Discriminating between Posidonia oceanica meadows and sand substratum using multibeam sonar

High-resolution, multibeam sonar (MBS) (455 kHz) was used to identify two typologies of seabed 8 m deep: Posidonia oceanica meadow and sandy substratum. The results showed that the heterogeneity of the architecture of the P. oceanica canopy and the relatively simple morphology of a sandy substratum can be detected easily by statistical indices such as standard deviation or range-of-beam depth. Based on these indices, an automated classification was performed for seabed mapping. The overall classification accuracy was as high as 99 and 98% in October and January, respectively. The probability that P. oceanica in situ was omitted on the map was <7%, whereas the probability that an area classified as P. oceanica on the map did not correspond to the seagrass in situ was consistently negligible. Based on these results, high-resolution MBS can be considered to be an accurate tool for mapping P. oceanica and sand substrata, and its discriminating power seems to be independent of season (autumn or winter).