Wendy Diruit1, Mélanie Barda-Chatain2, Mathieu Helias1, Sara Terrin3, Thomas Burel1, Touria Bajjouk4, Anthony Le Bris5, Sophie Richier5, Erwan Ar Gall1
1 Univ Brest, CNRS, IRD, Ifremer, LEMAR, 29280 Plouzané, Brittany, France
2 Université de Pau et des Pays de l’Adour, Campus d’Anglet (Montaury), 64600 Anglet, France
3 Sorbonne Université, Campus Pierre et Marie Curie, 75005 Paris, France
4 Ifremer, DYNECO/LEBCO, 29280 Plouzané, Brittany, France
5 Centre d’Etude et de Valorisation des Algues (CEVA), 22610 Pleubian, Brittany, France
Temperate rocky shores are highly diverse and dynamic ecosystems threatened by anthropogenic and climate change driven pressures. The primary producers developing are mainly canopy-forming Phaeophyceae, commonly monitored to detect ecological changes. As remote sensing is increasingly used to better understand the functioning of coastal ecosystems, this work aimed at comparing in situ data to vegetation indices defined through two resolution remote imagery approaches, satellite (50 cm) and UAV (2 cm) on two sites of Brittany (NW France). Covers of dominating seaweeds were characterized in the field and by remote sensing in 24 sampling spots (2.7 m² each) according to 4 vertically distributed bathymetric levels dominated by Phaeophyceae: 1- Pelvetia canaliculata and Fucus spiralis (high shore), 2- Ascophyllum nodosum (mid shore), 3- Fucus serratus (low shore) and 4- Himanthalia elongata and Laminaria digitata (sublittoral fringe). Three vegetation indices were determined from imagery: Normalized Difference Vegetation Index (NDVI), Vegetation Cover Index (VCI) and Pigment Index (IP). In situ covers were significantly different from remote sensing indices in the high shore, but were otherwise similar in the subsequent bathymetric levels. Both VCI and IP values were unaffected by the resolution. On the contrary, NDVI calculation was dissimilar according to the resolution considered and whatever the bathymetric level. To conclude, the three vegetation indices offered coherent results suitable for the study of macroalgal-dominated habitats, fluctuating logically according to the bathymetric level. Even if low resolution seems acceptable to roughly describe seaweed covers, a high resolution appeared yet needed when fully monitoring macroalgal communities’ structure.