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Mapping rocky subtidal habitats: An analysis of method reliabilityJacques Populus 1 , Stevenn Lamarche 3 , Anouar Hamdi 2 , Michaël Vasquez 1 & Touria Bajjouk 11 Ifremer, Dyneco/AG, BP70, 29280 Plouzané, Francejpopulus@ifremer.fr, mvasquez@ifremer.fr, tbajjouk@ifremer.fr2 Chemin de Montoulieu, 64800 Nay, Franceanouarhamdi76@gmail.com3 Beti, 105 rue de Siam, 29200 Brests.lamarche@gmail.comAbstractThe objective of the EU INTERREG IVB project MeshAtlantic is to test and harmonise seabed surveyprotocols and interpretation standards across the region.To produce a benthic habitat map of the Natura 2000 site of Roches de Penmarc’h surveys were conductedusing optical and acoustic tools combined with samples and observations. Data from both ancillary and bespokesources were combined to make a seabed substrate map, a prerequisite to the comprehensive habitat map. In theshallow area the emphasis was on the use of a predictive model using a LiDAR-derived DTM and its derivativesto inform visual interpretation and achieve rock substrate identification.Given the variety of data sources used, confidence assessment was deemed important. Due to their scarcityall field data were necessary for interpretation, ruling out possibilities of statistical assessment. Therefore amethod was proposed to score confidence visually during the interpretation process by assigning scores to eachpolygon.IntroductionThe recent enforcement of the European Habitat Directive, the current designation of more marine protectedareas, along with the development of innovative techniques such as LiDAR, gave impulse to research in coastalseabed habitat mapping and rocky seabeds in particular. Upon running the first hydrographic LiDAR surveys, itquickly appeared that, owing to the high sounding density, there was a way to identify rock from LiDARtopographic features. Méléder et al. (2010) did an attempt to use LiDAR data to retrieve bottom types. Morerecently Gorman et al. (2012) used LiDAR associated with acoustic mosaics to determine the rock layer extentin the Bay of Morlaix in Brittany.The LiDAR detection capability is limited: (i) by light penetration into water, which restricts its use toreasonably clear waters, (ii) by seabed ruggedness, which may make identification uncertain in case of low lyingreefs, (iii) by the quality of the terrain model and its derivatives. However, given its high rate of coverage andprovided more developments are made to automate seabed type identification, this technique is quite promisingas an alternative to time-consuming field surveys in inshore coastal zones.SurveysThe marine protected area of Roches de Penmarc’h, France, is a very exposed coastal stretch with a rockyseabed platform extending from the shore to the 100 m deep circalittoral zone. The strategy used to make thehabitat map was first to produce a seabed substrate map and secondly to incorporate biological fieldobservations and samples to produce the final habitat map (MESH, 2008).A LiDAR survey of the inshore area provided full coverage to a depth of approximately 15 m below chartdatum. In deeper infralittoral and circalittoral zones, side scan sonar was used but due to limited fundingcomplete coverage of the 450 km² could not be achieved, which resulted in empty corridors. LiDAR cloud datawere first interpolated with geostatistics using ordinary kriging and yielded a 5 m resolution bathymetric model(Figure 1, top left). Sonar swath tracks were corrected with navigation data and mosaiced into georeferencedgrey level images.151