MAPPING OF POSIDONIA-OCEANICA USING AERIAL PHOTOGRAPHS AND SIDE-SCAN SONAR - APPLICATION OFF THE ISLAND OF CORSICA (FRANCE)

In light of the essential role that seagrass beds play from an ecologi cal and sedimentary point of view, mapping of these organisms should b e carried out in order to clearly assess their distribution. Although a large number seagrass beds have been mapped, these generally only ta ke into consideration the shallower formations (0-15 m) or those which are geographically very confined (<5000 ha) due to the inherent diffi culties involved in mapping such ecosystems. Posidonia oceanica is a m arine phanerogam endemic to the Mediterranean. The wide bathymetric ra nge (0-50 m) of this species increases the difficulties involved in it s mapping. The combined use of image processing of aerial photographs for the shallower layers (0-15 or 20 m in regions of very sheltered wa ters) and of side scan sonar for the deeper depths (20-50 m) is a part icularly suitable approach. The implementation of these techniques alo ng the Corsican coast (approximately 127 000 ha) allowed the mapping o f assemblages and bottom-types present from the surface to a depth of 50 m with a resolution of 5 m. The results reveal that P. oceanica sea grass beds occupy a surface area of approximately 62 000 ha with a mea n lower bathymetric limit of about 32 m. A comparison of these results with other regions of the Mediterranean confirms the extent of distri bution of this species along the Corsican coast (49.1% from 0 to 50 m vs 16.3% along the French continental coast). The surface area of the Mediterranean coastline potentially occupied by P. oceanica seagrass b eds can be estimated to be between 2.5 and 5 million hectares. (C) 199 8 Academic Press.