In general, the coralline red algae are the deepest living of any of the marine algae, having been seen growing at depths in excess of 300 m (from a submersible) in the clear waters of the Caribbean (Littler et al., 1986, 1991). Free-living coralline algae (rhodoliths) in tropical waters can usually be 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), Brittany and elsewhere (Norway, Scotland and amongst seagrasses in the Mediterranean), maerl occurs intertidally, generally only near the extreme low-water mark.

Depth range of living maerl beds in the British Isles, Europe and elsewhere

The light levels under which maerl can thrive are suggested by the depth ranges in which it grows (see table above), in areas where it is subject to a particular water clarity (Figure). In the Mediterranean, where water is of oceanic quality, some maerl beds are found below 100 m (Jacquotte, 1962). The Outer Galway Bay, where maerl grows down to 30 m, receives warm, high salinity southern water of North Atlantic Drift (and occasionally Lusitanian) origin (O'Brien, 1977; O'Connor et al., 1993). In Mannin Bay (where water clarity is much reduced compared to the Outer Galway Bay due to the occurrence of coastal water), Bosence (1976) found that dense maerl beds were restricted to less than 8 m depth by light penetration. He reported that light was the limiting factor for maerl growth in Mannin Bay. Growth was best at 1-8 m, and ceased below 16 m at 12-13 °C. In the British Isles, maerl beds have been recorded to 27 m (Irvine & Chamberlain, 1994), but are most frequently reported at depths of 1-10 m. On the Channel and Atlantic coasts of France, few maerl beds are deeper than 20 m, probably due to the turbidity of the coastal waters (Giraud & Cabioch, 1979).

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