Please cite this article in press as: Montefalcone, M., et al., Evaluating change in seagrass meadows: A time-framed comparison of Side Scan Sonar maps. Aquat. Bot. (2011), doi:10.1016/j.aquabot.2011.05.009 ARTICLE IN PRESS G Model AQBOT-2391; No. of Pages 9 Aquatic Botany xxx (2011) xxx–xxx Contents lists available at ScienceDirect Aquatic Botany j ourna l ho me page: www.elsevier.com/locate/aquabot Evaluating change in seagrass meadows: A time-framed comparison of Side Scan Sonar maps Monica Montefalcone a,* , Alessio Rovere a,b , Valeriano Parravicini a,b , Giancarlo Albertelli a , Carla Morri a , Carlo Nike Bianchi a a DipTeRis, Department for the study of the Territory and of its Resources, University of Genoa, Corso Europa 26, 16132 Genoa, Italy b SEAMap srl Environmental Consultancy, via Greto di Cornigliano 6R, 16152 Genoa, Italy a r t i c l e i n f o Article history: Received 20 July 2010 Received in revised form 24 May 2011 Accepted 30 May 2011 Available online xxx Keywords: Seagrass Mapping Side Scan Sonar Posidonia oceanica Diachronic analysis Mediterranean Sea a b s t r a c t Side Scan Sonar (SSS) is a common method for mapping seagrass meadows that allows to define meadow extent and typology. Should sonogram interpretation be efficient, repeated mapping would allow evaluation of spatial meadow dynamics through time. Here, maps of Posidonia oceanica (L.) Delile meadows from western Liguria (NW Mediterranean Sea) produced in 1990, 1991 and 2006 through SSS surveys, were processed with GIS (Geographical Information System) and compared by means of diachronic analysis to establish concordances and discordances. While any difference between maps of 1990 and 2006 might provide information about change in seagrass cover with time, the availability of maps taken at only one year of distance (1991 vs 1990) offered the unique opportunity to evalu- ate the efficiency of SSS mapping, as the slow dynamics of P. oceanica makes significant change over such a short time scale unrealistic in absence of any large-scale environmental catastrophe. Results showed obvious differences in P. oceanica meadow distribution among the three periods in most of the areas. The largest differences were found between the maps of 1990 and 1991. This discrepancy was probably due to erroneous interpretation of small, fragmented shallow-water meadows on sono- grams. Errors in sonogram interpretation combine with inefficient positioning in old cartographies to blur the evaluation of change over large time scales (i.e. between 1990 and 2006). Extensive sea- truthing based on a rigorous design is mandatory for efficient acoustic mapping of seagrass meadows. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Marine cartography plays a key role for basic research, con- servation and monitoring of marine ecosystems and for coastal management purposes. The benthos is generally the target of marine habitat mapping because of its greater structural com- plexity, stability over time and space, and ability to record environmental alterations as compared to open sea environments (Bianchi et al., 2004). Benthic cartography has a long tradition in the Mediterranean Sea (Molinier and Picard, 1960; Meinesz, 1985) and assessment of the status of coastal ecosystems has often involved seagrass (Montefalcone et al., 2007a, 2007b, 2009; Boudouresque et al., 2009; Waycott et al., 2009). Collection of reference state inventories and baseline maps describing the current extent and condition of seagrass meadows is priority in the light of the proper management * Corresponding author. Tel.: +39 010 3538584; fax: +39 010 3538140. E-mail address: montefalcone@dipteris.unige.it (M. Montefalcone). of such ecologically important ecosystems (Leriche et al., 2006). A vast array of methods can be used for mapping seagrass. The choice of which tool to use relies on the characteristics of the studied area, the degree of accuracy required and the cost-effectiveness in relation both to the time spent and the data acquisition scale (Pasqualini et al., 2005 and reference therein). The last two decades have seen rapid developments in the use of digital data collected by remote sensing instruments (i.e. satellite imagery, aerial photography, acoustic devices). Aerial photography, in conjunction with image processing techniques, represents one of the most efficient methods in terms of cost, rapidity and reliability (Pasqualini et al., 1998, 1999), as it provides objective information on entire meadows at a territorial spatial scale (Dekker et al., 2005). While aerial and satellite imageries have been demonstrated to be the best methods to draw the borders of shallow meadows, they are not adequate for deeper meadows when water turbidity pre- vents light penetration across the water column. Recent progress in acoustic data acquisition and processing, together with improve- ment in positional accuracy and computing power (McGonigle et al., 2009), made the acoustic techniques, such as Side Scan Sonar (hereafter SSS), echo sounders and multibeam, satisfactory alter- 0304-3770/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.aquabot.2011.05.009