What is it?
Coastal saltmarshes comprise the upper, vegetated portions of intertidal mudflats, lying approximately between mean high water neap tides and mean high water spring tides.
Saltmarshes are usually restricted to comparatively sheltered locations in five main situations:
in saline lagoons
behind barrier islands
at the heads of sea lochs
on beach plains
The development of saltmarsh vegetation is dependent on the presence of intertidal mudflats. Saltmarsh vegetation consists of a limited number of halophytic (salt tolerant) species adapted to regular immersion by the tides. A natural saltmarsh system shows a clear zonation [does this word exist?] according to the frequency of inundation. At the lowest level, the pioneer glassworts Salicornia spp can withstand immersion by as many as 600 tides per year, while transitional species of the upper marsh can withstand only occasional inundation.
The communities of stabilised saltmarsh can be divided into species-poor low-mid marsh, and the more diverse communities of the mid-upper marsh. On traditionally grazed sites, saltmarsh vegetation is shorter and dominated by grasses. At the upper tidal limits, true saltmarsh communities are replaced by drift-line, swamp or transitional communities that can only withstand occasional inundation. Saltmarsh communities are additionally affected by differences in climate, the particle size of the sediment and, within estuaries, by decreasing salinity in the upper reaches.
Saltmarshes are an important resource for wading birds and wildfowl. They act as high tide refuges for birds feeding on adjacent mudflats, as breeding sites for waders, gulls and terns, and as a source of food for passerine birds, particularly in autumn and winter. In winter, grazed saltmarshes are used as feeding grounds by large flocks of wild ducks and geese.
Areas with high structural and plant diversity, particularly where freshwater seepages provide a transition from fresh to brackish conditions, are particularly important for invertebrates.
Saltmarshes also provide sheltered nursery sites for several species of fish.
The situation in the South East
|Extent in England|| app. 32,500 ha
|Extent in the SE region|| 2,500 ha
|Percentage UK resource in the SE||8%|
|Extent covered by SSSI designation|| data to be added
Rate of Change
It is estimated that 20% of the saltmarsh resource in Kent and Essex was lost between 1973 and 1988. The best available information suggests that saltmarshes in the UK are being lost to erosion at a rate of 100ha a year.
|County||1998 extent (ha)||2008 extent (ha)|
|Hampshire||2,660||data to be added|
|Isle of Wight|| 160
|| data to be added
|Kent||1,390|| datato be added
|Sussex||815|| data to be added
1998 data taken from 'The Biodiversity of South East England - An Audit and Assessment' published by the Wildlife Trusts of South East England and the RSPB, South East and Central Regions
2008 data taken from regional or national BAP habitat inventory. Please note that some of the changes listed here are due to improvements in mapping and habitat definition.
For the 2008 report on intertidal habitats please follow this link.
The main threats to this habitat are:
Land claim. Large-scale saltmarsh land claim schemes for agriculture are now rare. Piecemeal smaller scale land claim for industry, port facilities, transport infrastructure and waste disposal is still comparatively common, and marina development on saltmarsh sites occurs occasionally. Such developments usually affect the more botanically diverse upper marsh and landward transition zones.
Erosion and 'coastal squeeze'. Erosion of the seaward edge of saltmarshes occurs widely in the high-energy locations of the larger estuaries as a result of coastal processes.
Accretion. Accretion and development of saltmarsh is occurring on parts of the British coastline, notably in North West England where sediments are comparatively coarse and isostatic uplift largely negates sea level rise. However, this accretion is not sufficient to offset the national net loss of saltmarsh, and in many cases the newly created habitats differ from those being lost due to regional differences.
Sediment dynamics. Local sediment budgets may be affected by coast protection works, or by changes in estuary morphology caused by land claim, dredging of shipping channels and the impacts of flood defence works over the years.
Cord grass. The small cordgrass, Spartina maritima, is the only species of cordgrass native to Great Britain. The smooth cordgrass, S. alterniflora, is a naturalised alien that was introduced to the UK in the 1820s. This introduction led to its subsequent crossing with S. maritima resulting in both a sterile hybrid, Townsend?s cordgrass S. townsendii, and a fertile hybrid, commoncordgrass S. anglica. The latter readily colonises mudflats and has spread around the coast. It has also been extensively planted to aid stabilisation of mudflats and as a prelude to land-claim. Common cordgrass often produces extensive monoculture swards of much less intrinsic value to wildlife, and in many areas is considered to be a threat to bird feeding grounds on mudflats. As a result, attempts have been made to control it at several locations, although in some areas it is undergoing dieback for reasons not fully understood.
Grazing. Grazing has a marked effect on the structure and composition of saltmarsh vegetation by reducing the height of the vegetation and the diversity of plant and invertebrate species. Intensive grazing creates a sward attractive to wintering and passage wildfowl and waders, while less intense grazing produces a tussocky structure which favours breeding waders. In recent decades, some grazed saltmarshes have been abandoned, leading to domination of the mid to upper marsh by rank grasses. Intensive grazing is considered to be a problem in some areas.
Other human influences. Saltmarshes are affected by a range of other human influences including waste tipping, pollution, drowning by barrage construction, and military activity. Turf cutting is a traditional activity in some areas. Oil pollution can potentially destroy saltmarsh vegetation and while it usually recovers, sediment may be lost during the period of die-back. The effects of recreational pressure are not well understood but may be locally significant. Agricultural improvement (re-seeding and draining) has affected the upper edge and transition zones of some saltmarshes in the past and may still occur on a small scale. Eutrophication due to sewage effluent and agricultural fertiliser run-off has caused local problems of algal growth on saltmarshes.
Vision for coastal saltmarsh
The South East Biodiversity Forum’s vision for this habitat is that there should be:
No further loss of existing habitat
Good management, including visitor management, on all extant sites
No damage to site integrity from activities arising outside the sites, eg. inadequately managed public access
Re-creation of sand dunes on appropriate sites to restore some past losses, including the linking up of fragmented sites
Greater public appreciation of sand dunes and their specialist wildlife, including greater awareness of the impacts of human pressures, such as dog-walking, mountain-biking, dumping of waste
Creation of alternative green space around important sand dunes under pressure for increasing new housing
How we can deliver this vision
- MoD Integrated Land Management Plans (www.defence-estates.mod.uk/conservation/2_biodiversity.php)
- Provision of land management advice by statutory (Natural England) and non-statutory agencies (NGOs)
- Agreements under Higher Level Stewardship
- Project funding (SITA Trust, WREN etc)
- Site management plans
- Minerals after-use (http://www.afterminerals.com/)
- Land purchase/management agreements by NGOs
- River restoration linked to Water Framework Directive Programme of measures
- Catchment Sensitive Farming tackling diffuse pollution issue