Marine Biodiversity Data Portal - NI

Correspondence with existing habitats

OSPAR habitat: Maerl beds

Habitats Directive Annex I: Large shallow inlets and bays

Habitats Directive Annex I: Sandbanks which are slightly covered by seawater all the time

Description: Maerl is a collective term for several species of calcified red seaweed. It grows as unattached nodules on the seabed, and can form extensive beds in favourable conditions. Maerl is slow-growing, but over long periods its dead calcareous skeleton can accumulate into deep deposits (an important habitat in its own right), overlain by a thin layer of pink, living maerl.

Maerl beds typically develop where there is some tidal flow, such as close to Sketrick Island in Strangford Lough. Beds may also develop in more open areas where a combination of wave action and tidal streams are sufficient to remove fine sediments, but not strong enough to break the brittle maerl branches.

In Northern Ireland the distributions of the three maerl bed-forming species is not entirely clear due to difficulties with identification in the field. The three reported species are Lithothamnion corallioides, Lithothamnion glaciale and Phymatolithon calcareum.

Maerl beds are an important habitat for a wide variety of marine animals and plants which live amongst or are attached to its branches, or burrow in the coarse gravel of dead maerl beneath the top living layer. Along the Antrim coast, maerl beds can be associated with brittlestar beds. Due to the fragility of maerl, the beds are easily damaged.

In Northern Ireland maerl beds can be found along the east coast, especially at Cushendun Bay. They have also been recorded from Church Bay, Rathlin Island and within Strangford Lough and Carlingford Lough.

Associated biotopes:

• SS.SMp.Mrl – Maerl beds
• SS.SMp.Mrl.Lcor - Lithothamnion corallioides maerl beds on infralittoral muddy gravel
• SS.SMp.Mrl.Pcal - Phymatolithon calcareum maerl beds in infralittoral clean gravel or coarse sand
• SS.SMp.Mrl.Pcal.R - Phymatolithon calcareum maerl beds with red seaweeds in shallow infralittoral clean gravel or coarse sand
• SS.SMp.Mrl.Pcal.Nmix - Phymatolithon calcareum maerl beds with Neopentadactyla mixta and other echinoderms in deeper infralittoral clean gravel or coarse sand

• EUNIS code: A5.51, A5.513, A5.511, A5.111 and A5.5112

Priority species: Lithothamnion corallioides, Schmitzia neapolitana, Schmitzia hiscockiana, Cruoria cruoriaeformis, Atractophora hypnoides, Capnea sanguinea, Pecten maximus

Threats:

• Commercial extraction for use as a soil conditioner on acidic ground, as an animal food additive, for the filtration of acid drinking water and in pharmaceutical and cosmetic products.
• Scallop dredging has been identified as the biggest impact on maerl beds of both maerl, by breaking and burying the thin layer of living maerl, and the associated species. Other types of mobile fishing gear are also likely to damage the living layer of maerl on top of the bed.
• Heavy anchors and mooring chains could cause considerable damage to maerl beds.
• Eutrophication, which causes smothering of the maerl by excess growth of other seaweeds and increased sedimentation.
• Finfish farms nutrient and chemical discharges that can affect the fauna associated with maerl beds may be affected.
• Mussel aquaculture could cause damage to maerl beds as the production of pseudo-faeces could smother the live maerl.
• Obstruction to water flow building of barrages, causeways and bridges are potential blockages to water flow, causing fine sediment particles to accumulate between the maerl fragments and smother the bed.

References:

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