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Holon et al 2015-85 years
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
auteurs : F. Holon a, P. Boissery b, A. Guilbert a, E. Freschet a, J. Deter a, c,
publié dans Estuarine Coastal Shelf Science
Résumé : Shallow Posidonia oceanica beds (0 to 15 m), the most common seagrass in the Mediterranean, were
mapped from aerial photographs dating from the 1920’s and from 2012 along 800 km of coastline in
South-Eastern France (Provence-Alpes-C^ote-d’Azur region). Changes in P. oceanica bed spatial distribution
(limits and extent) 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 photographs. The comparison showed that 73% of the shallow limits have declined. Considering
spatial extent, 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 artificial coastlines
but 44% remained with undetermined causes (invisible pressures and/or mixed effects). The analysis of
96 coastal facilities creating the artificial (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.