@phdthesis{92745, type = "Thesis", year = "2022", title = "Structure spatiale à multi-échelles de la biodiversité benthique des monts sous-marins et pentes insulaires à partir de l’analyse d’images : approches méthodologiques et rôle de l’habitat, Multiscale spatial patterns of benthic biodiversity on seamounts and island slopes from image analysis: methodological approaches and role of habitat", journal = "", editor = "", volume = "", number = "", pages = "", author = "Hanafi Portier Melissa", url = "https://archimer.ifremer.fr/doc/00815/92745/", organization = "", address = "FRANCE", school = "Université de Bretagne Occidentale", abstract = "
Seamounts display high habitat heterogeneity. However, the multiscale spatial structure of their community and the environmental factors that explain it are poorly documented. In a context where these habitats are subject to various anthropogenic pressures (e.g., industrial fishing, mining of rare metals), it is urgent to fill these knowledge gaps. This implies using underwater imaging methods and a multiscale sampling plan. However, the deep-sea fauna remains largely unknown and one of the major difficulties is to robustly identify the organisms visible on the images.The objectives of this thesis are to improve the methods for characterising the visible fauna on bottom images and to use these methods to describe the spatial structure of the megabenthic communities of seamounts and island slopes of the Mozambique Channel (Indian Ocean), and finally, to identify the environmental factors that explain this structure at regional and local scales.The comparison of biodiversity data assessed from dredges/trawls and in situ images in various benthic habitats shows differences according to the method (image/physical sampling), the habitats (soft/rocky bottom) and the taxa. These methods are complementary and we propose recommendations to improve the identification of fauna on images, notably with identification keys adapted to the images. The taxonomic identification of sponges and cnidarians on images does not exceed the rank of the order or class. For these habitat-forming organisms, we propose to characterize diversity using a morpho-functional approach. These methods were deployed on towed camera transects on the summits and upper slopes of four seamounts and the slopes of two islands in the Mozambique Channel and combined with quantitative environmental data. Densities can vary by a factor of 10 between sites, favoured by strong currents. High levels of beta diversity are explained by the diversity of the substrate, and of different rock types. The communities are very heterogeneous within sites (a mount or an island slope) and between sites which are 4 to 1400 km apart. The factors that contribute to explain this heterogeneity are hydrology (current, primary production) (15%; > km), geomorphology (6.5%; > km), topography (7.9%; 60-500 m), and substrate (9.5%; 60 m). The morpho-functional approach on cnidarians and sponges allowed the assessment of the spatial structure of their morphological diversity and how it responds to the environment, as well as its role on the structure of communities of associated organisms. This method provides indicators of the structure of vulnerable habitats. This work provides initial results on seamount communities in the Mozambique Channel, and could be used to implement management strategies, particularly in the context of the marine protected areas of Mayotte and the Glorieuses archipelago.
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