Description of (Fucus distichus) and (Fucus spiralis) f. (nana) on extremely exposed upper eulittoral rock
Extremely exposed gently or steeply sloping upper shore bedrock which supports a mixture of the wracks Fucus distichus and Fucus spiralis f. nana, the latter often at the top of the zone. On some sites F. distichus dominates and F. spiralis is not present. Other seaweeds normally found on exposed coasts are common in this biotope. These include ephemeral species such as the foliose red Porphyra umbilicalis and the green Enteromorpha spp. The winkles Melarhaphe neritoides and Littorina saxatilis can be found grazing on the bedrock or on the fucoids, while red crusts of Hildenbrandia rubra and the mussel Mytilus edulis are restricted to moist cracks and crevices. A sparse covering of the black lichens Verrucaria maura and Verrucaria mucosa can be found in the upper part of this biotope competing for space with barnacle Semibalanus balanoides and the limpet Patella vulgata. This biotope is very rare and restricted to the far north and west coasts. Situation: This mixed band of F. distichus and F. spiralis f. nana is generally found between the Verrucaria maura and Porphyra spp. zone (B3.1131 or B3.1132) above, and the M. edulis and barnacle zone below (A1.111). It may also occur above a red algal zone consisting of Mastocarpus stellatus as recorded on Barra (Scotland). Temporal variation: Due to the occurrence of this biotope on very exposed coasts a certain amount of fluctuation between this biotope and unit B3.1131 from year to year is to be expected. More information is required to validate this hypothesis. There can be seasonal fluctuations in the density of ephemeral green algae.