Multi-scale remote sensing of microbial mats in an atoll environment

Microbial mats are encountered in many coastal environments. They are generally constituted by stratified layers produced by the development of various micro-organisms. A current research programme aims to assess the biotechnological potential of the microbial communities in South Pacific atolls. As part of this project, the characterisation of mats was examined in Rangiroa atoll (French Polynesia) using high resolution (20-30 m) multi-spectral images (SPOT HRV and Landsat ETM+), hyperspectral imagery (CASI (Compact Airborne Spectrographic Imager), 27 bands, 5.5 × 1 m) and in situ reflectance spectra. At atoll scale, mats are successfully inventoried among different geomorphological environments by fuzzy classification/segmentation of the 20-30 m resolution multi-spectral data. Laboratory biochemical analysis (not described here) highlighted and identified the most promising mats for their biotechnological potential. The spectral signatures of these remarkable mats are described at two scales. Major reflectance features of the microbial community were identified by peaks in fourth-derivatives analysis, discriminating the surface layers dominated by the cyanobacteria Schizothrix sp. (orange mats), Scytonema sp. (grey-black mats) and Phormidium sp. (green mats). At mat scale, CASI-derived spectral signatures from heterogeneous 5.5m² pixels including various microbial communities, vegetation, sand and water did not provide fine optical distinctions between mats because of mixing effects. This multi-scale analysis provides optical and geomorphological criteria to locate interesting stratified mats in other atolls.