Methodological approach for hydroacoustic seabed characterization in Serrana Bank (Seaflower Biosphere Reserve)

Hydroacoustic methods provide rapid and extensive access to seabed characteristics, which is not possible using other underwater technologies. Currently, hydroacoustics are widely used to determine seabed topography and sediment type, but significant developments have not occurred for detecting and classifying structures above the seabed, such as coral reefs and marine meadows, which are very important ecosystems. In the present study, we propose a new methodological approach to characterize the seabed at Serrana Bank (14.27 degrees-14.48 degrees N and 80.10 degrees-80.23 degrees W), located in the Colombian Basin of the Caribbean Sea, by combining acoustic information (split-beam echosounder) and underwater imagery. In all, 49 acoustic transects were created using a BioSonics DT-X scientific echosounder (with a 38-kHz split-beam transducer and a FURUNO differential GPS) during the Seaflower Scientific Expedition HI (Colombian Ocean Commission, Colciencias) from August 12-15, 2016. Using cluster analysis with the Spearman correlation distance, 14 different Typical Acoustic Reflectance Profiles (TARPs) were identified at a distance of 0.80. These TARPs result from the natural acoustic signals produced by the sediment types and ecological structures present on the seabed. When comparing these TARPs with the photographic records and previous ecological maps, various ecological structures above the seabed were acoustically identified, such as coral debris, octocorals, macroalgae meadows, and several associations among coral and different sizes of macroalgae. The potential of this approach to generate a low-cost, automated method for mapping seabed ecosystems and to enable feasible assessments in marine protected areas is discussed.