Maerl-forming species

Characteristic features of most common maerl species

Distribution

There are a large number of seaweeds that deposit calcium salts within their tissues - many of these are crust-forming members of the Rhodophyceae. Of these crust-forming red algae, a proportion of the species may also be found free-living as maerl, not attached to the rock or pebble substratum. Coralline species contributing to maerl beds seem to be those capable of growing on lightweight, mobile substrata and/or continuing to grow as mobile portions of thallus after becoming detached (Irvine & Chamberlain, 1994). Several species in several genera can form maerl beds; these are differentially distributed, as summarised in Table 1 below. Under appropriate conditions live, branched maerl can build up to 2 (-10) m above the surrounding sea floor (J. Hall-Spencer, pers. comm.), sometimes occupying thousands of square metres (Irvine & Chamberlain, 1994). Deposits of unattached coralline algae are found in both tropical and temperate seas of the world (Bosence, 1983b).

Three main species of free-living coralline algae are reported to occur in European waters, with at least a further six (to eight) species known to contribute to deposits in certain areas (linked table). Phymatolithon calcareum is often the most abundant maerl species, with other species usually found only as minor elements of the maerl bed.

Lithophyllum incrustans can also occur as a maerl component, and the spectacular large Lithophyllum thalli found in western Ireland (e.g. Irvine & Chamberlain, 1994: fig. 23) may be attributable to this species. Recently Basso (e.g. 1995a, 1995b) has begun a taxonomic investigation of maerl-forming species in the Mediterranean. Varieties of L. corallioides have previously been recognized (Cabioch, 1969) but are now considered taxonomically superfluous (although they may have ecological significance).

Classification of maerl biotopes requires that the physical descriptors of the site (water depth, substratum composition, wave exposure regime, salinity and tidal currents) are known as well as the species of maerl-forming algae. The main species of maerl-forming algae can be difficult to tell apart without extensive experience of identification of coralline algae. The characteristic features of the three most common species of maerl in the British Isles, abstracted from Irvine & Chamberlain (1994) are summarised in the table below (further details given in the Appendix).

In practice, a combination of the surface texture (glossy for L. corallioides only) and the colour is most useful, and distinguishes quite well between L. corallioides and P. calcareum, although it is problematic for discriminating between L. glaciale and P. calcareum. The chalky surface of P. calcareum may be diagnostic in cases for which these latter two species are likely to be confused. To be certain, one must examine sterile thalli microscopically as identification by eye is unreliable.

Distribution of European maerl species is currently being reviewed as part of the EC MAST-funded BIOMAERL programme (J. Hall-Spencer, pers. comm.). Although patchily distributed, maerl beds are found throughout the Mediterranean Sea, with important beds in Algeria (Feldmann, 1943), at Marseilles (Huvé, 1956), in Corsica and Sardinia, and in the Aegean (Jacquotte, 1962). Maerl is common on the Atlantic coasts, from Norway and Denmark in the north to Portugal in the south (extending to Morocco and Mauritania on the African coast). It is particularly abundant in Brittany (see J. Cabioch, 1966, 1969, 1970). Spanish maerl deposits are confined mainly to the Ria de Vigo and Ria de Arosa (Galicia, NW Spain) (Adey & McKibbin, 1970; J. Hall-Spencer, pers. comm.). In Ireland, maerl is widely distributed in the south and south-west (e.g. Galway Bay, Bantry Bay, Roaringwater Bay). Phymatolithon calcareum is the most widespread maerl species in Europe. Maerl is absent from large areas of Europe, such as most of the North Sea, the Baltic, the Irish Sea and eastern English Channel, presumably due to environmental constraints.

Figure - the main sites where maerl has been studied in Europe.

Maerl beds are patchily distributed around the coasts of the UK. They are nearly all on exposed west coasts of Britain, where there are no major rivers carrying large quantitites of suspended sediment. The dominant coastal rocks are crystalline (T. Scoffin, pers. comm.), especially in west Scotland and west Ireland, so that weathered fine terrigenous sediments are generally absent (with the exception of reworked glacial deposits). Maerl beds are typically associated with sounds or estuaries with currents but protection from strong waves.

In southern Britain, maerl beds consist of Phymatolithon calcareum and Lithothamnion corallioides. Lithothamnion corallioides is replaced in Scotland by L. glaciale (Hall-Spencer, 1995b). Phymatolithon calcareum is both the most widely distributed and the most abundant maerl species in the UK.

In Scotland, maerl is widespread along the west coasts, in the Western Isles and Orkney and Shetland. It is known from the north coast (Loch Eriboll) but is absent from east coasts.

In Wales it is restricted to a small area of Milford Haven and small patches around the Pembrokeshire Islands and Lleyn peninsula.

In England also maerl is rare. Living maerl (including L. corallioides) grows on the St Mawes Bank in the Fal Estuary, the largest known area of the biotope in England. Maerl has also been reported from the mouth of the Helford River. Deep deposits of dead plants (described as sub-fossil) are known in other parts of Carrick Roads and in Falmouth Bay and these show that maerl formerly covered a much wider area. Maerl beds are also reported from Dorset (Phymatolithon calcareum; Irvine & Chamberlain, 1994) and small amounts occur in the Isles of Scilly and Lundy.

In Northern Ireland, extensive maerl beds are found on the north-east coasts at Garron Point and Ballygalley Head, whilst scattered maerl has been recorded from a number of sites including Church Bay, Ringfad Point, Cushendun Bay and Carlingford Lough. A thin maerl bed of small extent is present in Strangford Lough but it has not been investigated in detail (Erwin et al., 1986). On the open coast maerl is found from approximately 10 m to 35 m, with dense beds at 15-25 m, whilst in Carlingford Lough it is in 2-5 m. In both cases the tidal streams are 2-4 knots. Morton (1994) cites a 19th century record of Lithothamnion corallioides from Belfast Lough but this is unlikely to be present now due to subsequent industrial development, the dredging of navigational channels and recent levels of sewage pollution (see Brown et al., 1997).  The linked figure  shows the main sites where maerl has been studied in the UK and Ireland. In the UK, maerl beds occur in three of the 12 project demonstration site candidate SACs, and in the Fal and Helford candidate SAC (see table below). Of these sites the best studied are the Sound of Arisaig and the Fal and Helford. The extensive maerl beds in the Sound of Arisaig have been mapped by acoustic remote sensing, and sampled biologically by diving and diver-operated corers (e.g. Howson, 1994; Davies & Hall-Spencer, 1996). The Fal and Helford maerl bed on St Mawes bank has a long history of biological study, principally by a group from Portsmouth University (e.g. Blunden et al., 1981, 1997; Farnham & Jephson, 1977; Farnham & Bishop, 1985) and by the NCC (Rostron, 1985) and English Nature (Davies & Sotheran, 1995). The extent of the relatively small live bed and the very extensive dead beds have been surveyed, and both fauna and flora investigated by divers. The maerl has been chemically characterised. As noted above, the Strangford Lough maerl beds have not been studied in detail and the only published work concerns the molluscan fauna (Nunn, 1992).

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