Two components of the substratum are relevant here, rock type and substratum stability.

In intertidal waters rock type is important because, amongst other factors, it influences water retention and radiative heating (see Hartnoll, 1983 for references and figure below). Subtidally these are not significant, but surface texture, erosion and rock hardness are factors of obvious relevance. A smooth surface will provide a different environment from one pitted with cracks and crevices. Friable eroding rocks are colonised by pioneer species, and the community characteristic of firm surfaces does not develop (Rubin, 1980). Very soft rocks, such as soft chalk or clay, allow the development of 'soft rock communities' characterised by boring molluscs (piddocks) or boring polychaetes (Polydora) (Hiscock, 1979; Wood, 1989).

Substratum stability is determined by whether it is comprised of bedrock, or of loose boulders or stones. The mobility of boulders and stones will be a function of wave exposure, and mobility of the substratum will selectively impact CFT species. Marked differences between the communities of bedrock and adjacent loose rocks has been recorded (e.g. Knight-Jones & Jones, 1955), and in the MNCR classification distinct biotopes are recognised for bedrock and loose substratum. Mobile substrata under exposed conditions have a community characterised by serpulid worms, barnacles and bryozoan crusts (Bunker & Hiscock, 1987; Dipper, 1983; Hiscock, 1981; Howson, 1988; Mitchell et al., 1983) rather than by the larger more delicate species which feature on the adjacent bedrock. Larger boulders which are not regularly displaced will provide a variety of cryptic environments on their undersides, but these will not be surveyed by the standard methods in use.

Dry weight biomass of plants and animals on rock surfaces of different slopes on Port Erin Breakwater (from Hartnoll, 1983).

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