Wadden area: endangered ecotopes

Many ecotopes in the Wadden area are endangered.

Ecotopes in the Wadden area and their threat status

Ecotope	Area	Quality	Potential for recovery1)
Migrating sand banks	not endangered	not endangered
Complete tidal channel system	not endangered	endangered
Complete tidal flats system	not endangered	greatly endangered	difficult
Tidal watershed	not endangered	endangered	under certain conditions
Saltmars - natural saltmarsh - island saltmarsh - land reclamation saltmarsh - vegetated beach	critical not endangered endangered greatly endangered	critical endangered endangered greatly endangered	difficult under certain conditions under certain conditions under certain conditions
Migrating island	critical	critical	under certain conditions
Transitional zone between: - marsh and saltmarsh - Pleistocene and tidal area - dunes and tidal area	disappeared disappeared critical	disappeared disappeared critical	difficult difficult difficult
Sheltered bay	critical	greatly endangered	under certain conditions
Estuaries	disappeared	critical	difficult
Flood-prone estuarine areas	disappeared	disappeared	under certain conditions
Biogenic structures: - mussel beds - oyster beds - reefs of Polychaeta tubes - forests of Lanice conchilega tubes - eelgrass beds exposed at low tide - sublittoral eelgrass beds - high densities of shellfish	critical disappeared disappeared not endangered greatly endangered disappeared endangered	critical disappeared disappeared not endangered endangered disappeared endangered	difficult difficult difficult very difficult
Hard substrate (natural)	greatly endangered	greatly endangered	under certain conditions
1) very difficult = > 150 years difficult = 15 - 150 years
Source: Alterra			Alterra/NC/Oct02

Trends

The Wadden area is one of the most pristine areas in the Netherlands. In the tidal area the variation in duration of submergence, and in flow and salinity has created a great variety of ecotopes, many of which have shrunk in area or have disappeared. Furthermore, the quality of certain ecotopes is threatened. Though some ecotopes could be restored, others would be difficult to rescue. The most important reasons for the changes are the harvesting of shellfish, water pollution, sand abstraction, sand supplementation, coastal defences and dredging.

Description of Wadden area ecotopes

Migrating sandbanks like Het Rif are unvegetated "islands". A complete tidal channel system consists of a tidal inlet, ebb and flood channels, main channels and side channels, some of which have a dead end in sand flats or mudflats. A complete tidal flat system contains a diversity of flats. In tidal inlets these flats consist of coarse sand, but along the coast of the mainland or at tidal watersheds the flats are mudflats. Some tidal flats are exposed for only a short time, others are submerged only in storms. If the flood tide flows inland on either side of an island, where the two flood currents meet a tidal watershed is created.

Saltmarshes form naturally because vegetation establishes and traps silt from the floodwater. The lower-lying areas of saltmarsh are submerged at high water, i.e. twice daily, whereas the higher-lying areas are submerged only at spring tide or during storms.

Island saltmarshes have formed on the Wadden islands as a result of the construction of barriers to trap drifting sand. Along the coast of Friesland and Groningen the creation of saltmarsh has been greatly stimulated by constructing barriers and willow fences along the dikes; these are land reclamation saltmarshes.

Vegetated beaches are the higher-lying vegetated parts of the beach where dunes do not form. Migrating islands are islands that are vegetated but unstabilised and therefore generally uninhabited.

Other ecotopes are the transitions between saltmarsh and marsh, between the higher-lying sandy soils and the tidal area, and the sharp transitions between dune areas and tidal area, without any intervening saltmarsh.

Where rivers discharge into the sea, sheltered bays and estuaries where there is a gradual transition from saline to fresh water may occur. During storms, large areas along the river become inundated by seawater, resulting in the formation of characteristic flood-prone estuarine areas.

There is a separate class for structures formed by organisms (biogenic structures). These include mussel beds, oyster beds, the tubes of Polychaetae, and eelgrass beds. Natural hard substrates are formed by pebbles and shingle that occur naturally.