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Salt Marshes
Salt marshes are wetland areas that are found in the intertidal zone of low-lying coastlines.
They are flooded intermittently and contain a variety of salt-tolerant (halophytic) plants.
These plants help build up the coastline by increasing deposition. Salt marshes show a
typical change in plant species (known as succession), both over time and over space. This
change is known as a halosere. They are naturally very productive ecosystems but are
under increasing pressure as a result of human activity. As they occur in low-lying areas,
they are located in areas that may contain much human activity.
Formation and development of salt marsh
Salt marshes are intertidal areas of fine sediment transported by water and stabilised by vegetation.
Once vegetation has established the rate of sedimentation, accretion often increases. In addition,
organic matter is added to the marsh surface. This occurs due to the accumulation of litter on the
sediment surface and root growth below the surface.
There are four elements necessary for the initial development and growth of a salt marsh:
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A relatively stable area of sediment covered by the tide for a short time.
A supply of suitable sediment available.
Low water velocities for some of the sediment to be deposited.
A supply of seeds for the establishment of vegetation cover.
There are approximately 40 species of higher plants that are found in British salt marshes, but individual
salt marshes commonly have between ten and twenty species.
Major types of salt marsh
There are three major types of salt marsh:
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Barrier-connected salt marshes that develop in the lee of spits or barrier islands – for example
in north Norfolk.
Foreland salt marshes that develop in front of sheltered alluvial coastal plains (such as areas
protected by a bay or offshore banks). Examples of this type of marsh are seen around the
Wash and at Dengie in Essex.
Estuarine salt marshes, usually found where rivers gradually merge into the open sea. This
type of marsh is associated with the larger rivers of the east coast of Britain such as the
Thames and the Humber.
Salt marshes are found all around the coastline of Great Britain but they vary considerably in character
between essentially lowland areas and upland areas. Lowland marshes are associated with the major
estuaries and inlets in low-lying geographic areas – notably around the Wash, in Essex, north Kent, the
Solent, the Severn estuary, the Welsh estuaries, Liverpool Bay and the Solway Firth. The upland areas
have a scattered distribution of mainly small isolated marshes, either associated with minor estuaries or,
commonly, at the head of sea lochs where there is shelter from wave action. The higher rainfall in the
Northwest reduces salinity levels sufficiently to enable species normally found in the upper marsh to
occur lower down in the marsh, occasionally in pioneer situations.
There are at least four main salt marsh zones and an upper transition zone. However, one or more of
these zones may be absent in any area. UK salt marsh zones are generally as follows:
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Pioneer – open communities with one or more of Spartina, Salicorni or Aster. This zone is
covered by all tides except the lowest neap tides.
Low marsh – generally closed communities with Puccinellia and Atriplex as well as the
previous species. This zone is covered by most tides.
Middle marsh – generally closed communities with Limonium and/or Plantago, as well as the
previous species. This zone is covered only by spring tides.
High marsh – generally closed communities with one or more of Festuca, Armeria, as well as
the previous species. This zone is covered only by highest spring tides.
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Transition zone – vegetation intermediate between the high marsh and adjoining nonhalophytic areas. This zone is covered only occasionally by tidal surges during extreme storm
events.
In areas exposed to high wave energy, middle to high marsh can occur well above the level of normal
spring tides. In areas restricted by the existence of a sea wall (for example East Anglia), the higher zone
is virtually absent and the transition zone appears in a line along the sea wall.
Figure 1.
Salt marshes are highly productive ecosystems receiving nutrients from river systems and being cleansed of debris by tidal
action.
Plant nutrients
Root growth can be restricted both by raised salinity and low oxygen concentrations in the soil, and this
can restrict the ability of the plant to acquire sufficient quantities of phosphorus and nitrogen. Salinity
influences the germination of seeds and seed-root longation. Studies have shown good correlation
between productivity and phosphorus levels, but no correlation with nitrogen levels. Both elements are
crucial for good plant growth and even a slight shortage of either of them will lead to the concentrations
of the other becoming the limiting factor.
Higher plants
The range of higher plants that are able to tolerate the effects of salinity and inundation in UK salt
marshes is limited. Species found on the lower marsh are restricted to those tolerant of inundation and
salinity. The pioneer communities are, in general, structurally simple with just a few species and limited
ground cover. The surface of the ground beneath the plants is generally bare. In places where the
vegetation is denser, there may be an increase in accretion leading to the development of raised
mounds with the space between these mounds being colonised gradually. With the development of
more mature plant communities, the ground cover becomes almost complete. Any bare ground that may
occur from time to time is usually colonised by individuals of one or more of the pioneer annual species.
The vegetation structure of a mature salt marsh depends very much on whether grazing takes place. In
the UK, the vegetation of ungrazed marshes is generally 0.5–1 metre tall. Grazed salt marsh vegetation
has a very characteristic appearance, forming a dense mat of vegetation often less than 0.1 metre in
height. Trampling can produce similar effects to that of grazing, but generally there are fewer species in
a grazed sward than in a trampled sward.
Succession
The primary colonisation of a salt marsh is dependent on the arrival of sufficient quantities of the seeds
of key colonising plant species, such as annual species like Salicornia, or perennial grasses such as
Spartina. These plants are tolerant of being covered by salt water for long periods. As the salt marsh
develops, the accumulation of new material raises the surface level of the new marsh in relation to the
sea and this reduces the frequency and duration of tidal inundation. This enables species less tolerant
of inundation to colonise, and more complex plant communities of mature salt marsh to gradually
develop. Development of mature salt marsh depends on sediment supply, and the rate of sedimentation
typically takes between 40 and 80 years.
The colonisation of mudflats by pioneer salt marsh species is the critical first step in the development of
salt marsh. Most marshes still have a limited pioneer zone. Like many other pioneer communities, the
general appearance is one of scattered individual plants with considerable bare ground between them.
It is crucial for the germination and establishment of the plants to have a period of a few days with little
or no disturbance from the action of the tide. A period of stormy weather while the seedlings are still
small may result in substantial losses through erosion. A seedling’s success depends on its ability to
extend its root to a sufficient depth in the soil, so as to anchor it securely against the action of the tide.
The deposition of fresh sediment will benefit this process. Some pioneer salt marsh species also show
enhanced root and shoot growth rates when rates of sedimentation are increased.
A typical salt marsh site is subjected to a range of changes in water levels. The distribution of the salt
marsh vegetation is determined by these water levels. Vegetation will only become established and
survive in places where the ground surface is more often exposed than flooded. The surface needs to
be significantly above the mean sea level. The bottom of the pioneer zone is only submerged for around
40 per cent of the time. In contrast, the high and middle marshes are only submerged for about ten per
cent of the time, being reached by only the higher spring tides. In between these two extremes, the
marsh can be divided into pioneer marsh, which is covered by virtually all the neap tides, and low
marsh, covered by only the spring tides.
Nutrient requirements of pioneer communities
The upper mudflats on which the pioneer salt marsh develops are relatively low in nutrients, and it is
crucial for the plants colonising these areas to be able to absorb nutrients from the soil and from the
water whenever they are available. The increase in the organic content of the soil, partly by the
incorporation of plant litter and other organic matter into the soil and partly by the root growth of the
pioneer plant species, is a key element of the establishment and development of salt marsh
communities.
Figure 2.
The denser grass vegetation such as Spartina (cord grass) and rice grass increases the deposition of mud, thereby building
up the salt marsh.
Water movements
Water velocity over the marsh surface is generally lower because it is slowed by the roughness of the
salt marsh vegetation. This reduction in the velocity of the water flow is crucial to sediment accretion.
Two processes are involved. First, the vegetation impedes the water flow, particularly near the surface
of the marsh, enabling suspended sediment to settle. Second, the vegetation cover reduces the resuspension of material and potential loss of the sediment that has been deposited.
Development of closed plant communities
Once vegetation is more or less permanently established, the next stage is the increase in the density of
shoots or individuals to a stage when the cover of vegetation over the ground is more or less complete.
This process can occur either with pioneer annuals such as Salicornia or with colonising perennials such
as Spartina. The presence of a closed or nearly-closed canopy increases rates of sedimentation. The
next stage in salt marsh development is the arrival of salt marsh plant species that are less tolerant of
flooding and inundation than the pioneer species.
Maturation of vegetation
The development of the lower marsh communities is marked by an increasing diversity. The lower
marsh is a more stable community than the pioneer marsh and this can be seen by the arrival of some
perennial species. There are various other perennial salt marsh species which usually follow on quite
quickly, particularly the woody sub-shrub Atriplex. The next stage is the development of the middle
marsh with the establishment of such species as Limonium, Plantago and Triglochin. These are all
rosette perennials, long-lived species but with relative low seed production and, therefore, slow rates of
spread.
Most of the species characteristic of the pioneer and lower marsh can also survive, usually as scattered
small individuals, in the middle marsh. The transition to high marsh communities is not found
everywhere. In much of south and east Britain, the presence of a sea wall impedes the development of
high marsh communities. High marsh communities are generally found only where there is a natural and
gradual slope from middle marsh through to non-saline areas.
Primary productivity
The salt marshes of Essex and Norfolk have a net annual primary productivity (NPP) of 450–500 g/m2
per year, similar to those from France and the Netherlands. There is moderately high productivity in the
pioneer marsh, then a fall in productivity with the development of lower marsh, but after this a steady
increase in productivity as the salt marsh develops into mature middle marsh, until finally there is a
reduction in productivity in the high marsh.
Figure 3. Table of Net Annual Primary Productivity.
Marsh zone
Main species Range NAPP Average g/m2 per year
Pioneer marsh Salicornia
233–849
465
Pioneer marsh Suaeda
1032
1032
Lower marsh
Puccinellia
317–701
485
Middle marsh
Puccinellia
417–824
586
Middle marsh
Atriplex
631–1708
1137
High marsh
Elymus
362–1260
593
Functions and values
Salt marshes have a high gross and net primary productivity. This is due to deposition of nutrients, and
the abundance of water and oxygen. This offsets the salinity, widely fluctuating temperatures and
alternate flooding and drying. Removal of detritus, and shelter found along marsh edges, make salt
marshes important as nursery areas for many commercially important fish and shellfish.
The value of salt marshes
Wildlife and nature conservation
The value of salt marshes for wildlife conservation has been known for many years. The diversity of
fauna and flora reflects interactions between the marine environment and the terrestrial element. Salt
marshes provide feeding, roosting and nesting areas for a wide range of bird species. Notable species
are oystercatchers, lapwing and redshank. Salt marshes are particularly important for wintering wildfowl
such as Brent geese and widgeon. Other conservation issues include:
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Overgrazing – The historical decline in salt marshes has been dramatic. The principal cause for
this decline is the drainage, or reclamation, of agricultural polders or pasture. Reclamation
continues to threaten many salt marshes, and an estimated 50 per cent of the invertebrate
fauna of marshes is now threatened by coastal protection measurements. Overgrazing has
been identified as a problem at Mont St Michel, the Wadden Sea area, Donana, Camargue and
Orbetello. A wide variety of human uses presents specific threats to salt marsh communities,
including dumping, excavation, infilling, aquaculture, tourism, hunting, and changes in the
water regime through civil engineering projects and land reclamation. These stresses are
particularly damaging to marsh habitats in seas such as the Caspian and Black Sea, where the
spatial distribution of different species is limited by water salinity.
Pollution – The heavy metal cadmium, found in the sediments of the Sado estuary in Portugal,
is readily taken up by the leaves of Atriplex. Cadmium in the leaves then represents a potential
health hazard to grazing animals. Selenium and heavy metals, including copper, nickel, zinc,
lead, cadmium and iron, have been shown to accumulate in the litter of common salt marsh
plant species.
Human uses and their impact on salt marshes – Humans have been exploiting salt marshes for
many millennia, but initially this exploitation was at the level of subsistence fishing and shellfish
gathering. Over the last few hundred years, man has had a major impact on salt marshes
through the construction of tidal walls. These have enabled the conversion of salt marsh to
cattle grazing, and, more recently, for growing agricultural crops. This process is called
reclamation as most of the present salt marsh areas were dry land shortly after the last ice age
and were later submerged by rising sea levels.
Flood protection
Salt marshes play a major role in flood defence by providing a dynamic buffer between the land and the
sea. Salt marshes form effective buffers to wave action, through the reduction in water depth over the
marsh and the increased friction of the vegetated surface reducing the wave height. The effectiveness of
this system of buffering will depend on the width of marsh that is present. For example, in Essex it has
been shown that where the salt marshes are 80 metres wide, a 3-metre-high sea wall will give adequate
protection. If there are 30 metres of marshes then a 5-metre wall is needed. However, if there were no
marshes in front of the wall, a 12-metre wall would be required.
The Wadden Sea
The Wadden Sea is the body of water lying between the coast of northwestern Europe, and the North
Sea. It is one of the largest tidal wetland areas in the world, providing habitats for many waterfowl and
shore birds – up to 12 million creatures of 50 different species each year. The rich and varied ecosystem
is also a critical spawning and feeding area for 102 species of North Sea fish, as well as for the harbour
seal, the grey seal and the bottlenose dolphin.
Today, the Wadden Sea is fringed by one of the most industrialised regions of Europe and the
construction of port facilities and embankments has resulted in the substantial loss of habitats. During
the last five decades, for example, more than 33 per cent of all salt marshes have been lost to
embankments. Many remaining areas are exposed to other stresses such as dumping, excavation,
infilling, aquaculture, tourism, hunting and changes to the water regime through civil engineering
projects.
Pollution by urban, industrial and agricultural runoff and sewage are also triggering community changes,
with algal growth smothering the rooted halophytes in nutrient-rich waters. Unfortunately, the threats to
the Wadden Sea are not local – and local efforts alone will not be sufficient to protect it indefinitely.
Oil spill
After the Amoco Cadiz oil tanker was wrecked off the coast of Brittany in March 1978, a total of around
220,000 tonnes of oil were spilt. Following the accident, over 4,500 oiled birds of 33 species were
recovered from beaches, and it was suggested that many more had actually died but had not been
recovered. The most severely affected species were puffins, razorbills and guillemots. Extensive areas
of salt marsh were damaged and some animals, such as the heart urchin, were almost completely wiped
out. The affected coastline was also being intensively used for oyster culture, which was severely
disrupted by the spill. A high initial mortality was accompanied by heavy contamination that resulted in
shellfish being unsuitable for consumption.
Figure 4. Table detailing protection and threats for European coastal salt marshes.
Area
(km2)
Protected area
(%)
Area threatened by reclamation
(%)
Denmark
81
30
0
France (excluding
Camargue)
148
3
35
France (Camargue)
650
Reserve
N/A
Germany
188
55
60
Italy
740
30
Not known
Ireland
175
4
5
The Netherlands
129
100
47
Portugal
92
28
80
Spain (Donana)
400
Reserve
N/A
UK
314
50
40
Natural changes and sea walls
The isostatic adjustment of the British Isles following the removal of ice from the Scottish Highlands
after the last ice age means that Scotland is rising while southern Britain is gradually sinking. In the last
few decades, this process has been augmented by the rise in sea level as a result of climate change. In
completely natural situations, salt marsh can adapt to rising sea levels by a process of erosion at the
seaward edge and the re-deposition of this material in the higher areas of the marsh. By this process,
the marsh can effectively move landwards and up the coastal slope. This can still happen in remote
areas of the north and west where there is no barrier at the landward edge of the marsh to limit the
processes. However, particularly in the south of England where the problem of rising sea levels is at its
greatest, the presence of fixed sea walls means that marsh cannot re-form in a new position at a higher
level further landwards. Effectively, the marshes are progressively being lost in a process of coastal
squeeze.
Figure 5.
Here bare mud is surrounded by patches of discontinuous salt marsh. The mud has been colonised by Enteromorpha (algae)
and occasional grass species such as Samphire and Spartina (cord grass).
Conclusion
Salt marshes are highly productive ecosystems, which display a great deal of biodiversity. Nevertheless,
they are under great pressure from development, ranging from industrial land use to tourism and
recreation. Salt marshes offer valuable protection from coastal flooding, so their removal is all the more
serious. Although their importance is increasingly recognized, so far there have been limited attempts to
re-establish salt marshes. Moreover, attempts to protect coastlines from flooding, by means of seawalls
and other defences, allow less land for salt marshes to develop in. In certain places, notably the
southern part of Britain, natural processes such as isostatic adjustment mean that salt marshes are
gradually being covered by rising sea levels. All in all, salt marshes are under a great deal of pressure.