The habitats of the Wadden Sea demonstrate in a fascinating way how physical forces and biological activities interact to generate conditions for life in a fragile balance. Major habitats are arranged along an offshore-inshore gradient and from deep tidal inlets up to the highest dunes.

The spectacular dunes on the barrier islands are evidence of the ongoing contest between aeolian mobilisation of sand and biotic stabilisation.

Salt marshes grow out of the sea and form a dense vegetation trapping deposits during inundation, while at the same time waves erode the edges.

Mussels attach to each other, accumulate sediment over the years and successively provide habitats for more and more species, until in a severe storm or during winter floes of ice scour it all away.

Less obvious are the habitats maintained by the activities of lugworms, which by their continuous recycling of surface sediments keep a sand flat sandy and prevent it from becoming a mud flat. Without this unseen work of worms, about 3,000 km² of rippled sand would be smooth and slimy.

The offshore belt

The offshore belt of the Wadden Sea has no tidal flats and drops off smoothly towards the open North Sea.

There is a continuous exchange of both water and sediment within the tidal area. The sediment supply from the offshore belt is vital for the resilience of the coast when responding to changes in tidal area, sea level and to disturbances caused by storm surges.

Phytoplankton blooms often start in this belt because turbidity is low enough to allow for sufficient light and nutrient concentrations are high. Through the tidal channels and inlets this offshore primary production reaches the inshore zoobenthos. Also, larvae of benthic fauna and fish drift from the offshore belt further inshore.

Shrimp, fish, diving birds, seals and harbour porpoises readily commute between offshore and inshore zones. In severe winters, the offshore belt provides an important refuge for the survival of populations otherwise confined to the tidal area.

 

The tidal area

The tidal area includes the most characteristic habitats of the Wadden Sea - the tidal flats, subtidal shoals and gullies of the back-barrier regions. The tidal flats up to the horizon are a phenomenon that cannot be found anywhere else on such a large scale.

The tidal flats of the Wadden Sea form the largest unbroken stretch of mud and sand flats in the world. At low tide, the tidal flats of about half of the tidal area are exposed. The other half are subtidal shoals and deep gullies which branch into ever smaller creeks and runnels across the tidal flats.

 

The Subtidal area

Within the tidal area, the subtidal shoals and gullies serve as a refuge for the intertidal fauna when conditions become harsh. In particular, the young crabs, shrimp and fish soon begin to migrate to the subtidal area with the ebbing tide into the subtidal zone and then return with the next flood. Some, like the shore crab (Carcinus maenas), hibernate in the subtidal zone but from spring onwards they begin to commute with the tides between subtidal and intertidal zones.

In the back-barrier subtidal zone, species which cannot endure low tide exposure join species which mainly occur in the intertidal zone. Sponges, tunicates and colonial hydrozoan polyps are mostly confined to subtidal shoals. The most beautiful colonies are formed by polyps of the species Sertularia cupressina. In the past, these were dredged, dried and then stained in bright colours to use for decorative purposes, until substitutes made out of plastic replaced them on the market.

The subtidal fringe and low intertidal zone are the primary sites for beds of suspension feeders, mussels and oysters in particular. Here, the entire volume of tidal waters is filtered every two weeks. Mussels and oysters also stabilise the bottom and accrete fine sediments, accumulate large amounts of shell material, provide attachment for algae and sessile invertebrates, and provide shelter for mobile invertebrates and fish.

The Tidal Flats

Twice a day a spectacle occurs. Land seems to slowly rise from the sea and is then engulfed again by the flooding waters. The bottom of the sea meets the horizon and invites the observer to take a walk on the seafloor. However, the walker has to be cautious. Numerous runnels, some creeks and deep gullies intersecting the tidal flats may block the way. Depending on the wind, the flood may return sooner than expected. Therefore, guided tours are offered to the visitor to explain not only the tides but also to reveal the secrets of hidden life in the marine sediments under our feet.

The sediment surface is almost completely covered with microscopic algae and bacterial colonies. Intertidal sea grass beds may also accumulate fine particles. However, most leaves are shed in autumn and then waves re-suspend the intermittent accretion. The most extensive seagrass meadows occur in the Northern Wadden Sea on approximately 10% of the tidal flat area, and these represent the largest intertidal seagrass beds in Europe.

A large proportion of the tidal flats of the Wadden Sea consist of wave-rippled sands. This habitat is maintained by the constant sediment reworking of lugworms. This is reminiscent of Darwin who described the role of earthworms in the shaping of the English landscape. Similarly, lugworms shape the appearance of the tidal flats and the spatial relationship between mud and sand flats in the Wadden Sea. Their faecal mounds with coiled strings of sand are the most characteristic feature of the tidal flats in the Wadden Sea. The total population may comprise about one billion worms, which is considered to be the largest number worldwide. Lugworms recycle the upper layer of the sediment 10-20 times per year through their guts and prevent clogging of the interstices of sand with organic material. They also irrigate their burrows with water from above and build up an oxic environment in an otherwise anoxic sediment. This increases bacterial activity and the permeable sand functions as an effective filter for the tidal waters

Estuaries

There are five main estuaries in the Wadden Sea region: the Varde A estuary in Denmark and the Eider, Elbe and Weser estuaries in Germany and Dutch-German Ems estuary.

Estuaries are tidally influenced transition zones between marine and riverine environments. Around the globe, estuaries and deltas constitute the main coastal wetlands. The Wadden Sea is different in this regard. Although estuarine habitats are present, they are not a dominant feature and are small in size relative to the marine parts of the Wadden Sea. 

Nevertheless, they are highly relevant to the Wadden Sea ecosystem for various reasons:

1. they supply riverine inputs such as nutrients and toxic substances,
2. they are pathways for diadromous fish such as flounder (Platichthys flesus), smelt (Osmerus eperlanus) and eel (Anguilla anguilla), 
3. they form a specific habitat characterised by a strong variability of salinity, tidal range and turbidity.

From an ecological point of view, they are important for the migration of a number of species and, additionally, they are inhabited by various obligate brackish-water species making them of special importance for conservation purposes. However, compared to the Wadden Sea World Heritage, the estuaries have been strongly altered by human activities and only some parts are protected as nature reserves.

Estuarium Varde Å (foto: John Frikke)

Salt marshes

The Wadden Sea salt marshes form the transition zone between sea and land. They are naturally open grasslands with habitat specific plants of great beauty and diversity.

Salt marshes can be rich in flowers, exhibit a diverse mixture of specialised plants and generalists adapted to disturbed regimes, or they can be completely dominated by one or two grass species.

In general, diversity increases from the lowest part, the pioneer zone, which is flooded during each tide, to the rarely submerged upper salt marsh belt. Ranges of salt marsh plant populations are generally limited in the seaward direction by their ability to withstand marine inundations. In the landward direction they tend to be limited by competition, particularly shading by other plants.

Accordingly, the most specialised salt marsh plants are to be found in the lower zone, while the upper salt marsh also includes widely tolerant generalist plants. which may be common outside salt marshes as well. 

Plants in a salt marsh either adjust to salinity or regulate the salt content in their cells. Some of the salt marsh halophytes are succulents (i.e. Salicornia spp., Suaeda maritima), others are capable of excreting salt through special glands (i.e., Limonium vulgare, Spartina anglica), or salt bladder cells which fill with salt, then die or burst, releasing salt from the plant (i.e., Atriplex spp.). Still others simply seem to accumulate salt in their leaves until they die at the end of the season (i.e., Juncus gerardi). Under conditions of rising sea levels, salt marshes persist as a habitat by accretion if the sedimentation rate is similar to the sea level rise. Accretion is accomplished by inorganicsediments imported during inundations from the seaward tidal flats and by organic matter

 

Beaches and dunes

Beaches and coastal dunes together constitute one habitat system. Sand blown in the landward direction from the dry parts of beaches becomes trapped by various pioneer plants forming embryonic dunes.

In this way, dune heights of 20 m are exceeded. Above that height, wind forces become too strong for marram grass to slow down sand transport, and bare migrant dunes arise. These usually travel from west to east in response to the prevailing wind direction. Migrant dunes may reach the lee side of barrier islands, supplying beaches and tidal flats there with new sand.

Ecologically, beaches and dunes are linked to the other habitats, not only by sand transport but in particular by birds, which rely on beaches and dunes for foraging, nesting and resting habitats. Two red-list bird species prefer to nest on plains of dissipative beaches and among cusps of reflective beaches: Kentish plover (Charadrius alexandrinus) and little tern (Sterna albifrons). Their survival is threatened because they unfortunately prefer the same beaches as nesting sites that are most attractive for recreation. In winter, snow buntings (Plectrophenax nivalis) are common visitors of the upper washlines.

Coastal dunes develop where sand is mobilised at dry beaches and blown landwards. The sand is trapped by plants, which give rise to a succession of dunes from embryonic to white, grey and brown dunes. This dry dune vegetation alternates with wet dune vegetation in the dune valleys (slacks). With a few exceptions (e.g. Eiderstedt peninsula, Skallingen) the dune habitat is confined to the Wadden Sea barrier islands. Dry dune vegetation dominates with 85% over wet dune slack vegetation types.

Photo: embryonic dunes on beach,  (Photo: Martin Stock)