Subtidal Chalk

The geology of chalk cliffs frequently results in subaerial and marine erosion producing vertical cliff faces and extensive wave-cut platforms. The chalk seascape often continues below the low water mark, where subtidal sea caves and reef habitats are formed. Turbidity and exposure determine the species found, but these areas can be rich in sessile species such as sea squirts and burrowing species such as piddocks. Rare species of sponge have been found in sea caves formed in chalk.

Tompot Blenny on chalk reef

Photo: Gerald Legg

Why is it important?

  • Subtidal chalk can have a diversity of animals living on the seabed.
  • Sheltered rocky habitats can provide a refuge for the Native Oyster, itself a priority species
  • Chalk reefs can provide important feeding and nursery areas for a number of crustaceans and fish
  • The chalk reef structures are able to act as a natural coastal defence

Subtidal Chalk in Sussex

The most extensive areas of sublittoral chalk in Britain occur in Kent and Sussex, as such there are many important areas of the coast of Sussex including off Brighton, Eastbourne and Worthing.

What are the threats?

  • Coastal works such as defences can interfere with the natural exchange of material and cause losses to subtidal habitat
  • Pollution and nutrients can change the species composition including producing algal blooms
  • Fishing gear, anchoring, and harvesting of piddocks can damage subtidal reefs
  • Non-native species including Wireweed Sargassum muticum, Polysiphonia harveyi (a red alga) and Wakame Undaria pinnatifida can displace native species

Some associated species

  • Ballan Wrasse Labrus bergylta
  • Leopard-spotted Goby Thorogobius ephippiatus
  • Wrinkle Rock Borer Hiatella arctica
  • Yellow Boring Sponge Cliona celata
  • Oarweed Laminaria digitata

Corkwing Wrasse

Photo: Sally Sharrock/Sussex Wildlife Trust

Links to more information

The Marine Life Information Network