MBDP-NI: Priority habitat

Tide-swept Channels

Tide-swept Channels

Correspondence with existing habitats

Habitats Directive Annex I: Reefs and large shallow inlets and bays

Description: In this habitat action plan, the term ‘tidal rapids’ is used to cover a broad range of high energy environments including deep tidal streams and tide-swept habitats. Wherever they occur, strong tidal streams result in characteristic marine communities rich in diversity, nourished by a constantly renewed food source brought in on each tide. The marine life associated with these habitats is abundant in animals that are fixed on or in the seabed, and typically include soft corals, hydroids (sea firs), bryozoans (sea mats), large sponges, anemones, mussels and brittlestars in dense beds. In shallow water, bedrock and boulders often support kelp and sea oak plants, which grow very long in the tidal currents, and have a variety of animals growing on them. Other smaller red and brown seaweeds grow on cobbles and pebbles, many of these being characteristic of tide-swept situations. The Dorn in Strangford Lough, the tidal rapids at Ballydorn and also the rapids near Sketrick Island are noted for their diversity of flora and fauna and for displaying a marked emergence phenomenon.

Coarse gravel is a more difficult habitat for animals to colonise, as it is constantly moving, yet even here there are typical animals, such as sea cucumbers, worms and burrowing anemones. Maerl beds are also closely identified with the conditions found in tidal narrows, examples of this can be found near Portaferry and also at Sketrick in Stranford Lough. This habitat also is found in deeper water, such as between islands.

Tidal streams in the North Channel are accelerated around Rathlin Island (e.g. tide-swept boulders alongside the Lochgarry) and The Maidens (e.g. North of West Maiden) and provide conditions for a distinctive community, unrecorded elsewhere in the UK or Ireland. This community is characterised by long-lived, deepwater hydroids such as Diphasia alata, Diphasia nigra, Diphasia margareta, Polyplumaria flabellata, Lytocarpia myriophyllum and the sponges Axinella dissimilis, Axinella infundibuliformis, Clathria barleei, Spongionella pulchella, Lissodendoryx jenjonesae and Hymedesmia spp.

Strangford Lough also has a long rapids system with very strong tidal streams up to 8 knots.

Associated biotopes:

Priority species: Diphasia alata, Diphasia nigra, Polyplumaria flabellata, Lytocarpia myriophyllum, Axinella dissimilis, Clathria barleei, Spongionella pulchella, Lissodendoryx jenjonesae, Hymedesmia rathlinia

Threats:

Obstruction to the water flow (e.g. ferries running the entrance to sea lochs, bridges, causeways).
Tidal power generation (in conjunction with bridge construction) change the ecology of the lochs considerably through restriction of seawater influence and consequent changes in salinity. The effects on the connecting rapids can also be expected to be drastic.
Fishing: rapids often have dense beds of animals, for example mussels, which may become attractive for exploitation in the future. Rapids can be a sanctuary for crustaceans because strong tidal currents make creeling difficult.
Water pollution: Although the currents in rapids may quickly disperse one-off sources of pollution, chronic continuing pollution could affect sensitive marine life.
Non-native species: Non-native species can have a significant impact on this habitat replacing native species. An example of this is the spread of the non-native seaweed Sargassum muticum competes with native species such as the sea oak Halidrys siliquosa. The carpet ascidian has recently gained a strong hold near Paddy’s Point in Strangford lough and also around Sketrick Isalnd and the tidal rapids there. The carpet ascidian Didemnum vexillum overgrows and smothers other marine life.

References:

This habitat description has been adapted from the 1994 UK BAP Action Plan for Sabellaria spinulosa.

Marine Biodiversity Data Portal - NI

Tide-swept Channels