Geographical comparisons between Maerl Beds

Maerl epiflora and fauna has been compared between and within geographical areas by several workers. For example, Jacquotte (1962) reported that the fauna of maerl beds in the Mediterranean was more diverse than the maerl beds of Brittany. However, these rough comparisons were based on very limited data. Given the paucity of detailed information concerning the biodiversity of European maerl beds, it is difficult to say which exhibit the highest diversity. As a general rule, Mediterranean biotopes have a high species diversity but low productivity due to low nutrient concentrations (Boudouresque, 1993). On Atlantic coasts, preliminary comparisons of maerl biodiversity are possible for seaweeds and molluscs which have been relatively well studied.

For example, a total of 123 live species of mollusc were found by Nunn (1992) in maerl beds in Scotland and Ireland, while Hall-Spencer (1998) found 130 species on two very small areas of maerl in the Clyde (see table below; summary table). Data in the table below indicates that maerl biotopes can exhibit impressive levels of biodiversity. Those from Galway Bay seem to be particularly species-rich for algae compared with the west of Scotland. Although this may be related to the small number of samples examined from Scotland, maerl beds off Coll and Tiree, Hebrides, undoubtedly support a much reduced epifloral community compared with that found on the maerl beds of the more southerly Galway Bay. Factors that are probably involved include the northern distributional limits of some species lying to the south of the Hebrides, the lack of Lithothamnion corallioides, as this species tended to support a higher diversity of algae in Galway Bay than did Phymatolithon calcareum beds, and the greater wave action to which these beds are exposed. High wave-exposure is correleated with low species richness (see Local variations section below). The relatively low overall total for the Fal is a result of a single period of collecting, by contrast with the extended sampling in Galway Bay. There is no clear pattern for the molluscs.

Comparison of species diversity (ie richness) reported by different authors, for algae and molluscs in maerl beds of the British Isles

Samples collected by Maggs were all equivalent (300 cm3); see summary table for more details


Location and depth range

(m below chart datum, where available)

Number of species (total recorded or no. found in limited samples)






Maggs, 1983a

2 sites in Galway Bay; 5 and 10 m 147 (total); mean per summer sample set, 51-58 (n = 6)
Maggs in Dipper, 1981 Coll & Tiree, Hebrides; 14-19 m. 28-35 per sample (n = 3)
Maggs in Dipper, 1981 Coll & Tiree, Hebrides; 9 m. 43 (in samples)
Hall-Spencer, 1998 Clyde Sea area; 10 m 57 (total)
Davies & Hall-Spencer, 1996 Sound of Arisaig, 2-21 m 85 (total)
Howson, 1990; Howson et al., 1994 Arisaig area 42 (total)
Maggs in Rostron, 1988 St Mawes Bank, Fal; 7 m 60 (total); 44-50 per sample (n = 3)



Keegan, 1974

Galway Bay; 0-30 m 59 (total)
Nunn, 1992 Strangford Lough; c. 8 m 78 (total)
Nunn, 1992 Galway Bay 56 (total)
Nunn, 1992 W. Scotland 42, 17, 14 (in samples)
Hall-Spencer, 1998 Clyde Sea area; 10 m 130 (total)
Davies & Hall-Spencer, 1996 Sound of Arisaig; 2-21 m 72 (total)
Howson, 1990; Howson et al., 1994 Arisaig area 32 (total)

Comparisons can also be made in terms of species composition, rather than simply species richness. Farnham & Jephson (1977) reported that the maerl beds at Falmouth supported a seaweed flora similar to that found across the Channel in Brittany. Blunden et al. (1981) gave more details of the maerl epiflora, and described the algae from three maerl bed sites near Galway (at Finavarra, near Carna and in Kilkieran Bay). These also showed similarities with the epiflora in Brittany, including some of the most characteristic species of the Brittany maerl beds.

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