In the British Isles maerl is composed of three species of coralline algae growing loose in beds of fragmented nodules in the sub-littoral. The species generally involved are: Lithothamnion corallioides,Lithothamnion glaciale and Phymatolithon calcareum.
Maerl is dredged from the sea floor and crushed to form a powder. It is still harvested around the coasts of Brittany in France and Bantry Bay, Ireland, and is a popular fertilizer for organic gardening. 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).
Chemical analysis of maerl showed that it contained 32.1% CaCO3 and 3.1% MgCO3 (dry weight).
The distibution of maerl is controlled mainly by light (and water clarity), temperature and water motion. Maerl has no tolerance for desiccation. 
An early reference to maerl was made by John Ray 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 . The maerl-forming species Lithothamion corallioides and Phymatolithon calcareum are listed in Annex V of the EC Habitats Directive .
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, and readily available alternative products (e.g., garden lime) make modern day exploitation questionable.
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