In brief
A salt marsh is a coastal wetland of salt-tolerant plants on soft mud or sand, flooded by the tide. Marsh plants grow in stripes set by how often they are flooded. Marshes shelter young fish, clean water, slow waves and store carbon, but they are squeezed between rising seas and human land use.
What it is
Marshes form in sheltered places: behind barrier islands, in river mouths and in bays. Fine sediment brought by rivers and tides settles where the water is calm. Plants then trap more sediment and the mud builds up until it is high enough for more plants to grow. The surface is cut by winding tidal creeks that fill and drain with each tide.
Plants that can live in salty, waterlogged soil are called halophytes. They have special features: some store water in fleshy stems, some push salt out through glands on their leaves, and some have hollow tissue that carries air to roots in oxygen-poor mud.
Plants in stripes
Marsh plants sort themselves by how often they are flooded and how much salt they can handle. Tall cordgrass grows at the muddy seaward edge, where the tide reaches daily. Glasswort, a fleshy green plant, fills the middle. Higher up, where only the biggest tides arrive, finer grasses and flowers such as sea lavender appear. Where the zone meets dry land, rushes and shrubs are found.
| Zone | Flooding | Typical plants | What decides who lives here |
|---|---|---|---|
| Low marsh | Daily, for hours | Cordgrass | Tolerates long flooding and soft, airless mud |
| Mid marsh | Most high tides | Glasswort, small grasses | Tolerates salt that builds up in drier mud |
| High marsh | Only the biggest tides | Fine grasses, sea lavender | Needs fewer floods; competes well for light |
| Upper edge | Rarely; fresher soil | Rushes, shrubs | Salt-sensitive plants can survive here |
Open bare patches called pans collect salty water that evaporates in the sun, making very salty ground that few plants can bear.
What a marsh does
- Nursery and feeding ground. Young fish and crabs shelter among the stems and feed on the marsh surface when it floods. Scientists still debate how much food a marsh exports to the open sea, but its value as habitat is well established.
- Filter. Slow water drops its mud, and plants and soil microbes take up some of the nutrients washed off the land. Microbes in waterlogged mud can turn nitrate into nitrogen gas.
- Cushion. Dense stems slow waves and lower their energy before they reach the land behind.
- Store. Waterlogged soil breaks plant matter down very slowly, so carbon stays locked in the mud. Carbon stored in coastal wetlands such as marshes, mangroves and seagrass is called blue carbon.
How a marsh keeps up with the sea
A marsh can grow upward. Sediment settles on the surface and roots and dead plant matter add volume, so the surface rises. If the sea rises slowly enough and enough sediment arrives, a marsh can keep pace. If the sea rises faster than the marsh can build, the lowest zone drowns and turns to open mud or water. If the marsh cannot move inland, because a sea wall or road blocks it, it is lost. This squeeze between the sea and a hard barrier is called coastal squeeze.
Pressures and responses
- Drainage and embankments. Marshes have been drained for farmland and ports in many countries.
- Too many nutrients. Fertiliser runoff can change which plants thrive and can fuel algae.
- Sea-level rise and sinking land. Both lower the marsh in relation to the tide.
- Managed realignment. Moving a sea wall back so that the tide can flood farmland again can create new marsh. Where it has been tried, the new marsh takes years to develop the plants and animals of an old one.
Field exercise
Walk from the water’s edge toward the land on a boardwalk or path and note where the plant type changes. Draw a simple strip map with each band labelled and a rough guess at its width. Stay on paths: marsh mud can be deep and soft, and creeks fill fast on a rising tide.
Check yourself
Read the question, think of your answer, then open it with a tap or the Enter key.
What is a halophyte?
A plant that can live in salty soil.
Why do marsh plants grow in stripes?
Each kind tolerates a different amount of flooding and salt, so the plants sort themselves by height above the tide.
Name three jobs a marsh does for the coast.
Any three of: nursery for young fish, filter for the water, cushion against waves, store of carbon.
What is coastal squeeze?
Loss of marsh caught between a rising sea and a hard barrier, such as a sea wall, that stops it from moving inland.
What is blue carbon?
Carbon stored in coastal wetlands such as marshes, mangroves and seagrass meadows.
What happens in managed realignment?
A defence is moved back so that the sea can flood land behind it, which can create new marsh.
Basis of the facts
Where these facts come from. Marsh formation, zonation and plant adaptation follow standard texts such as Adam (1990). The ecosystem services listed (habitat, nutrient processing, wave attenuation, carbon storage) are reviewed in Barbier and others (2011) and in McLeod and others (2011), the paper that set out the blue carbon idea. The debate about how much material marshes export is noted as a debate; no figure is quoted. Species named are examples for temperate coasts. See the sources list.