THE WADDEN SEA
A detailed description of the status of
the Wadden Sea can be found in the Quality
Status Report 1999.
The Wadden Sea is a shallow sea extending along the North Sea
coasts of The Netherlands, Germany and Denmark (see map).
It is a highly dynamic ecosystem with tidal channels, sands,
mud flats, salt marshes, beaches, dunes, river mouths and a transition
zone to the North Sea, the offshore zone.
The area of the trilateral cooperation of The Netherlands, Germany
and Denmark, the so-called Cooperation Area is 13,500 km2 large.
The transition zone to the North Sea covers about 4000 km2 ,
the islands about 1,000 km2, the tidal area some 7,500 km2 ,
the salt marshes and summer polders some 350 km2. The four estuaries,
the Varde Å, the Elbe, the Weser and the Ems have a total
surface area of 260 km2. Also some areas on the mainland, which
are important for birds, are part of the cooperation area and
cover about 250 km2.
Most parts of the Wadden Sea, in particular in The Netherlands
and Lower Saxony, are sheltered by barrier islands and contain
smaller or wider areas of intertidal flats. Between the Weser
estuary and the island Amrum, the area is relatively broad and
open to the North Sea. Because of embankments only four large
sheltered bays have remained in the total area; the Ho Bugt in
Denmark, the Jadebusen and the Leybucht in Lower Saxony and the
Dollard in the Dutch-German border area.
Twenty three islands with sand dunes, as well as fourteen high
sands without dunes form a barrier to the North Sea.
The present form of the Wadden Sea is the result of both natural
forces and action by man.
Of greatest influence on the shape and functioning are the daily
tides. Twice a day, on average, 15 km3 of sea water enter the
Wadden Sea. This doubles the volume to some 30 km3. With the
water from the North Sea, large amounts of sand and silt are
imported which settle in places with little water movement. During
low tides large parts of the Wadden Sea emerge. These so-called
tidal flats cover about two-thirds of the tidal area and are
one of its most characteristic features. Nowhere in the world
can such a large unbroken stretch of tidal flats be found. They
account for 60 percent of all tidal areas in Europe and North
Africa.
Since the middle ages Man has changed the Wadden Sea landscape:
dikes were built and land reclaimed.
The natural 'wandering' of the islands as a result of accretion
and erosion has been considerably reduced during the last century
through the construction of dikes and groynes and through beach
nourishment.
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The
biological importance of the Wadden Sea
The importance of
the Wadden Sea as habitat for birds, seals, shellfish and fish
species stems from the high growth rate of algae, the so-called
primary production. Two factors are essential for the high primary
production. Because the water is shallow, there is sufficient
light for algae to grow. Secondly the water of the Wadden Sea
contains many nutrients which are also essential for algal growth.
In addition algae are imported from the North Sea. These account
for almost half of the food resources in the Wadden Sea.
The Wadden Sea ecosystem is very dynamic with regular and unexpected
changes from one extreme situation to another. Factors such as
temperature with the possibility of ice, salinity, storms, waves
and currents vary greatly. Only species, which have adapted to
these extreme conditions can exist here. That is why the Wadden
Sea species, and consequently the ecosystem itself, have a large
potential for survival.
The Wadden Sea provides a multitude of transitional zones to
the land, the sea and freshwater environment, which is the basis
for an exceptional species richness. This includes 2,000 species
of spiders, insects and other invertebrates in the salt marshes
and 1,800 in the marine and brackish areas. Among these organisms,
there is a high degree of ecological specialization.
On the tidal flats, to the contrary, only a few species of flora
and fauna have adopted to the extreme environment. Of these however
exceptionally high numbers can be found. The high biological
productivity of the tidal flats is comparable with that of tropical
rain forests.
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Birds
The Wadden Sea is vital for about 50 bird species originating
from a large part of the northern hemisphere. Among these are
many rare and threatened species. The area is of international
importance for at least 52 geographically distinct populations
of 41 species. In about 20 populations more than half of the
individuals utilize the Wadden Sea at some stage of their annual
life cycle. For about 10 species almost the entire populations
occurs in the Wadden Sea.
Every year an average of 10 to 12 million birds pass through
this area on their migration route from the breeding grounds
in Siberia, Iceland Greenland and Northeast Canada to their wintering
grounds in Europe and Africa. They feed on the tidal flats, which
are the most nutritious areas of the Wadden Sea.
For more than 30 species of birds, the Wadden Sea is an indispensable
reproduction area. For some endangered species, like little tern
(Sterna albifrons) and kentish plover (Charadrius alexandrinus),
the Wadden Sea has special significance. The most important breeding
areas are the salt marshes, and, to a lesser extent, the dunes
and beach plains of the islands. Typical Wadden Sea birds are
redshank (Tringa totanus), black-tailed godwit (Limosa
limosa), oystercatcher (Haematopus ostralegus), ringed
plover (Charadrius hiaticula), avocet (Recurvirostra
avosetta) and a number of species of ducks, geese, gulls
and terns.
Recent report on the status of birds are
published in the "Wadden Sea Ecosystem" (see list
of publications)
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Seals
The common seal is the most numerous native marine mammal species
in the Wadden Sea. To date some 10,000 individuals of seals live
in the entire Wadden Sea. This figure is based on aerial surveys.
At the beginning of the aerial counts in 1960 there were only
5,000 individuals. In the period 1960 to 1974, the seal population
further decreased to a minimum of 3,600 animals. Since then,
probably supported by the ban of seal hunting in the 60s and
70s, the number of seals stabilized or increased slightly with
the exception of the Dutch seal stock. The total Wadden Sea seal
population increased in size until 1988. In that year a virus
epidemic reduced the population by an estimated 60 percent from
10,000 to 4,000 animals. The population has recovered very well
since (see also Quality
Status Report 1999 and recent seal
counts)
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