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 arenaria).

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 (1200 m-2).

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 diversity.

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, depositional sub-systems.

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 classification.

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.

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