Evaluating the possible role of bottom currents and internal waves in shaping seafloor morphology in a mesophotic reef

Mesophotic reefs are located in low light conditions which, depending on the region, are usually found in water depths greater than ~30 m. They are less affected by ocean warming than reefs found in shallower water depths 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 modelled current velocity) and water column seismic reflection data. The mesophotic reefs are built as a series of asymmetric ridges (higher slope on the north-east-facing flanks) with coarse-grained biogenic sand to gravel within the valleys between the ridges. Accumulation of sediment appears to be increased at the south-west-facing slope of the ridges compared to the north-east-facing slope. Two types of sedimentary bedforms are identified on the sediment surface: north–south elongated linear bedforms (classified as dunes) and intersecting smaller bedforms forming chevron shapes. At the western margin of the ridge, a moat-drift contourite system is identified. The oceanographic data and modelling results show a northward-flowing along-slope current at the sea surface. The contourites and ridge asymmetry indicate a northward-flowing bottom current. The observed dunes and chevron bedforms are attributed to the effects of bottom currents, internal waves and their interactions. In conclusion, the geometries of the mesophotic reefs and the distribution of the sediment around them are interpreted to be controlled by internal waves and bottom currents.