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

red algae Red algae, or Rhodophyta (, ; ), make up one of the oldest groups of eukaryotic algae. The Rhodophyta comprises one of the largest Phylum, phyla of algae, containing over 7,000 recognized species within over 900 Genus, genera amidst ongoing taxon ...

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). The terms rhodolith and maerl are used in very similar ways. A study in 2023 clarifies that maerl refers to only living, branched coralline thalli, while rhodolith includes unattached coralline red algae, both dead and alive.

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 (or 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 ...

, 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 an archipelago in the Atlantic Ocean, 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 Hebrides, Inner and Outer Hebr ...

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 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 ( ) is a peninsula, historical country and cultural area in the north-west of modern France, covering the western part of what was known as Armorica in Roman Gaul. It became an Kingdom of Brittany, independent kingdom and then a Duch ...

France France, officially the French Republic, is a country located primarily in Western Europe. Overseas France, Its overseas regions and territories include French Guiana in South America, Saint Pierre and Miquelon in the Atlantic Ocean#North Atlan ...

and

Bantry Bay Bantry Bay () is a bay located in County Cork, Ireland. The bay runs approximately from northeast to southwest into the Atlantic Ocean. It is approximately 3-to-4 km (1.8-to-2.5 miles) wide at the head and wide at the entrance. Geograp ...

Ireland Ireland (, ; ; Ulster Scots dialect, Ulster-Scots: ) is an island in the North Atlantic Ocean, in Northwestern Europe. Geopolitically, the island is divided between the Republic of Ireland (officially Names of the Irish state, named Irelan ...

, 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 pres ...

. It was also dredged off

Falmouth, Cornwall Falmouth ( ; ) is a town, civil parish and port on the River Fal on the south coast of Cornwall, England, United Kingdom. Falmouth was founded in 1613 by the Killigrew family on a site near the existing Pendennis Castle. It developed as a po ...

, 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. A 2024 survey by researchers from the

University of Exeter The University of Exeter is a research university in the West Country of England, with its main campus in Exeter, Devon. Its predecessor institutions, St Luke's College, Exeter School of Science, Exeter School of Art, and the Camborne School of ...

, commissioned by Cornwall Council's Blue Natural Capital Project, found that the Fal and Helford Estuary Special Area of Conservation supports an 880 hectare bed of maerl. 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 forest Kelp forests are underwater areas with a high density of kelp, which covers a large part of the world's coastlines. Smaller areas of anchored kelp are called kelp beds. They are recognized as one of the most productive and dynamic ecosystems on E ...

s or

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

beds. Maerl beds provide a complex habitat for a wide range of

taxa In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

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 Cod (: cod) is the common name for the demersal fish genus ''Gadus'', belonging to the family (biology), family Gadidae. Cod is also used as part of the common name for a number of other fish species, and one species that belongs to genus ''Gad ...

Gadus morhua The Atlantic cod (: cod; ''Gadus morhua'') is a fish of the family Gadidae, widely consumed by humans. It is also commercially known as ''cod'' or ''codling''.saithe ''

Pollachius virens ''Pollachius virens'' is a species of marine fish in the genus '' Pollachius''. Together with '' P. pollachius'', it is generally referred to in the United States as pollock. It is commonly known in Britain as the coalfish, coley, or saithe ( or ...

'', Pollack ''

Pollachius pollachius ''Pollachius pollachius'' is a species of marine fish in the family Gadidae. FAO uses the English name pollack for this species, whereas in American English it is known as European pollock. Other vernacular names include ''lythe'', and in the ...

'' and juvenile

scallops Scallop () is a common name that encompasses various species of marine bivalve molluscs in the taxonomic family Pectinidae, the scallops. However, the common name "scallop" is also sometimes applied to species in other closely related famili ...

'' Aequipecten opercularis''.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 Fellow of the Royal Society, FRS (November 29, 1627 – January 17, 1705) was a Christian England, English Natural history, naturalist widely regarded as one of the earliest of the English parson-naturalists. Until 1670, he wrote his ...

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.

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 mat ...

, and readily available alternative products (e.g., garden lime) make modern day exploitation controversial.{{Cite journal , last1=Verma , first1=Audrey , last2=van der Wal , first2=René , last3=Fischer , first3=Anke , date=2017-10-01 , title=New Technological Interventions in Conservation Conflicts: Countering Emotions and Contested Knowledge , url=https://doi.org/10.1007/s10745-017-9936-z , journal=Human Ecology , language=en , volume=45 , issue=5 , pages=683–695 , doi=10.1007/s10745-017-9936-z , issn=1572-9915 , pmc=5680367 , pmid=29170591, bibcode=2017HumEc..45..683V

References

Corallinales Environment of Cornwall