Maerl is a collective name for Coralline

Coralline

Coralline means "resembling coral" and may refer to:* Coralline algae, whose fronds are covered with calcareous deposits.* Coralline rock, formed by the death of layers of coralline algae.* Sclerosponges, sometimes called "coralline sponges"....

 red algae

Red algae

The red algae are one of the oldest groups of eukaryotic algae, and also one of the largest, with about 5,000–6,000 species  of mostly multicellular, marine algae, including many notable seaweeds...

 with a certain growth habit. It accumulates as unattached particles and forms extensive beds in suitable sublittoral sites.

Description

In the British Isles

British Isles

The British Isles are a group of islands off the northwest coast of continental Europe that include the islands of Great Britain and Ireland and over six thousand smaller isles. There are two sovereign states located on the islands: the United Kingdom of Great Britain and Northern Ireland and...

 maerl is composed of three species of coralline algae

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...

 growing loose in beds of fragmented nodules in the sub-littoral. The species generally involved are: Lithothamnion corallioides,http://www.algaebase.org/speciesdetail.lasso?species_id=110&sk=0&from=results&-session=abv3:51909EC30a4171DDE6xoK2749EABLithothamnion glaciale and Phymatolithon calcareum.http://www.algaebase.org/CommonDetail.lasso?cname_id=583&sk=0&-session=abv3:51909EC30a4171DDE6xoK2749EAB

Maerl is dredged from the sea floor and crushed to form a powder. It is still harvested around the coasts of Brittany

Brittany

Brittany is a cultural and administrative region in the north-west of France. Previously a kingdom and then a duchy, Brittany was united to the Kingdom of France in 1532 as a province. Brittany has also been referred to as Less, Lesser or Little Britain...

 in France

France

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

 and Bantry Bay

Bantry Bay

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

, Ireland

Ireland

Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...

, and is a popular fertilizer for organic gardening. It was also dredged off Falmouth, Cornwall

Falmouth, Cornwall

Falmouth is a town, civil parish and port on the River Fal on the south coast of Cornwall, England, United Kingdom. It has a total resident population of 21,635.Falmouth is the terminus of the A39, which begins some 200 miles away in Bath, Somerset....

, 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).

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

History

An early reference to maerl was made by John Ray

John Ray

John Ray was an English naturalist, sometimes referred to as the father of English natural history. Until 1670, he wrote his name as John Wray. From then on, he used 'Ray', after "having ascertained that such had been the practice of his family before him".He published important works on botany,...

 in 1690 who reported it from Falmouth. Maerl is still harvested at Falmouth, as well as elsewhere. In Ireland, maerl is extracted from subfossil beds in Bantry Bay by Celtic Sea Minerals http://www.celticseaminerals.com. The maerl-forming species Lithothamion corallioides and Phymatolithon calcareum are listed in Annex V of the EC Habitats Directive http://www.forest-trends.org/biodiversityoffsetprogram/BBop%20library%202/Europe/Not%20Printed/Council%20Directive%201992%20NATURAL%20HABITATS%20Annex%20V.pdf.

Uses

Used as a soil conditioner, it is dredged from the sea floor and crushed to a powder.
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

Non-renewable resource

A non-renewable resource is a natural resource which cannot be produced, grown, generated, or used on a scale which can sustain its consumption rate, once depleted there is no more available for future needs. Also considered non-renewable are resources that are consumed much faster than nature...

, and readily available alternative products (e.g., garden lime) make modern day exploitation questionable.