Maerl refers to a collection of calcified marine algae, which grow as unattached nodules on the seabed. Over time the dead calcareous skeleton is overlain by a thin layer of pink, living maerl & extensive beds can cover the seabed.
- Site: Sheltered areas, with some tidal flow or open waters
- Main species: red algae (calcified)
- Maerl beds develop where there is some tidal flow, such as in the narrows & rapids of sea loughs, or in the straits & sounds between islands.
- Maerl beds can also form in open waters where wave action removes fine sediments but does not break or disperse the brittle maerl branches.
- Beds tend to form above 20m, extending to low tide level but live maerl has been found at 40m in the UK.
Current Status in UK & Northern Ireland
- Maerl beds in Northern Ireland are found in Strangford Lough & off the north-east coast of Co. Antrim.
- Extensive beds have been reported at Garron Point & Ballygally Head (Erwin et al., 1986). Scattered maerl has been reported from Church Bay, Ringfad Point, Cushendun Bay & Carlingford Lough. A previously undiscovered area of maerl bed was found in the centre of the Maidens (Sublittoral Survey of Northern Ireland, 2006). Other locations, extent & condition of maerl beds have not been fully studied.
- Phymatolithon calcareum is scarce in NI. It has been recorded from Rathlin Island & Garron Point.
- Slow growth rates & extreme longevity make maerl beds vulnerable to overexploitation. Maerl beds are under threat from direct physical disturbances such as scallop dredging, anchoring & mooring. They are also threatened by indirect physical disturbances such coastal construction, pollution, eutrophication from discharge of sewage & climate change (e.g. increased incidence of extreme weather events & sea level rise).
- Garron Plateau
- Rathlin Island [Church Bay]