Maerl (or marl) is a collective name for Coralline red algae with a certain growth habit.[1] It accumulates as unattached particles and forms extensive beds in suitable sublittoral sites.[2]
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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,[1]Lithothamnion glaciale and Phymatolithon calcareum.[3][2]
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).[4][5]
Chemical analysis of maerl showed that it contained 32.1% CaCO3 and 3.1% MgCO3 (dry weight).
Maerl has no tolerance for desiccation.[6]
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.[3] In Ireland, maerl is extracted from subfossil beds in Bantry Bay by Celtic Sea Minerals [3]. The maerl-forming species Lithothamion corallioides and Phymatolithon calcareum are listed in Annex V of the EC Habitats Directive [4].
Used as a soil conditioner, it is dredged from the sea floor and crushed to a powder.[7] 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.