Biodiversity & Conservation

Beds of dead maerl

Dead maerl beds

Beds of dead maerl
Distribution map

Dead maerl beds recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)

  • EC_Habitats
  • UK_BAP

Recorded distribution in Britain and Ireland

Maerl beds (live and dead) have a patchy distribution around the coast of the British Isles. They are widespread around the west coast of Scotland, in the Western Isles, Orkney and Shetland but restricted to Milford Haven, the Pembrokeshire Islands and the Llyn Peninsula in Wales. Beds are rare in England, reported from Dorset, the Isles of Scilly and Lundy but with extensive beds in the Fal Estuary and mouth of the Helford River. Extensive beds occur on the north east coast of Northern Ireland, and along the west coasts of Ireland (e.g. Galway Bay). In Europe, maerl beds are found in the Mediterranean, and on the Atlantic coast from Norway and Denmark south to Portugal, Morocco and Mauritania on the African coast (Birkett et al. 1998). Few of the distributional records distinguish between ‘live’ and ‘dead’ beds. The map (above) only shows confirmed records of dead beds off the Isle of Bute and in Falmouth Bay.

Habitat preferences

Temperature range preferences - Not relevant

Water clarity preferences - Data deficient

Limiting Nutrients - Data deficient

Other preferences - Not relevant

Additional information

Extensive maerl beds are restricted to areas of moderate to strong currents protected from strong wave action, e.g. in bays and inlets. Lithothamnion coralloides is more tolerant of low water flow that Phymatolithon calcareum, so that the species composition varies with water flow. The depth at which live maerl can grow depends on light availability (and hence water clarity), and in the British Isles maerl can occur at 30m but larger beds occur at 15m or less. Species composition is also dependant on temperature. While maerl occurs from the tropics to Norway, Lithothamnion coralloides is restricted to southern waters while Lithothamnion glaciale is particularly abundant in Scotland (Birkett et al. 1998). Maerl beds are normally found at full salinity but can tolerate reduced salinities. Live maerls are intolerant of desiccation, so are rarely found or form beds in the intertidal (Birkett et al., 1998, Wilson et al., 2004).

The presence of ‘dead’ maerl beds is dependent of the prior growth and development of live maerl beds over decades or thousands of years. Therefore, dead maerl beds can only occur in areas that are presently, or were previously suitable for the growth of live maerl.

This review can be cited as follows:

Tyler-Walters, H. 2013. Beds of dead maerl. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 27/11/2015]. Available from: <>