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Plans | Habitats | Maerl beds

Habitat Action Plan

Maerl beds

Current Status

Physical and biological status

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 in the narrows and rapids of sea lochs, or the straits and sounds between islands. Beds may also develop in more open areas where wave action is sufficient to remove fine sediments, but not strong enough to break the brittle maerl branches. Live maerl has been found at depths of 40 m, but beds are typically much shallower, above 20 m and extending up to the low tide level.
Maerl beds are found off the southern and western coasts of the British Isles, north to Shetland, but 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 also occur in other western European waters, from the Mediterranean to Scandinavia.
The distributions of the three main maerl bed-forming species in the UK are not entirely clear because of problems with identification in the field. Phymatolithon calcareum occurs throughout British waters, while Lithothamnion glaciale is a northern species with its southern limits at Lundy in the Bristol Channel and in the North Sea, off Yorkshire. Lithothamnion corallioides has caused the most problems with identification, but appears to be a south-western species with Scottish records as yet unconfirmed. Currently, it is known to occur in less than 15 of the ten km squares for the UK as defined by JNCC.
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. Maerl beds, because of the wide geographical range over which they occur, have a wide range of associated animals and plants, with species diversity tending to be greater in the south and west. Due to the fragility of maerl, the beds are easily damaged and have probably declined substantially in some areas.

Links with other action plans

Reference should be made to the habitat action plans produced for saline lagoons and tidal rapids. In particular, attention needs to be drawn to operations that may damage benthic habitats.

Current factors affecting the habitat

Maerl is of commercial value 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. In 1978 a licence was issued by the Crown Estate Commissioners (CEC) to dredge 30,000 tonnes per year of dead maerl from the Fal Estuary. The area dredged avoids the live maerl of the St Mawes Bank. An exploratory licence was awarded to a company to remove 20 tonnes of maerl off Barra, but was not subsequently taken up. A licence has been granted by the CEC under the Government View Procedure, in Wyre Sound, Orkney, for the experimental dredging of 4,000 cubic metres a year for five years. A condition of the licence was the establishment of a monitoring programme agreed with SNH.
Scallop dredging has been identified as the biggest impact on maerl beds in the Clyde, causing serious decline 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.
Maerl communities in Brittany have been damaged by eutrophication, which has caused smothering of the maerl by excess growth of other seaweeds and increased sedimentation. Finfish farms discharge large amounts of nutrients into sea lochs, derived from uneaten food and waste materials. Finfish farms also routinely use chemicals which are specifically toxic to fish lice and other crustaceans and molluscs. When such chemicals disperse in the marine environment there is the possibility that fauna associated with maerl beds may be affected.
Maerl beds rely on water movement to disperse fine sediment particles, which would otherwise accumulate between the maerl fragments and smother the bed. Any obstruction to the water flow can be expected to have adverse effects on the maerl and its associated fauna and flora. The building of barrages, causeways and bridges are potential blockages to water flow, particularly in sea lochs and between islands.

Current Action

Legal status

No maerl species are specifically listed for protection under the Wildlife and Countryside Act 1981 or the Wildlife (NI) Order 1985. However, maerl is mentioned in the JNCC guidelines for selection of intertidal SSSIs as a component of the tidal rapids part of saline lagoons. The guidelines also list 'tide-swept algae' as a community of at least national importance, which could include maerl on the lower shore.
As most maerl beds are subtidal, they cannot normally be included within SSSIs in England and Wales, or ASSIs in Northern Ireland, as the lower limit of SSSI and ASSI designations is usually the Mean Low Water mark. However, in Scotland, the planning boundary is normally the Mean Low Water of Spring tides, which could include maerl where it occurs in the subtidal fringe. This happens at a few sites, for instance at Taynish on the shores of Loch Sween, Argyll, where the SSSI boundary (but not the National Nature Reserve boundary) extends to Mean Low Water of Spring tides and includes the rapids, which are of high marine interest. At best SSSI designation can only afford limited protection to a very small proportion of the total maerl habitat.
All three of the statutory Marine Nature Reserves in Britain - Skomer in Wales, Lundy in England and Strangford Lough in Northern Ireland - contain maerl, although none have particularly well-developed beds.
Maerl beds are covered by four different habitat types in Annex I of the EC Habitats Directive: 'Sandbanks which are slightly covered by seawater at all times'; 'Large shallow bays and inlets'; 'Estuaries' and the priority habitat 'Lagoons'. For the first of these, the JNCC interpretation manual specifically mentions maerl beds, for which Special Areas of Conservation (SACs) have been selected to cover the geographical and ecological range of variation.
Annex V of the Habitats Directive lists two maerl species, Lithothamnium (sic) corallioides and Phymatolithon calcareum, as species of community interest whose taking in the wild and exploitation may be subject to management measures. However, Lithothamnion glaciale, an important constituent of maerl beds in the north, is not included in Annex V.
The current list of candidate SACs includes the Sound of Arisaig, selected particularly for its extensive series of maerl beds. Loch Maddy and The Vadills also contain maerl communities. In Northern Ireland, Strangford Lough (a cSAC and statutory Marine Nature Reserve) contains maerl. In England, the Fal and Helford cSAC includes the largest beds in south-west Britain. The Pembrokeshire Islands cSAC also includes maerl communities.
Other areas with maerl beds are included in the list of 29 Scottish Marine Consultation Areas. Although this is a non-statutory designation used by SNH to denote areas of special marine interest, it is used in planning consultations, particularly over the siting of fish farms.
Discharges to the sea are controlled by a number of EC Directives, including the Dangerous Substances, Shellfish (Waters), Integrated Pollution Control, Urban Waste Water Treatment, and Bathing Waters Directives. The forthcoming Water Framework Directive will also be relevant. The 1992 Convention for the Protection of the Marine Environment of the North East Atlantic (OSPAR) and North Sea Conference declarations are also important. These commitments provide powers to regulate discharges to the sea and have set targets and quality standards to marine waters. An extensive set of standards covering many metals, pesticides and other toxic, persistent and bioaccumulative substances, and nutrients have been set under UK legislation.

Management, research and guidance

The surveys undertaken throughout Britain by JNCC`s Marine Nature Conservation Review (MNCR) and subsequent surveys commissioned by country agencies have identified maerl bed sites, and described and classified the communities within them, based on conspicuous macrofauna.
The University Marine Biological Station, Millport, has coordinated a three-year EU-funded project looking at maerl bed biodiversity, function, structure and anthropogenic impacts at sites in Scotland (Clyde), Brittany, Galicia, Alicante and Malta. This project has identified numerous impacts threatening this habitat.
An EU-funded review of literature and extraction of information, which could be of use to managers of protected sites, has been conducted by Queen`s University, Belfast.
Specific studies have been carried out at some sites with maerl in conjunction with the impact of road and bridge schemes (eg Skye Bridge).

Action plan objectives and targets

Maintain the geographical range of maerl beds and associated plant and animal communities in the UK subject to best available information.
Maintain the variety and quality of maerl beds and associated plant and animal communities in the UK subject to best available information.

Proposed actions with lead agencies

Policy and legislation

Give consideration to proposing the inclusion of maerl beds under Annex 1 of the EC Habitats Directive when the opportunity arises. This could be in their own right or as a specific component of large shallow inlets and bays (other action plans also call for the inclusion of sea lochs as a specific Habitats Directive category). Protection can be afforded to a limited number of sites at present using the existing Habitats Directive categories (large shallow bays and inlets; estuaries; sandbanks slightly covered with water all the time; lagoons). (ACTION: JNCC)
Consider proposing the addition of Lithothamnion glaciale to Annex V of the Habitats Directive (species of community interest whose taking in the wild and exploitation may be subject to management measures). This will allow consistency with the present listing of the other two UK maerl bed-forming species. (ACTION: DETR, JNCC)
Ensure that fishing policy takes account of the potential impact of operations with mobile gear on maerl beds and seeks to avoid or minimise operations. (ACTION: DANI, EHS, MAFF, NAW, SE, SFCs)
In view of the present extent of known maerl beds there should be a presumption against the granting of new extraction licences. (ACTION: CEC, DETR, DoE(NI), Duchy of Cornwall, LAs, MAFF, SE)

Site safeguard and management

Include in all new extraction licences criteria for the continued existence of maerl beds. (ACTION: CEC, Duchy of Cornwall, MAFF, SE)
Re-negotiate existing extraction licences to ensure the continued existence of maerl beds and their associated species. (ACTION: CEC, Duchy of Cornwall, MAFF, SE)
Include by 2005 provision for the maintenance of the extent and health of maerl bed communities in management plans for SACs where these include maerl beds. (ACTION: All relevant authorities)
Ensure that planning for aquaculture and other operations, which may cause eutrophication and smothering does not adversely affect the conservation requirements of important maerl beds. (ACTION: CEC, DETR, EA, EHS, LAs, NAW, SE, SEPA)
Ensure that road, bridge, energy and other construction schemes which might affect maerl beds do not risk damage to their conservation interest. (ACTION: CCW, DETR, DTI, EHS, NE, LAs, SE, SNH)
Take account of the conservation requirements for maerl bed communities in the development and implementation of coastal zone management plans and ensure they are not managed in isolation from other habitats and communities in these areas. (ACTION: All relevant authorities)
Ensure that fishing operations do not adversely affect the conservation interests of maerl beds within designated sites. (ACTION: CCW, DANI, DETR, NE, MAFF, LAs, SE, SFCs, SNH)


Advise government on the best maerl beds for inclusion in the UK network of protected sites, thus ensuring that the full range of habitat and associated community types is represented. This includes co-operation with other EU countries to ensure an adequate EU-wide network of sites. (ACTION: JNCC)
Provide advice to local authorities and planners on minimising impacts of plans and operations on maerl bed communities by 2000. Consideration should be given to their importance from the early stages of planning, as there may often be a simple engineering or siting solution to preserving the features of interest. (ACTION: CCW, DETR, EHS, NE, NAW, SE, SNH)


None proposed.

Research and monitoring

Complete survey and recording of the extent, quality and composition of maerl bed habitats by 2005. (ACTION: CCW, EHS, NE, SNH)
Monitor the recovery of sites after previously consented construction works to establish the effectiveness of mitigation measures (eg Skye Bridge). For new construction projects prior environmental assessment is required and monitoring should be costed in before the start of development. (ACTION: LAs, NAW, SE, SNH)
Establish a monitoring programme by 2005 that will enable progress towards the objective of this plan to be properly assessed. (ACTION: CCW, EHS, NE, SNH)

Communications and publicity

Implement the communication of information on maerl beds and their importance to planners, coastal users and the general public. This should include their importance as a Scottish sea loch feature and their international importance in the context of the European distribution of this habitat. Attention should also be given to the variety and diversity of maerl and the abundance of associated marine life and communities. (ACTION: CCW, EHS, NE, LAs, SNH)
Provide information on maerl beds for relevant SACs. This should place more emphasis on local features of interest and should encourage local pride and ‘ownership’ of the resource. (ACTION: All relevant authorities)
Write popular articles for relevant publications on the importance of maerl beds. (ACTION: CCW, EHS, NE, SNH)


The successful implementation of this habitat action plan will have resource implications for both the public and private sectors. The data in the table below provide an estimate of the current expenditure on the habitat and the likely additional resource costs. These additional costs are based on the annual average over 5 and 10 years. The total expenditure for these time periods is also given. Almost all the costs will relate to the public sector, although some costs (eg for research) will be met by the private sector/non-governmental organisations).
  Current expenditure 1st 5 yrs to 2004/2005 Next 10 yrs to 2014/2015
Current expenditure /£000/Yr      
Total average annual cost /£000/Yr   54.1 29.7
Total expenditure to 2005/£000   270.5  
Total expenditure 2005 to 2014/£000     297

Key references

Adey, W.H. & Adey, P. 1973. Studies on the biosystematics and ecology of the epilithic crustose Corallinaceae of the British Isles. British Phycological Journal, 8 , 343-407.
Birkett, D.A., Maggs, C.A. & Dring, M.J. 1998. Maerl (volume V). An overview of dynamic and sensitivity characteristics for conservation management of marine SACs. Scottish Association for Marine Science/UK Marine SACs Project.
Costello, M.J., Tierney, P. & Emblow, C. 1997. Observations on maerl records made during the Biomar survey. Abstracts: Irish maerl workshop, Martin Ryan Institute/University College Galway, 30 May 1997.
Davies, J. 1991. Benthic marine ecosystems in Great Britain: a review of current knowledge. Western Channel and Bristol Channel and approaches. MNCR sectors 8 and 9. Nature Conservation Council, CSD Report, No. 1173 (Marine Nature Conservation Review Report, No. MNCR/OR/9).
Hall-Spencer, J. 1998. Conservation issues relating to maerl beds as habitats for molluscs. Journal of Conchology Special Publication, 2 , 271-286.
Howson, C.M. 1990. Survey of Scottish sea lochs. Sea Lochs of Arisaig and Moidart. Nature Conservancy Council, CSD Report No. 1086.
Howson, C.M., Connor, D.W. & Holt, R.H.F. 1994. The Scottish sealochs - an account of surveys undertaken for the Marine Nature Conservation Review. (Contractor: University Marine Biological Station, Millport). Joint Nature Conservation Committee Report No. 164 (Marine Nature Conservation Review Report MNCR/SR/27).
Irvine, L.M. & Chamberlain, Y.M. 1994. Seaweeds of the British Isles. Volume 1 Rhodophyta Part 2B Corallinales, Hildenbrandiales. HMSO, London.
Scott, R. & Moore, P.G. 1996. The Status and distribution of maerl in Scotland with regard to the EC Habitats Directive. A desk study prepared for Scottish Natural Heritage. University Marine Biological Station, Millport.

Local implementation

The following LBAPs are working on Maerl beds:

Purbeck LBAP Dorset Biodiversity Initiative Cornwall’s Biodiversity vol 1, 2 and 3 Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute Local Biodiversity Action Plan for Argyll and Bute

Publication details

Originally published in: UK Biodiversity Group Tranche 2 Action Plans - Volume V: Maritime species and habitats (October 1999, Tranche 2, Vol V, p161)
© Joint Nature Conservation Committee 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010