To date, little work has been carried out explicitly from the point of view of community structure, although a study is currently underway in Britanny as part of the BIOMAERL programme. However, considerable relevant information is available from studies of maerl species and of other organisms.

Many coralline algae produce chemicals which promote the settlement of the larvae of certain herbivorous invertebrates. The herbivores then graze off the epiphytic, and often fast-growing, algae which might otherwise overgrow the coralline algae, competing for light and nutrients. Another strategy for maintaining epiphyte-free surfaces of coralline algae has recently demonstrated in Japanese representatives of the crustose genus Lithophyllum (Suzuki et al., 1998). The allelopathic production of chemicals by the crust directly prevents overgrowth by epiphytes.

The presence of herbivores associated with corallines can generate patchiness in the survival of dominant seaweeds. In addition to the ecological importance of live maerl beds, which is described below, dead maerl contributes in two ways. Firstly, dead maerl supports diverse communities, although these are generally reported to be less rich than those in live maerl beds (Keegan, 1974). Secondly, maerl is one of the sources of subtidal and beach-forming calcareous sediments. In Scotland, maerl can form up to 4% of calcareous sediments (Farrow et al., 1978).

There are numerous features of maerl that contribute to its value as a habitat for other marine species (Nunn, 1992):

• It provides a surface to which other seaweeds e.g. Plocamium cartilagineum can attach. Other organisms, e.g. Aplysia punctata and rissoids, then feed on these seaweeds.
• It can be grazed itself by organisms such as Tectura (Acmaea) virginea.
• The algal film and detritus can also be grazed by e.g. Jujubinus montagui.
• It provides attachment sites for animals which in turn are food for others, e.g. Antedon bifida, hydroids, bryozoans.
• The infauna in maerl beds includes many bivalves, e.g. Mya truncata, Dosinia exoleta.
• Its loose structure provides shelter, e.g. for small gastropods.

Bosence (1979) carried out a community analysis of all animals associated with maerl in Mannin Bay, Galway (see table below), classifying them into vagile (i.e. mobile) epifauna, sessile epifauna, burrowing infauna and boring infauna, and further indicated their trophic group (herbivore, carnivore/scavenger, deposit feeder, suspension feeder, commensal). The maerl bank community was characterized by abundant vagile epifauna. Gastropods were common in the lattice formed by the maerl, the most abundant species being the herbivores Bittium reticulatum and Gibbula cineraria. Small decapod crustaceans such as Porcellana longicornis and Galathea squamifera could move within the maerl lattice, while larger species formed burrows or swam over the surface. More recently, Grall & Glémarec (1997) have examined the community structure of maerl at control and impacted (eutrophicated or harvested) sites in Brittany using multivariate analysis (see Sensitivity to human activities).

Both floristic and faunistic studies have focussed on biodiversity aspects of maerl communities, as discussed in the next chapter. Some of the epifloral species listed in a summary table may be key to the integrity of the maerl bed, either physically binding the maerl or biologically interacting within the biotope.

Although bare maerl substratum occurs throughout the year, competion for space between crustose species is high. The chemical and growth rate interactions between crustose algae in competing for space have been investigated (Fletcher, 1975; Maggs, 1983a) and some crustose species are known to slough epithelial layers as a means of reducing epiphyte cover. These mechanisms make for continual shifts in the population of the epiflora and promote the diversity of the maerl biotope flora. Changes in the environment of the maerl biotope, particularly any which influenced the interactions of the coralline species, might affect settlement of the epiflora, changing the species mix, probably reducing the epifloral diversity and possibly resulting in the dispersion of the maerl bed. Alternatively, reduction of epiphytism by some species could enhance the growth rate of maerl due to increased penetration of light to the maerl thalli.

Several species of red and green filamentous algae are common borers into maerl (Cabioch, 1969), and may contribute to the breakup of maerl thalli. As noted above (under Reproduction), the most important maerl-forming species, Phymatolithon calcareum, rarely produces conceptacles. The main way maerl beds of this species build up is through fragmentation. J. Hall-Spencer (pers. comm.) has noted that it can colonise new areas of sedimentary substrata by transport of live thalli attached to algae - particularly Laminaria saccharina and Phycodrys rubens in Scotland. These large algae can transport maerl over considerable distances after storms.

The various maerl species can be regarded as keystone species within the maerl beds in which they occur because the community depends on their biological and structural characteristics. However, the integrity of some forms of maerl bank in turn requires at least some elements of the rich epiflora associated with it, and interactions with invertebrate grazers are also very important in keeping open substratum clear for settlement by algal and animal species. It should be pointed out here that some of the deeper Scottish maerl beds are floristically poor so that this does not apply to them (J. Hall-Spencer, pers. comm.).

In general, maerl beds form a fragile and easily disturbed habitat for a rich assemblage of seaweeds and invertebrates. Under some conditions, they can be relatively stable communities over long timescales. In Northern Norway, for example, although the maerl beds have fluctuated with glaciation-related changes in the relative sealevel and shore position, the oldest layers within the accumulated sediments have been ¹⁴C dated to about 6000 years old (Freiwald et al., 1991). Individual pieces of dead maerl in the Sound of Iona, Scotland, were dated at c. 4000 years old (Farrow, 1983).

Both Jacquotte (1962) and Cabioch (1969) discussed the importance of various prostrate algae in stabilising the maerl deposits by the formation of stolons and secondary attachments (see table below). These growths apparently act as an effective means of vegetative reproduction for these prostrate species, several of which were never observed with reproductive organs. The morphology of Gelidiella calcicola (as Gelidiella sp. in Cabioch, 1969), which is largely confined to maerl, seems to have evolved in response to the maerl habitat. Unlike other gelidiacean algae, it forms no erect axes - all axes bend down at the tips and reattach to the maerl by specialised peg-like holdfasts that penetrate into the maerl.

In general, the seasonal stabilisation of maerl beds is advantageous, permitting the summer growth of many larger algae, but clearly, if the structure became permanently bound together by excessive algal turfs, this could affect the nature of the maerl bed detrimentally. It may be significant that the alien red alga Polysiphonia setacea, which stabilises maerl beds in the Mediterranean, is currently increasing greatly in abundance and may soon affect the majority of Mediterranean maerl beds.

Invertebrates are also important in the structural integrity of maerl. The bivalves Modiolus modiolus and Limaria hians bind maerl together with their byssal threads. Deep burrowers and tube dwellers (e.g. Cerianthus, Sabella, Chaetopterus and Upogebia) can stabilise surface sediments. Crabs (Cancer pagurus) and starfish (Asterias rubens) dig pitfall traps to catch prey.

Suggestions have been made that maerl beds may be important nursery areas for commercially valuable molluscs and crustaceans. However, maerl has been little-studied as a habitat for the juvenile stages of demersal and pelagic fish species. Divers visiting maerl beds or collecting samples for maerl studies have commented on the large numbers of small individuals of many species that can be seen, and certainly the open structure of a maerl bed would provide a secure habitat for juveniles as well as a wide range of flora and fauna as food for them.

The nursery interpretation of maerl biotopes is rather controversial (e.g. in south-west Ireland no nursery activity was observed during maerl bed surveys; S. de Grave, pers. comm.) but there is some good evidence that maerl beds are nurseries for at least a few species. In Co. Clare, maerl deposits are known to act as nursery grounds for the black sea urchin Paracentrotus lividus. Juvenile urchins can be obtained for aquaculture purposes by dredging small quantities of maerl and removing the urchins using benzocaine (Minchin, 1997). In these maerl beds, densities of more than 1600 individuals per square metre of surface area (down through the depth of the maerl) have been counted (Keegan, 1974). In France, juvenile scallops have been collected experimentally from spat collectors placed over maerl (Thouzeau, 1991). Similarly, the presence and abundance of scallop spat in benthic samples from the west of Scotland (Sound of Raasay) was apparently correlated with the presence of maerl (D. McKay, pers. comm.).

Spatial competition between flora and fauna was not generally noted as a major factor of population structure control (Hily et al., 1992) in the maerl beds of the rade de Brest. However, at a few locations the abundance of large suspension feeders (e.g. the ascidian Phallusia mamillata) was such that they occupied more than half the available surface area. In sites such as these it was noted that opportunistic algae were best adapted to compete for space. Bosence (1979) described competition for space between encrusting algae and animals in Mannin Bay. Bryozoans and foraminiferans were overgrown by coralline algae, whereas Halichondria, Anemonia sulcata and serpulids overgrew the living maerl.

The presence of both generalist and specialist herbivores is essential for the health of maerl beds. Generalist herbivores graze off epiphytic algae which might otherwise shade the coralline algae. There is constant erosion of the surface of the maerl by sea urchins and specialist grazers such as the small limpet Tectura virginea. Around the UK, T. virginea, which also feeds on shell-boring algae (Farrow & Clokie, 1979), is one of the main grazers on maerl. Very large populations may be found and it is likely that these small limpets settle selectively on coralline algae, as has been shown for Haliotis species (abalone) by Morse & Morse (1984). The surface of the maerl is kept clear of microalgae and algal sporelings by the feeding activities of Tectura, so that bare substratum is always available. The radula action also wears away the surface layers of maerl thalli creating a clear and more easily penetrable surface for settlement of algal spores.

Population densities of Sphaerechinus granularis of 2-3 m^-2 were found to affect the algal cover, on small temporal and spatial scales, on maerl beds in the rade de Brest (Hily et al., 1992) but on a larger scale and longer time span, it was suggested that the grazing pressure was not of an intensity to modify the species composition of the assemblage. Maggs (1983a), however, reported that the high diversity of algae on maerl in Galway Bay (50-80 species of epiphytic algae per sample depending on sample size (300 cm³ or 1500 cm³)) might be due in part to the reduced grazing pressure relative to hard substrata. The microtopography of the maerl itself provides some protection from grazers in that the interlocking, branched shapes restrict access to larger grazing species.

Boring polychaetes and sponges probably affect production rates and may be involved in maerl fragmentation. The most conspicuous borer into live algae is the polychaete Polydora, which is thought to bore both mechanically and by chemical activity (Bosence, 1979).

One of the principal substrata in several maerl biotopes is mollusc shells, present usually as shell gravel, but also as variable quantities of intact shells. Intact shells are favoured by large species of algae, such as young kelps. In the rade de Brest, the population dynamics, particularly the mortality rates, of the shelled molluscan species in the maerl beds had an indirect effect on the algal population (Hily et al., 1992). The dead shells formed a major substratum for the algae, but as the attached biomass increased, the shell/algal assembly became more buoyant and susceptible to transport by tide and wave currents, thus moving the shell support shoreward and removing the attached species of algae from the population of the maerl bed.

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