The aim of the EC Habitats Directive 92/43/EEC is to promote the maintenance of biodiversity by requiring Member States to take measures to maintain or restore natural habitats and wild species listed on the Annexes to the Directive at a favourable conservation status, introducing robust protection for those habitats and species of European importance. Special Areas of Conservation (SACs) are strictly protected sites designated under the Habitats Directive. In the UK coastal lagoons show a wide range of geographical and ecological variation; five main sub-types have been identified in the UK, on the basis of their physiography, as meeting the definition of the Annex I habitat type. These are graded from A to D. - A Outstanding examples of the feature in a European context. - B Excellent examples of the feature, significantly above the threshold for Sites of Special Scientific Interest (SSSI) notification but of somewhat lower value than grade A sites. - C Examples of the feature which are of at least national importance (i.e. usually above the threshold for SSSI notification on terrestrial sites) but not significantly above this. These features are not the primary reason for SACs being selected. - D Features of below SSSI quality occurring on SACs These are non-qualifying features (“non-significant presence”), indicated by a letter D, but this is not a formal global grade. Cemlyn lagoon on the north coast of Anglesey, north Wales, is considered to be the best example of a saline coastal lagoon in Wales (Graded A) and forms part of the Cemlyn Bay SAC. The lagoon is separated from the sea by a shingle bank with a narrow channel at the western end, across which a sluice system was built in the 1930s. Seawater exchange occurs mainly through the sluice and by percolation through the shingle bank, although in extreme storms coinciding with spring tides waves break over the top of the shingle bank. Cemlyn lagoon supports a relatively diverse set of species, several of which are specific to lagoons, including the bryozoan Conopeum seurati, the lagoon cockle Cerastoderma glaucum and the lagoonal mud-snail Ventrosia ventrosa. Cemlyn lagoon is also the only site in Wales where the lagoonal isopod Idotea chelipes has been recorded. A number of uncommon plant species are found within the lagoon, including the brackish water-crowfoot Ranunculus baudotii and beaked tasselweed Ruppia maritima. The purpose of this data capture is to aid in the monitoring and condition assessment of this Annex I feature. This dataset is a collation of data from subtidal benthic soft sediment sampling carried out within Cemlyn lagoon.