Just beyond the coast in many parts of the world is a strip called a “march.” It’s definitely country. But the sea or even the water in the mouth of a river still leaves its mark on it. Because: If the tide rises a little higher, the salt water of the sea or the brackish water in estuaries like the Elbe flood this country a little. Only plants that can tolerate the high salt content of the soil grow there, such as sea aster and cedar grass. A salt marsh is created by the sea.
As the sea level rises, the higher tides also deposit more suspended matter there, and the alluvium gets a little bit higher. In this way, the marsh can practically grow with the sea level. However, this only works if the tides only slowly reach a higher level. On the other hand, if sea levels rise rapidly, as is expected by the end of this century, this new alluvium could sink a little deeper under its own weight and eventually submerge, report Neil Saintilan of Macquarie University in Sydney, Australia, and his team now in the journal Science.
Such a loss affects an important but also sensitive ecosystem that has been heavily influenced by people on the coasts for centuries. There, in northern Germany and Holland, dikes have long protected the alluvial land that has formed behind the coast over the course of thousands of years. This marshland is no longer affected by the tides.
Behind the dikes, the villages are safe from storm surges and the farms till a very rich soil that brings very good harvests. However, the salt marshes in front of the dikes are much narrower than they used to be.
“If the sea level rises, we also observe that some salt marshes grow with it,” explains Hans-Ulrich Rösner, who heads the Wadden Sea office of the nature conservation organization WWF in Husum. The biologist and ornithologist was not involved in the current study. The reason for this effect are the tides, which are a little different every day and therefore allow the high tide to run up more or less.
In addition, winds and currents change the tides. Depending on the altitude, the sea reaches this strip of shore behind the coast almost every day or only a few times a year and deposits salt there. A salt marsh is created in which plants such as andelgrass, harlequin, sea lavender and salt aster grow. At the points where the coast is a little higher, the flood comes only a few times a year. There is less salt in the soil there, so other plants also flourish.
However, the floods not only carry salt into these active marshes, they also bring suspended matter with them. These are formed from the soil carried away by waves and storm surges from the coasts and beaches of the Wadden Sea islands and also contain a lot of organic material.
In a salt marsh, however, the growth slows down the flood and the light material slowly sinks to the bottom. As a result of this frequent sprinkling, the soils of many salt marshes are slowly growing upwards and are thus at least able to keep pace with the rising sea level due to climate change.
If these salt marshes are no longer artificially drained and grazed, but are left to their own devices, a number of bird species also benefit enormously: the animals are protected from predators in the high vegetation there, and on average more chicks hatch from each nest because the predators have a worse clutch Find. The administration of the Schleswig-Holstein Wadden Sea National Park concludes from several studies that the young animals also find more insect food in the dense vegetation than on the grazed areas. On the Hamburger Hallig, for example, 35 pairs of redshanks bred in 1991, after the end of grazing there were already 190 pairs in 1999. Gadwall, shoveler, tufted and eider ducks, as well as shelducks, like songbirds, prefer areas that are not used for grazing. The salt marshes that are not grazed also hold back the sediments more and thus grow upwards faster.
However, the active marshland does not grow on all coasts. “In the southeast of England, many salt marshes often break off at their edges and are then washed into the sea by the currents,” says WWF expert Hans-Ulrich Rösner, describing the situation on the flat coasts of Great Britain. “Many salt marshes on these coasts have already been lost there,” explains the biologist.
Neil Saintilan and his team have now identified another factor in such salt marsh losses. To do this, they examined 97 areas with such tidal marshland on four continents: If the currents in the salt marshes and in brackish water areas in the estuaries of the rivers deposit more and more sediment, a critical limit can also be exceeded.
Then the sediments are compressed under their own weight, causing the salt marsh to sink downwards. The race against rising sea levels then threatens to be lost.
In this situation, the salt marshes have only one alternative, they can migrate inland, Neil Saintilan and his group explain: When sea levels rise, the storm surges flood some higher regions that they had not previously reached. There, too, they deposit sediments, fertilize the soil and allow it to grow. But only as long as the floods don’t meet any obstacles. These, in turn, are often made by humans, are called “dykes”, and are intended to protect the settlements, including their belongings, and the farmland behind them.
Even then, the salt marshes can still have a chance, as the 800-hectare Wallasea Peninsula on the coast of the English county of Essex, north of the Thames Estuary, shows. There, the endangered dike on the coast was relocated to the hinterland. The land between the old and new barriers has been left to the tides. In this way, mud flats and salt marshes were created on 275 hectares, where many waders and migratory birds rest.
“Even on the German North Sea, it is important to carry out coastal protection as naturally as possible,” says WWF expert Hans-Ulrich Rösner.