Atlantic maerl beds

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.

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats