Or M. Bialik , Omri Gadol, Aaron Micallef, Christian Betzler , Hagai Nativ, Shai Einbinder, Yizhaq Makovsky

Mesophotic reefs are located in low light conditions which, depending on the region, found in water depths greater than ~30 m. They are less affected by ocean warming than reefs found in shallower water depth and thus might become increasingly important for the sustainability of marine biodiversity. Here we explore the physical mechanisms controlling the sediment distribution around a coralligenous mesophotic reef. A detailed survey of the Bustan HaGalil Ridge offshore Israel (30 to 50 m water depth) was carried out using an autonomous underwater vehicle with an interferometric synthetic aperture sonar, a multibeam echo sounder, and a sub-bottom-profiler. The data were combined with surface sediment samples, underwater photography, oceanographic data (measured and modeled current velocity) and water column seismic reflection data. The mesophotic reefs are built as a series of asymmetric ridges (higher slope on the northeast-facing flanks) with coarse-grained biogenic sand to gravel within the valleys between the ridges. Two types of sedimentary bedforms are identified on the sediment surface - north-south elongated linear bedforms; and intersecting smaller bedforms, forming chevron shapes. Additionally, at the western margin of the ridge, a moat-drift contourite system is identified. The oceanographic data and modeling indicate that the northward flowing regional along slope current is strong enough to transport sediment. This explains the contourites, the ridge asymmetry and the increased accumulation of sediment at the southwest-facing slope of the ridges. The elongated linear bedforms are interpreted to be ripples attributed to internal waves, which are imaged in the water column off the shelf at the same depth as the bedforms. The chevron bedforms are suggested to be created by the interference of the bottom current with internal waves. In conclusion, the geometries of the mesophotic reefs, and the distribution of the sediment around them are controlled by internal waves and bottom currents.