The Hidden Value of Salt Marshes

Along many stretches of the British coast lies a landscape that most visitors hurry past. Salt marshes, the flat, muddy areas of grass and low plants found between the land and the open sea, are flooded by the tide once or twice a day and can look dull and uninviting from a distance. For a long time they were regarded as wasteland, good for little except grazing sheep. Scientists now believe, however, that these unremarkable places are among the most valuable habitats on the planet.

Salt marshes form in sheltered places such as estuaries and the backs of sandbanks, where the water moves slowly enough for fine mud to settle. Once the mud surface is high enough to be exposed for part of the day, a few hardy plants are able to take root. These pioneer species can tolerate salt that would kill most garden plants, and their roots and stems trap more sediment each time the tide comes in. Over decades, the marsh gradually rises, and a wider range of plants, including sea lavender and sea aster, moves in to colonise the higher ground.

Wildlife thrives in this constantly changing environment. The creeks that wind through the marsh act as nurseries for young fish such as bass and flounder, which shelter there from larger predators before heading out to sea. In winter, huge flocks of wading birds and geese arrive from the Arctic to feed on the worms and snails buried in the mud. Ruth Fennimore, a coastal ecologist who has surveyed marshes on the east coast for twenty years, says that a single square metre of marsh mud can contain thousands of small creatures, most of them invisible to the casual observer.

Marshes are also valuable to people who study the past. Because mud is laid down in thin layers, year after year, a core drilled from a marsh can reveal how sea levels, storms and pollution have changed over several centuries. Fennimore's team has found traces of industrial waste from the nineteenth century preserved deep in the mud, as well as pollen grains that show which plants once grew nearby. Such records help scientists understand how coasts responded to past changes and how they might respond in the future.

Salt marshes also provide a service that has only recently been fully appreciated: they protect the coast. When storm waves reach a marsh, the dense vegetation slows them down and absorbs much of their energy, so that by the time the water reaches the sea wall behind, it is far less powerful. Engineers have calculated that a sea wall with a wide marsh in front of it can be built lower and more cheaply than one that faces the open sea directly. In addition, the plants and mud store large amounts of carbon, locking it away for centuries in a way that helps to slow climate change.

Despite these benefits, Britain has lost a large proportion of its salt marshes over the past few hundred years. Much of the damage was deliberate: farmers built embankments to keep the sea out and turned the marshes into fields for growing crops. Ports and industrial sites were constructed on reclaimed marshland, and in some areas rising sea levels are now squeezing the remaining marshes against fixed defences, leaving them with nowhere to move inland. Fennimore describes this process as a slow disappearance that rarely makes the news.

In recent years, a new approach known as managed realignment has begun to reverse some of these losses. Instead of repairing old sea walls, engineers deliberately breach them in carefully chosen places and allow the tide to flood the land behind. Within a few years, mud builds up and marsh plants return. Such schemes are not always popular: farmers may lose land that their families have worked for generations, and some residents fear that letting the sea in will put their homes at risk. Organisers now spend months consulting local communities before any work begins.

Fennimore is cautiously optimistic about the future. New marshes take time to become as rich in wildlife as old ones, she admits, and some species may take decades to return. But she believes that public attitudes are changing, helped by guided walks and school visits that introduce people to the hidden life of the mud. 'People protect what they understand,' she says, 'and once they have seen a marsh at high tide, full of birds, they rarely think of it as wasteland again.'