Description of Foliose red seaweeds with dense [Dictyota dichotoma] and/or [Dictyopteris membranacea] on exposed lower infralittoral rock
A dense turf of foliose red seaweeds mixed with a dense turf of the foliose brown seaweeds Dictyota dichotoma and/or Dictyopteris membranacea on exposed and moderately exposed lower infralittoral rock, generally at or below the lower limit of the kelp zone. In some areas the lower infralittoral is subject to a moderate amount of scour from nearby sand. D. dichotoma is relatively tolerant of such scour and in such areas a zone forms with other sand-tolerant seaweeds. D. membranacea is confined to south-western coasts. Typically brown seaweeds dominate the seabed or are at least in equal abundance to the red seaweeds, some of which may also form dense stands such as Plocamium cartilagineum, Calliblepharis ciliata, Cryptopleura ramosa, Bonnemaisonia asparagoides, Heterosiphonia plumosa, Delesseria sanguinea and Brongniartella byssoides. The urchin Echinus esculentus can be found grazing the rock surface which can be covered in coralline algae. The anthozoans Caryophyllia smithii and Alcyonium digitatum are usually present in this biotope along with the tube-building worm Pomatoceros sp. which is more common in sand-scoured areas. The starfish Asterias rubens and Henricia sp. and sponge crusts including Cliona celata can also be found here. D. dichotoma also occurs in the kelp park, and records should only be assigned to this biotope where kelp such as Laminaria hyperborea is sparse or absent and a relatively high density of D. dichotoma and/or D. membranacea is present. Situation: This biotope usually occurs at or below the lower limit of kelp L. hyperborea (units A3.1152 or A3.214). In south-west England a zone of mixed kelp forest L. hyperborea and Laminaria ochroleuca may occur above the dense foliose algae (unit A3.1153). Unit A3.1161 marks the lower limit of the lower infralittoral zone. Temporal variation: Like many of the red seaweeds found in this biotope the dominant brown seaweeds D. membranacea and D. dichotoma have annual fronds which tend to die back in the autumn and regenerate again in the spring. This produces a seasonal change in the density of the seaweed cover, which is substantially reduced over winter months and reaches its most dense between April and September.