Maerl

Maerl (also known as rhodolith) a collective name for non-geniculate coralline red algae with a certain growth habit. Maerl grows at a rate of ∼1mm per year. It accumulates as unattached particles and forms extensive beds in suitable sublittoral sites. The term maerl originally refers to the branched growth form of Lemoine (1910) and the term rhodolith is sedimentalogical or genetic term for both the nodular and branched growth forms (Basso et. al, 2015).

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. Maerl beds occur in the photic zone, 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 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.[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).

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

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