Ascophyllum nodosum on full salinity mid eulittoral rock
|Marine Habitat Classification for Britain and Ireland Version 97.06
|Bedrock, or stable boulders and cobbles, in the mid-eulittoral zone of sheltered to very sheltered shores, typically in fully marine or near fully marine conditions, are characterised by a dense canopy of Ascophyllum nodosum. Fucus vesiculosus also occurs and in some places may co-dominate the canopy. Such mixed canopies occur when clearings are formed in the Ascophyllum, since F. vesiculosus is able to colonise such clearings more rapidly. Ascophyllum (which may live for up to 25 years) will, however, eventually out-compete any F. vesiculosus. Such changes in the overlying canopy have little effect on the under-storey species. Beneath the canopy, filamentous and foliose red algae, including Mastocarpus stellatus and Chondrus crispus, and the green alga Cladophora rupestris, occur in moderate to low densities. The Ascophyllum is generally epiphytised by Polysiphonia lanosa (compare with Asc.VS). Very steep and vertical surfaces are often characterised by barnacles and limpets (BPat.Sem), but by small fucoids in areas of extreme shelter. Large numbers of the winkle Littorina obtusata may be present. This biotope usually lies between the Fucus spiralis (Fspi) and Fucus serratus (Fser) zones, although on some shores a narrow zone of Fucus vesiculosus (Fves) may occur immediately above the Ascophyllum. With increasing wave exposure the Ascophyllum canopy is replaced by F. vesiculosus (Fves). Asc.Asc may also occur on moderately exposed shores, where there is localised shelter.
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