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

Mesophotic reefs are found in water depths greater than 30m and as such are, for the most part, removed from most direct wave activity, notably in the Mediterranean Sea. There, they are protected from anthropogenic impact and serve as a possible refuge for marine biodiversity in a warming ocean. Yet, like any reef, their growth is modulated by the distribution of the sediment they produce and supply of nutrients. Here we explore the mechanisms governing the sediment distribution around a coralligenous mesophotic reef system as a case study of the long-term governing mechanisms in this environment. A detailed survey of a mesophotic reef offshore Israel was carried out using acoustic imaging from an autonomous underwater vehicle. This survey generated a highly detailed (cm-scale) resolution dataset with wide area coverage of the sediment distribution patterns around the reef. The mesophotic reefs are built as a series of asymmetric ridges with biogenic sediment filling the depressions between them. Two types of sediment bedforms were identified in the sediment fill - elongated linear bedforms aligned north-south; and intersecting smaller bedforms aligned NW-SE and NE-SW, forming chevron-like shapes. Both types of bedforms are composed of coarse-grained material (sand to gravel) and their distribution is partially related to their location relative to the hard substrate. The bedform present and the ridge asymmetry points to a prominent role of a south-to-north bottom current. These current supplies nutrients to the reefs, but also loads sediment on their southern side, leading to asymmetry. Additionally, the chevron bedforms are attributed to interactions with the rocky substrate and internal waves. These internal waves likely also play a role in modulating the reefs’ growth. The geometries and growth of mesophotic reefs, and the distribution of the sediment around them, are controlled by the combination of both internal waves and bottom currents.