Description of [Laminaria saccharina] with foliose red seaweeds and ascidians on sheltered tide-swept infralittoral rock
Sheltered, tide-swept rock in south-western Britain tends to be restricted to estuarine conditions, where variable salinity and increased turbidity have a significant effect on the biota. Due to the turbidity of the water, the infralittoral zone is restricted to very shallow depths. Unlike the tide-swept channels in sealochs, which support a mixed kelp canopy, the rock in these estuaries is characterised by Laminaria saccharina alone, occurring in relatively low abundance (Frequent). The brown alga Desmarestia ligulata can occur in this biotope, though never dense, along with the non-native brown seaweed Sargassum muticum. Beneath the sparse kelp, cobbles and boulders, often surrounded by sediment, are encrusted by fauna and often a dense turf of red seaweed. The foliose red seaweeds associated with this biotope include Callophyllis laciniata, Nitophyllum punctatum, Kallymenia reniformis, Gracilaria gracilis, Gymnogongrus crenulatus, Hypoglossum hypoglossoides, Rhodophyllis divaricata, Chylocladia verticillata, Cryptopleura ramosa and Erythroglossum laciniatum as well as the filamentous Ceramium nodulosum and Pterothamnion plumula. Green seaweeds Ulva lactuca, Bryopsis plumosa and Cladophora spp. may be locally abundant. The dominating faunal species vary from site to site but include sponges such as Halichondria panicea, Esperiopsis fucorum, Dysidea fragilis and Hymeniacidon perleve as well as ascidians, particularly Dendrodoa grossularia and Morchellium argus, which can cover the rocks. Also present is the anthozoan Anemonia viridis, the barnacle Balanus crenatus and the tube-building polychaete Pomatoceros triqueter. The hydroid Plumularia setacea can cover rocks and seaweed fronds Of the range of solitary ascidians found in the north-west, only Ascidiella aspersa tends also to be present in these south-western inlets. There is also a general absence of echinoderms. Where there is vertical rock present, it tends to support more fauna, including barnacles Balanus crenatus, the ascidians Clavelina lepadiformis and Botryllus schlosseri and sometines the featherstar Antedon bifida. Where soft rock allows, such as the limestone in Plymouth Sound, rock-boring organisms such as Polydora sp. may be locally abundant. Sheltered, tide-swept rock is generally restricted to the narrows or tidal rapids of marine inlets. The clear tide-swept waters of Scottish sealochs are significantly different to the marine inlets of south-west Britain. This biotope deals with the latter. Situation: This biotope generally occurs on rocky outcrops interspersed by sediment areas. Where the rock extends into deeper water, beyond the limit of kelp, sponges and ascidians tend to dominate these sheltered, tide-swept circalittoral sites (A4.251); also Alcyonium digitatum with sponges and Nemertesia antennina (A4.131).