The impact of 85 years of coastal development on shallow seagrass beds (Posidonia oceanica L. (Delile)) in South Eastern France: A slow but steady loss without recovery

International audience; The impact of 85 years of coastal development on shallow seagrass beds (Posidonia oceanica L. (Delile)) in South Eastern France: a slow but steady loss without recovery Abstract: Shallow Posidonia oceanica beds (0 to-15 m), the most common seagrass in the Mediterranean, were mapped from aerial pictures dating from the 1920's and from 2012 along 800 km of coastline in SouthEastern France (Provence-Alpes-Côte-d'Azur region). Changes in P. oceanica beds spatial distribution (limits and areal extents) during these 85 years were analyzed in terms of concordance (remaining areas), positive discordance (expanding areas) or negative discordance (lost areas). Lost areas were linked with direct or indirect impacts of coastal development (artificialized coastlines (namely harbours, ports of refuge, landfills, artificial beaches, groynes and pontoons, submarine pipelines and aquatic farms) visible on the pictures. The comparison showed that 73 % of the shallow limits have declined. Considering areal extents, remaining seagrass meadows areas accounted for the major part (85 %), while lost areas accounted for 13 % and expanding areas for 1.1%. Lost areas were mainly linked with artificialized coastlines but 44 % remained with undetermined causes (invisible pressures and/or mixed effects). The analysis of 96 coastal facilities participating to the artificialized (namely man-made) coastlines showed that the highest impact over the longest distance (5 km) was caused by harbours. Only artificial beaches had such a distant impact. Pontoons were the least surrounded by lost seagrass meadows areas. These quantitative data offer important information for marine conservation.