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Paper title Topographic analysis of intertidal polychaete reefs (Sabellaria alveolata) using very high resolution UAV remote sensing
Authors
  1. Guillaume Brunier University of Nantes
  2. Simon Oiry Nantes Université Speaker
  3. Pierre Gernez Nantes Université, Institut des Substances et Organismes de la Mer, ISOMER, UR 2160 Nantes, France
  4. Stanislas Dubois IFREMER, Centre de Bretagne
  5. Laurent Barillé University of Nantes
Form of presentation Poster
Topics
  • C4. HAPs/UAVs
    • C4.01 Innovative UAV applications
Abstract text In temperate regions of Western Europe, the polychaete Sabellaria alveolata (L.) build extensive intertidal reefs of several hectares on soft-bottom substrates. These reefs are protected by the European Habitat Directive EEC/92/43 as biogenic structures hosting high biodiversity and providing ecological functions such as protection against coastal erosion. Monitoring their health status is therefore mandatory. These reefs are characterized by complex three-dimensional structures composed of hummocks and platforms, either in development or degradation phases. Their high heterogeneity in physical shape and spectral optical properties is a challenge for accurate observation.
As an alternative to time-consuming field campaigns, an Unmanned Aerial Vehicle (UAV) survey was carried out over Noirmoutier Island (France), where the second-largest European reef is located in a tidal delta. Structure-from-motion (SfM) photogrammetry coupled with multispectral images was used to describe and quantify the reef topography and colonization by bivalves and macroalgae. A DJI Phantom 4 Multispectral UAV provided a very resolute and accurate topographic dataset at 5 cm/pixel resolution for the Digital Surface Model (DSM) and 2.63 cm/pixel resolution for the multispectral orthomosaic images. The reef footprint was mapped using the combination of two topographic indices: the Topographic Openness and the Topographic Position Index. The reef structures covered an area of 8.15 ha, with 89 % corresponding to the main reef composed of connected and continuous biogenic structures, 7.6% of large isolated structures with a projecting surface< 60 m², and 4.4 % of small isolated reef clumps < 2 m². To further describe the topographic complexity of the reef, the Geomorphon landform classification was used. The spatial distribution of tabular platforms considered as a healthy reef status (by opposition to a degraded status) was mapped with a proxy comparing the reef volume to a theoretical tabular-shaped reef volume. Epibionts colonizing the reef (macroalgae, mussels, and oysters) were also mapped by combining multi-spectral indices such as NDVI and simple bands ratio with topographic indices. A confusion matrix showed that macroalgae and mussels were satisfactorily identified but that oysters could not be detected by an automated procedure due to the complexity of their spectral reflectance.
The topographic indices used in this work should now be further exploited to propose a health index for these large soft-bottom intertidal reefs, monitored by environmental agencies in charge of managing and conserving this protected habitat. It is not known if these topographic methods are transferable to high resolution (0.4 to 0.8 m) stereo images from satellites such as Pleiades, Pleiades-neo, IKONOS, or Worldview solutions. Mapping from stereo-satellite images will be tested on the largest Sabellaria alveolata intertidal reef in Europe, in the bay of Mont Saint-Michel (France). This work will be done in the ESA project BiCOME (Biodiversity of the Coastal Ocean: Monitoring with Earth Observation).