The degree of exposure of a beach to wave action largely determines particle size, sorting and beach gradient (King, 1959) and these, in turn, have a dominant effect on the animal communities within the sand (Swedmark, 1964; Mclntyre, 1969). It is likely, however, that the main turnover of energy, as indicated by the rapid removal of oxygen (Brafield, 1964; Hamilton & Greenfield, 1967), is due to bacteria and algae. On sheltered muddy beaches, motile diatoms can be of considerable importance (Aleem, 1950; Perkins, 1960) but for a relatively exposed beach at Firemore, Loch Ewe, on the west coast of Scotland, Fig. 1, there was negligible unattached plant material, living or dead, and the primary production by the diatoms attached to the sand grains was low (Steele & Baird, 1968). Further, these diatoms did not show evidence of heterotrophy (Munro & Brock, 1968; Mclntyre, Munro & Steele, 1970) yet, at low water mark on this beach, the populations of attached diatoms are relatively uniform to a depth of 20–30 cm in the sand and show no sign of pigment degradation or decreased photosynthetic ability. These conditions at low water mark differ from those at a water depth of 5–10 m where chlorophyll concentration in the upper 5 cm of the sand is higher but where pigment degradation and blackening of the sand, indicating reducing conditions, are usually found below 10 cm depth of sand.