Saltmarsh and mudflats
What is it?
In coastal areas sheltered from waves, slow moving tides gently lap over a flat expanse of fine mud. Covered at high tide and exposed during low tide mudflats support a wealth of life; bristle worms, bivalves (molluscs with two hinged shells) and mud snails - food for flocks of wading birds.
Towards land, in the absence of manmade structures, mudflats become saltmarshes - first vegetated with succulent samphire (harvested at some sites and considered a delicacy) and then with cord-grasses, sea purslane, sea aster and sea lavender as the mud becomes drier. Creeks fan out through the saltmarsh, channelling the water up to flood the lower marsh at high tide and draining away rapidly as the tide drops.
Why is it like this?
Mudflats form within the shelter of estuaries or natural harbours, where fine silt and clay sediments carried in by the gentle inward movement of the tides (and in estuaries the slow outward flow of the river) settle. The mud is very fertile thanks to its high content of organic material, making mudflats ideal for hosts of filter-feeding and scavenging invertebrates.
When the accumulating mud rises above the water surface on average tides (half way between spring and neap tides), saltmarsh plants can colonise. These capture more sediment and stabilise the substrate – cord-grasses can trap 10cm of sediment a year – allowing the marsh to keep building for as long as it is still low enough to be inundated by the higher tides.
Distribution in the UK
Found in sheltered estuaries and natural harbours around the UK.
What to look for
Visit southern saltmarshes in late July and August to see lilac-tinged expanses of flowering sea lavender – look out also for the pretty, soft leaves and delicate pink flowers of the scarce marsh-mallow at the very top of the saltmarsh. On the grassier, grazed saltmarshes of north-west England and Galloway, huge flocks of wintering wildfowl can be spectacular - Visit the east coasts in late autumn and winter to see tens of thousands of wintering waders pushed by the rising tide onto the saltmarshes to roost.
Saltmarshes have been historically threatened, having been ‘reclaimed’ from the sea for use as farmland as early as Roman times, but modern developments threaten to squeeze them out of existence. Trapped between increasingly frequent storms and rising seas, and hard sea defences that prevent them from moving inland, saltmarshes are being eroded in many places.
As well as being a vital habitat for thousands of waders and waterfowl, the role that saltmarshes play in defending our coasts from erosion by waves is increasingly recognised, and efforts are being made to protect and restore them. Although saltmarshes tend to need little conservation management, many have a long tradition of low-intensity grazing, which is important in maintaining their diversity. Re-alignment of sea-defences can allow for expansion of this habitat (for example at the Essex Wildlife Trust’s Abbotts Hall farm on the Blackwater Estuary).