The seabed of the Celtic Seas ecoregion is primarily comprised of sediments with extensive areas of mixed sediments:
- coarse and sandy to muddy areas on the Malin shelf
- coarse and mixed sediments with some muddy patches in the Irish Sea
- coarse, rocky, and sandy to muddy sands in the Celtic Sea
Areas of rock and hard substratum are present in the northern and
inshore parts of the ecoregion.
Threatened and declining habitats in the Celtic Seas according to OSPAR:
Carbonate mounds
Coral gardens
Deep-sea sponge aggregations
Intertidal Mytilus edulis beds on mixed and sandy sediments
Intertidal mudflats
Lophelia pertusa reefs
Maerl beds
Modiolus modiolus beds
Oceanic ridges with hydrothermal vents/fields
Ostrea edulis beds
Sabellaria spinulosa reefs
Seamounts
Sea-pen and burrowing megafauna communities
Zostera beds