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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
Reference |
Location and depth range
(m below chart datum, where available) |
Number of species (total recorded or no.
found in limited samples) |
Algae |
|
|
| 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) |
| Molluscs |
|
|
| 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.
Next Section
References
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