Biodiversity & Conservation

The ecological relationships of maerl beds can be very complex. The maerl thalli provide considerable surface area to which both flora and fauna can attach. The maerl nodules themselves may be directly grazed by species like Tectura virginea. The surface film of microalgae and detritus can also be grazed by animals such as Psammechinus miliaris. The loose structure permits water circulation and oxygenation to considerable depth. As a consequence of this loose structure, maerl provides shelter for a wide of fauna e.g. molluscs (Hall-Spencer, 1998) and amphipods (Grave de, 1999). The loose structure also permits animals to burrow to considerable depths (at least 60 cm) within the gravel.

Seasonal changes in the biotope potentially include variations in the amount of ephemeral algae growing on and over the maerl. Temperature controls the development of reproductive conceptacles in Lithothamnion glaciale (Adey, 1970).

Habitat complexity is bought about by the branching nature of the maerl and the open spaces between nodules that occur. However, the maerl nodules are frequently loose and mobile preventing colonization by many species. In some instances, the branched nodules can also interlock creating a more stable environment. Species such as Saccharina latissima and Lanice conchilega may also help to bind the maerl together. The highly branching nature of the maerl thalli permits oxygenation and circulation of water deep within the sediment. This is exploited by many burrowing animals and deep burrowing (to 68 cm) fauna are a notable feature of maerl beds (Hall-Spencer & Atkinson, 1999). Most surveys under-record the species in the biotope, primarily because the vast majority of species live below the maerl surface. Maerl in general is known as a particularly diverse habitat with over 150 macro-algal species and 500 benthic faunal species recorded (Birkett et al., 1998(a)).

Maerl beds may contain dead as well as live nodules. Productivity will depend on the relative proportions of dead and live nodules. In the British Isles this biotope tends to occur in shallow waters down to 10 metres in depth where algal primary productivity may be boosted by the occurrence of epiphytic algae. Some maerl beds may have very high faunal densities and in these, secondary production may be very high.

The main recruitment mechanism of Lithothamnion glaciale is uncertain. Individual plants have reproductive conceptacles (whether sexual or asexual is unclear) during the winter months and are sterile in summer (Hall-Spencer, 1994 cited in Birkett et al., 1998). Recruitment may occur via planktonic dispersal of sexual or asexual propagules. Vegetative growth and division of maerl nodules also forms a propagation mechanism in the biotope. In fact, in the British Isles this may be the only form of propagation in the species Phymatolithon calcareum (also a minor component of this biotope) and Lithothamnion corallioides. Hall-Spencer (pers. comm.) has observed that colonization of new locations by maerl can be mediated by a 'rafting' process where maerl thalli are bound up with other sessile organisms that are displaced and carried by currents (e.g. Saccharina latissima (studied as Laminaria saccharina) holdfasts after storms). Of the other species typically found in this biotope , Psammechinus miliaris is highly fecund with relatively long lived planktonic larva that may disperse and colonize from long distances. Ophiothrix fragilis also has a long-lived planktonic larva that can disperse over distance.

Lithothamnium glaciale is very slow growing (although faster than other maerl species (Irvine & Chamberlain, 1994). Individual plants are estimated to live from between 10-50 years (Adey, 1970) and would need a long period for populations to expand into a 'bed'. Development into a thick bed with the associated interstices and open structure important for the development of the associated community would take even longer. Maerl beds are known to be extremely long lived with life-span of the habitat being 6000 years or more (Birkett et al., 1998(a)). Within the biotope, the community is dependent on the growth of a surface veneer of photosynthetically active maerl thalli.

Outward appearances of the biotope may be misleading with respect to dominant trophic groups. Grall & Glémarec (1997) found that dominant trophic groups in maerl beds varied according to the assessment criteria used. In terms of species richness, carnivores were most dominant, while detritivores were the most abundant and surface deposit feeders had the highest biomass.