Seabed characterization off the Bradano river mouth (North-western Ionian Sea) by acoustic investigations and sampling.

The continental shelf edge in the north-western sector of the Ionian sea has been undergoing progressive retreat due to the entrenchment of the Taranto canyon upper reach (Meo et al., 2017) and, more recently, the coastline has been withdrawing as well, owing to a reduced sediment yield from the main rivers. Off the Bradano river mouth - a Site of Community Importance since 2014 - the break in slope is at about -30 m, less than 2 km from the shoreline. Along such a narrow and unsteady infralittoral strip, a surprisingly wide meadow of Cymodocea nodosa (Ucria) Asch. has been reported by Appolloni et al. (2013) through scuba diving. C. nodosa is one of the most common phanerogams in the Mediterranean Sea, colonizing the low-energy sandy/muddy seabed, between the 5-20 m depth range and it is considered a pioneer species (D'Angelo and Fiorentino, 2012). Despite its common occurrence, few descriptions of the acoustic facies by side scan sonar investigations are available in literature. The aim of this work is to describe the outstanding aspect of the seabed in the area colonized by the C. nodosa meadows by means of acoustic investigations and sedimentological data. The meadows consist of two wide fields, north and south the Bradano River mouth, 4 and 2 km2 wide, respectively, about 1 km far from the coastline, from -5 m to -20 m. The Side Sonar Sonar images show hundreds of dark circular patches, ranging between 10 m and 60 m in diameter, with a high contrasting backscatter against the weak acoustic response of the soft fine-grained sediment of estuarine origin. The spotty trait of the seabed is accounted for by the patches of C. nodosa in proximity of the lower limit. At lower depths, between -5 m and -9 m, a dark belt with a high backscatter is detected indicating that, in this case, the C. nodosa forms the characteristic meadow habitat, with a remarkable density according to the visual inspections (Appolloni et al., 2013). The sediment collected by means of Shipek bucket (Belfiore et al., 1981) show a muddy seabed with the benthic foraminiferal assemblages mainly characterized by the euryhaline species Ammonia beccarii, Aubignyna perlucida, Nonion depressulum, Protelphidium granosum and Quinqueloculina seminulum, which testify the influence of brackish waters. The preservation of C. nodosa meadows in the submerged sectors of the Bradano river mouth is of overriding importance for the protection of the habitat in the SIC area and to mitigate the retreat of the coastline.