Seaweed communities on full salinity Atlantic littoral rock

Description (English)

This habitat type encompasses littoral rock habitats dominated by seaweeds. The seaweed community varies depending on the wave exposure and tidal currents.
In areas of high energy, the physical stresses caused by wave action often results in dwarf forms of the individual seaweeds. The strong holdfasts and short tufts structure of the wracks Fucus distichus and Fucus spiralis f. nana allow these fucoids to survive on extremely exposed shores in the north and north-west (MA1-231). Another seaweed able to tolerate the wave-wash is the red seaweed Corallina officinalis, which can form a dense turf on the mid to lower shore (MA1-232). The wrack Himanthalia elongata occurs on the lower shore and can extend on to moderately exposed shores (MA1-233). The red seaweed Mastocarpus stellatus is common on both exposed and moderately exposed shores, where it may form a dense turf (particularly on vertical or overhanging rock faces, MA1-235). Very exposed to moderately exposed lower eulittoral rock can support a pure stand of the red seaweed Palmaria palmata. It is found either as a dense band or in large patches above the main sublittoral fringe (MA1-234). Exposed to moderately exposed lower eulittoral rock characterised by extensive areas or a distinct band of Osmundea pinnatifida (MA1-236). Outcrops of fossilised peat in the eulittoral are soft enough to allow a variety of piddocks, such as Barnea candida and Petricola pholadiformis, to bore into them (MA1-237). This biotope is rare. Other species such as the anemone Halichondria panicea, the barnacle Semibalanus balanoides, the limpet Patella vulgata, the mussel Mytilus edulis and the whelk Nucella lapillus can be present as well, but they are never dominant as in MA1-22. There is also a higher number of seaweeds present including the red Palmaria palmata, Lomentaria articulata, Ceramium spp. and the brown seaweeds Laminaria digitata and Fucus serratus. The green seaweeds Enteromorpha intestinalis, Ulva lactuca and Cladophora rupestris are occasionally present.
In tide-swept conditions on sheltered to extremely sheltered mid eulittoral to lower eulittoral rocky shores, such as narrow channels in sea lochs, the middle shore can be dominated by the wrack Ascophyllum nodosum (MA1-238), while Fucus serratus is dominating the lower shore (MA1-239, MA1-23A). The high levels of water movement encourages a rich associated fauna including several filter-feeding groups. These include the sponges Grantia compressa, Halichondria panicea and Hymeniacidon perleve which frequently occur on steep and overhanging faces of boulders and bedrock. It also includes the sea squirts Dendrodoa grossularia and Ascidiella scabra, which occur on steep surfaces and beneath boulders. Hydroids such as the pink Clava multicornis can form colonies on A. nodosum while Dynamena pumila is more often found on Fucus vesiculosus or F. serratus. Underneath the canopy formed by the brown seaweeds is a diverse community of the red seaweeds Gelidium pusillum, Chondrus crispus, Lomentaria articulata, Membranoptera alata and coralline crusts, but the green seaweeds Enteromorpha intestinalis, Ulva lactuca and Cladophora rupestris can be present. The filamentous red seaweed Polysiphonia lanosa can usually be found growing on A. nodosum. On the rock beneath are the limpet Patella vulgata and the barnacle Semibalanus balanoides, while the crab Carcinus maenas and a variety of winkles including Littorina littorea, Littorina mariae and Littorina obtusata can be found on or among the boulders. The whelk Nucella lapillus can either be found in cracks and crevices.
Situation: Sheltered tide-swept shores (i.e. estuaries and sea lochs) below the Fucus spiralis and F. vesiculosus band and above the kelp dominated zone in the sublittoral.
Dense blankets of fucoid seaweeds dominate in sheltered to extremely sheltered rocky shores and/or in locally sheltered patches on exposed to moderately exposed rocky shores. Typically, the wrack Pelvetia canaliculata (MA1-23B) occurs on the upper shore, with the wrack Fucus spiralis (MA1-23C) below. The middle shore is dominated by vast areas of the wrack Ascophyllum nodosum or the wrack Fucus vesiculosus (MA1-23D, MA1-23E) or a mixture of both. The wrack Fucus serratus covers lower shore bedrock and boulders (MA1-23F). Sheltered to very sheltered mixed substrata (pebbles and cobbles overlying muddy sand and gravel) shores can support fucoid communities (MA1-23C2; MA1-23D2; MA1-23E2; MA1-23F2).
Ephemeral seaweeds can occur on disturbed littoral rock in the lower to upper shore. Dominant green seaweeds include Enteromorpha intestinalis, Ulva lactuca and the red seaweeds Rhodothamniella floridula and Porphyra purpurea. Winkles such as Littorina littorea and Littorina saxatilis, the limpet Patella vulgata and the barnacles Semibalanus balanoides can occur, though usually in low abundance. The crab Carcinus maenas can be found where boulders are present, while the barnacle Elminius modestus is usually present on sites subject to variable salinity. On moderately exposed shores, the biotope is Enteromorpha spp. on freshwater-influenced or unstable upper shore rock (MA1-23G) or P. purpurea and/or Enteromorpha spp. on sand-scoured mid to lower eulittoral rock (MA1-23H). These are biotopes with a low species diversity and the relatively high number of species in the characterising species list are due to a variation in the species composition from site to site, not to high species richness on individual sites.

Source: EUNIS habitat classification