Substratum nature is an important factor in the spatial distribution of algal and faunal assemblages in general (Hily et al., 1992). Maerl beds can be found in association with a range of different sediments, varying in size from fine mud to coarse gravel and pebbles (as shown in the MNCR biotopes classification, and in the Appendix). Jacquotte (1962), L. Cabioch (1968), J. Cabioch (1969, 1970) and Bosence (1976) all considered that the importance of sediments with a low proportion of fine sands and muds had previously been overestimated; in the Mediterranean maerl is often mixed with fine mud. On the contrary, some maerl species or morphological forms show a preference for finer substrata (J. Cabioch, 1969).

The density at which numbers of living thalli are to be found on different underlying substrata has apparently not been investigated in detail. In the sound of Iona (Cucci, 1979) estimated that about 22,000 thalli m^-2 were present in the surface layer of the maerl bed, but that the proportion of living thalli varied in different areas of the sound. Keegan (1974) reported that animals associated with maerl differed according to whether it was on a soft bottom, supporting burrowing animals e.g. Mya arenaria, or hard, dominated by brittlestars and crinoids. Some of the underlying substratum may be mixed with maerl, or there may be no obviously terrigenous material present, the maerl bank consisting solely of living maerl overlying deposits of dead maerl.

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