Maerl off the coast of theIsle of ArranCalcified remains of maerl, on the "coral beach" in theIsle of SkyeMaerl inLanildut.Pieces of popcorn-like maerl from Fuerteventura island.
Maerl (alsorhodolith) is a collective name for non-geniculatecorallinered algae with a certain growth habit.[1] Maerl grows at a rate of c. 1 mm per year.[2] It accumulates as unattached particles and forms extensive beds in suitable sublittoral sites.[3] The term maerl originally refers to the branched growth form of Lemoine (1910)[4] andrhodolith is a sedimentological or genetic term for both the nodular and branched growth forms (Basso et al., 2015).[5] 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.[6]
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.[7] The distribution of maerl is dependent on water movement, light and salinity concentration.[8] Maerl beds occur in thephotic zone, and can be found to around 30 m depth in the British Isles and up to 120 m deep in the Mediterranean.[9] 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.[10]
Maerl is dredged from the sea floor and crushed to form a powder. It is still harvested around the coasts ofBrittany inFrance andBantry Bay,Ireland, and is a popular fertilizer fororganic gardening. It was also dredged offFalmouth, Cornwall, but this ceased in 2004.
Scientists investigated Falmouth maerl and found thatL. corallioides predominated down to 6 m andP. calcareum from 6–10 m.[14][15] A 2024 survey by researchers from theUniversity of Exeter, commissioned by Cornwall Council's Blue Natural Capital Project, found that theFal and Helford Estuary Special Area of Conservation supports an 880 hectare bed of maerl.[16][17][18]
Maerl fragments, Trá an Doilin, Maerl Beach (incorrectly known as "Coral Strand") Carraroe, County Galway, Ireland
Chemical analysis of maerl showed that it contained 32.1%CaCO3 and 3.1% MgCO3 (dry weight).
The ecology of maerl habitats has received very little attention in contrast to other marine ecosystems such askelp forests orsea grass beds.[19] Maerl beds provide a complex habitat for a wide range oftaxa[20] with a variety of niches that support high associated invertebrate and algal biodiversity.[8]Maerl beds act as nursery areas for the juvenile stages of commercial species such as juvenilecodGadus morhua,saithePollachius virens,PollackPollachius pollachius[21] and juvenilescallopsAequipecten opercularis.[22] 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.[9][22]
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.[19] Large scale maerl extraction over the past 40 years has removed and degraded maerl beds.[24] In Cornwall, England, maerl has been extracted since the 1970s, but was banned in 2005 by Falmouth Harbour Commissioners.[25]
An early reference to maerl was made byJohn Ray in 1690 who reported it from Falmouth. In Ireland, maerl is extracted from subfossil beds in Bantry Bay by Celtic Sea Minerals.[26] The maerl-forming speciesLithothamion corallioides andPhymatolithon calcareum are listed in Annex V of the EC Habitats Directive.[27]
Used as a soil conditioner, it is dredged from the sea floor and crushed to a powder.[28]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 anon-renewable resource, and readily available alternative products (e.g., garden lime) make modern day exploitation controversial.[29][30][31]
^Steneck, R. S. (1986). "The Ecology of Coralline Algal Crusts: Convergent Patterns and Adaptative Strategies".Annual Review of Ecology and Systematics.17:273–303.doi:10.1146/annurev.es.17.110186.001421.JSTOR2096997.
^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
^Lemoine (1910). "Répartition et mode de vie du maërl ( Lithothamnium calcareum) aux environs de Concarneau (Finistère)".Annales de l'Institut Océanographique.1:1–29.
^Grall, J., Le Loc'h, F., Guyonnet, B., and Riera, P. (2006) "Community structure and food web based on stable isotopes (δ15N and δ13C) analysis of a North Eastern Atlantic maerl bed".Journal of Experimental Marine Biology and Ecology 338, 1–15 [Online] Available at:http://horizon.documentation.ird.fr/exl-doc/vcc/2006/11/010037649.pdf[permanent dead link]
^abWilson, 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. [Online] Available at:http://www.qub.ac.uk/bb-old/provan/pdf/charmaine2.pdf
^Blunden, G; Campbell, S A; Smith, J R; Guiry, M D; Hession, C C; Griffin, R L (1997). "Chemical and physical characterization of calcified red algal deposits known as maërl".J. Applied. Phycol.9 (1):11–17.Bibcode:1997JAPco...9...11B.doi:10.1023/A:1007965325442.S2CID43607799.
^abNelson, W (2009). "Calcified macroalgae – critical to coastal ecosystems and vulnerable to change: a review".Journal of Marine and Freshwater Research.60 (8):187–801.doi:10.1071/MF08335.
^Steller, D.L., Riosmena-Rodríguez, R., Foster, M.S., Roberts, C.A. (2003). "Rhodolith bed diversity in the Gulf of California: the importance of rhodolith structure and consequences of disturbance".Aquatic Conservation: Marine Freshwater Ecosystem 13, 5–20. [Online] Available at:http://onlinelibrary.wiley.com/doi/10.1002/aqc.564/pdf
^abKamenos, 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. [Online] Available at:http://icesjms.oxfordjournals.org/content/61/3/422.short
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