Topographic Analysis of Intertidal Polychaete Reefs (Sabellaria alveolata) at a Very High Spatial Resolution

In temperate coastal regions of Western Europe, the polychaete Sabellaria alveolata (Linné) builds large intertidal reefs several hectares on soft-bottom substrates. These are protected by European Habitat Directive EEC/92/43 under status biogenic structures hosting a high biodiversity and providing ecological functions such as protection against erosion. As an alternative to time-consuming field campaigns, UAV-based Structure-from-Motion photogrammetric survey was carried out in October 2020 over Noirmoutier Island (France) where second-largest known reef is located tidal delta. A DJI Phantom 4 Multispectral UAV provided topographic dataset at very resolutions 5 cm/pixel for Digital Surface Model (DSM) 2.63 multispectral orthomosaic images. The footprint mapped using combination two indices: Topographic Openness Index Position Index. covered area 8.15 ha, with 89% corresponding main composed connected continuous structures, 7.6% isolated (<60 m2), 4.4% small clumps (<2 m2). To further describe complexity reef, Geomorphon landform classification used. spatial distribution tabular platforms considered healthy stage contrast degraded proxy that consists comparing volume theoretical tabular-shaped volume. Epibionts colonizing (macroalgae, mussels, oysters) were also combining indices Normalised Difference Vegetation simple band ratios indices. confusion matrix showed macroalgae mussels satisfactorily identified but oysters could not be detected automated procedure due their spectral complexity. used this work should now exploited propose health index these reefs.