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laag water tussen texel en vlieland, foto fitis, sytske dijksen

Wadden Sea

It is one of the last large tidal flat regions in the world, the Wadden Sea. The immense sea extends from the Dutch Den Helder to the Danish Esbjerg. The Wadden Sea is unique. The plants and animals that live here are true diehards - they must cope with a Wadden Sea that is constantly changing between high and low tide. At one moment you see extensive mudflats while at another moment everything is under water. The weather is often rugged and difficult to predict. If you want to survive in this environment, then you must adapt. No wonder that the plants and animals living in and around the Wadden Sea are so extraordinary.

  • Formation of the Wadden Sea

    The Wadden Sea has not always been a sea. It only started to take on its present appearance several thousand years ago. The coastline has constantly changed, as it still does today.

    The formation of the Wadden Sea goes back to the last glacial period which ended around 10,000 years ago. In those days, the North Sea was mostly dried up. You could walk to England. For the 100,000 years that the glacial period lasted, strong westerly winds blew sand in the direction of Denmark, Germany and the Netherlands. Around 10,000 years ago, the North Sea filled again with water. That was at the beginning of the warmer period following the glacial period, the Holocene. Wind and waves helped form a wall of sand along the entire coast of the southern North Sea. Extensive peaty grounds lay behind this wall.

    Openings in the wall of sand occurred over the following thousands of years, forming sea channels. The peat grounds behind the wall eroded where the sea advanced. It took a long time before the channels were so large that they connected together and developed into a true coastal sea.

  • Creeks, channels and streams
    priel, foto fitis, sytske dijksen

    During warm, peaceful summery days, it's hard to imagine that the Wadden Sea can also turn into a rough sea. The strong currents ensure that no two days are the same. The creeks, channels and streams, the major and minor water currents, are constantly shifting. During flood, the seawater first flows through the major channels, eventually flooding the flats. During ebb, the water retreats along the many creeks back into the major channels.

    The tidal flats also shift around. Tidal flats consist of sand or mud. The size of the particles and the motion of the water determines when the sand and where the mud settles to the bottom. Larger, heavier sand particles settle where water is still flowing, such as by sea inlets and beaches. The very fine mud particles only settle to the bottom when there is no motion during high tide.

    As bare as they may appear, tidal flats are teeming with life. Besides the brown-golden layer on the surface is made up of algae. Worms and shellfish live burrowed in the bottom. Some flats are covered with seaweed or eelgrass.

  • How the Wadden Sea got its name
    wadlopers, foto fitis, sytske dijksen

    Twice a day, half of the Wadden Sea is exposed and that is due to the tides. At one moment, you see only water, while several hours later you see immense tidal flats. The Wadden Sea got its name from the fact that it's possible to walk over the tidal flats and wade through the channels. It's the wadable sea.

  • The tides
    Duitse waddeneilanden, foto fitis, sytske dijksen

    The tide is one of the most influential aspects of the Wadden Sea. Thanks to the tide, we have high and low tide in the wadden region twice a day. In between high and low tide, there is ebb; in between low and high tide there is flood.

    The rise and fall of the seawater causes currents. The greater the difference between high and low tide, the stronger the current. In the Netherlands, the difference between high and low tide is less than in Germany and Denmark. That means that the currents are not as strong in the Netherlands. You can see this effect by the size of the individual Wadden Islands. The German and Danish islands are much smaller than the Dutch. You can also see the strength of the tides by the sea inlets between the islands. The greater the difference between high and low tide, the greater the amount of water that needs to pass through the inlet and the wider it gets. The sea inlets in the Netherlands are much narrower than in Germany and Denmark.

  • Slack water areas
    Schorren en wantij, foto fitis, sytske dijksen

    A strange phenomenon that we see in the Wadden Sea is the slack water areas. These are the places where sea currents collide behind an island or sandbank. The currents lose all their strength and the water seems to stagnate. Very fine material which has been carried by the sea, such as mud, now has the chance to sink to the bottom. In that way, new land can form. One example is the tidal flats by the Schorren on Texel.

    Slack water areas are the shallowest parts of the Wadden Sea, where ships with little draught are just able to cruise during high tide. During low tide, these areas are where tidal flat hikers cross from one mudflat to the next.

  • Two meals a day
    opkomende vloed, foto fitis, syytske dijksen

    Not only is the tide responsible for how the wadden region looks. That rise and fall of the sea is also very important for the plants and animals that live here. During flood, the Wadden Sea receives a new supply of nutrients. This food forms the basis for a complicated food web. The very tiny microscopic plants and bacteria consume the nutrients, which in turn are eaten by animals, which are consumed by larger animals. Without the supply of nutrients two times a day, there would be much less life found in the Wadden Sea. Therefore, the tide is of vital importance.

  • During good and bad times
    afval, foto fitis, sytske dijksen

    Unfortunately, the tide doesn't only bring good things to the wadden region. Beside nutrients, all kinds of poisonous materials are carried into the region, such as pesticides, oil and marine litter. Every once in awhile, large amounts of poisonous material suddenly appears in the area. That usually means there has been an accident with a ship, where containers with toxic material have fallen overboard. For example in 1994, thousands of bags with herbicides fell overboard. Luckily, only a few of these bags reached the Wadden Sea.

  • Life in the Wadden Sea

    The Wadden Sea is a nutrient-rich area thanks to the influence of the tide. Nutrient-rich areas are rich in plants and animals. However in the Wadden Sea, there are not all that many different species of plants and animals. The reason lies by the extreme circumstances. The mudflats are first exposed, while a few hours later they are under water. The temperature can suddenly change, the oxygen and saline content vary constantly. As plant or animal, you have to be able to withstand all these things!

    The largest animals in the Wadden Sea are the seals. They don't just use the Wadden Sea to look for food; they also use it to give birth and as haul-out area. Fish from the North Sea, such as sole and plaice, use the Wadden Sea as nursery. The young fish grow up there.

    On and around the tidal banks, there are lots of animals, such as worms, shrimp and crabs. Many of the birds are dependent upon these large amounts. More than 12 million of these feathery friends make use of the Wadden Sea every year. Most of them are migrating birds. Twice a year, they forage for food in the muddy tidal flats in order to gain sufficient strength for a long journey. Should the Wadden Sea run into problems, then the birds also run into problems. 

  • Exotic wadden life
    harig spookkreeftje, foto fitis, sytske dijksen

    In the Wadden Sea, there are not only plant and animal species that have lived here for thousands of years. There are also species that from origin don't belong here. These are species from other marine areas. Biologists usually call them exotics. They were either released by people or arrived unintentionally in ballast water or on the hull of ships. The fact that the seawater is warming up makes it easier for a number of species to survive in the Wadden Sea. Every year, at least one new species of plant or animal is discovered in the wadden region. In the table below, you find an overview of several exotics in the Wadden Sea.

    English nameScientific nameFirst find
     (sea squirt) Molgula socialis * 2009
     (seaweed) Antithamnionella spirographidis 2009
    (tube worm) Ficopomatus enigmaticus 2009
    (tube worm) Cf Neodexiospira brasiliensis 2009
    (sea squirt) Aplidium glabrum * 2009
    (sea squirt) Botrylloides viocaceus 2009
    Velvet horn Codium fragile subsp. atlanticum 2009
    (red algae) Ceramiaceae sp * 2009
    Wakame Undaria pinnatifida 2008
    (sea squirt) Didemnum vexillum kott, 2002 2008
    Brush-clawed shore crab Hemigrapsus takanoi 2006
    warty comb jelly or sea walnut Mnemiopsis leidyi 2006
    amphipod Caprella mutica 2005
    Asian shore crab Hemigrapsus sanguineus 2004
    (bryozoan) Bugula stolonifera 1993
    (polychaete-worm) Marenzelleria viridis 1983
    Sargassum Sargassum muticum 1980
    Atlantic jackknife clam Ensis directus 1979
    Stalked sea squirt Styela clava 1974
    (acorn barnacle) Elminius modestus Jaren ’50 (20ste eeuw)
    Common slipper shell Crepidula fornicata 1939
    Chinese mitten crab Eriocheir sinensis Jaren ’30 (20ste eeuw)
    Pacific oyster Crassostrea gigas 1928
    Orange anemone Diadumene cincta 1925
    Tube amphipod Jassa marmorata unknown
    (sponge) Haliclona (Soestella) xena * unknown
     (red seaweed) Ceramium cimbricum * unknown
     (seaweed) Gracilaria vermiculophylla unknown
    (red seaweed) Polysiphonia harveyi unknown
    Ulva or Sea lettuce Ulva pertusa unknown
    (acorn barnacle) Balanus improvisus * unknown
     (tanaid crustacean) Sinelobus stanfordi * unknown
    Soft-shell clam or sand gaper Mya arenaria Middle Ages
    * It is unclear whether this species was originally found in the Wadden Sea. Further studies are needed.
  • Practically disappeared
    schol, foto fitis, sytske dijksen

    Unfortunately, the number of plant and animal species is not just increasing. Some species have almost disappeared. For example, eelgrass. You used to find large fields of this saltwater plant in the Dutch Wadden Sea. Nowadays, there are only a few places where it can be found. With the disappearance of the eelgrass fields, the broadnosed pipefish also disappeared from the Wadden Sea. A number of other plant and animal species now found here are having difficulties. They threaten to disappear from the region due to overfishing and climate change, for example. Flounder, plaice, eel and whiting have been declining greatly.

  • World Heritage
    wad bij terschelling, foto fitis, sytske dijksen

    In June 2009, the Dutch and German parts of the Wadden Sea were added to Unesco World Heritage List. The Danish part of the Wadden Sea was added in June 2014. This means that the entire Wadden Sea now has the World Heritage status. Being on this list underlines the necessity for special protection due to its high natural value for the world. The new status means recognition of this exceptional region and a prognosis for a higher influx of tourists, which helps to strengthen the economy. The areas on the landside of the dikes on the inhabited Wadden Islands and mainland are not part of the World Heritage region.