Phymatolithon calcareum maerl beds in infralittoral clean gravel or coarse sand
|Within Vocab||Marine Habitat Classification for Britain and Ireland Version 97.06|
|Definition||Maerl beds characterised by Phymatolithon calcareum in gravels and sands. Associated epiphytes include red algae such as Cryptopleura ramosa, Brongniartella byssoides and Plocamium cartilagineum with Desmarestia spp. and Dictyota dichotoma also very often present. Algal species may be anchored to the maerl or to dead bivalve shells amongst the maerl. Polychaetes, such as Chaetopterus variopedatus, and the gastropods Gibbula magus and Gibbula cineraria may be present. Liocarcinus depurator and Liocarcinus corrugatus are often present, although they may be under-recorded; it would seem likely that robust infaunal bivalves such as Circomphalus casina, Mya truncata and Dosinia exoleta are more widespread than available data currently suggests. IGS.Phy contains two distinct entities depending on depth: a shallower type with red seaweeds (IGS.Phy.R) and a lower infralittoral entity with notably less epiphytic seaweeds (IGS.Phy.HEc). It seems likely that stable wave-sheltered maerl beds with low currents may be separable from IGS.Phy; having a generally thinner layer of maerl overlying a sandy /muddy substratum with a diverse cover of epiphytes (e.g. Bosence 1976; Blunden et al. 1977; 1981; Davies & Hall-Spencer 1996) but insufficient data currently exists on a national scale. Wave and current-exposed maerl beds, where thicker depths of maerl accumulate, frequently occur as waves and ridge / furrows arrangements (see Bosence 1976; Blunden et al. 1977; 1981; Irvine & Chamberlain 1994). At some sites where IGS.Phy occurs, there may be significant patches of maerl gravel containing the rare burrowing anemone Halcampoides elongatus; this may be a separate biotope, but insufficient data exists at present. Northern maerl beds in the UK do not appear to contain L. corallioides but in south-west England and Ireland L. corallioides may occur to some extent in IGS.Phy as well as IMX.Lcor, where it dominates.|
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