Zostera noltii seagrass beds form valuable extensive habitats in the intertidal zone of bays and estuaries of the North Atlantic Ocean. Being submitted to natural and anthropogenic pressures, long term change assessment becomes thus critical for relevant protection and management practices. Identification and macro-scale mapping of these seagrass beds have been performed with visible and near infra-red remote sensing data using airborne and satellite sensors. The methodology was based on above-ground leaf reflectance spectrum analysis, from 400 to 900 nm. Quantitative relationships between biomass and vegetation indices have been obtained from spectral libraries of different biomasses, built with a spectroradiometer in the laboratory, and taking into account the sediment background influences and leaf phenology. This spectroradiometric approach was necessary to interpret and process multi and hyperspectral images (respectively SPOT and HySpex) of Bourgneuf Bay (2°10'W, 46°57'N). Z. noltii bed extent globally increased from 1991 to 2011, probably due to a natural accretion of the tidal flats, although a sharp decline was detected between 2006 and 2009, which remains to be explained.
