Red List habitat classification > RL - Marine habitats > RLNEA - Atlantic > NEAA5.51 Atlantic maerl beds

Atlantic maerl beds

Quick facts

Red List habitat type code NEAA5.51
Threat status
Europe Vulnerable
EU Vulnerable
Relation to
Source European Red List habitat factsheet
European Red List of habitats reports
European Red List of habitats (Excel table)


Maerl is a collective term for various species of non-jointed coralline red algae (Corallinophycidae) that live unattached to the seabed. These species can form extensive beds, mostly on coarse clean gravel and clean sand or on muddy mixed sediments, either on the open coast, in tide-swept channels or in sheltered areas of marine inlets with weak current. Wave and current-exposed maerl beds, where thicker depths of maerl accumulate, frequently appear as waves and ridge-and-furrows arrangements. As maerl requires light to photosynthesize, the depth of live beds is determined by water turbidity, being recorded from the lower shore to depths of 40 m or more. Water movement also appears to be a key physical environmental factor affecting the distribution of maerl and hence the formation of maerl beds.

North East Atlantic maerl beds are typically composed of both living and dead maerl of varying proportions. Extensive maerl beds formed during the late Holocene sea level rise on the west facing Atlantic coastlines of the British Isles, Scandinavia, France and Spain. Some are believed to have been tens of kilometres across and several meters in thick. Maerl is slow growing with growth rates for presently existing free living maerl in northwest Spain and western Ireland have been calculated to vary from 0.10- 1.00 mm/yr and in Norway from 0.05 - 0.15 mm/yr or up to 1.0 mm/yr.

The fauna and flora associated with maerl beds often constitute highly diverse communities, which may be attached to the surface of the maerl, on areas of exposed sediment, between the interstices of both living and dead maerl, and within the underlying sediment. They include foliose and filamentous seaweeds, hydroids, bryozoans, gastropod and bivalve molluscs, anemones, echinoderms and polychaete worms. Beds typically support high numbers of macroalgal species, with 349 species recorded, representing around 30% of the total seaweed diversity of the NE Atlantic. Similarly, over 2,500 macrobenthic species have been found associated with NE Atlantic maerl beds which coincidentally constitute around 30% of the total number of coastal invertebrate species in the area.

The structural complexity of maerl beds are known to provide important nursery areas for fish and shellfish species such as cod and edible crustaceans at a critical phase in their life histories, as well as a refuge and feeding area for commercially important shellfish brood stock (e.g. Ensis spp, Pecten maximus and Venus verrucosa). There is some evidence to suggest that coralline algae produce physical and chemical cues that encourage the settlement and recruitment of planktonic juvenile stages of many invertebrate species, while providing the prospect of higher growth potential.

Indicators of quality:

Both biotic and abiotic indicators have been used to describe marine habitat quality. These include: the presence of characteristic species as well as those which are sensitive to the pressures the habitat may face; water quality parameters; levels of exposure to particular pressure, and more integrated indices which describe habitat structure and function, such as trophic index, or successional stages of development in habitats that have a natural cycle of change over time.

There are no commonly agreed indicators of quality for this habitat, although particular parameters may have been set in certain situations e.g. protected features within Natura 2000 sites, where reference values have been determined and applied on a location-specific basis. The overall quality and continued occurrence of this habitat is, however, largely dependent on the presence of coralline red algae which creates the biogenic structural complexity on which the characteristic associated communities depend. In the UK and France the proportion (%) of live maerl coverage is routinely used as a quality indicator for selected beds. The density and the maintenance of a viable population of maerl-forming species is a key indicator of habitat quality, together with the visual evidence of presence or absence of physical damage.

Other quality indicators currently being developed specifically for maerl beds include the detection of community shifts associated with quality decline (e.g. from clean maerl gravel with low silt and abundant suspension-feeding bivalves, to muddy sand dominated by deposit feeders and omnivores), the reduction in the thickness of live maerl cover, opportunist species dominance and overgrowth by the slipper limpet Crepidula fornicata.

Characteristic species
For full habitat description, please download the habitat factsheet.

Threat status

Synthesis of Red List assessment

Maerl beds have declined in both extent and quality in the North East Atlantic. Recorded maerl bed declines include off the west coast of Scotland, related to the expansion of the scallop fishing industry, and a similar situation in Ireland. Extraction of both living and fossil deposits has depleted beds in the Fal Estuary in England and at least four maerl beds in Brittany (France) have been completely destroyed by this activity. In Galicia (north west Spain), mussel farming is implicated in the deterioration of maerl bed complexity and biodiversity through increased sedimentation. A further decline in the condition of this habitat is expected in the next decade due primarily to commercial extraction, mariculture and demersal fishing activities. Because of the slow growth rate of maerl, recovery of areas where maerl has been removed is not considered possible. Recovery of damaged maerl beds is also very slow and likely to take centuries replenish to themselves. Consequently, if damaging activities continue there is a high likelihood of collapse of this habitat.
This habitat does not have a restricted geographical distribution or occur in a few locations but it has been assessed as Vulnerable in the EU 28 and EU 28 + because of current and likely future continuing declines in quality and quantity.
Red List Category Red List Criteria
Vulnerable A1, C/D1
Red List Category Red List Criteria
Vulnerable A1, C/D1

Confidence in the assessment

Red List of habitat categories and criteria descriptions

Pressures and threats

  • Biological resource use other than agriculture & forestry
    • Marine and Freshwater Aquaculture
    • Intensive fish farming, intensification
    • Suspension culture
    • Fishing and harvesting aquatic resources
    • Professional active fishing
    • Benthic or demersal trawling
    • Benthic dredging
  • Human intrusions and disturbances
    • Other human intrusions and disturbances
    • Shallow surface abrasion/ Mechanical damage to seabed surface
  • Invasive, other problematic species and genes
    • Invasive non-native species
  • Natural System modifications
    • Human induced changes in hydraulic conditions
    • Extraction of sea-floor and subsoil minerals (e.g. sand, gravel, rock, oil, gas)

Habitat restoration potential

No. Maerl thalli are fragile and therefore inherently susceptible to seabed abrasion, although they are resilient to natural movement by currents and waves. It is important not to change the natural disturbance regime or increase compaction of maerl as this can kill live thalli through burial in sediment or crush the brittle thalli, reducing their length and complexity. Large live thalli can range from 10s-100s of years old (and dead thalli 100s-1000s of years old) and with increasing maerl size, the three-dimensional complexity of the thalli increases which leads to increases in the diversity of associated flora and fauna. The size of maerl thalli decreases in the presence of activities that abrade or compact the seabed resulting in associated biodiversity loss.

Trends in extent

Average current trend in quantity

Decreasing Unknown
EU28 EU28+

Trends in quality

Average current trend in quality

Decreasing Unknown
EU28 EU28+

Conservation and management needs

Prohibiting the direct extraction of maerl would be of immediate benefit to this habitat. Other important conservation and management measures would be to stop the use of mobile demersal gears and other types of fishing activity where this habitat occurs because of the damaging effects of abrasion. Sufficient distance between maerl beds and aquaculture facilities is also important. Mussel aquaculture, for example contributes large amounts of fine sediment and detritus to the bottom, causing the burial and death of the maerl thalli by reduction of gas exchange around them.

List of conservation and management needs

  • Measures related to wetland, freshwater and coastal habitats
    • Restoring/Improving water quality
  • Measures related to spatial planning
    • Establish protected areas/sites
    • Legal protection of habitats and species
  • Measures related to hunting, taking and fishing and species management
    • Regulation/Management of fishery in marine and brackish systems
    • Specific single species or species group management measures
  • Measures related to special resouce use
    • Regulating/Managing exploitation of natural resources on sea


For each habitat a distribution map was produced from a wide variety of sources indicating known and potential occurrences of the habitat in 10x10 km grids within Europe. Occurrences in grid cells were given in two classes: actual distribution from relatively reliable sources (surveys, expert knowledge), and potential distribution based on models or less reliable indicators. Please download the fact sheet to see the map.

Geographic occurrence and trends

Seas Present or presence uncertain Current area of habitat (Km2) Recent trend in quantity (last 50 years) Recent trend in quality (last 50 years)
Bay of Biscay and the Iberian Coast Present Unknown Decreasing Decreasing
Celtic Seas
Greater North Sea

Extent of Occurrence, Area of Occupancy and habitat area

Extent of Occurrence (EOO) (Km2) Area of Occupancy (AOO) Current estimated Total Area Comment
EU28 4,149,537 199 unknown EOO and AOO have been calculated on the available data. Although this data set is known to be incomplete the figures exceed the thresholds for threatened status.
EU28+ >199 unknown EOO and AOO have been calculated on the available data. Although this data set is known to be incomplete the figures exceed the thresholds for threatened status.
AOO = the area occupied by a habitat measured in number of 10x10 km grid cells.
EOO = the area (km2) of the envelope around all occurrences of a habitat (calculated by a minimum convex polygon).

Characteristic species

Not available

Vegetation types

Relation to vegetation types (syntaxa)

Not available

Other classifications

Not available
European Environment Agency (EEA)
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1050 Copenhagen K
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