Received 24 July 2004 Accepted 28 December 2004 Copyright # 2006 John Wiley & Sons, Ltd. AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS Aquatic Conserv: Mar. Freshw. Ecosyst. 16: 29–42 (2006) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/aqc.688 A new synthetic index and a protocol for monitoring the status of Posidonia oceanica meadows: a case study at Sanremo (Ligurian Sea, NW Mediterranean) MONICA MONTEFALCONE a, *, GIANCARLO ALBERTELLI a , CARLO NIKE BIANCHI a , MAURO MARIANI b and CARLA MORRI a a DIP.TE.RIS, Dipartimento per lo Studio del Territorio e delle sue Risorse, Universita ` di Genova, Corso Europa 26, 16132 Genova, Italy b Acquario Civico e Stazione Idrobiologica di Milano, Viale Gadio 2, 20121 Milano, Italy ABSTRACT 1. The status of a Posidonia oceanica meadow in front of the town of Sanremo, Italy, was studied through a combined use of benthic mapping and synthetic indices. 2. Mapping was accomplished by integrating side scan sonar imagery and data collected by scuba diving along transects placed perpendicularly to the coastline. A thematic map (scale 1:5000) was produced. Extent of the meadow, occurrence of dead matte areas, and morphology of the lower limit (with new definition) are all described. 3. Two synthetic environmental indices were applied to transect data in order to quantify the status of the meadow: the Conservation Index and the Substitution Index. The former is related to the proportion of dead matte; the latter is a novel index measuring the amount of replacement of the ‘constructional’ seagrass P. oceanica by the ‘non-constructional’ seagrass Cymodocea nodosa. The potential of a ‘phase shift’ in Ligurian Sea seagrass meadows is discussed. 4. The approach here proposed, based on detailed mapping plus synthetic indices, may provide immediate information to evaluate the state of Mediterranean Posidonia oceanica for monitoring and management. Copyright # 2006 John Wiley & Sons, Ltd. KEY WORDS: Posidonia oceanica; Cymodocea nodosa; benthic cartography; Substitution Index; phase shift; environmental indices; Ligurian Sea; Mediterranean Sea INTRODUCTION The endemic species Posidonia oceanica (L.) Delile is the most important seagrass in Mediterranean coastal waters, extending from the surface to more than 40 m depth in exceptionally clear waters (Meinesz and *Correspondence to: M. Montefalcone, DIP.TE.RIS, Universita` di Genova, Corso Europa 26, 16132 Genova, Italy. E-mail: montefalcone@dipteris.unige.it