Intertidal Monitoring of Sabellaria alveolata Reefs in Pen Llyn ar Sarnau Special Areas of Conservation (SAC) (2004/2005) dataset https://portal.medin.org.uk/portal/start.php?tpc=007_NRW_DS102008 The Habitats Directive establishes that the management of Special Areas of Conservation (SACs) should aim to achieve the favourable conservation status of habitat and species features listed within its Annex I and Annex II. For SACs in Wales, the Countryside Council for Wales (CCW) is therefore required to report on a regular basis on whether features are in favourable conservation status. Honeycomb reef worm Sabellaria alveolata is a sedentary, tube-dwelling polychaete most commonly found on rocky substrata in the lower shore and shallow subtidal zones of marine and estuarine areas. The honeycomb reef worm is a UK BAP Priority Habitat (BAP habitats are now Habitats of Principal Importance/Priority Habitats). Dense congregations of S. alveolata tubes form large biogenic reef structures, which can be up to a metre thick and take the form of hummocks, sheets or more massive formations. This dataset combines the results of two monitoring events carried out on Sabellaria alveolata reef features within the Pen Llyn a'r Sarnau SAC during 2004 and 2005. In 2004 the purpose of this data capture was to identify specific survey locations, measure the extent of S. alveolata reef along agreed transects, assess the quality of S. alveolata reef present. Map and make notes on any sightings of wireweed Sargassum muticum. In 2005 the purpose of this data capture was more detailed monitoring of selected reef areas to thoroughly characterise small patches of reef and make an assessment of the impacts of anthropogenic disturbance on reef diversity. 20081231