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.

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  Pollution has affected beds of larger seaweeds occurring on chalk seabeds, which have been replaced by rapid growth of sea lettuce and mussel beds.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.

For the official habitat definition please see the documents listed below.


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

Subtidal chalk fact


Official definition

UK Biodiversity Action Plan; Priority Habitat Descriptions. BRIG (ed. Ant Maddock) 2008 (updated December 2011) 


Further information

JNCC biotope classification - Piddocks with sparse associated fauna in sublittoral very soft chalk or clay

JNCC - UK BAP Priority Species and Habitats

JNCC EUNIS habitat correlations table