Vadehavet – UNESCO Verdensarv

Tidal Flats Full of Life

At first glance, the tidal flats may appear barren and lifeless, with their vast, exposed stretches of sand. Yet beneath the surface lies a teeming abundance of life, playing a crucial role in the entire Wadden Sea ecosystem. The flats are home to algae as well as a wide range of benthic organisms. While it is not a treasure trove of gold, it is a true storehouse of food for birds and fish. Compared to an average seabed, the tidal flats contain ten times more animals by weight. Covering a total area of 4,700 km², the tidal flats form a vast, natural pantry.

The conditions for this rich animal life are primarily created by the tides, which twice a day bring nutrient‑rich water into the shallow Wadden Sea. The foundation of life is a high concentration of microscopic algae, especially diatoms, which form the first link in the food chain. Diatoms live suspended in the water, on the surface of the tidal flats, and within the sediment itself. They are also what give the flats their brown colour. On just 1 cm², there can be up to 2.5 million diatoms, which serve as the primary food source for bottom‑dwelling animals such as lugworms, snails, and small crustaceans. These organisms, in turn, become food for birds and other larger animals.

The rich life of the tidal flats depends on the right balance between sandy sediment and clay—known as mud. The greatest abundance of animals is found in areas where mud makes up up to one fifth of the seabed. Where the ground consists of pure sand or pure mud, there are only few animals. Most of the animals living in the tidal flats are quite small, but they occur in vast numbers. There can be more than 100 nematodes in just a single cubic centimetre of sediment. On the surface of the flats lives the mud snail, measuring only a few millimetres and therefore easily overlooked. Yet it is extremely abundant: on just one square metre, there can be up to 120,000 snails.

The Tides – The Pulse of the Wadden Sea

Twice a day, the Wadden Sea changes dramatically—from kilometres of life‑filled mudflats to a sea surface rippled with waves. The Wadden Sea is, above all, a product of the tides, which have shaped the area’s characteristic landscape of islands, tidal flats, and marshlands.

Over millennia, the water has alternated between flooding the flat coastal landscape at high tide and receding again at low tide, exposing vast areas of the seabed. This pulsing movement has, over time, carried large amounts of both sand and nutrients into the shallow waters and across the tidal flats. In this way, the changing water levels have created both the wide, white sandy beaches and the fertile marshlands.

The tides are driven by the gravitational pull of the Moon. There are precisely six and a quarter hours between high and low tide, meaning that the sea floods the Wadden Sea twice within a 25‑hour period—throughout the year. Approximately one billion cubic metres of water flow back and forth across the tidal flats slightly more than twice a day. The mud and sand carried by the water are distributed along the Wadden Sea coast. Sand, being heavier, settles as soon as the current loses speed near the shoreline. Mud, by contrast, only settles when the water becomes still further inland. This is why you will find sand along the coasts, while the nutrient‑rich clay and mud are deposited further inland in the tidal inlets.

A Food Source for 10–12 Million Birds

The tides supply nutrients and organisms to the tidal flats, forming the basis of a rich ecosystem. Each of the different habitat types plays an important role for a range of characteristic coastal bird species. Taken together, the Wadden Sea is both a vital breeding area and a key stopover site for numerous migratory birds that feed, moult, and rest in the area. Nearly half a million pairs of waterbirds breed in the Danish‑German‑Dutch Wadden Sea, and this unique coastal environment is used annually by 10–12 million migratory waterbirds from more than 40 different species.

The Wadden Sea is the most important wetland for migratory waterbirds in Europe and forms part of the East Atlantic Flyway, which connects diverse regions such as the High Arctic tundra of Siberia, Greenland, and northern Canada with tidal areas and coastal wetlands in West and southern Africa. The deep channels, tidal inlets, flats, and salt marshes of the Wadden Sea—together with the adjacent reclaimed marshlands—provide ideal stopover habitats for many populations of geese, ducks, waders, gulls, and terns.

Of particular importance are the tidal flats, which are key to the exceptionally high biological productivity that characterises the Wadden Sea. Nowhere else in Denmark can you experience such large flocks of rare and remarkable waterbirds, and there is always something to see throughout the year.

A Shared World Heritage

The Wadden Sea is listed as a UNESCO World Heritage Site. This means that the area is recognised as exceptionally unique, of universal value, and irreplaceable for the whole world. The Wadden Sea stretches from Ho Bay near Esbjerg all the way south to the Dutch town of Den Helder. This World Heritage designation is therefore shared by the three countries of Denmark, Germany, and the Netherlands.

Since 1982, the three countries have worked together to protect the Wadden Sea through the Trilateral Wadden Sea Cooperation. This collaboration includes joint efforts in nature monitoring and conservation, and has resulted in the Wadden Sea being designated a UNESCO World Heritage Site. The Wadden Sea enjoys a high level of protection, including through EU Natura 2000 regulations, and is designated as a nature and wildlife reserve with extensive conservation measures in place.