Local variations in Maerl biotope biodiversity
Two maerl beds in the same area may differ markedly from each other, not only in the
most common live maerl species forming the bed, but also in the flora and fauna associated
with the bed. This can be illustrated for epifloral species by a comparison of the data
obtained for two maerl beds in Galway Bay (see summary table). The variation in both
floral and faunal composition of maerl beds is dramatic and can be seen by comparing
species lists from different sites. The species composition of the biotopes is also known
to change over periods of years. Furthermore, the number of individuals of a species and
the biomass density of a species and the reproductive habits of individual species will
also vary from site to site.
J. Cabioch (1969) noted that, in addition to the seasonal changes within a maerl bed,
there were local variations in maerl epiflora in the Baie de Morlaix. These variations
were associated with the prevailing environmental conditions and the principal maerl
species of which the beds were composed. In maerl beds composed of finely branching L.
corallioides (var. minima) Audouinella floridula dominated, together
with Aglaozonia parvula and Plocamium cartilagineum. The more open maerl
beds formed by more coarsely branched L. corallioides (var. corallioides)
and P. calcareum supported a wider variety of abundant species, some of which were
only found in the absence of sand mixed in with the maerl.
Fazakerley & Guiry (1998) related the algal species diversity of maerl beds at five
sites in Co. Galway to wave exposure. Diversity, measured as the the number of epiphytic
species per maerl thallus (n=50 for each site), was very significantly higher at the two
wave-sheltered sites compared with two paired wave-exposed sites in the same bays.
Keegan (1974) compared the fauna associated with a series of maerl beds in Galway Bay.
The maerl species was identified as Lithothamnion corallioides var. corallioides.
Samples of the fauna associated with the maerl were collected using a suction dredge.
Maerl beds were divided into the following categories:
1. Intertidal maerl bank. The areas of the bank composed of maerl debris were
relatively compacted and strikingly barren of animal life. In contrast, the living maerl
was loosely accumulated and supported a large sub-surface community. Local aggregations of
Paracentrotus lividus were reported, reaching densities within the layers of the
deposit of over 1600 m-2 surface area.
2. Subtidal maerl bank. A thick blanket of living maerl on a wave-exposed
sloping bottom (Kilkieran Bay), with stong tidal flows. An open lattice formation of the
maerl permitted a great depth of habitat available to the infaunal species. The water
depth was variable.
3. Maerl on soft ground, the thin covering of maerl being swept into wide, shallow
ridges by the strong tidal currents. There was no maerl cover in the troughs. Water depth
16 m. Most of the animals were living in the top 25 cm of the sediment although some,
particularly large individuals, extended to a depth of more than 50 cm (e.g. Mya
4. Maerl on hard ground, transient deposits of maerl debris in high current areas, and
less tide-swept areas with stable banks of maerl and gravel. Water depth 17 m. Dominated
by Antedon bifida and Ophiocomina nigra, with dense aggregations of crinoids
5. Unstable maerl debris, mixed with some sediment.
5 a. Formed into ripples by wave action reflecting the most recent storm activity. The
storm ripples become flattened by the tidal currents, filling in the troughs. Water depth
17 m. A relatively poor faunal diversity with heavy settlement of flora and faunal species
on any stable objects.
5 b. Transitory accumulation of maerl within a channel subject to very strong tidal
flows which render the maerl mobile. Formed from the erosion of material from mobile dunes
of live and dead maerl, piled up to 5 m in height. Water depth 19 m. A restricted faunal
6. Mixed maerl and muddy sand. A patchwork of deposits and rocky outcrops, the latter
causing interruptions to the current flow and so giving rise to a range of sheltered,
There is only a limited amount of correlation possible between Keegan's classification
and that of the MNCR biotopes classification (Connor et al., 1997), because the
Galway Bay maerl was predominantly L. corallioides whereas most of the MNCR
biotopes were found in Scotland or characterised by the rarer Irish maerl species Lithophyllum
fasciculatum and L. dentatum. Nevertheless, both schemes recognise the
importance of general habitat features (e.g depth, wave exposure) for maerl bed
In a desk study, Scott & Moore (1996) drew attention to the fact that, contrary to
previous suggestions, the diversity of species found in a maerl bed is not necessarily
correlated with the proportion of live maerl within that bed. This conflicts to some
degree with Keegan's findings that the degradation of granulometry detrimentally affected
diversity, making the maerl less different from gravel substrata. It is likely that the
lack of direct correlation in Scott & Moore's analysis was due to the variability of
other important factors, which were not compared, and a multivariate analysis of their
data would be a better indication of the importance of live versus dead maerl.