Résumé
Marine habitat maps are important tools for managers who need updated and accurate data to implement suitable management actions in the marine environment. While shallow-water habitats can be mapped using high-resolution satellite imagery, deep-water habitats are difficult to survey and tend to be poorly mapped, especially over large regions. This project aimed to map the coral reef lagoon of Mayotte, a French island of the Southwestern Indian Ocean, down to 150 m depth and over an area of 1406 km². The entire exclusive economic zone (EEZ) of Mayotte is managed by a marine park. Mapping was carried out by visual interpretation of satellite and aerial remote sensing data for shallow-water habitats. For deeper waters, the bathymetric data were generated by merging aerial Lidar surveys and acoustic data. Historical biological data were gathered through bibliographic review (2164 points) and completed by 659 ground truth points using underwater video, visual census from scuba-diving and snorkeling. Kriging was used as a spatial interpolation method to assess the spatial distribution of living corals cover. A total of 236 habitat classes were defined and mapped. Analyses revealed that the region around Mayotte consists of 478 km² of bio-constructed reefs, including 83,414 patch reefs (69 km²). In order to better characterize coral habitat – a keystone habitat – spatial community structure was studied with non-metric multidimensional scaling (nMDS) ordination and tested with permutational multivariate analysis (PERMANOVA). Each reef geomorphological feature presents diverse benthic morphotypes. Areas with higher living coral cover (50%) are located in the inner lagoon patch reef complex (northeast), the double barrier reef (south) and the western fringing reef. Conversely, the health of the fringing reefs appears to have been degraded in the northeast and east of Mayotte, and in enclosed bays. The composition of benthic assemblages varies significantly among water mass and among geomorphological structures. The best preserved reefs are characterised by Acropora communities and encrusting calcareous algae, while degraded reefs typically include other coral genera (e.g., Porites, Montipora, Echinopora) and present higher proportions of algal turf.The detailed habitat maps created in this project became an essential management tool for this region, and the approach used in Mayotte could be applied to other similar contexts. Such a detailed mapping of marine habitats in terms of depth range and extent was a first in a French overseas territory, encouraging further research on these ecosystems for conservation purposes.