The use of very high resolution images for studying Posidonia oceanica reefs

Posidonia oceanica is an endemic Mediterranean seagrass that forms wide and dense meadows from the surface up to about 40 m depth. This species can develop a biogenic structure called matte, a typical terraced formation built up by itself, consisting of intertwined rhizomes, roots and sediment, which may allow shoots to reach the sea surface forming reefs (récif barrière), considered natural monuments. Posidonia oceanica reefs are particularly exposed to the ongoing increase in temperature and in storm frequency and intensity due to climate change, with negative repercussions on their conservation. Much more attention must be paid to the implementation of monitoring tools able to detect early signs of reef regression. In this study, the distribution of P. oceanica reefs located along southern coasts of Sicily (Italy, Mediterranean Sea) was investigated. A remote sensing approach was used to assess reef extension, morphological features (e.g., atolls), upper limit and occurrence of dead matte. In particular, very high-resolution drone images (~ 2 cm pixel size), combined with GPS field data, were used for 2D- and 3D-reconstruction of P. oceanica reefs. The 3D-model allowed to estimate bathymetrical distribution of P. oceanica reef enabling to achieve a more detailed and complete understanding of the P. oceanica reef architecture