Tidal characteristics

Wave action

A key physical environmental factor affecting the distribution of maerl and the biotope type is the occurrence of currents. These can be the result of tides, river influence or due to density differences arising from variation in salinity in the virtually tide-less Mediterranean, as well as the ripple-forming bottom currents resulting from wave action. In the bay of Morlaix (L. Cabioch, 1968) maerl deposits are often confined to areas where current speeds are increased by passing over rocky outcrops or sills. In areas where the maerl beds are subjected to strong tidal currents or wave oscillation they may develop into a large scale ripple pattern, with differential distributions of live and dead thalli (and of epiflora and epifauna) between the tops of the ridges and the bottoms of the gullies. These maerl megaripples have been described and illustrated by Bosence (1976) and Hall-Spencer (1995a).

The upper limit of the living maerl is generally defined by the astronomical low-water mark since maerl thalli cannot survive desiccation. Unlike many seaweeds, maerl-forming species have a very poor ability to withstand emersion, probably only for a few minutes. It is also possible that a coincidence of the lowest tides, clearest water and mid-day sunshine could allow sub-surface irradiances which are above the tolerance limit for the maerl species or the consolidating epiphytes of the maerl bed. Tidal regime, in combination with the local water clarity, may likewise affect the lower limit of maerl by its effect on irradiance (i.e. large tidal amplitude decreases irradiance by increasing the depth of water at high tide). Currents induced by tidal movements are likely to be very important in influencing maerl distribution, in terms of the maerl biotope complex in general and the particular species present. In Galway Bay, where there are extensive live maerl beds in three main areas, only one area (in the Inner Bay) lacks moderate or strong currents. Tidal flow rates at spring tides on maerl beds in Greatman's Bay, Galway, were measured at over 10 cm s^-1 at the surface of the maerl bed (Maggs, 1983a). Silt-free deposits of Phymatolithon calcareum are found at 20-30 m inshore from the Aran Islands, where the tide flow between the islands results in fairly strong bottom currents. At the only known site where large maerl banks occur intertidally, in western Ireland (Muckinish, Co. Clare), very strong tidal currents keep the maerl mobile, as large maerl waves, so that no individual maerl thallus is emersed more than briefly at low water.

In Mannin Bay, Bosence (1976) found that dense maerl beds were restricted to less wave-exposed parts of the bay. In moderately wave-exposed and sheltered areas, different morphological forms of maerl develop under different degrees of wave action. Bosence (1976, 1983b) showed that branching of maerl is a sensitive indicator of hydraulic conditions: more stable (discoid) forms were found in areas with higher exposure to water movement whereas ellipsoid forms were less stable and occurred where there was less water movement. Wave action, like currents, can create flat areas of maerl or, more often, small ridges or megaripples in a ridge and furrow system.

Wave exposure has effects on species composition also. In Galway Bay, maerl deposits on the wave-exposed northern shore of the bay are dominated by Phymatolithon calcareum, while in the inner bay, the silty beds sheltered from tidal streams and wave action are formed mostly of Lithothamnion corallioides.

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