Sublittoral hard-substratum communities off Orkney and St Abbs (Scotland)

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Abstract
During the summer of 1990 the sublittoral communities on hard substrata were investigated at 32 localities off Orkney and St Abbs (Scotland). The environment in both places has an open coast character, with small seasonal fluctuations in temperature and salinity and a high transparency of the water. The major governing environmental factors are the available amount of daylight and the degree of exposure to water movement. For the photic zone two different communities, dominated by foliose algae, are described. Under exposed conditions a widespread community is found, dominated by red algae in the middle structural layer and characterized by a dense canopy of Laminaria hyperborea. Under more sheltered conditions a community occurs in which brown and green algae become more abundant in the middle structural layer, while the top layer is formed by Laminaria saccharina and Saccorhiza polyschides. In the aphotic zone, under exposed conditions, a community is found which consists of a number of variants, with a dominance of the suspension feeders Pomatoceros triqueter and Alcyonium digitatum or bryozoans and hydrozoans. These variants are found in different regions, depths and angles of inclination of the substrata. Comparable variants have also been found off Helgoland (German Bight). Under sheltered conditions the suspension feeders become of minor importance and the community is dominated by encrusting red algae. In a surge tunnel in the photic zone off St Abbs Head, a community dominated by Dendrodoa grossularia and Clathrina coriacea is present.