###### Abstract  
In this work, multibeam echosounder (MBES) and dual frequency sidescan sonar (SSS) data are combined to map the shallow (5-100 m) benthic habitats of the National Marine Park of Zakynthos (NMPZ), Greece, a Marine Protected Area (MPA). NMPZ hosts extensive prairies of the protected Mediterranean phanerogams _Posidonia oceanica_ and _Cymodcea nodosa_, as well as reefs and sandbanks. Seafloor characterization is achieved using the multi-frequency acoustic backscatter of: (a) the two simultaneous frequencies of the SSS (100 and 400 kHz) and (b) the MBES (180 kHz), as well as the MBES bathymetry. Overall, these high-resolution datasets cover an area of 84 km\({}^{2}\) with ground coverage varying from 50% to 100%. Image texture, terrain and backscatter angular response analyses are applied to the above, to extract a range of statistical features. Those have different spatial densities and so they are combined through an object-based approach based on the full-coverage 100-kHz SSS mosaic. Supervised classification is applied to data models composed of operationally meaningful combinations between the above features, reflecting single-sonar or multi-sonar mapping scenarios. Classification results are validated against a detailed expert interpretation habitat map making use of extensive ground-truth data. The relative gain of one system or one feature extraction method or another are thoroughly examined. The frequency-dependent separation of benthic habitats showcases the potentials of multi-frequency backscatter and bathymetry from different sonars, improving evidence-based interpretations of shallow benthic habitats. National Marine Park of Zakynthos (NMPZ); sidescan sonar; multibeam echosounder; multi-frequency acoustic backscatter; benthic habitat mapping; seagrass mapping; object-based image analysis (OBIA); angular backscatter responses; marine protected area (MPA) 2019 11 461 461 10.3390/rs11040461 10.3390/rs11040461 10.  
our knowledge regarding the spatial extent, the geographical range and the ecological characteristics of the resource or the habitat of interest [3]. Thus, accurate and high-resolution habitat mapping can be considered as a prerequisite in spatial marine management and conservation planning. In recent years the urgent need for accurate habitat maps of the seafloor has been largely acknowledged within the framework of the relative legislation such as the European Habitats Directive (92/43/EEC) and Marine Strategy Framework Directive (MSFD; 2008/56/EC). Likewise, marine spatial planning, systematic conservation planning, biodiversity and marine resources management also demand good and comprehensive spatio-temporal knowledge of the seascape and the habitat types [4; 5]. Swath mapping echosounder is the preferred and most efficient way to rapidly map benthic habitats at all ranges and depths [6; 7; 8]. A significant part of the survey efforts rests on acquiring acoustic backscatter intensity of the seafloor with sidescan sonars (SSS) and, more recently, multibeam echosounders (MBES). Variations in surveying approaches limit repeatability and comparability of the mapping results, but MBES offers the opportunity of building standardized procedures for automated seafloor mapping [9; 10]. There is a growing literature that makes use of MBES for benthic habitat mapping [11; 12], taking advantage of the wide spectrum of information that it provides, including co-registered backscatter and bathymetry and angular backscatter intensity. Bathymetric and backscatter texture features are long being used for quantitative analysis of the seabed components [13; 14], but also Backscatter Angular Response Analysis is gaining popularity over the years and tends to be a popular procedure for seafloor characterization [15; 16; 17; 18; 19]. Among the disadvantages of traditional MBES is the relatively low resolution that they provide, although newer systems (e.g., interferometric sonars) tend to compete with SSS systems in spatial sample density in shallow waters.