Saltmarsh is found on the upper part of the mud, which the sea reaches only when the tide is high. It is covered in plants that can cope with salt and with being regularly underwater. Salt marshes start life as mudflats. In areas of sheltered water, like a harbour, the sediment held in the water settles out and builds up. As plants arrive and grow their roots help to stick the mud particles together and trap even more sediment so the mudflats become more stable. As the mudflats build up, different types of plants can grow and live there creating a salt marsh habitat made up of blocks of flat low growing vegetation with narrow channels between.
The development of mudflats and saltmarsh over time is known as succession.
Saltmarshes are important areas for small creatures such as worms, shrimps and shellfish, fish, wading birds and wildfowl. They provide nursery areas for fish, food for waders and wildfowl and nesting sites for waders and seabirds. Many of the plants growing on saltmarsh are not found anywhere else.
Farm animals may graze on the upper parts of the saltmarsh.
Saltmarshes may help with defence against the sea as they can reduce the force of the waves hitting sea walls.
Saltmarsh in Chichester Harbour
Chichester Harbour has the 7th largest area of saltmarsh in Britain.
There are about 10 noteworthy saltmarsh sites in the Harbour although none is very big. The largest is at Gutner. Saltmarsh is a very productive habitat. Although very few animals eat the saltmarsh plants because they are tough and rather salty, the muddy channels contain a rich mixture of decaying matter, providing food for crabs, mud shrimps (Corophium spp) snails (Periwinkles) and worms, which in turn feed the wading birds.
At Itchenor, a walk westwards past the boatyard will take you to an area of typical saltmarsh which is quite easy to explore.
On the lower levels, washed by the tide twice every day there is mainly Glasswort (a pioneer species). At the top, covered in seawater only a few times a year, the plants are mainly grasses, for example Sea Couch, rushes and sedges.
Other typical saltmarsh plants are Sea Lavender, Sea Aster and Sea Purslane.
When the high tide covers the mud flats birds such as Curlew, Godwits, Redshank and Dunlin, which feed on creatures in the mud, need somewhere like the saltmarsh to roost, rest and preen.
Crabs hide in the channels. A variety of beetles, flies and moths also live in saltmarsh.
Moving up the shore from the mudflats the saltmarsh starts and it shows a clear zonation according to how often it gets covered in sea water. At the lowest level the pioneer glassworts Salicornia spp can withstand immersion by as many as 600 tides per year, whereas species of the upper marsh can only withstand occasional inundation. The lower marsh has the fewest species, the upper marsh a much more diverse community. Where the saltmarsh is grazed the vegetation is shorter and there are more grasses.
These are the first organisms to arrive in a new habitat. Pioneer plants may be very quick growing like some weeds or tolerant of difficult growing conditions, like the seaward end of a saltmarsh. However, once the mud in a saltmarsh starts to build up, other less hardy species will move in and the pioneer may be squeezed out. Glasswort and Annual Seablite are found on the lower part of the saltmarsh, but Sea Lavender, Sea Purslane and Saltmarsh Grass take over further up. However, if there is a hollow or pool in this higher area you will find Salicornia and Seablite again.
Threats to Saltmarsh
Sea level rise - the sea is rising due to climate change and also because the land along the south east of England is tilting towards the sea.
Sea defences - defences to protect the land from the rising sea may be built on saltmarsh or they may change the movement of the sediment necessary to maintain saltmarshes and mudflats.
Dredging - maintains the channels but may also affect the movement of sediment and hence the state of the saltmarsh.
Coastal squeeze - ideally saltmarsh need to be able to ‘move’ in response to changing conditions. Many saltmarshes are being 'squeezed' between the rising sea and fixed flood defence walls.
Erosion - wave action (including wash from boats) can damage and erode the marsh.
Disturbance by people - recreational use, for example by trampling and creating informal footpaths, can damage saltmarsh.
Land claim for farming or building - since medieval times, saltmarshes have been enclosed for agricultural use or destroyed to make way for building ports, harbours and other infrastructure. Nowadays this happens only in special cases.
Pollution from land or sea; oil, sewage, fertilizers, run off from old waste tipping - oil pollution can damage saltmarsh vegetation and whilst it usually recovers, sediment may be lost during the period of die-back. Water pollution from sewage and fertilizers can lead to eutrophication. This is the excessive growth of green algae, which may cause local problems of smothering on saltmarshes.
Grazing - can be beneficial if it controls coarse grasses, but sometimes it reduces the height of the vegetation and the diversity of plant and invertebrate species. This makes it less attractive as a breeding place for wading birds although they still use it in winter and when passing by on migration. Intensive grazing is considered to be a problem in some areas.
Colonisation by cordgrass - the small cordgrass, Spartina maritima, is the only species of cordgrass native to Great Britain. S. alterniflora, was introduced to the UK in the 1820s resulting in the hybrid cord grasses Spartina townsendii and S.anglica, which have invaded and dominated most of the marshes along the south coast. Common cordgrass S. anglica, helps to stabilise mudflats but in many areas it is considered to be a threat to bird feeding grounds. As a result, attempts have been made to control it at several locations. However some areas it is dying back for reasons not fully understood, and this is exposing the mudflats to erosion and saltmarsh is being lost.
New saltmarsh can be created. The sea can be allowed to flood fields which will eventually turn to saltmarsh. This is good for wildlife and for defence against the sea.
Some important saltmarsh species in Chichester Harbour
- Sea Heath - Frankenia laevis
- Golden Samphire - Inula crithmoides
- Sea Lavender - Limonium vulgare
- Lax-flowered Sea Lavender - Limonium humile
- Saltmarsh Grass - Puccinellia maritima
- Glasswort - Salicornia species
- Perennial Glasswort - Sarcocornia perennis
- Sea Wormwood - Seriphidium maritimum
- Saltmarsh Rush - Juncus gerardii
- Sea Aster - Aster tripolium
- Sea Plantain - Plantago maritima
- Sea Arrowgrass - Triglochin maritimum
- Leaf beetle - Crepidodera impressa
- Matthew's Wainscot - Mythimna favicolor
- Crescent Striped - Apamea oblonga
- Starwort - Cucullia asteria
- Bass- Dicentrarchus labrax