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Subtidal chalk
The chalk coasts of the south-east and Yorkshire are very different from the harder rock coastlines of western and northern Britain. The chalk seabed can continue below the low water mark, and the largest areas of these underwater chalk seascapes are most found in Kent and Sussex.
Sea slug on chalk reef -
© Paula Lightfoot
In the shallows less than about 5m deep, there may be little living on the seabed, as the chalk is crumbly and easily washed away, and the water can be very murky. Large seaweeds like kelp do not thrive here, although red seaweeds may survive. The fauna tends to be dominated by burrowing piddock shells, sponges and worms. Edible crabs and velvet swimming crabs may also be found in chalk seascapes.
In deeper water, the chalk environment becomes more important to marine life, particularly where it forms reefs and sea caves, in which rare species of sponge have been found.
Human impacts on these habitats include coastal defence and other works, with major ports and harbour developments having the biggest impact on offshore chalk seascapes. Small-scale fisheries can also cause damage to soft chalk reefs. Pollution has affected beds of larger seaweeds occurring on chalk seabeds, which have been replaced by rapid growth of sea lettuce and mussel beds. Native British species along the English Channel have been displaced by the invasion of non-native plants, including the Japanese seaweed (known as ‘japweed’ or wireweed), which has spread widely since its accidental introduction to the Solent in the 1970s.
Brief description of European distribution
Underwater habitats of chalk are rare in Europe, with those occurring on the southern and eastern coasts of England accounting for the greatest proportion.
Conservation status / need
UKBAP Priority Habitat
Listed in Annex 1 of the Habitats Directive: Reefs
Occurs in marine Special Areas of Conservation, designated for their reef habitats.
Further information sources
Priority habitat descriptions
(PDF 1.5Mb) (UKBAP)
Piddocks with a sparse associated fauna in sublittoral very soft chalk or clay (JNCC)
Interesting fact
Chalk was formed on the bed of a tropical sea that covered most of England during the Cretaceous Period, over 65 million years ago. It is a pure limestone, created from the skeletons of microscopic plants called coccoliths.
