Habitat type 1170 on the Cleaver Bank
The habitat type ‘Open-sea reefs’ is taken to mean reefs consisting of rocks and boulders as well as structures formed by living organisms that stick up out of the sediment on the seabed. Oyster banks, mussel banks and peatbeds are all examples of these biogenic structures. Their accretion can be similar in some respects to the accretion on cobbles and gravel. No natural rocky bottoms occur in the Netherlands, with the possible exception of the Borkum Reef Ground (where research is still underway). The hard substrata at sea formed by dikes, shipwrecks, oil and gas installations, windmills and the like, are not part of this habitat type.
Although, strictly speaking, reefs in the Netherlands have no plant communities, there are occurrences of various species of algae, including red, brown and green algae. On the Cleaver Bank some of the hard bottom consists of crustose calcareous red algae. To distinguish it from reefs of biogenic origin, the name ‘Open-sea reefs’ is used to refer to the reef site of the Cleaver Bank. Before long, the Borkum Reef Ground may be considered to be another example of this habitat type, if additional research provides sufficient grounds for this idea.
A minimum requirement for reefs not created by living organisms, such as the Cleaver Bank, is that they consist of rocks, boulders or cobbles ‘generally greater than 64 mm’. If they support a zonation of benthic communities of algae and animal species, their quality is good; without these communities, their quality is moderate.
Species that cling to a hard subsurface – such as mussels or oysters – appear to show the same behaviour on gravel and cobbles measuring 8 mm to 64 mm. Small cobbles, gravel and shells of this size are only included in this habitat type if organisms are actually clinging to them. Thus, the situations in question are those in which the biotic community extends from the cobbles to surrounding smaller cobbles, coarse gravel and shells.
In the European context, ‘Open-sea reefs’ is one of the most important habitat types. Compared with the extensive reefs of gravel and cobble in other parts of the North Sea, the Dutch reef is not greatly significant in terms of its size. On the other hand, the Cleaver Bank is the Netherlands’ only example of this habitat type and it has a very particular biodiversity.
Habitat type 1170 occurs in the Netherlands at the Natura 2000 site of Cleaver Bank, which has been notified to the EU. At present, no profile document has been finalised for this habitat type. However, some building blocks for compiling this profile document are available in the report entitled, Conservation Objectives for Natura 2000 sites (SACs and SPAs) in the Dutch sector of the North Sea (pdf: 177 pages).
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