||Range of conditions
||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)
||Exposed (IGS.Mrl), Moderately exposed (IGS.Mrl), Sheltered,
Very sheltered (IMX.MrlMx)
||Strong (IGS.Mrl), Moderately strong (IGS.Mrl), Weak, Very
||Gravels (IGS.Mrl), Clean gravels (IGS.Mrl), Muddy gravels
||0-20 m (IGS.Mrl), 0-10 m (IMX.MrlMx)
||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.
||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.
||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.
||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 .