequent tidal floods and therefore have the opportunity for more sediment deposition to occur.[17][22] Species at higher elevations can benefit from a greater chance of inundation at the highest tides when increased water depths and marsh surface flows can penetrate into the marsh interior.[19]

Human impacts

Spartina alterniflora
Spartina alterniflora (Saltmarsh Cordgrass). Native to the eastern seaboard of the United States. Considered a noxious weed in the Pacific Northwest

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 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.[5][23][24] The indirect effects of human activities such as nitrogen loading also play a major role in the salt marsh area. Salt marshes can suffer from dieback in the high marsh and die-off in the low marsh.

Land reclamation

Reclamation of land for agriculture by converting marshland to upland was historically a common practice.[5] Dikes were often built to allow for this shift in land change and to provide flood protection further inland. In recent times intertidal flats have also been reclaimed.[25] For centuries, livestock such as sheep and cattle grazed on the highly fertile salt marsh land.[1][26] 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.[6] 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 recognized.

In the Blyth estuary in Suffolk in eastern England, the mid-estuary reclamations (Angel and Bulcamp marshes) that were abandoned in the 1940s have been replaced by tidal flats with compacted soils from agricultural use overlain with a thin veneer of mud. Little vegetation colonisation has occurred in the last 60–75 years and has been attributed to a combination of surface elevations too low for pioneer species to develop, and poor drainage from the compacted agricultural soils acting as an aquaclude.[27] Terrestrial soils of this nature need to adjust from fresh to saline interstitial water by a change in the chemistry and the structure of the soil, accompanied with fresh deposition of estuarine sediment, before salt marsh vegetation can establish.[11] The vegetation structure, species richness, and plant community composition of salt marshes naturally regenerated on reclaimed agricultural land can be compared to adjacent reference salt marshes to assess the success of marsh regeneration.[28]

Upstream agriculture

Cultivation of land upstream from the salt marsh can introduce increased silt inputs and raise the rate of primary sediment accretion on the tidal flats, so that pioneer species can spread further onto the flats and grow rapidly upwards out of the level of tidal inundation. As a result, marsh surfaces in this regime may have an extensive cliff at their seaward edge.[29] At the Plum Island estuary, Massachusetts (U.S.A), stratigraphic cores revealed that during the 18th and 19th century the marsh prograded over subtidal and mudflat environments to increase in area from 6 km2 to 9 km2 after European settlers deforested the land uptream and increased the rate of sediment supply.[30]

Urban development and nitrogen loading

Chaetomorpha linum-salt marsh algae
Chaetomorpha linum is a common marine algae found in the salt marsh.

The conversion of marshland to upland for agriculture has in the past century been overshadowed by conversion for urban development. Coastal cities worldwide have encroached onto former salt marshes and in the U.S. the growth of cities looked to salt marshes for waste disposal sites. Estuarine pollution from organic, inorganic, and toxic substances from urban development or industrialisation is a worldwide problem[25] and the sediment in salt marshes may entrain this pollution with toxic effects on floral and faunal species.[29] Urban development of salt marshes has slowed since about 1970 owing to growing awareness by environmental groups that they provide beneficial ecosystem services.[5] They are highly productive ecosystems, and when net productivity is measured in g m−2 yr−1 they are equalled only by tropical rainforests.[25] Additionally, they can help reduce wave erosion on sea walls designed to protect low-lying areas of land from wave erosion.[11]

De-naturalisation of the landward boundaries of salt marshes from urban or industrial enchroachment can have negative effects. In the Avon-Heathcote estuary/Ihutai, New Zealand, species abundance and the physical properties of the surrounding margins were strongly linked, and the majority of salt marsh was found to be living along areas with natural margins in the Avon and Heathcote river outlets; conversely, artificial margins contained little marsh vegetation and restricted landward retreat.[31] The remaining marshes surrounding these urban areas are also 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.[15]

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[15][32] 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.[5] 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.[15] 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.[5][33][34] 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.

Sea level rise

Due to the melting of Arctic sea ice and thermal expansion of the oceans, as a result of global warming, sea levels have begun to rise. As with all coastlines, this rise in water levels is predicted to negatively affect salt marshes, by flooding and eroding them.[35][8] The sea level rise causes more open water zones within the salt marsh. These zones cause erosion along their edges, further eroding the marsh into open water until the whole marsh disintegrates.[36]

While salt marshes are susceptible to threats concerning sea level rise, they are also an extremely dynamic coastal ecosystem. Salt marshes may in fact have the capability to keep pace with a rising sea level, by 2100, mean sea level could see increases between 0.6m to 1.1m.[37] Marshes are susceptible to both erosion and accretion, which play a role in a what is called a bio-geomorphic feedback.[38] Salt marsh vegetation captures sediment to stay in the system which in turn allows for the plants to grow better and thus the plants are better at trapping sediment and accumulate more organic matter. This positive feedback loop potentially allows for salt marsh bed level rates to keep pace with rising sea level rates.[37] However, this feedback is also dependent on other factors like productivity of the vegetation, sediment supply, land subsidence, biomass accumulation, and magnitude and frequency of storms.[37] In a study published by U.S.N. Best in 2018,[37] they found that bioaccumulation was the number one factor in a salt marsh's ability to keep up with SLR rates. The salt marsh's resilience depends upon its increase in bed level rate being greater than that of sea levels increasing rate, otherwise the marsh will be overtaken and drowned.

Biomass accumulation can be measured in the form of above-ground organic biomass accumulation, and below-ground inorganic accumulation by means of sediment trapping and sediment settling from suspension.[39] Salt marsh vegetation helps to increase sediment settling because it slows current velocities, disrupts turbulent eddies, and helps to dissipate wave energy. Marsh plant species are known for the tolerance of increased salt exposure due to the common inundation of marshlands. These types of plants are called halophytes. Halophytes are a crucial part of salt marsh biodiversity and their potential to adjust to elevated sea levels. With elevated sea levels, salt marsh vegetation would likely be more exposed to more frequent inundation rates and they must be adaptable or tolerant of the consequential increased salinity levels and anaerobic conditions. There is a common elevation (above the sea level) limit for these plants to survive, where anywhere below the optimal line would lead to anoxic soils due to constant submergence and too high above this line would mean harmful soil salinity levels due to the high rate of evapotranspiration as a result of decreased submergence.[39] Along with the vertical accretion of sediment and biomass, the accommodation space for marsh land growth must also be considered. Accommodation space is the land available for additional sediments to accumulate and marsh vegetation to colonize laterally.[40] This lateral accommodation space is often limited by anthropogenic structures such as coastal roads, sea walls, and other forms of development of coastal lands. A study by Lisa M. Schile, published in 2014,[41] found that across a range of sea level rise rates, marshlands with high plant productivity were resistant against sea level rises but all reached a pinnacle point where accommodation space was necessary for continued survival. The presence of accommodation space allows for new mid/high habitat to form, and for marshes to escape complete inundation.

Mosquito control

Earlier in the 20th century, it was believed that draining salt marshes would help reduce 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, despite some efforts to refill the ditches.[42]

Crab herbivory and bioturbation

Tunnelling mud crab
The tunnelling mud crab Helice crassa of New Zealand fills a special niche in the salt marsh ecosystem.

Increased nitrogen uptake by marsh species into their leaves can prompt greater rates of length-specific leaf growth, and increase the herbivory rates of crabs. The burrowing crab Neohelice granulata frequents SW Atlantic salt marshes where high density populations can be found among populations of the marsh species Spartina densiflora and Sarcocornia perennis. In Mar Chiquita lagoon, north of Mar del Plata, Argentina, Neohelice granulata herbivory increased as a likely response to the increased nutrient value of the leaves of fertilised Spartina densiflora plots, compared to non-fertilised plots. Regardless of whether the plots were fertilised or not, grazing by Neohelice granulata also reduced the length specific leaf growth rates of the leaves in summer, while increasing their length-specific senescence rates. This may have been assisted by the increased fungal effectiveness on the wounds left by the crabs.[43]

The salt marshes of Cape Cod, Massachusetts (U.S.A), are experiencing creek bank die-offs of Spartina spp. (cordgrass) that has been attributed to herbivory by the crab Sesarma reticulatum. At 12 surveyed Cape Cod salt marsh sites, 10% – 90% of creek banks experienced die-off of cordgrass in association with a highly denuded substrate and high density of crab burrows. Populations of Sesarma reticulatum are increasing, possibly as a result of the degradation of the coastal food web in the region.[44] The bare areas left by the intense grazing of cordgrass by Sesarma reticulatum at Cape Cod are suitable for occupation by another burrowing crab, Uca pugnax, which are not known to consume live macrophytes. The intense bioturbation of salt marsh sediments from this crab's burrowing activity has been shown to dramatically reduce the success of Spartina alterniflora and Suaeda maritima seed germination and established seedling survival, either by burial or exposure of seeds, or uprooting or burial of established seedlings.[45] However, bioturbation by crabs may also have a positive effect. In New Zealand, the tunnelling mud crab Helice crassa has been given the stately name of an 'ecosystem engineer' for its ability to construct new habitats and alter the access of nutrients to other species. Their burrows provide an avenue for the transport of dissolved oxygen in the burrow water through the oxic sediment of the burrow walls and into the surrounding anoxic sediment, which creates the perfect habitat for special nitrogen cycling bacteria. These nitrate reducing (denitrifying) bacteria quickly consume the dissolved oxygen entering into the burrow walls to create the oxic mud layer that is thinner than that at the mud surface. This allows a more direct diffusion path for the export of nitrogen (in the form of gaseous nitrogen (N2)) into the flushing tidal water.[46]

Restoration and management

Salicornia
Glasswort (Salicornia spp.) a species endemic to the high marsh zone.

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.[24] They are now protected by legislation in many countries to look after these ecologically important habitats.[47] 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.[5]

Bakker et al. (1997)[48] 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 to its original structure and the natural tidal cycles are shifted due to land changes. The second option suggested by Bakker et al. (1997)[48] 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.[47] Marshes in their pioneer stages of development will recover more rapidly than mature marshes[47] 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.

Phragmites australis Schilfrohr
Common reed (Phragmites australis) an invasive species in degraded marshes in the northeastern United States.

This last approach is often the most practiced 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 ceased this practice,[34] 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.[34] 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.[34]

By 1980, a restoration programme was put in place that has now been running for over 20 years.[34] 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.[47] Once a better understanding of these processes is acquired, and not just locally, but over a global scale, then more sound and practical management and restoration efforts can be implemented to preserve these valuable marshes and restore them 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.[47]

Atlantic ribbed mussel
Atlantic ribbed mussel, found in the low marsh

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.[24] 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 State Marine Conservation Area in San Diego, where a "friends" group worked for over a decade in trying to prevent the area from being developed.[49] 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.[49]

Research methods

There is a diverse range and combination of methodologies employed to understand the hydrological dynamics in salt marshes and their ability to trap and accrete sediment. Sediment traps are often used to measure rates of marsh surface accretion when short term deployments (e.g. less than one month) are required. These circular traps consist of pre-weighed filters that are anchored to the marsh surface, then dried in a laboratory and re-weighed to determine the total deposited sediment.[19][20] For longer term studies (e.g. more than one year) researchers may prefer to measure sediment accretion with marker horizon plots. Marker horizons consist of a mineral such as feldspar that is buried at a known depth within wetland substrates to record the increase in overlying substrate over long time periods.[22] In order to gauge the amount of sediment suspended in the water column, manual or automated samples of tidal water can be poured through pre-weighed filters in a laboratory then dried to determine the amount of sediment per volume of water.[20] Another method for estimating suspended sediment concentrations is by measuring the turbidity of the water using optical backscatter probes, which can be calibrated against water samples containing a known suspended sediment concentration to establish a regression relationship between the two.[17] Marsh surface elevations may be measured with a stadia rod and transit,[20] electronic theodolite,[19] Real-Time Kinematic Global Positioning System,[17] laser level[22] or electronic distance meter (total station). Hydrological dynamics include water depth, measured automatically with a pressure transducer,[19][20][22] or with a marked wooden stake,[18] and water velocity, often using electromagnetic current meters.[18][20]

See also

References

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Further reading

External links

Asian short-toed lark

The Asian short-toed lark (Alaudala cheleensis) is a species of lark in the Alaudidae family. It is found from south-central to eastern Asia.

Blackwater River (Massachusetts–New Hampshire)

The Blackwater River is a 3.1-mile-long (5.0 km) tidal inlet in northeastern Massachusetts and southeastern New Hampshire in the United States.The river forms in a salt marsh in the northeastern corner of Salisbury, Massachusetts, by the convergence of the Little River and Dead Creek. Heading north, the river quickly enters Seabrook, New Hampshire and continues to flow through salt marsh until it reaches Hampton Harbor, northwest of Seabrook Beach, where it joins the Hampton River.

Browns River (New Hampshire)

The Browns River is a 2.9 miles (4.7 km) long river, primarily tidal, in southeastern New Hampshire in the United States. It is part of the largest salt marsh in New Hampshire, covering over 3,800 acres (15 km2).The river rises in the town of Seabrook just east of U.S. Route 1 and quickly enters the salt marsh and tidewater. For most of its length, the river forms the boundary between Seabrook and Hampton Falls. The river runs along the north side of Seabrook Station Nuclear Power Plant, then ends in Hampton Harbor, where it joins the Hampton River.

Ellisville Harbor State Park

Ellisville Harbor State Park is a nature preserve and public recreation area located in the village of Ellisville, Massachusetts, on the western shore of Cape Cod Bay. Natural features of the coastal property include a barrier beach, sphagnum bog, salt marsh, rolling meadows, and red pine forest habitats. Scenic features include views of the South Shore coastline, small fishing boats, harbor seals seen offshore during fall and winter, and birds attracted to the park's bog as both habitat and migration stopover. The state park is managed by the Department of Conservation and Recreation.

Great Rann of Kutch

The Great Rann of Kutch is a salt marsh in the Thar Desert in the Kutch District of Gujarat, India. It is about 7500 sq km (2900 sq miles) in area and is reputed to be one of the largest salt deserts in the world. This area has been inhabited by the Kutchi people.The Hindi word is derived from Sanskrit/Vedic word iriṇa (इरिण) attested in the Rigveda and Mahabharata.

Halophyte

A halophyte is a salt-tolerant plant that grows in waters of high salinity, coming into contact with saline water through its roots or by salt spray, such as in saline semi-deserts, mangrove swamps, marshes and sloughs and seashores. These plants do not prefer saline environments but because of their ability to cope with high salinity in various ways they face much less competition in these areas. The word derives from Ancient Greek ἅλας (halas) 'salt' and φυτόν (phyton) 'plant'. An example of a halophyte is the salt marsh grass Spartina alterniflora (smooth cordgrass). Relatively few plant species are halophytes—perhaps only 2% of all plant species.

The large majority of plant species are glycophytes, which are not salt-tolerant and are damaged fairly easily by high salinity.

Hampton Falls River

The Hampton Falls River is a 5.6 mile (9.0 km) long river in southeastern New Hampshire in the United States. Its lower reaches are tidal, as part of the Hampton salt marsh close to the Atlantic Ocean.

The river rises in the southeast corner of Kensington, New Hampshire and travels east into Seabrook. The river approximately follows the boundary between Seabrook and Hampton Falls, crossing it three times. It passes Weares Mill and eventually heads northeast into Hampton Falls. Approaching the town center, it passes through the chain of three Dodge Ponds, dropping 10 feet over the "Hampton Falls" before entering the final one. Crossing under U.S. Route 1, the river enters the Hampton salt marsh, where it ends at the Hampton River.

Keyhaven

Keyhaven is a hamlet on the south coast of England in the county of Hampshire. It is a fishing village, but the trade has been in decline for a period of years and its main draw now is tourism, especially sailing.

Lamb and mutton

Lamb, hogget and mutton are the meat of domestic sheep (species Ovis aries) at different ages.

In general, a sheep in its first year is called a lamb, and its meat is also called lamb. The meat of a juvenile sheep older than one year is hogget; outside the United States this is also a term for the living animal. The meat of an adult sheep is mutton, a term only used for the meat, not the living animals. In the Indian subcontinent, the term mutton is also used to refer to goat meat.Lamb is the most expensive of the three types, and in recent decades sheep meat is increasingly only retailed as "lamb", sometimes stretching the accepted distinctions given above. The stronger-tasting mutton is now hard to find in many areas, despite the efforts of the Mutton Renaissance Campaign in the UK. In Australia, the term prime lamb is often used to refer to lambs raised for meat. Other languages, for example French, Spanish, Italian and Arabic, make similar, or even more detailed, distinctions among sheep meats by age and sometimes by sex and diet, though these languages do not always use different words to refer to the animal and its meat — for example, lechazo in Spanish refers to meat from milk-fed (unweaned) lambs.

Little River (New Hampshire Atlantic coast)

The Little River is a 4.6 mile long (7.4 km) river located in southeastern New Hampshire in the United States. It is located entirely in the town of North Hampton, and it flows directly into the Atlantic Ocean, south of Little Boars Head.

The river rises in a forested wetland in the northern part of North Hampton and flows south, passing under U.S. 1. The river turns southeast, passes through Mill Pond and under NH 111, and enters the Little River salt marsh. The river enters the Atlantic Ocean by passing under NH 1A, using a new, large culvert installed by the New Hampshire Coastal Program in an effort to improve tidal flow into the salt marsh.

Lloyd Center for the Environment

The Lloyd Center for the Environment is a non-profit organization that provides educational programs on aquatic environments in southeastern New England in the United States.

The Lloyd Center’s 55 acres (220,000 m2) of estuary and maritime forest in South Dartmouth, Massachusetts, was donated to the Dartmouth Natural Resources Trust in 1978 by Karen Lloyd as a living memorial to her mother, Katharine Nordell Lloyd. Lloyd Center programming began in 1981. Five walking trails offer scenic views of the Slocum River, Buzzards Bay, Demarest Lloyd State Park, Mishaum Point, and Cuttyhunk Island. The Chaypee Woods Trail snakes its way through stone walls that hearken back to farming days when dairy cattle, ducks, and chickens were raised there. Steep steps and slopes mix with the vistas of the salt marsh, oak-hickory forest, freshwater wetlands, kettleholes, moraine, and a Native American midden site.

Point Molate Marsh

Point Molate Marsh is a salt marsh on the western shoreline of the San Pablo Peninsula in Richmond, California. The area is environmentally valuable land as it is largely untouched and isolated from nearby urban development. The marsh was once used as a Chinese shrimp camp. It is habitat to important endangered species, especially the Ridgway's rail and salt marsh harvest mouse. Harbor seals also make use of the marsh which is on San Francisco Bay. It is located between Point San Pablo and Point Molate.

Quincy Shore Reservation

Quincy Shore Reservation is a public recreation area and protected shoreline on Quincy Bay, Boston Harbor, in Quincy, Massachusetts. Its primary attraction is a 2.3-mile-long (3.7 km) beach, accessible along its entire length by Quincy Shore Drive. The largest beach on Boston Harbor, it is known locally as Wollaston Beach, named for the adjacent Wollaston neighborhood.

The reservation is part of the Metropolitan Park System of Greater Boston and was established in 1899. Also included in the reservation are Moswetuset Hummock, site of the first encounter of Plymouth Colony commander Myles Standish with the local native sachem, or leader, Chickatawbut in 1621 and cited as a source for the name of Massachusetts; and Caddy Park, a preserved salt marsh with nearby picnic facilities.

Rumney Marsh Reservation

Rumney Marsh Reservation is a Massachusetts state park occupying over 600 acres (240 ha) in the town of Saugus and city of Revere. The salt marsh is located within the Saugus and Pines River estuary and provides habitat for many different migratory birds and marine life. The park is managed by the Massachusetts Department of Conservation and Recreation.

Salt marsh harvest mouse

The salt marsh harvest mouse (Reithrodontomys raviventris), also known as the red-bellied harvest mouse and sometimes called the saltmarsh harvest mouse, is an endangered rodent endemic to the San Francisco Bay Area salt marshes in California. There are two distinct subspecies, both endangered and listed together on federal and state endangered species lists. The northern subspecies (Reithrodontomys raviventris halicoetes) is lighter in color and inhabits the northern marshes of the bay, and the southern subspecies (Reithrodontomys raviventris raviventris) lives in the East and South Bay marshes. They are both quite similar in appearance to their congener species, the [Western harvest mouse, R. megalotis], to which they are not closely related. Genetic studies of the northern subspecies have revealed that the salt marsh harvest mouse is most closely related to the plains harvest mouse, R. montanus, (), which occurs now in the Midwest]. Its endangered designation is due to its limited range, historic decline in population and continuing threat of habitat loss due to development encroachment at the perimeter of San Francisco Bay.

Salt pannes and pools

Salt pannes and pools are water retaining depressions located within salt and brackish marshes. Pools tend to retain water during the summer months between high tides, whereas pannes generally do not. Salt pannes generally start when a mat of organic debris (known as wrack) is deposited upon existing vegetation, killing it. This creates a slight depression in the surrounding vegetation which retains water for varying periods of time. Upon successive cycles of inundation and evaporation the panne develops an increased salinity greater than that of the larger body of water. This increased salinity dictates the type of flora and fauna able to grow within the panne. Salt pools are also secondary formations, though the exact mechanism(s) of formation are not well understood; some have predicted they will increase in size and abundance in the future due to rising sea levels.

Salt pannes and pools are unique microhabitats dominated by various species of halophytes, benthic plants and varying estuarine marine life that vary considerably in composition due to a variety of factors:

Substrate type: affects the ability of the depression to hold water.

depth and diameter: affect water temperature and evaporation rate in the depression. A shallow and wide pool will evaporate at a greater rate than a pool of the same volume of water which is deeper and has a smaller surface area. Evaporation rate also affects salinity, the higher the evaporation rate the higher the salinity, with rates as high as a third greater than ocean water.

location within the intertidal zone, whether high marsh or low marsh and distance from the mean low tide mark which affects the length and duration of inundation until the depression is subject to evaporation as well as length of time until the rising tide replenishes the water volume.These factors affect the types of species which can survive within the various types of salt pannes and pools.

Variants of salt pannes and pools:

Low salt marsh

Low salt marsh panneUsually devoid of vegetation, that may be present include smooth cordgrass (Spartina alterniflora), marine algae such as knotted wrack (Ascophyllum nodosum) and rockweeds (Fucus spp.). The substrate is typically soft, silty mud.

High salt marsh

Arrow-grass (forb) panneBriefly flooded, very shallow with a moderate amount of vegetation usually dominated by Arrow grass (Triglochin maritimum), with the deeper sections possibly remaining unvegetated.

Smooth cord-grass (short form) panneShallow anaerobic depressions with poor drainage, poor water quality due to low nutrient levels and high concentrations of sulfides and similar compounds which inhibit plant growth. Short form (6-12" tall)smooth cord-grass (Spartina alterniflora) is the dominant plant species. Typically found on the high salt marsh, but can occasionally be found on the upper margins of low salt marsh.

Salt marsh mosquito panne

Minimal vegetation often found on the upper half of the high salt marsh. It is typically deeper than forb and smooth cord-grass pannes. Usually flooded by the higher of the two spring tides, retains water for 2–3 weeks later until drying out. The female eastern salt marsh mosquito (Aedes sollicitans) lays eggs on the exposed surface. The eggs lay dormant until the next time the panne floods.

Widgeon grass (Ruppia maritima) - marsh minnow deepwater pool

Pools on the high salt marsh that are semi-permanently and permanently flooded. They are able to sustain populations of Sheephead minnow (Cyprinodon variegatus variegatus), Mummichogs, (Fundulus heteroclitus), and other species of small fish which may become trapped in the pools and benthic species of vegetation. Occasioanally can be found at the upper edge of the low salt marsh.

South Cape Beach State Park

South Cape Beach State Park is a Massachusetts state park located in the town of Mashpee. It is part of the Waquoit Bay National Estuarine Research Reserve. The park is situated between Waquoit Bay and Vineyard Sound and features barrier beach and dunes, salt marsh, scrub oak and pitch pine woodland and kettle ponds and is managed by the Department of Conservation and Recreation.

Tigris–Euphrates river system

The Tigris and Euphrates, with their tributaries, form a major river system in Western Asia. From sources originating in eastern Turkey, they flow by/through Syria through Iraq into the Persian Gulf. The system is part of the Palearctic Tigris–Euphrates ecoregion, which includes Iraq and parts of Turkey, Syria, Iran, Saudi Arabia, Kuwait, and Jordan.

From their sources and upper courses in the mountains of eastern Anatolia, the rivers descend through valleys and gorges to the uplands of Syria and northern Iraq and then to the alluvial plain of central Iraq. The rivers flow in a south-easterly direction through the central plain and combine at Al-Qurnah to form the Shatt al-Arab and discharge into the Persian Gulf.The region has historical importance as part of the Fertile Crescent region, in which civilization is believed to have first emerged.

Westport Town Farm

Westport Town Farm is a 40-acre (16 ha) open space preserve and historic farm complex located in Westport, Massachusetts along the bracken East Branch of the Westport River. The property, owned by the town of Westport and managed by the land conservation non-profit organization The Trustees of Reservations through contract since 2007, was once the town's poor farm and local infirmary.

The preserve includes hiking trails, working farmland, salt marsh frontage, an antique farmhouse, dairy barn, corn crib, and stone walls dating back to Colonial times. It is open to hiking, picnicing, cross country skiing, canoeing, and kayaking. The preserve trailhead is located on Drift Road in Westport.

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