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