Ascophyllum nodosum on full salinity mid eulittoral rock

Description (English)

Bedrock, stable boulders and cobbles in the mid-eulittoral zone of moderately exposed to extremely sheltered shores, in fully marine conditions, characterised by a dense canopy of the wrack Ascophyllum nodosum. Another wrack Fucus vesiculosus may in some places co-dominate the canopy. The hydroid Dynamena pumila can form colonies on the wracks F. vesiculosus and Fucus serratus. Variations in the ratio of A. nodosum and F. vesiculosus in the overlying canopy have little effect on the under-storey species. Beneath the canopy are a diverse array of filamentous and foliose red seaweeds, including Mastocarpus stellatus, Chondrus crispus, Gelidium pusillum and coralline crusts. The filamentous red seaweed Polysiphonia lanosa is usually present on A. nodosum as an epiphyte. A few green seaweeds including Cladophora rupestris and Enteromorpha spp. are also present in moderate to low densities. On the bedrock and boulders beneath the seaweed canopy is a fauna including the barnacle Semibalanus balanoides, the limpet Patella vulgata, tube-forming spirorbid polychaetes and the anemone Actinia equina. The latter can be present in damp cracks and crevices. On and among the seaweeds are mobile species including the winkles Littorina littorea and Littorina obtusata, the whelk Nucella lapillus or even the crab Carcinus maenas. At the top of the A. nodosum zone there might be the occasional presence of the olive green lichen Verrucaria mucosa.
Situation: This biotope is usually found between the wrack Fucus spiralis (unit A1.312) and F. serratus dominated biotopes (A1 315), although on some shores a narrow zone of F. vesiculosus (A1.313) may occur immediately above the A. nodosum. With increasing wave exposure the A. nodosum canopy is replaced by F. vesiculosus (A1.213; A1.313). This unit can occur on more exposed shores, where there is localised shelter.
Temporal variation: A. nodosum can reach an age of 25 years on sheltered shores and the communities are, once established, usually very stable. F. vesiculosus or F. serratus can occur in patches where the A. nodosum has been removed.

Source: EUNIS habitat classification