This chapter presents an overview of the ecological requirements and physical attributes for maerl, all of which affect its distribution. Different environmental factors may influence maerl distribution interactively or synergistically - for example the interaction between water clarity and the presence of fine sediments. Although few studies have been made of the environmental requirements of maerl beds in the UK, the environmental factors leading to the observed distribution patterns of maerl beds in Europe have previously been discussed by Jacquotte (1962), L. Cabioch (1968), J. Cabioch (1969, 1970) and Bosence (1976). Bosence (1976) carried out a valuable study of some environmental aspects of the ecology of maerl in Mannin Bay, County Galway.

Detailed measurements of environmental data are current being made at several European maerl beds as part of the BIOMAERL programme. These includec omprehensive sampling not only of the benthic environment of each maerl ground (including bathymetry, macro- and micro-architecture of the maerl surface, granulometry, calcimetry, organic matter content, pH and Eh profile), but also of the water column overlying the BIOMAERL study sites (including measurements of seawater temperature, salinity, Secchi disc transparency, seston content, current speed as well as chlorophyll and photosynthetically active radiation on occasion). Edaphic measurements have been collected seasonally over one to two years. Preliminary results of these studies are in broad agreement with data presented here.

We suggest that the ecological niche of both Lithothamnion corallioides and Phymatolithon calcareum is relatively narrow and subject to many controlling environmental factors. Moderate current and wave action on the one hand, but moderate turbidity and sedimentation on the other, are antagonistic conditions which help to explain the limited spatial distribution of these species in shallow coastal temperate waters.

The most significant environmental factors affecting the distribution of maerl are water movement - both currents and wave action - and the interactive effects of depth and water quality. On a geographical scale, the distribution of maerl species is strongly influenced by the temperature regime. Other physical factors, such as water chemistry, are of relatively minor importance. Maerl beds are found on a very broad range of underlying substrata.

Water movement

Light, depth and water clarity

Temperature

Chemistry

Substratum

References