The Cleaver Bank is a Habitat Directive site or SAC in the category of ‘Open-sea reefs’. It is a marine site of approx. 1,235 km² that lies some 160 km to the north-west of Den Helder.
The Cleaver Bank is the only site in the Dutch North Sea where considerable quantities of gravel lie on the surface and larger cobbles with a specific covering of calcareous red algae also occur. The site came into being as the terminal moraine of a glacier dating from the last Ice Age. The Cleaver Bank is split in two from north-west to south-east by a 60-metre deep channel called the Botney Cut. Here, in particular, many harbour porpoises, as well as minke whales and white-beaked dolphins, can be found in summer.
Owing to the great depth of the Cleaver Bank (30-50 m), the bottom is disturbed by wave action only in very heavy weather. The gravel is relatively poor in silt and the water is so clear that sufficient light penetrates even to a depth of 40 m to enable the growth of crustose calcareous red algae. Calcareous red algae are primarily marine algae that are characterised by their red or reddish purple colour. Their cell walls can be high in calcium. Owing to this, they often form a large part of coral reefs. Some 400 genera are known, among a total of 4,000 species, most of which live in the sea.
Owing to the variety of marine bottoms, there is a great diversity of benthos species at the site. Benthos is the collective name for all organisms that live in, on, and just above the seabed. These are crustaceans, bristleworms, molluscs and echinoderms, as well as thread algae, diatoms and bacteria. The word benthos comes from the Greek language and means depth (of the sea). Benthos is usually differentiated by the size of the organisms (macro-, meio- and microbenthos), or by the environment in which these live (endo-, epi- and hyperbenthos – respectively in, on, and above the bottom). The biodiversity of the macrobenthos on the Cleaver Bank is among the highest in the EEZ: 44% of the species here occur exclusively on the Cleaver Bank.