3D structure of reefs, pilot study for improving Mayotte’s reef biodiversity management (Structure 3D des RÉcifs, étude pilote pour l’amélioration de gestion de la BIOdiversité récifale de MAyotte)

Mayotte

Coral reefs represent emblematic ecosystems both in terms of local natural heritage and throughout the Indian Ocean. Natural disruptions and various anthropogenic pressures (tourist traffic, coastal development, etc.) endanger the state of these ecosystems, their biodiversity and the goods and services they provide to the local population. The most precise reef monitoring program in Mayotte (GCRMN) uses standard descriptors, but does not consider the reefs three-dimensional nature. The application of underwater photogrammetry and the calculation of 3D descriptors will thus make it possible to complete the assessment of the reefs’ health state. Given the high precision of the models and the 3D analysis, this technique is the most suitable for quantifying possible impacts to the reef structure and proves to be the most relevant for monitoring areas that will be disturbed by coastal development projects, but also due to tourist activities.

REBIOMA-3D proposes, through an underwater photogrammetry pilot study, a morphobiological and physical analysis of six reef areas of Mayotte: 3 located in a reserve and 3 subject to possible impacts due to urban development. The objective is to do a “P0” and to determine any degradation linked to anthropogenic impacts, as well as to recommend management measures in order to conserve their biodiversity and ecosystem services. The specific objectives of the project are: Objective 1. Apply the underwater photogrammetry technique on the reef areas of Mayotte to study their 3D characteristics, and calculate new habitat descriptors. Objective 2. Identify anthropogenic impacts on the reef structure (physical destruction, loss of structural complexity, macro-waste). Objective 3. Propose conservation measures and recommend management measures, in close consultation with the managers. Objective 4. Raise the awareness of the local population regarding the importance of preserving coral reefs. The project ultimately proposes a “Point 0” (baseline) for monitoring.

According to UNESCO, 50% of coral reefs could disappear by 2030. In addition to their major role for biodiversity, they provide many ecosystem services to more than 500 million people (tourism, fishing, coast protection). The structural complexity of coral reefs as well as other 3D features (e.g. refuge capacity, fractal dimension, slope) represent fundamental aspects of reef functioning. Despite their importance, few monitoring programs take them into account to determine the health of reefs. Underwater photogrammetry allows the creation of three-dimensional reef models at different spatial scales, ranging from coral colony to reef landscapes. 3D reconstruction by underwater photogrammetry also allows precise temporal monitoring of reef communities. In addition to the calculation of conventionally used descriptors (coverage percentages, diversity, etc.), photogrammetric analyses make it possible to define new descriptors of the reefs’ 3D physical characteristics such as structural complexity, volumes of coral colonies, refuge capacity, roughness, slope of benthic and landscape components.