Tidal rapids are strong tidal streams that result from a constriction in the coastline including the entrance to or within the length of an enclosed water body, such as a sea lough & between or around islands.

Key Characteristics

  • Site: Shallower than 5 metres, high energy environments
  • Main Species: Corals, hydroids, bryozoans, sponges, anemones, mussels, brittle stars & algae

Habitat Description

  • Tidal rapids are usually found in water shallower than 5 metres but they may also occur in deeper areas.
  • They often support diverse populations of sessile fauna colonising the substrate that is beyond the abrasive effect of long kelp fronds. The strong tidal streams support diverse marine communities, which are supplied with a constant renewed food source, brought in on each tide. The communities are particularly rich where there is mixing of warm water with colder, nutrient rich current (NIEA, 2003).
  • This habitat includes high energy environments from rocky seabeds to gravel in deep tidal streams & tide-swept areas.

Current Status in UK & Northern Ireland

  • In Northern Ireland tidal rapids occur in Strangford Lough: the Narrows & the Dorn NNR.
  • Tidal rapids occur at the Maidens & in the entrance to Larne Lough. They also occur at the outer coast of Rathlin Island, the entrance to Carlingford Lough, Dundrum channel, Killough harbour mouth, the entrance to Lough Foyle between Greencastle & Magilligan Point & the outer coast of the Skerries (Boaden, pers.comm)
  • Due to the strong tidal streams, human activities such as fishing are impracticable. Therefore, no major disturbances to tidal rapids are thought to occur in NI (Erwin, pers.comm; NIEA, 2003). They are, however, subjected to small, localised impacts including specimen which often involves overturning boulders. This exposes communities under the boulders to desiccation & predation. They are also subjected to pollution, effects of climate change such as a rise in sea levels, warming of water & more frequent extreme events. Introduced species, such as Sargassum muticum,may disrupt tidal rapids. They are also affected by changes in the tidal regime & construction of seawalls, bridges, barrages, causeways & tidal power schemes.

Designated Sites

  • Larne Lough
  • Lough Foyle
  • Rathlin Island

Biological Importance & Priority Species

  • Tidal rapids are often the most species-rich parts of sea lochs.
  • The marine species associated with tidal rapids are diverse, and include red & brown algae, soft corals, hydroids, bryozoans, large sponges, anemones, mussels & brittle stars.
  • In shallow water, bedrock & boulders support kelp & sea oak plants, which also support various animals.

Further Information