Saltmarsh & Estuaries
Saltmarsh restoration involves the re-assembly of saltmarsh plant communities on formerly reclaimed land after managed re-alignment. Restoration of species and ecosystems is a central part of biodiversity conservation. At present this is done mostly by establishing communities of plant species to resemble the target communities of conservation interest. The ultimate aim is to create complex ecosystems that resemble the targets in terms of composition of plant, animal and microbial communities: ecosystem function and stability.
According to research by the Centre for Ecology and Hydrology, results have shown that, with fairly minded intervention, plants and animals rapidly colonise newly accreted sediments. However, recreated sites differ in species composition and structure from natural marshes and can take several decades to converge, if at all.
Coastal saltmarshes are made up of upper, vegetated portions of intertidal mudflats, lying approximately between mean high water neap tides and mean high water spring tides. They are usually restricted to comparatively sheltered locations in five main situations:
- In estuaries
- In saline lagoons
- Behind barrier islands
- At the heads of sea lochs
- On beach plains
The environmental importance of mudflats should not be underestimated. Whilst the species richness is relatively low – only about 20 species including lugworms, ragworms, crustaceans, bivalves and snails – they occur in vast numbers. Because of this, mudflats are incredibly important as feeding grounds, particularly for birds that use them as refueling stops when migrating between their breeding and wintering grounds. Colonisation of new mudflats can be relatively rapid, reflected by bird usage. A high degree of bird usage can occur within 2 years and by 5 years after restoration species richness can be comparable to “natural” areas.
The development of saltmarsh vegetation is dependent on the presence of intertidal mudflats. Saltmarsh vegetation consists of a small number of halophytic (salt tolerant) species adapted to regular immersion by the tides. A natural saltmarsh system shows a clear zonation according to the frequency of tidal immersion. At the lowest level, pioneer glassworts can withstand immersion by as many as 600 tides per year. Transition species of the upper marsh can withstand occasional immersion. Transitional species of the upper marsh can only withstand a few inundations.
If there is already a source of propagules within the site, saltmarsh development is likely to be fairly rapid. Birds can quickly adapt to the new conditions. The type of habitat created by saltmarsh restoration depends on the relative land levels and how often it would be covered the sea. If the land is very low, it would be flooded during most tides. As a result, the main habitat would be mudflats. As the land level height increases and land is exposed for 2-3 days a month, pioneer glasswort can develop. The higher the land, the less often it is covered by sea water. As a result, the number and diversity of plants increases. As soon as vegetation has established, it captures sediment left as the tides moves over it, building up land levels. This enables the saltmarsh to grow.
The main threats to saltmarshes are:
- Land claim. Large scale saltmarsh land claim schemes are now rare. Some smaller scale land claim for industry, port facilities and transport infrastructure and waste disposal is still reasonably common. Marina development on saltmarsh sites still takes place. These developments usually affect the more botanically diverse upper marsh and landward transition zones.
- Erosion and “coastal squeeze”. Erosion of the sea edge of saltmarsh 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 in parts of the UK coastline – mainly in NW England. Here the sediments are coarse compared to the rest of the UK. Isostatic uplift largely negates sea level rise. However, this does not offset the national trend of saltmarsh loss.
- 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 Britain. The smooth cordgrass, S. Alterniflora, is a naturalised alien that was introduced to the UK in the 1820’s. This introduction led to it’s inevitable crossing with S. maritime, which resulted in both a sterile hybrid, Townsend’s cordgrass S. townsendii, and a fertile hybrid, common cordgrass 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 cord grass in many areas is thought to be a threat to bird feeding grounds on mudflats.
- Grazing. Grazing has a marked effect on the vegetation and diversity of plant and invertebrate species. Intensive grazing creates a sward attractive to wintering and passage wildfoul and waders. Less intensive grazing gives rise to a tussocky structure which favours breeding waders. In recent decades, some grazed saltmarshes have been abandoned leading to domination of the mid to upper marshes by rank grasses. Intensive grazing is thought to be a problem in some areas.
- Other human involvement. Wastetipping, pollution, drowning by barrage construction, and military activity. Turf cutting is a traditional activity in some areas. Oil pollution can damage saltmarsh vegetation – and despite the apparent recovery, sediment may be lost during the die-back period. We often read about the effects of humans using the sea as a tipping ground which not only has an effect on wildlife, but also on vegetation. Sewage effluent and agricultural fertiliser run-off has caused problems of algal growth in some of the UK coastal saltmarshes.
If you have a Saltmarsh restoration or mitigation project, then OpenSpace can help. Call us on 01228 711841 or email firstname.lastname@example.org . Our team of experts will be delighted to hear from you to discuss ways we can bring your project to fruition.