Marine Biodiversity Data Portal - NI

Correspondence with existing habitats

Part of 1994 BAP habitat: Littoral and sublittoral chalk

No OSPAR equivalent

Habitats Directive Annex I: Reefs

Description: The chalk formations of Northern Ireland belong to the Ulster White Limestone Formation from the Upper Cretaceous period, with exposures on the County Antrim coast. It differs to the chalk typically exposed on the English coast in that it is extremely hard and of low porosity. The Northern Ireland chalk forms extremely hard, low porosity deposits with subsequent erosion forming cobble and boulder spreads with subtidal reefs. At Rathlin Island there are spectacular, deep subtidal cliffs affected by strong tidal currents and extensive underwater exposures of chalk, and sublittoral caves are known to be present in chalk down to at least 75 m depth. These caves support rich populations of rare species.

The sublittoral chalk community/species assemblage on Rathlin Island is not represented by any existing biotope, the assemblage is characterised by the sponges Spongosorites calcicola, Dercitus bucklandi, Stelletta grubii, Stryphnus ponderosus, Desmacella cf. annexa, the cnidarians Caryophyllia inornata, Alcyonium hibernicum, Parazoanthus axinellae, Parazoanthus anguicomus, Edwardsiella carnea and the ascidians Didemnum spp.

At Garron Point the sublittoral chalk in shallow subtidal 5-10 m has the piddock Pholadidea loscombiana. At Larrybane Head, near Ballintoy, there are gullies cut into limestone rock with Hiatella arctica. Good examples of subtidal sites in Northern Ireland are the Ruecallan Archway and west of Derginan Point. A good intertidal example can be found at Glenarm.

Associated biotopes:

• CR.MCR.SfR – Soft rock communities
• EUNIS code: A4.23

  Three biotopes have been identified within this complex: Pid, Pol and Hia:

• CR.MCR.SfR.Pid Piddocks with sparse associated fauna in sublittoral very soft chalk or clay
• CR.MCR.SfR.Pol Polydora sp. Tubes on moderately exposed soft sublittoral rock
• CR.MCR.SfR.Hia Hiatella-bored vertical sublittoral limestone rock
• EUNIS code: A4.231, A4.232 and A4.233

• IR.MIR.KR.HiaSw – Hiatella arctica and seaweeds on vertical limestone/chalk
• EUNIS code: A3.217

Priority species: Spongosorites calcicola, Caryophyllia inornata, Alcyonium hibernicum, Parazoanthus axinellae, Parazoanthus anguicomus

Threats:

• Coastal defences and other works: This causes a heavier impact to littoral chalk communities; however, alteration of chalk has occurred at lower shore and subtidal levels (e.g. Church Bay and harbour at Rathlin).
• Extraction of chalk for construction.
• Pollution and eutrophication: The deterioration of water quality by pollutants and nutrients has caused respectively the replacement of fucoid-dominated biotopes by mussel-dominated biotopes, and the occurrence of nuisance Enteromorpha spp. blooms.
• Boulder turning for winkles.
• Small-scale fisheries and harvesting of piddocks in some parts of the UK: Damage to subtidal reefs.

References:

This habitat description has been adapted from the 1994 UK BAP Action Plan for Littoral and Sublittoral chalk, and the 2005 NI Action plan for Littoral and Sublittoral chalk.