1 USE OF A HIGH-RESOLUTION PROFILING SONAR FOR SEAGRASS DETECTION

Abstract: Seagrasses are flowering plants that develop extensive underwater meadows and play a key role in the coastal ecosystem. In the last few years, several techniques have been developed to map and monitor seagrass beds in order to protect them. We present here the results of a survey using a profiling sonar- the Sediment Imager Sonar (SIS)- and a towed video sledge to study a Zostera marina bed off Calshot, southern UK. The survey aimed to test the instruments for seagrass detection and describe the area for the first time. Data processing from the SIS and video data will be presented along with correlation of the 2 instruments. From the SIS data, the bed elevation was recorded as the strongest backscatter along a beam. The presence of seagrass was indicated as a high backscatter intensity above the seabed. For each sweep, four parameters were calculated from the SIS data: depth of the seabed, Seagrass Index (used to assess seagrass presence), canopy height and vegetation patchiness. From the video, Zostera density was estimated together with macroalgae abundance and bottom type. The results of an algorithm developed to detect seagrass from the SIS data were tested against video data. Patchiness calculated from the SIS data was strongly correlated to seagrass density evaluated from the video, indicating that the parameter could be used for seagrass detection. Canopy height evaluated from the acoustic data was consistent with field observations. The SIS therefore proved to be a useful instrument for surveying seagrass beds although it needs to be tested over other seagrass types