Maerl beds

Maerl beds © Keith Hiscock

Maerl is a collective term for several species of red seaweed, with hard, chalky skeletons. It is rock hard and, unlike other seaweeds, it grows as unattached rounded nodules or short, branched shapes on the seabed.  Like all seaweeds, maerl needs sunlight to grow, and it only occurs to a depth of about 20m. 

Maerl can form large beds, when the conditions are right – a fast tidal flow or sufficient wave action to remove fine sediments, but not strong enough to break the brittle maerl branches.  Within these beds, layers of dead maerl build up with a thin layer of pink, living maerl on the top.

Maerl beds are an important habitat for many different types of marine life, which live amongst or are attached to the surface of Maerl, or burrow in the coarse gravel of dead maerl beneath the top living layer. Maerl beds can be of importance to sustainable fisheries, providing nursery grounds for commercial species of fish and shellfish. 

Due to the fragility of maerl, the beds are easily damaged and have probably declined substantially in some areas.  Pressures on maerl beds include scallop dredging, bottom trawling, aquaculture and pollution. Maerl beds are very slow to develop and are unlikely to return if removed or lost. As such, they should be treated as a non-renewable resource.

In England, the Fal and Helford Special Area of Conservation includes the largest maerl beds in south-west Britain.

For the official habitat definition please see the documents listed below.


European distribution

Maerl beds are found off the southern and western coasts of Britain and Ireland, as far north as Shetland, and are particularly well developed around the Scottish islands and in sea loch narrows, around Orkney, and in the south in the Fal Estuary.  Maerl beds occur within the wider north-east Atlantic, as far as Iceland, and also in the Mediterranean.


Conservation status/need

Maerl beds fact

  • This is a UK BAP Priority Habitat (BAP habitats are now Habitats of Principal Importance/Priority Habitats).
  • Listed in Annex I of the Habitats Directive
  • OSPAR List of Threatened and/or Declining Species and Habitats (Region III – Celtic Sea)
  • Certain types of maerl species found within the beds are UKBAP Priority Species, and species of principal importance for the purpose of conservation of biodiversity under the Natural Environment and Rural Communities Act 2006.


Official definition

UK Biodiversity Action Plan; Priority Habitat Descriptions. BRIG (ed. Ant Maddock) 2008 (updated December 2011)

Descriptions of habitats on the OSPAR list of threatened and/or declining species and habitats (OSPAR agreement 2008/07).


Further information

UK Biodiversity Action Plan - Maerl beds

OSPAR Commission – Background Document for Maerl beds

JNCC - UK BAP Priority Species and Habitats

Marine Life Information Network - Maerl beds

JNCC EUNIS habitat correlations table