Salinity

Nutrients

Calcium

Several aspects of water chemistry may influence the development of maerl beds. Although in the past low salinity was thought to favour maerl, this idea has been superseded by the results of salinity measurements made near the maerl surface, which show that maerl beds grow in fully saline waters and do not tolerate strongly reduced salinity. Elevated nutrient levels do not appear to affect maerl beds, and high calcium is advantageous for maerl.

It was previously thought that the occurrence of maerl beds was related to depression of salinity, since maerl beds were commonly found near estuaries (e.g. Joubin, 1910). However, Jacquotte (1962), L. Cabioch (1968), J. Cabioch (1969, 1970) and Bosence (1976) showed that although the surface salinity in the vicinity of maerl beds in France and Ireland is often low, the bottom water is generally fully saline. In Galway Bay, the maerl beds are subject to fully saline water for most of the year, bottom salinity being measured as between 34.4 psu (practical salinity units ~ ppt) and 34.8 psu, but reduced to about 30.0 psu in February and April (Maggs, 1983a). King & Schramm (1982) found that growth of some maerl species is impaired at salinities beow 24 psu.

Lithothamnion glaciale differs from the other maerl species, in that it can tolerate variable salinities in Scottish sealochs, where the biotope IGS.Lgl is found (Connor et al., 1997). Actual salinity measurements at the biotope surface are not available.

Tolerance of elevated nutrient levels has been suggested by J. Cabioch (1969) on the basis of field observations of maerl distribution in Britanny, but experimental studies are still sparse. Recently, Grall & Glémarec (1997) have shown that maerl beds in the bay of Brest are functionally intact, in terms of diversity and species richness, under eutrophicated conditions, although growth of ephemeral algae is promoted.

King & Schramm (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 the salinity per se. They found an optimum uptake of calcium carbonate at 30 psu.

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