Temperature has long been known to be the primary determinant of species composition on a geographical scale, because the boundaries of biogeographical regions are associated with isotherms (Lüning, 1990). Maerl biotopes occur in a wide range of temperature regimes, from the tropics to northern Norway, but the species composition of the maerl beds is greatly influenced by temperature. Adey & Adey (1973) showed that the distribution of coralline algal species in the North Atlantic could be correlated with temperature/habitat boundaries. An obvious temperature-related maerl phenomenon in the UK is the absence of Lithothamnion corallioides from Scotland, either because winter temperatures occasionally drop below the minimum survival temperature of this species (between 2 and 5°C) or because temperatures do not remain high enough for long enough to support sufficient annual growth (Appendix). Laboratory studies on Spanish maerl (Appendix) showed that Phymatolithon calcareum survived down to 2° C, dying at 0.4°C, and the optimum temperature for growth was 15°C. L. corallioides had a higher minimum survival temperature, dying at 2°C, and surviving without growth at 5°C. Temperature also appears to confine Lithothamnion glaciale to northern parts of the British Isles. Hall-Spencer (1994) showed that L. glaciale only produced reproductive conceptacles in winter when water temperatures were below 9°C.

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