Maerl beds

Compiled by: Mats Lindegarth, Sweden

 

1. Extract from OSPAR Case Reports:

Initial List of Threatened and/or Declining Species & Habitats in the OSPAR Maritime Area (OSPAR 2006)

OSPAR definition for habitat mapping

“Maerl” is a collective term for several species of calcified red seaweed (e.g. Phymatolithon calcareum, Lithothamnion glaciale, Lithothamnion corallioides and Lithophyllum fasciculatum) which live unattached on sediments. In favourable conditions, these species can form extensive beds, typically 30% cover or more, mostly in coarse clean sediments of gravels and clean sands or muddy mixed sediments, which occur either on the open coast or in tide-swept channels of marine inlets, where it grows as unattached nodules or ‘rhodoliths’. Maerl beds have been recorded from a variety of depths, ranging from the lower shore to 30m depth. As maerl requires light to photosynthesize, depth is determined by water turbidity. In fully marine conditions the dominant species is typically P. calcareum, whilst under variable salinity conditions such as sealochs, beds of L. glaciale may develop. Maerl beds have been recorded off the southern and western coasts of the British Isles, north to Shetland, in France and other western European waters.

 

OSPAR management considerations

The main management measure which would assist the conservation of this habitat is protection from physical damage. This would require halting direct extraction from maerl beds. A recently concluded four year EU project on maerl in Europe has recommended a presumption of protection of all maerl beds as they are effectively non-renewable resources. Other proposals from this work include the prohibition on the use of towed gear on maerl grounds, moratoria on the issue of further permits for the siting of aquaculture units above maerl grounds.

 

2. Additional HELCOM information

2.1. Description of the habitat

Maerl beds are benthic habitats consisting of unattached particles (maximum diameter ≈5cm) of calcareous red algae of the genera Lithothamnion and Phymatolithon in gravel and sand. Areas where maerl occur are generally well ventilated with low levels of turbidity at depths of 17-22 m.

2.2. Distribution (past and present)

Known areas where maerl beds occur are on offshore banks in the Kattegat (e.g. Lilla Middelgrund and Fladen). The presence of dead maerl at some offshore banks indicates that the habitat must have been more widespread in the past. Maerl beds occur patchily but regularly under similar environmental conditions in full marine areas.

2.3. Importance (sub-regional, Baltic-wide, global)

Because of their restricted distribution, maerl beds of the Kattegat are considered to be of Baltic-wide importance in the HELCOM area. Animals associated with maerl beds and their surroundings include many rare decapod crustaceans, such as Corystes cassivelaunus and Thia scutellata, and echinoderms, such as Ophiothrix fragilis and Ophiocomina nigra.

2.4. Status of threat/decline

The status of threat and/or decline is poorly known. Increased pressure from offshore developments (e.g. wind-farm development) may affect the distribution of the habitat. From a Baltic perspective the biotope is rare and must therefore be considered “potentially endangered”.Nomination of maerl beds to be placed on the OSPAR list cited sensitivity, ecological significance and decline. Information was also provided on threat (for further information see OSPAR 2006).

2.5. Threat/decline factors

Extraction, offshore wind-farms, destructive fishing methods and eutrophication causing increased turbidity.

2.6. Options for improvement

A biotope inventory and a compilation of existing data will increase the knowledge about the distribution of the habitat in the Kattegat. This will allow detailed mapping and therefore more appropriate assessments of potential environmental impacts to the habitat with respect to extraction, offshore installations and fishing. As recommended within OPSAR (OSPAR 2006), protection from any physical damage would be the most important management measure.

Maerl beds on offshore banks are part of the European Habitats Directive Annex I Habitats “1110 (Sandbanks which are slightly covered by sea water all the time)”. As such they are considered to be of community interest which requires the designation of special areas of conservation on a European scale.

2.7. References

HELCOM (1998). Red List of Marine and Coastal Biotopes and Biotope Complexes of the Baltic Sea, Belt Sea and Kattegat - Including a comprehensive description and classification system for all Baltic Marine and Coastal Biotopes. HELCOM-Baltic Sea Environ–ment Proceedings 75, Helsinki Commission. 115 pp.

Jackson, A. (2006). Phymatolithon calcareum maerl beds with hydroids and echinoderms in deeper infralittoral clean gravel or coarse sand. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 13/06/2007]. Available from: http://www.marlin.ac.uk

Naturvårdsverket (2006).  Inventeringar av marina naturtyper på utsjöbankar. Rapport 5567. ISBN 91-620-5576-3. 69 pp.  Available at www.naturvardsverket.se (in Swedish).

OSPAR (2006). OSPAR Case Reports for the Initial List of Threatened and/or Declining Species and Habitats in the OSPAR Maritime Area. Publication no. 276, pp 125-128. (http://www.ospar.org/eng/html/welcome.html).