Habitat requirements

Habitat factor Range of conditions
Salinity Full, Variable (IGS.Mrl)

The salinity tolerance of maerl is species specific with Phymatolithon calcareum, Lithothamnion corallioides and Lithophyllum sp. usually associated with full salinity areas and Lithothamnion glaciale with variable salinities such as in Scottish sealochs. Maerl beds in Galway Bay, Ireland are subject to fully saline water for most of the year, bottom salinity being measured as between 34.4 ‰ and 34.8 ‰. However, in February and April the salinity was reduced to about 30 ‰ (Birkett et al 1998)

Wave exposure Exposed (IGS.Mrl), Moderately exposed (IGS.Mrl), Sheltered, Very sheltered (IMX.MrlMx)
Tidal streams Strong (IGS.Mrl), Moderately strong (IGS.Mrl), Weak, Very weak
Substratum Gravels (IGS.Mrl), Clean gravels (IGS.Mrl), Muddy gravels (IMX.MrlMx)
Zone Infralittoral
Depth range 0-20 m (IGS.Mrl), 0-10 m (IMX.MrlMx)
Temperature Maerl biotopes occur in a wide range of temperature regimes, from the tropics to northern Norway, but the species composition and distribution of the maerl beds is greatly influenced by temperature. The most obvious temperature-related phenomenon in the UK is the absence of Lithothamnion corallioides from Scotland, either because winter temperatures occasionally drop below the minimum survival temperature of this species (between 2-5oC) or because temperatures do not remain high enough for long enough to support sufficient annual growth. Laboratory studies on Spanish maerl (Adey & Mckibben 1970) showed that Phymatolithon calcareum survived down to 2oC, dying at 0.4oC, and that the optimum growth was at 15oC. Lithothamnion corallioides had a higher minimum survival temperature, dying at 2oC, and surviving without growth at 5oC.
Water quality The light levels under which maerl can grow are suggested by the depth ranges in which it grows. Maerl found in tropical waters is usually found at depths below the range of the reef -binding coralline algae associated with coral reefs. At the other extreme of the habitat range, at a few sites in western Ireland (e.g. Mannin Bay, part of Killary Harbour and Muckinish) and Brittany, France maerl occurs intertidally, generally only near the extreme low-water mark.
Nutrients Cabioch (1969) has suggested tolerance of elevated nutrient levels on the basis of field observations of maerl distribution in Brittany, France; however experimental studies are lacking.
Calcium King & Scramm (1982) reported that the salient factor affecting growth of maerl in culture experiments using various salinity growth media was the calcium ionic concentration, rather than salinity per se. They found an optimum uptake of calcium carbonate at 30 ‰.

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