This article describes the habitat of salt marshes. It gives an introduction to the characteristics, distribution, evolution, zonation, succession, biota, threats, functioning and adaptations of the organisms that live in salt marshes.
In 2008 the European Union commissioned a series of Habitat Management Models for several of the more important communities. Included is a model for habitat 1330 "Atlantic salt meadows" (Glauco-Puccinellietalia maritimae).
Visit the European Union web site at  to download a copy of the Habitat Management Model for the above community.
Salt marshes are defined as natural or semi-natural terrestrial halophytic ecosystems that occur in the intertidal zone between the land and the sea and that are covered by salty or brackish water for at least part of the time. They can be considered, in some way, as the analogue of mangroves in temperate and arctic regions. The dominant flora is composed of halophytic plants such as grasses, shrubs and herbs. The flora is locally rather species poor, but the global species diversity is high, with over 500 salt marsh plant species known. Salt marshes are normally associated with mud flats but also occur on sand flats. These mud flats are sometimes dominated by algae and covered with algal mats. They are periodically flooded by the tide, the height of which can vary from several centimetres in enclosed seas (such as the Baltic Sea) to several metres in open seas along the ocean margins. A network of meandering tidal creeks ensures the drainage of seawater. Through these channels, sediments, detritus, dissolved nutrients, plankton and small fishes are flushed in and out the salt marshes.
Salt marshes are ubiquitous in estuarine systems in temperate zones all over the world. They also occur in deltas and rias but seldom on open coasts, because the development is inhibited by wave action. Although sediment is a prerequisite for their growth in height and width, salt marsh communities can occur in areas with limited or no sediment supply. Examples include seawater-drenched cliffs and slopes on exposed coasts, at the head of sea lochs and rocky beaches (Doody 2008). Salt marshes can be found from the Arctic region, over Europe, Africa, America, Asia to the coast of Australia. The most extensive development of salt marshes occurs in estuaries with a moderate climate, large tidal range, abundant fine-grained sediments and sheltered locations where particles can settle out of the water column.
Salt marshes evolve over time from young marshes to old marshes. The natural young marshes in eastern USA are vegetated for the largest part with low marsh cordgrass Spartina alterniflora. Nutrients are transported by tidal currents through the tidal channels. This allows the grasses to grow thickly and profuse, so weakening the effect of waves and tidal currents and increasing the deposition rate of mud. Erosion is reduced by the roots and rhizomes of the plants. At the time that the marsh surface builds up above the high water level, high marsh species invade, outcompete and replace the low marsh plants. The most stress-tolerant plant species occupy the lower reaches of the marshes while competitive dominants occupy the upper elevations that are less stressful. When the extent of the low and high marshes is about equal, the ecosystem is in a mature stage of development. The ongoing deposition of mud converts most of the low marshes into high marshes. Little water flows through the tidal channels of these elevated 'old marshes'. Deposition of sand and mud on these high marshes convert them into dry land that is disconnected from ocean influences. Lateral channel migration and wave attack at the base of marsh cliffs are the main mechanisms for erosion of mature salt marshes and their subsequent rejuvenation cycle, see also Dynamics, threats and management of salt marshes and French case studies: Upper tidal flat evolution in the bay of Mont-Saint-Michel (NW France).
Requirements for development
The requirements for development of salt marshes are:
- They need fine-grained sediments.
- There may be no strong waves or tidal currents.
- They need salty conditions to grow. They are halotolerant and have adaptations to these conditions.
- They need a temperate or cool temperature. Incidental freezing temperatures are not damaging the plants.
- They need a wide tidal range. This is important because it limits the erosion, makes deposition of sediments possible and causes a well-marked zonation.
Based on the topography and characteristic plant assemblages, salt marshes are classified as low, medium and high marshes. This classification is related to the number of tidal submergences per year (Adam 1990). The 'low marshes' extend from the mean low water neap tide to the mean high water spring tide. The low marshes are drained by tidal creeks that convey flood and ebb flow; ebb flow generally dominates because flood water also enters the marsh directly from the main channels. The 'high marshes' extend from the mean high water level to the highest springtide level. Flooded only by the highest tides and during storms, they are more like a terrestrial than a true marine environment. The plant community is more diverse than on the low marshes. Typical high marsh species are cordgrass Spartina patens, spike grass Distichlis spicata and species such as saltwort Salsola and seablite Suaeda. Distribution, density and activity of invertebrates mainly depend on protection, food and frequency of tidal flooding. 
Succession is the successive development in time of different vegetation types at one place. It is a complex process; the factors determining zonation and succession in salt marshes are discussed more in detail in Adam (1990, pages 49-57), Gray (1992) and Packham & Willis (1997, pages 107-114 ).
The mudflat colonization starts with unicellular algae such as diatoms sticking the sand together by production of mucus. This causes a brownish biofilm on the substrate.
After this stage, filamentous algae contribute to the further fixation of sediments. These algae are mostly blue-green algae, Cyanophyta and Chlorophyta. Small gastropods can feed and develop in huge quantities on it. Locally, the filamentous alga Vaucheria can form banquettes or elevations. Brown algae can be associated with this stage.
A next stage is the germination of species such as the glasswort Salicornia. The seeds germinate after partial desalination of the soil by rain. Sedimentation between and around the glassworts contributes to elevating and stabilizing the substrate. Other species such as Spartina maritima and Spartina anglica compete for the same place. S. maritima is an indigenous species of continental Europe and S. anglica is imported from the British Islands. The hybridisation and invasion of "Spartina" spp is a worldwide phenomenon.
In Europe, other plants also occur. These are the herbs common saltmarsh-grass (Puccinellia maritima), sea plantain (Plantago maritima), sea arrow-grass (Triglochin maritinum), sea aster (Aster tripolium), danish scurvy-grass (Cochlearia danica), common sea-lavender (Limonium vulgare), sea purslane (Halimione portulacoides), the red alga Bostrychia scorpioides and Catenella caespitosa. . Juncus maritimus is another species that can occur in salt marshes.
Initial plant colonizers play an important role in the recovery of salt marsh vegetation from disturbance events. They provide shading of the substrate of bare areas and reduce salt accumulation in the soils and thereby facilitate colonization by many other plant species.
Plants and animals living in low salt marshes must have adaptations to deal with the harsh physical stressors found in this intertidal habitat, including high salt concentrations, intense heat, and low oxygen in waterlogged soils. Some typical adaptations are discussed below.
The saline environment causes waterstress. Plants have to take up water against the osmotic pressure. To overcome the negative osmotic pressure, they generate a negative hydrostatic pressure (by transpiration processes). They have thin, fleshy leaves and are sensitive to extra stress such as pollution. Anatomically, the plants are adapted through strong lignification, a well-developed epidermis and succulent leaves and stems. Evaporation can be limited by thin leaves with scale-like hairs. Physiologically, plants are adapted by accumulating salt in their tissues. In this way, normal osmosis is possible. Other plants have salt gland cells on the lower surface of the leaves and excrete the salt from its tissue.
Salt marsh plants have to deal with an anoxic environment. The tissue of the plants requires oxygen for respiration. Gas diffusion between sediment particles only occurs in soils that are not waterlogged. Even when the surface water is saturated with oxygen, its concentration in the soil is too low because of the slow oxygen diffusion. Many salt marsh plants deal with low soil oxygen levels by shunting oxygen down to their roots through straw-like vascular tissue called aerenchyma. Roots are superficial systems because of the anoxic sediments. They consist of perennial thick roots with a corky layer and without root hairs. To fix the substrate, short-lived, thin and highly branched roots with numerous root hairs are developed to absorb nutrients.
Nitrogen limitation can also play a role in the development of salt marsh vegetation, even though nitrogen levels can be very high. The reason is that concentrations of sulfide and sodium ions are often high too and interfere with nitrogen uptake by plants.
Salt marshes have a whole range of functions. They play an important role as sediment trap; in this way they help stabilizing the coastline. Salt marshes improve the water quality by filtering water and retaining excess nutrients, toxic chemicals and disease-causing organisms. They remove nitrates and phosphates from rivers and streams which receive waste water effluents. Another function is water supply regulation by recharge and discharge of groundwater. Salt marshes are an important habitat, offering nursery grounds and shelter for larvae and other small organisms and providing food and nesting areas for wading birds and other organisms.
Salt marshes are home to many small mammals, small fishes, birds, insects, spiders and marine invertebrates. Marine invertebrates include crustaceans such as amphipods and isopods, sea anemones, shrimps, crabs, turtles, mollusks and snails. Fishes such as sticklebacks, silversides, eels and flounders are found in the waters of the salt marshes. The benthic community consists of mollusks, polychaeta and oligochaeta. Salt marshes are important breeding, feeding and overwintering grounds for waterfowl. These waterfowl consist of ducks, herons, sharptailed sparrows, Eurasian oystercatchers, reed bunting, etc. In Saudi Arabia, salt marshes are grazing places for wild dromedaries.
Although the local diversity of plants and animals found in salt marshes is comparatively low, the abundance of organisms that do occur in marshes is very high. The abundance per square meter of one species of fiddler crab or snail can reach 100–400 individuals, at times over 1000, and over larger spatial scales the density of one species of non-insect invertebrate (mussels, crabs and snails) can often reach 50,000,000 per square kilometer.
Freshwater tidal marshes have a high biodiversity but do not harbor many endemic species. Organisms typical of freshwater tidal marshes are boatmen, flies, mosquitoes and snails. There are also mollusks, ducks, geese, muskrats, raccoons, mink and other small mammals. Some species are seasonal visitors.
The total number and area of salt marshes has been declining for many years. The main cause is embanking, which removes the habitat from tidal inundation. This normally occurs in areas where the soil level is high enough and the area large enough for embankment to take place. The salt marsh is destroyed when converted to other uses, notably conversion to intensive agriculture.
In south east England and in France a special type of enclosed saltmarshes exist that are protected as a semi-natural habitat. Once enclosed, the saltmarsh still has salt water inlets with creeks and other features retained with little interference. Dispersed grazing or cropping for hay are often the only uses and with traditional management it develops into a wildlife habitat of some significance. The term Coastal grazing marsh (in France: prairie subhalophile) is used to describe this habitat.
Reclamation for harbour development and other infrastructures completely destroys the habitat and with it any opportunities for restoration. Areas outside the reclaimed zones still can produce new marshes if there is sufficient supply of new sediment and conditions suitable for growth of new plants.  However, climate change and the associated sea level rise are diminishing the opportunities for salt marsh development and the "coastal squeeze" process takes place in many areas, especially around the southern North Sea .
Other threats include:
- Over-grazing (especially farm animals, but also grazing by wild geese, crabs and snails)
- Eutrophication by agricultural effluents
- Coastal erosion
- Industrial pollution and waste water
- Altered hydrologic regimes
- Species invasions (especially invasion of Spartina anglica and Erytrigia in eutrophic salt marshes)
- Climate change (sela level rise and increased temperatures and frequency of intense drought).
Case-study: Land van Saeftinghe
The tidal area ‘Drowned land of Saeftinghe’ (Verdronken land van Saeftinghe) is located near the border between the Netherlands and Belgium, a few kilometers downstream Antwerp in the estuary of the Western Scheldt. It is an official nature reserve since 1976. Because of this legal protection, permits are compulsary for every intervention and strict entrance restrictions are applied. The land is situated at the transition zone of the Western Scheldt where the river Scheldt meets the saline North Sea water. Before the storm surge of 1570, the land was a fertile polder. The area has a surface of 3,484 hectares. Almost 70 % of the area is overgrown by salt marsh vegetation. The remaining 30 % consists of mud flats, sandbanks and a network of channels. Each tide, the brackish water overflows a large part of the area. The unique vegetation is fully adapted to this. The area is an ideal breeding, rest and wintering place for huge quantities of birds. Since 1996 it is a special protected area for birds (1979 Directive 79/409/EEC on the conservation of wild birds) of international importance.
In the past, a few dikes were created to promote the silting up. The northern dike connects a few hillocks (artificial elevations of earth) with the dike. These dikes are still recognizable and are used as wandering path. The hillocks were used by shepherds when the tide became too high. The flora consists of approximately 50 wild plant species. Algae are not abundant in Saeftinghe because there is too little light that penetrates in the water. Organic matter and a lot of silt make the water turbid. Higher plants are more important in this area. One of the most common plants is pickle weed (Salicornia), together with other salt marsh plants such as English scurvy-grass (Cochlearia anglica) and common sea-lavender (Limonium vulgare).
- Dynamics, threats and management of salt marshes
- Spatial and temporal variability of salt marshes
- Biogeomorphology of coastal systems
- Natural shore protecting barriers
Salt marsh restoration guide
For information of the management and restoration of saltmarshes in the UK see DEFRA Saltmarsh management manual
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- Credit Eric Coppejans - http://www.vliz.be/imis/imis.php?module=person&persid=134
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- Eric Coppejans – Course Biodiversity of aquatic food webs: from algae to marine mammals UGent
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