Underwater Scuba Photogrammetry VS. MBES Acoustic Sounding: how to integrate multiscale data for a better understanding of Coralligenous outcrops

Abstract

<p>Coralligenous (C) includes calcareous build-ups of biogenic origin, characterised by the association of calcareous algae and several invertebrates. This habitat is one of the most important at the Mediterranean scale; it is a hot-spot of biodiversity thriving from shallow waters down to the limit of the mesophotic zone. The Italian project “CRESCIBLUREEF - Grown in the blue: new technologies for the knowledge and conservation of the Mediterranean reefs” aims at studying  peculiar C outcrops found along a depth gradient offshore Marzamemi village (SE Sicily). </p> <p>During the first project expedition (June 2021), we performed a Multibeam Echosounder (MBES) survey of the target area by using a R2Sonic 2022 system. A new 17 km2 high-resolution morpho-bathymetric map was realised, which interpretation led to the identification and classification of five major habitats, including different C morphotypes. C habitat in the form of banks was found mainly distributed between 30 and 35 m of water depth (w.d.). This investigation allowed us to observe and quantify the overall C distribution along a depth gradient spanning between 20 and 100 m of w.d., giving us a broad-scale perspective of its extension at the seafloor. </p> <p>A third marine survey (September 2021) was focused on collecting video and still images by using a Sony α Alpha 7ii reflex coupled with the Easydive Leo3 Wi housing and the Easydive illumination system Smart Sea - Gold Plus 7000 Lumen, through scuba diving. Data collection was performed over selected areas suitable for the application of underwater photogrammetry, taking into account the presence of C build-ups  (as confirmed by the interpretation of the MBES dataset) and the operational depth (i.e.: no more than 35 m of w.d.). Data collected by adopting this technique and processed using Structure-from-Motion (SfM) algorithms allowed us to get more information at the community level of such complex habitats, coupling the seafloor scale with the smaller scale obtained by direct observations. </p> <p>In this work, our intention is to improve the understanding of the geospatial context of Coralligenous distribution and extent from a multiscale perspective. Specifically, we want to show how eco-geomorphological indexes calculated using the high-resolution outputs of the C photogrammetry (3D meshes, DTMs, and orthomosaics) may be used to perform resolute investigations of such habitat on a broader scale, by considering their spatial distribution extrapolated from the MBES data.</p>