Kelp and seaweed communities on sediment-affected or disturbed Atlantic infralittoral rock

Where Saccorhiza polyschides and other opportunistic kelps occur, scour-tolerant red seaweeds including Corallina officinalis, Kallymenia reniformis, Plocamium cartilagineum, Chondrus crispus, Dilsea carnosa and encrusting coralline algae are often present beneath the kelp. Foliose red seaweeds such as Callophyllis laciniata, Cryptopleura ramosa and Palmaria palmata also occur in this habitat. P. palmata and Delesseria sanguinea often occur as epiphytes on the stipes of L. hyperborea, when it is present. 

Where there are seasonally disturbed unstable boulders and cobbles in very shallow water the brown seaweeds Chorda filum and S.latissima can dominate with Desmarestia aculeata and encrusting coralline algae also typical. Other red seaweeds, which can be found here, include Chondria dasyphylla, Brongniartella byssoides, Polysiphonia elongata, Ceramium nodolosum, Cystoclonium purpureum, Heterosiphonia plumosa, Rhodomela confervoides and Plocamium cartilagineum.The brown seaweeds Punctaria sp. and Cladostephus spongiosus are generally present.

Where there are dense stands of Halidrys siliquosa  it can be mixed with the foliose brown seaweed; Dictyota dichotoma and kelp such as Saccharina latissima and Laminaria hyperborea. Below the canopy is an undergrowth of red seaweeds, that are tolerant of sand-scour such as Phyllophora crispa, Phyllophora pseudoceranoides, Rhodomela confervoides, Corallina officinalis and Chondrus crispus. Other red seaweeds such as Plocamium cartilagineum, Calliblepharis ciliata, Cryptopleura ramosa, Delesseria sanguinea, Heterosiphonia plumosa, Dilsea carnosa, Hypoglossum hypoglossoides and Brongniartella byssoides may be locally abundant, particularly in the summer months. There may be a rich epibiota on H. siliquosa, including the hydroid Aglaophenia pluma, ascidians such as Botryllus schlosseri. There is generally a sparse faunal component colonising the boulders and cobbles, comprising the tube-building polychaete Pomatoceros triqueter, the crab Cancer pagurus, the starfish Asterias rubens, the gastropod Gibbula cineraria and the sea anthozoan Urticina felina. 

The faunal component of this biotope is typically sparse - the starfish Asterias rubens and the crabs Pagurus bernhardus and Necora puber are amongst the most conspicuous animals. The bryozoan crust Electra pilosa colonise many of the algae along with the ascidian Botryllus schlosseri. Occasionally, the polychaete Lanice conchilega may occur in the sand between pebbles, and the anthozoan Urticina felina may be found amongst pockets of gravel along with the gastropod Gibbula cineraria. At some sites the rock beneath the algae can be occupied by the tube-building polychaete Pomatoceros triqueter.