Mapping of Posidonia oceanica beds around Elba Island (western Mediterranean) with integration of direct and indirect methods

Direct and indirect methods were used for mapping Posidonia oceanica beds a round Elba Island. Side scan sonar and a visual infra red scanner were used to detect the extension of beds. Also, direct observations were adopted to assess the bed density and to calibrate the indirect methods. Samples of P . oceanica were collected and phenological variables and macroalgal epiphyt es were studied to assess the health of the beds. P. oceanica around Elba I sland covered an area of 3 680 ha. The largest beds occupied the four main gulfs of the island, characterized by gentle slopes and sandy bottoms. Ther e, the beds showed uniform cover while a patchy structure was common on the rocky and steep bottoms of the western and south-eastern coasts of the isl and. P. oceanica beds at Elba Island demonstrated a good state of health. T he epiphytic algal assemblage of the leaves was dominated by species genera lly found on this habitat, while, on the rhizomes, we observed a high cover ing of turf-forming species. The methods adopted allowed accurate maps at a scale of 1:10000. The integration of data obtained both by direct and indi rect methods and the precision of the positioning system were particularly important. (C) 2000 Ifremer/CNRS/IRD/Editions scientifiques et medicales El sevier SAS.