Maerl (also rhodolith) is a collective name for non-geniculate coralline

red algae Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing. The majority ...

with a certain growth habit. Maerl grows at a rate of c. 1 mm per year. It accumulates as unattached particles and forms extensive beds in suitable sublittoral sites.Vize, S.; Blake, C.; Hinojosa, G. and Maggs, C.A. 2003. The distribution and composition of maerl beds in Northern Ireland. ''PMNHS Newsletter'' No.13 p.26 The term maerl originally refers to the branched growth form of Lemoine (1910) and ''rhodolith'' is a sedimentological or genetic term for both the nodular and branched growth forms (Basso et al., 2015).

Description

In Europe maerl beds occur throughout the Mediterranean, along most of the Atlantic coast from Portugal to Norway, and in the English Channel, Irish Sea and North Sea. The distribution of maerl is dependent on water movement, light and salinity concentration.Wilson, S., Blake, C., Berges, J.A., and Maggs, C.A. (2004) "Environmental tolerances of free-living coralline algae (maerl): implications for European marine conservation". ''Journal of Biological Conservation'' 120, 279–289. nlineAvailable at: http://www.qub.ac.uk/bb-old/provan/pdf/charmaine2.pdf Maerl beds occur in the

photic zone The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological pro ...

, and can be found to around 30 m depth in the British Isles and up to 120 m deep in the Mediterranean. Maerl deposits can reach up to 10 m thick, but are usually much thinner; carbon dating has shown that they can be more than 5500 years old. In the

British Isles The British Isles are a group of islands in the North Atlantic Ocean off the north-western coast of continental Europe, consisting of the islands of Great Britain, Ireland, the Isle of Man, the Inner and Outer Hebrides, the Northern Isl ...

maerl is composed of three species of

coralline algae Coralline algae are red algae in the order Corallinales. They are characterized by a thallus that is hard because of calcareous deposits contained within the cell walls. The colors of these algae are most typically pink, or some other shade of re ...

growing loose in beds of fragmented nodules in the sub-littoral. The species generally involved are: '' Lithothamnion corallioides'', ''Lithothamnion glaciale'' and ''

Phymatolithon ''Phymatolithon'' is a genus of non geniculate coralline red algae, known from the UK, and Australia. It is encrusting, flat, and unbranched; it has tetrasporangia and bisporangia borne in multiporate conceptacle Conceptacles are specialized ...

calcareum''.Irvine, L.M and Chamberlain, Y.M. 1994. ''Seaweeds of the British Isles.'' Volume ''1'', Part 2B. The Natural History Museum, London. Maerl is dredged from the sea floor and crushed to form a powder. It is still harvested around the coasts of

Brittany Brittany (; french: link=no, Bretagne ; br, Breizh, or ; Gallo: ''Bertaèyn'' ) is a peninsula, historical country and cultural area in the west of modern France, covering the western part of what was known as Armorica during the period ...

France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...

and Bantry Bay,

Ireland Ireland ( ; ga, Éire ; Ulster-Scots: ) is an island in the North Atlantic Ocean, in north-western Europe. It is separated from Great Britain to its east by the North Channel, the Irish Sea, and St George's Channel. Ireland is the s ...

, and is a popular fertilizer for

organic gardening Organic horticulture is the science and art of growing fruits, vegetables, flowers, or ornamental plants by following the essential principles of organic agriculture in soil building and conservation, pest management, and heirloom variety prese ...

. It was also dredged off Falmouth, Cornwall, but this ceased in 2004. Scientists investigated Falmouth maerl and found that ''L. corallioides'' predominated down to 6 m and ''P. calcareum'' from 6–10 m (Blunden ''et al.'', 1981). Coral Fragments, Coral Beach, Carraroe

Chemical analysis of maerl showed that it contained 32.1% CaCO₃ and 3.1% MgCO₃ (dry weight).

Ecology

The ecology of maerl habitats has received very little attention in contrast to other marine ecosystems such as kelp forests or

sea grass Seagrasses are the only flowering plants which grow in marine environments. There are about 60 species of fully marine seagrasses which belong to four families (Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), all in the or ...

beds. Maerl beds provide a complex habitat for a wide range of taxa with a variety of niches that support high associated invertebrate and algal biodiversity. Maerl beds act as nursery areas for the juvenile stages of commercial species such as juvenile cod '' Gadus morhua'', saithe '' Pollachius virens'', Pollack '' Pollachius pollachius'' and juvenile scallops ''

Aequipecten opercularis The queen scallop (''Aequipecten opercularis'') is a medium-sized species of scallop, an edible marine bivalve mollusk in the family Pectinidae, the scallops. It is found in the northeast Atlantic and is important in fisheries. Description At ...

''.Kamenos, N. A., Moore, P.G., Hall-Spencer, J.M. (2004b) Small-scale distribution of juvenile gadoids in shallow inshore waters; what role does maerl play? ''ICES Journal of Marine Science'' 61, 442–429. nlineAvailable at: http://icesjms.oxfordjournals.org/content/61/3/422.short Maerl beds offer physical refuge and protection from predation as well as productive feeding grounds but are easily damaged by dredging and towed fishing gear. Maerl has no tolerance for desiccation.

History

Maerl has been extracted for centuries mainly for use as an agricultural fertilizer. The amount extracted increased in the late 20th century and in 2000, maerl was extracted at c. 5,000 tonnes per year in Ireland and c. 500,000 tonnes per year in France. Large scale maerl extraction over the past 40 years has removed and degraded maerl beds. In Cornwall, England, maerl has been extracted since the 1970s, but was banned in 2005 by Falmouth Harbour Commissioners. An early reference to maerl was made by

John Ray John Ray FRS (29 November 1627 – 17 January 1705) was a Christian English naturalist widely regarded as one of the earliest of the English parson-naturalists. Until 1670, he wrote his name as John Wray. From then on, he used 'Ray', after ...

in 1690 who reported it from Falmouth. In Ireland, maerl is extracted from subfossil beds in Bantry Bay by Celtic Sea Minerals. The maerl-forming species ''Lithothamion corallioides'' and ''Phymatolithon calcareum'' are listed in Annex V of the EC Habitats Directive.Forest Trends

Uses

Used as a soil conditioner, it is dredged from the sea floor and crushed to a powder.Thomas, D. 2002. ''Seaweeds.'' Life Series. The Natural History Museum, London The slow growth of individual nodules and their accumulation in beds over a millennial timescale means that there is no possibility of maerl keeping up with dredging for this purpose. Maerl should be considered as a

non-renewable resource A non-renewable resource (also called a finite resource) is a natural resource that cannot be readily replaced by natural means at a pace quick enough to keep up with consumption. An example is carbon-based fossil fuels. The original organic ma ...

, and readily available alternative products (e.g., garden lime) make modern day exploitation controversial.{{citation needed, date=October 2018

References

Corallinales Environment of Cornwall