Ask most people where carbon is stored and they will picture rainforest. The UK's own coastline runs a quieter, muddier service — and it happens to be some of the country's richest wildlife habitat at the same time. Seagrass meadows and saltmarshes are blue carbon habitats: places where carbon is drawn out of the water and locked into sediment. Here is how they work, why they are under pressure, and what restoration looks like on the ground.

What "blue carbon" actually means

Blue carbon is the carbon captured and stored by coastal and marine ecosystems. The blue refers to the water, not the plants. Saltmarsh, seagrass, mangroves and similar habitats take carbon dioxide from the air and dissolved carbon from seawater, then bank it in two places: in living tissue such as leaves, roots and rhizomes, and in the sediment underneath.

The sediment store is the one that matters. In a healthy, waterlogged marsh or meadow, the soil is low in oxygen, so dead plant material breaks down very slowly. Layer after layer of silt and root matter builds up, and carbon that might otherwise return to the atmosphere stays put. On land, soils are aerated and often turned over; in tidal mud and shallow seabeds, decomposition slows to a crawl.

Seagrass: the UK's underwater meadows

Seagrasses are the only flowering plants that live in the sea. Around the UK coast you will mainly find common eelgrass (Zostera marina) and dwarf eelgrass in shallow, sheltered water — bays, estuaries and lagoons where enough light reaches the seabed. Eelgrass is not seaweed. It has roots, veins and tiny flowers, and where conditions suit it, it spreads into dense beds.

Those leaves slow the water down. Suspended sediment drops out and settles around the roots, which is why seagrass beds tend to be muddy and why they can accumulate carbon relatively quickly. The roots also bind the seabed together, so the meadow holds its ground when storms arrive.

Saltmarsh: the tidal sponge

Saltmarsh forms where tidal water is calm enough for fine silt to settle. Pioneer plants such as glasswort and cordgrass move in first, catching more sediment. As the ground rises, other species follow: sea aster, sea lavender, sea purslane, then rushes and grasses higher up. Over years, a marsh becomes a low, spongy platform that floods on spring tides and drains between them.

Because marsh grows upwards as sediment builds, it can keep pace with a slowly rising sea level — but only if it has room to move inland. Where a hard sea wall blocks that retreat, the marsh is squeezed between the wall and the rising water. Eventually it drowns, and the carbon it holds becomes exposed to erosion.

Why these habitats store carbon so well

• Waterlogged, low-oxygen sediment slows decomposition right down.
• Every tide brings more silt, burying older carbon deeper.
• Much of the carbon sits below ground in roots and rhizomes, where it is harder to disturb.
• Undisturbed, that carbon can stay locked away for a very long time.

The flip side is just as important. Drain a marsh, dredge a channel through a meadow, or trample the same patch of mud year after year and stored carbon is exposed to oxygen again. Saltmarshes also release some methane, a potent greenhouse gas, so the ledger is not perfectly clean — but a functioning marsh still stores far more than it loses.

The wildlife that depends on them

Seagrass beds work as nurseries. Juvenile pollack, bass and wrasse shelter between the blades, cuttlefish anchor their eggs to the leaves, and both spiny and short-snouted seahorses have been recorded in UK meadows. Prawns, crabs and brittlestars live in the root mat, and the whole system feeds the fish that feed the seals, cormorants and otters further up the food chain.

Saltmarsh is just as busy, in a different way. Wintering brent geese graze eelgrass and marsh plants. Curlew, redshank and lapwing probe the mud for invertebrates. The creeks that thread through a marsh are shelter for juvenile fish and crustaceans, and water voles and otters use them as routes and larders. On top of all that, marsh absorbs wave energy, which takes pressure off sea walls, and filters nutrients out of the water.

What restoration looks like in practice

Restoring these habitats is slow, unglamorous work, and most projects follow a similar sequence:

1. Survey and map. Find out what is there now and where the habitat used to be, using historical charts, aerial imagery, dive surveys and local knowledge.
2. Tackle the pressure first. Nutrient runoff, sewage discharges, anchoring, dredging and trampling will undo new planting. Fixing the cause comes before adding plants.
3. Replant or re-seed. Seagrass is grown from shoots or seeds in nurseries, then transplanted in small plots, sometimes with hessian bags or biodegradable mats to hold the sediment steady while roots establish.
4. Let the tide back in. For saltmarsh, managed realignment means breaching or removing a sea wall so the tide can reach low-lying land again. Marsh forms over years and wildlife moves in. Wallasea Island in Essex is a well-known example.
5. Monitor and adapt. Fixed transects, sediment cores, drone flights and volunteer counts show what is surviving and where to try next.
6. Involve people. Local fishers, schools, divers and walking groups often end up being the best long-term guardians of a site.

How you can help

• Join a survey. Wildlife trusts and marine conservation groups run seagrass and saltmarsh recording days. No specialist background needed for most of them.
• Mind your anchor. Anchor over sand or mud rather than seagrass, and use recognised moorings where they exist.
• Keep off the marsh. Footfall and dogs crush the plants and compact the sediment. Watch from the seaward edge or a boardwalk.
• Cut nutrient pollution. Go easy on garden fertiliser, keep drains clear of oils and chemicals, and report pollution incidents to the environment agency for your nation.
• Back a local project. Restoration runs on funding and volunteer hours, and both are usually in short supply.

Blue carbon is not an abstract accounting exercise. It is a hedge of eelgrass, a creek cutting through a marsh, and a series of small decisions about how we use the coast. Protect the habitat and the carbon and the wildlife look after themselves.

Photo: Franziska_Stier / Pixabay