French

Abstract

By their location along the coasts, coral reefs form a living natural barrier attenuating the energy of incoming waves, protecting coastal ecosystems and human populations from the risks of submersion, flooding and erosion. These reefs are facing increasing pressures and disturbances, leading to episodes of mortality of the corals constituting them and a deterioration of their status on a global scale. To monitor this state of health, remote sensing data, particularly aerial hyperspectral data, have long been used in Reunion Island and have shown their effectiveness. This data is expensive and therefore not very usable as management tools. This article shows the contribution of multispectral satellite data to monitor reefs, particularly with the rise of Very High Spatial Resolution. Here we test the ability of different spectral indices calculated on four types of multispectral satellite images (Sentinel-2, SPOT 6/7, Pléiades and Pléiades Neo) to detect living coral covers of coral reefs. We use Reunion as a case study. Thanks to historical and in-situ reference data we highlight that:
(3) Satellite images can detect living corals using the Blue Green Brightness Index BIBG (R2 > 0.60 on historical data)
(4) The new very high resolution Pléiades Neo satellite image, including a Deep Blue band, allows better detection with the Deep Blue Blue Brightness Index BIDBB (R2 = 0.63 on in-situ data).
The developed method applied to the index-image pair made it possible to generate an updated map of high-resolution coral covers of the entire Hermitage reef platform in Reunion.