Coralligenous habitat in the Mediterranean Sea: a geomorphological description from remote data

Sea floor mapping along the Apulia continental shelf (Italy) verified the abundance of autochthonous red algae build-ups, mapped as coralligenous habitats (CHs), in a water depth range of 5-100 m. In general, CHs were found to develop three dimensional structures, with a rigid framework and to represent an important geomorphological and sedimentological element on the Mediterranean shelf. Here, we provide the first geomorphological description of CHs (thus poorly categorized) using acoustic data obtained from Side Scan Sonar (SSS) and Multi Beam (MB) echosounder surveys, ground-truthed using a ROV and underwater camera. In SSS mosaics, CHs generally yielded intermediate to high backscatter in response to a rigid cavernous framework. According to the various shapes and the lateral continuity that coralligenous build-ups displayed in explored locations, two distinct textures were determined to be present. Various geomorphological expressions of CHs were noted within our dataset and in images obtained from MB bathymetry. We determined that coralligenous build-ups are typically represented by positive-relief structures that vary from isolated blocks (randomly scattered on a generally flat mobile soft bottom) to a field of blocks (adjacent or even coalescent), and/to ridge with several metres of lateral continuity. In most cases, CHs occurred on flat mobile soft bottom, thus representing an example of coralligenous de plateau. Our results characterize for the first time the CHs through seafloor mapping techniques, which demonstrated to represent an instrumental tool for their geomorphological characterization.