Upper infralittoral tide-swept channels of coarse sediment in full or variable salinity conditions support distinctive beds of Lithothamnion glaciale maerl 'rhodoliths'. Phymatolithon calcareum may also be present as a more minor maerl component. Associated fauna and flora may include species found in other types of maerl beds (and elsewhere), e.g. Pomatoceros triqueter, Cerianthus lloydii, Sabella pavonina, Chaetopterus variopedatus, Lanice conchilega, Mya truncata, Plocamium cartilagineum and Phycodrys rubens. Unit A5.512, however, also has a fauna that reflects the slightly reduced salinity conditions, e.g. Psammechinus miliaris is often present in high numbers along with other grazers such as chitons and Tectura spp. Hyas araneus, Ophiothrix fragilis, Ophiocomina nigra and the brown seaweed Dictyota dichotoma are also typically present at sites. In Scottish lagoons this biotope may show considerable variation but the community falls within the broad description defined here. Situation: This biotope can often be found at the upper end of Scottish sealochs where the variable salinity of the habitat may not be immediately obvious.