Just beyond the seawall are areas of saltmarsh, coastal wetlands that are flooded and drained by saltwater brought in by the tide. Saltmarsh is also known as Atlantic salt meadow.

Saltmarshes form along sheltered coastlines, such as estuaries, when mudflats reach the level of the average high tide and salt-tolerant (halophytic) plants, such as glassworts and cord grass, become established. These plants trap more sediments and help the saltmarsh to grow. As the height of the saltmarsh increases, so different plants become established.

Natural saltmarsh vegetation shows a clear zonation according to how often plants are covered by the tide. On the lower saltmarsh, glassworts, or Marsh samphire, predominate. Higher up, on drier ground, plants like sea aster and sea lavender flourish. These plants help increase the amount of sediment that is deposited on the land, through slowing water flow and trapping particles, provide food for fish and slow the movement of water back into the sea.

The rate at which the saltmarsh grows depends on the amount of sediment in the water, wave action and the topography of the shoreline; saltmarsh may be extensive on gently sloping shores, such as the Gwent Levels, or limited to only a few metres on steep shores. 

As the saltmarsh grows, deposited organic material and dead vegetation are buried under layers of sediment. Because there is little oxygen (anoxic conditions) in the fine silt, decomposition is very slow, so over time the amount or organic material increases. Saltmarshes are therefore an important carbon store.

Saltmarshes are characterised by a network of creeks and salt pans. The growth of plants in raised areas concentrates the flow of water into channels that eventually form deep creeks. The creeks channel incoming flood tides across the saltmarsh, ensuring an even distribution of sediments. They also act to slow the incoming water by frictional drag along the sides and bottom of the creek. As the tide rises, the creeks fill with water which eventually spills out over the adjacent marsh.

Depressions surrounded by plants (pans) hold water that gradually evaporates after high tide. These are the equivalent of rockpools.

Biodiversity

Although they are often considered to be species poor habitats, saltmarshes can support a range of specialist salt-loving (halophytic) plants and a diverse range of invertebrates, algae, fungi, fish, birds, amphibians and small mammals. This includes a range of invertebrates adapted to feed on salt-marsh plants, such as the sea aster mining bee (Colletes halophilus) and the saltmarsh aphid (Sipha littoralis), which feeds on common saltmarsh grass. In Britain, there are over 290 species of terrestrial invertebrates resident on saltmarsh, of which 148 are found in no other habitats.

Saltmarshes are particularly important for a range of bird species which use the marsh for breeding, roosting, feeding and moulting, including grazing geese and ducks, skylarks, reed bunting, and lapwing. Around 50% of the UK’s breeding population of redshank use saltmarsh. Redshank, along with snipe, also prefer to feed in the relatively sheltered marsh, rather than out on the exposed mudflats preferred by most waders.

Although they are important habitats in their own right, saltmarshes (and mudflats) form an important part of a complex of habitats that make up the estuarine ecosystem, one of the most productive ecosystems on earth.

First line of defence

Saltmarshes form a buffer zone between the sea and dry land, absorbing wave action and protecting the coast from erosion.

Seawalls are often constructed to keep out the highest tides from reclaimed land. Where saltmarshes exist, these walls are often simple earthen banks. Where the saltmarsh has disappeared, seawalls need to be heavily reinforced with massive stone blocks and concrete. Protecting or restoring saltmarsh can therefore significantly reduce the cost of sea defences.

Saltmarshes can also help filter pollutants from water, including herbicides, pesticides, organochlorines and heavy metals. Pollutants become attached to sediment particles that settle on the saltmarsh and subsequently become buried, effectively locking them away and reducing their harmful environmental effects.

Blue Carbon

‘Blue carbon’ refers to the carbon stored by marine habitats. Saltmarsh is considered one of the most important blue carbon habitats because of its ability to capture and store carbon in its vegetation, roots and rhizomes in a relatively short time.

As they grow, saltmarsh plants use atmospheric carbon dioxide (CO2) and photosynthesis to produce organic carbon. When they die, the plants become buried in layers of sediment. Because the sediment is often water-logged and low in oxygen, the plant material decays very slowly, effectively locking the carbon away for centuries.

Natural saltmarsh can store around 4 tonnes of carbon per hectare per year (tC/ha/yr). Restored saltmarsh is even more effective and can store up to 15tC/ha/yr, likely due to rapid sediment accumulation.

Protecting, restoring and creating saltmarshes can play a major part in absorbing and storing atmospheric carbon and reducing the impact of climate change.

Threats to saltmarsh

Since medieval times, many salt-marshes have have been reclaimed for farming. The enclosed land was originally converted to grazing marsh, but since the 1940s large areas have been agriculturally improved; drained, reseeded and intensively grazed, or converted to grow crops.

Many saltmarshes, including areas of the Gwent Levels, are still grazed by livestock, including sheep and cattle. Light grazing (2-3 sheep or 1 cattle per hectare) can mimic natural grazing and leads to benefits for biodiversity.

Conversely, heavy grazing can reduce the range of plants and invertebrates and lead to trampling of breeding birds’ nests, such as Lapwings. Removing grazing animals from a previously grazed marsh can lead to one or two plant species dominating, excluding other plant species from recolonising.

Saltmarsh Wildlife

Seen something interesting?

It’s important to record wildlife sightings with South East Wales Biological Recording Centre, which stores, manages and shares wildlife records from across the region.

Follow the link below to find out how to record your sightings.