Nature and importance of Maerl Beds

What maerl is and why it is important

Definitions

What maerl is and why it is important

Economic importance

Maerl has traditionally been harvested on a small scale in Europe by dredging for use as a soil conditioner or for various other purposes such as a treatment for acid drinking water. Industrial extraction as an animal food additive and to replace lime as an agricultural soil conditioner reached c. 600,000 tonnes of maerl per annum in the 1970s in France alone. Maerl extraction forms a major part of the French seaweed industry, both in terms of tonnage and value of harvest. In the UK up to 30,000 tonnes p. a. of maerl were harvested commercially in the Fal from 1975 to 1991.

Scientific importance

Within the United Kingdom, Scotland is home to many of the most extensive maerl beds in Europe. Maerl biotopes have high species diversity, which matches that in other marine biotopes studied by similar methods. There is a small group of species that appear to be confined to maerl biotopes; many other invertebrates and algae are found predominantly on maerl. The biotopes are fragile according to most recognized categories of fragility.

It should be emphasized that records of the presence or absence of maerl biotopes on European coasts are patchy. Within Europe, detailed studies of maerl biotopes have been undertaken only in the past 40 years and at only a handful of locations. In general, maerl beds have been better studied in Europe, particularly N. France, Norway and Ireland, than in the UK. Large, historically accessible maerl banks are relatively well recorded as a result of commercial interests. The locations of other maerl sites are known from the results of grab and dredge sampling during scientific research cruises. In more recent times, reports of maerl banks have been made by scuba divers. However, the extent of a maerl bed at any given location, its species composition, and the species associated with it, remain largely unknown.

Conservation significance

The value of maerl beds as a unique assemblage of biotopes is currently threatened by several types of human activity, e.g. large-scale commercial extraction, reduction of water quality by discharges, and the use of heavy demersal fishing gear. Part of the process designed to mitigate threats to these habitats is the designation of candidate Special Areas of Conservation (SACs). The maerl biotope complex has been selected as one of the biotope complexes to be reviewed scientifically in the UK Marine SACs Project for the reasons included in the list below. Assessment of the conservation value of maerl beds has previously been discussed by Hall-Spencer (1995a), who proposed most of the following points that must be taken into account:

Maerl beds have considerable conservation value because although maerl is confined to a very small proportion of European shallow sublittoral waters, each of the beds studied to date has been found to harbour a disproportionately high diversity and abundance of associated organisms in comparison with surrounding biotopes; some of these species are confined to the maerl habitat or rarely found elsewhere.

Some of the organisms that live within maerl beds are rare, unusual or poorly known.

Maerl biotopes, which are relatively scarce, are currently threatened by several types of human activity. The effects of habitat removal through offshore construction activities or the commercial extraction of maerl are irreversible over timescales relevant to humans. Other severe threats to maerl habitats include poor water quality and the use of demersal fishing gear such as scallop dredges.

The coralline algae that form the maerl are amongst the slowest-growing species in the North Atlantic so that any damage to the maerl beds may take decades to repair.

Large beds of free-living, unsegmented, coralline algae have occurred since the Miocene in diverse environments on continental shelves around the world. Since the coralline algae contain calcium carbonate, they fossilize fairly well and can be used as stratigraphic markers and as indicators of paleoenvironmental conditions (Foster et al., 1997).

Coralline algae may be one of the largest stores of carbon in the biosphere. All plants take up carbon during photosynthesis, but coralline algae deposit large amounts of carbon in their cell walls in the form of calcium carbonate.

Two of the more common maerl-forming species, Lithothamnion corallioides Crouan frat. and Phymatolithon calcareum (Pallas) W. Adey & McKibbin, are included in Annex V (b) of the EC Habitats Directive, 1992.

As part of the UK's response to the European Union Habitats Directive to protect habitats, maerl is identified in the JNCC interpretation of the EC Habitats Directive as a key habitat within the Annex I category >sand banks which are slightly covered by seawater at all times=.

Both Lithothamnion corallioides and Phymatolithon calcareum are on the long list of species in the UK Biodiversity Steering Group Report (Anon., 1995).

Maerl is the subject of a Habitat Action Plan under the UK Biodiversity Action Plan.

Maerl beds occur in three demonstration SACs within the UK (Table 3), while the Fal and Helford (Cornwall) candidate SAC includes the largest maerl bed in England.

Definitions

We provide here some definitions of the terms that will be used in this review.

Biotope

The habitat (i.e. the environment’s physical and chemical characteristics) together with its recurring associated community of species, operating together at a particular scale (Connor et al., 1997). The habitat encompasses the substratum and the particular conditions of wave exposure and other factors which contribute to the overall nature of the location. The term community refers to a similar association of species which regularly recurs in widely separated geographical locations.

Biotope complex/ biocoenosis

Group of biotopes with similar overall character that should be relatively easy to identify by non-specialists or by remote/rapid sensing methods (Connor et al., 1997).

Maerl

A Breton word (sometimes written marl), and refers to loose-lying, normally non-geniculate (i.e. unsegmented because they lack decalcified joints), coralline red algae. Attempts have been made to distinguish between branched, twig-like forms (maerl in the most frequently used sense) and nodules or rhodoliths (see below), which may or may not have a non-algal core.

Maerl beds

Composed of living or dead unattached corallines forming accumulations, with or without terrigenous material. This is the term most commonly used in the British Isles, although Irvine & Chamberlain (1994) refer to them as maerl-rhodolith beds. Elsewhere in the world, such beds are often called rhodolith beds (e.g. Steller & Foster, 1995). Dead maerl beds are often called maerl deposits.

Rhodolith

A general term covering nodules and unattached growths composed principally or entirely of coralline algae (Bosence, 1983a, 1983b) (meaning red stone). In this geological terminology, maerl is therefore a type of rhodolith. In biological usage in the British Isles, the term rhodolith is often reserved for corallines with a non-algal core (the plant may have grown to cover the shell or pebble originally colonised), but this terminology is not in accordance with that of Bosence (1983a, 1983b). There are intergrades between entirely algal growths and those those with non-algal cores, in both appearance and mode of formation, and strict definitions are probably not practical (Irvine & Chamberlain, 1994, p. 14). As noted above, in many parts of the world usage of the word rhodolith means that it can be read as a synonym for maerl in the British, biological, sense.

Thallus

The "body" of a seaweed.

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