Using vertical Sidescan Sonar as a tool for seagrass cartography N. Sánchez-Carnero a, * , D. Rodríguez-Pérez b , E. Couñago c , S. Aceña a , J. Freire a, 1 a Grupo de Recursos Marinos y Pesquerías, Universidade da Coruña, Rúa da Fraga 10, A Coruña 15008, Spain b Dept. Física Matemática y de Fluidos, Universidad Nacional de Educación a Distancia, Senda del Rey 9, Madrid 28040, Spain c Kartenn, Tecnologías para la Gestión Ambiental y Territorial, Capitan Juan Varela 35, A Coruña 15007, Spain article info Article history: Received 13 December 2011 Accepted 22 September 2012 Available online 5 October 2012 Keywords: Sidescan Sonar sea grass Posidonia canopy height distribution GIS classification systems abstract An acoustic method, a vertically oriented Sidescan Sonar (SSSv), is used to detect and map Posidonia oceanica meadows in the bay of Agua Amarga (SE of the Mediterranean coast of Spain). Sidescan sonar, among other active hydroacoustic techniques, has shown its ability to detect, map and monitor seagrass based on its acoustic backscatter; however, some limitations linked to its power based approach have been reported in the literature. Our method is based on the SSSv measurement of canopy height distribution, making the most use of the SSSv acoustic data and using existing algorithms as statistical mapping methods. The results show a spatially coherent and statistically consistent classification. The comparison with groundtruthing is difficult due to the steep variations in the seafloor cover found in the area of interest, nevertheless the validation is successful (proving low-order discrimination) in a zone with a large range of depth variations (025 m). Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The biological relevance of seagrass meadows worldwide has been widely studied and showed they are highly productive ecosystems of ecological and economical importance (see Pollard, 1984; Heck and Valentine, 2006). In the Mediterranean Sea, Pos- idonia oceanica is the most frequent seagrass (Den Hartog, 1970; Lipkin et al., 2003; Belzunce et al., 2005; Diaz-Almela et al., 2007), developing extensive underwater meadows, distributed up to 3040 m deep (Den Hartog, 1970; Boudouresque et al., 2006), although they can be found at 4050 m in very clear waters (Duarte, 1991; Olesen et al., 2002). These meadows play a funda- mental and varied role in the metabolism of the neritic system, not only regarding the production of living substances, shelter and reproductive grounds for species of economic interest, but also for the maintenance of abiotic equilibria such as sedimentation, concentration of dissolved gases and nutrients (Colantoni et al., 1982). In recent decades Posidonia oceanica has experienced a wide- spread decline throughout the Mediterranean Sea (Benedito et al., 1990; Sánchez-Lizaso et al., 1990; Marbà et al., 1996; Ardizzone et al., 2006; Boudouresque et al., 2006). The increase of the coast contamination and deterioration, boat anchoring, bottom trawling or climate change, among others, are causing P. oceanica meadows regression (Peirano et al., 2005; Diaz-Almela et al., 2007). This decline might result in a significant problem for ecosystem preservation. Because of the important role of underwater meadows in the marine environment and their value as bioindicator (Colantoni et al., 1982; Komatsu et al., 2003; Descamp et al., 2005), it is important to assess their spatial distribution and biomass and to identify their species composition. In this respect, several direct and indirect methods have been developed to evaluate the ecological status of the Posidonia oceanica and other species meadows (e.g. Giraud, 1977; Ott and Maurer, 1977; Fresi and Saggiomo, 1981; Meinesz et al., 1981; Belsher et al., 1988; Buia et al., 1992; Marbà et al., 1996; Boudouresque et al., 2000; Marcos Diego et al., 2000; Pasqualini et al., 2001; Di Maida et al., 2011). Various techniques have been used in the past to map and monitor seagrass. Direct sampling techniques provide accurate localised data, but are time and labour intensive and provide little insight into the spatial distribution. Optical remote sensing has been often the preferred method for seagrass mapping of intertidal areas, although in submerged areas, it is limited by water clarity, cloud coverage and sea surface roughness (Vis et al., 2003) and frequently results in systematic underestimation of the extent of seagrass (McCarthy and Sabol, 2000). Active hydroacoustic techniques have shown the ability to detect seagrass. The acoustic impedance difference between water * Corresponding author. E-mail addresses: noela.sanchezc@udc.es (N. Sánchez-Carnero), daniel@ dfmf.uned.es (D. Rodríguez-Pérez), elena.counago@kartenn.es (E. Couñago), s.acenam@udc.es (S. Aceña), jfreire@udc.es, juan.freire@gmail.com (J. Freire). 1 Present address: Barrabés Next, Serrano 16, Madrid 28001, Spain. Contents lists available at SciVerse ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss 0272-7714/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ecss.2012.09.015 Estuarine, Coastal and Shelf Science 115 (2012) 334e344