Kelp and seaweed communities on Atlantic infralittoral rock

Description (English)

Rocky habitats in the infralittoral zone dominated by kelp and seaweeds.
In areas exposed to extremely exposed wave action or strong tidal streams, typically the rock supports a community of kelp Laminaria hyperborea with foliose seaweeds and animals, the latter tending to become more prominent in areas of strongest water movement (MB1-213, MB1-215 and MB1-2152). The depth to which the kelp extends varies according to water clarity, exceptionally (e.g. St Kilda) reaching 45 m. In some areas, there may be a band of dense foliose seaweeds (reds or browns) below the main kelp zone (MB1-221). The sublitttoral fringe is characterised by dabberlocks Alaria esculenta (MB1-211). In very strong wave action the sublittoral fringe A. esculenta zone extends to 5 to 10 m depth, whilst at Rockall A. esculenta replaces L. hyperborea as the dominant kelp in the infralittoral zone (MB1-212).
Situation: Very exposed rocky coasts, from low water to depths up to 45m.
Temporal variation: Winter storms may remove patches of kelp, and fast-growing annuals may form a temporary forest (MB1-232).
On bedrock and stable boulders subject to moderate wave exposure, or moderately strong tidal streams there is typically a narrow band of kelp Laminaria digitata in the sublittoral fringe which lies above a Laminaria hyperborea forest and park. Associated with the kelp are communities of seaweeds, predominantly reds and including a greater variety of more delicate filamentous types than found on more exposed coasts (A3.11). The faunal component of the understorey is also less prominant than in A3.11.
On sheltered infralittoral rock exposed to strong tidal streams in the sublittoral fringe, dense Laminaria digitata is found together with erect seaweeds, sponges, ascidians and bryozoans (MB1-21E). Below this, on bedrock and stable boulders a canopy of mixed kelp (primarily Laminaria hyperborea and Laminaria saccharina) occurs with foliose red seaweeds, sponges and ascidians (MB1-21F). This biotope is typically found in the sheltered narrows and sills of Scottish sealochs. Mixed substrata of boulders, cobbles, pebbles and gravel, that also occurs in the tidal rapids of Scottish sealochs, supports a reduced kelp canopy (L. hyperborea and L. saccharina ; typically Frequent), with a rich red seaweed component and maerl at some sites (MB1-21G). In south-west Britain, sheltered, tide-swept rock is restricted to estuarine conditions where variable salinity and increased turbidity of the water have a significant effect on the biota, limiting the infralittoral zone to very shallow depths. Unlike the tide-swept channels in sealochs, the rock in these estuaries is characterised by a relatively low abundance of L. saccharina (< Common) with foliose red seaweeds, sponges and ascidians (A3.224). L. hyperborea is rarely present.

Source: EUNIS habitat classification