The mesophotic zone of the Mediterranean Sea: spatial definition, biodiversity, and predictive models

Giorgio Castellan
2022
Below the shallow waters up to where the sunlight turns out is where the mesophotic zone extends. Although long since recognized, the mesophotic zone has been largely ignored by the scientific community in favor of the shallower and deeper counterparts. The mesophotic zone is mostly defined as ranging between 30 and 150 m depth. However, these borders are necessarily imprecise due to variations in the penetration of light along the water column related to local factors, mainly solar radiation
more » ... cidence and water clarity. The Mediterranean Sea, for instance, represents a case in point, being characterized by complex oceanographic dynamics, strong climatic (e.g., rainfalls, sunlight), oceanographic (e.g., water temperature and salinity) and bio-geochemical (e.g., nutrients) gradients, which generate an alternation of temperate-or tropical-like situations within a limited spatial scale (about 4,000 km from the Strait of Gibraltar to the Gulf of Iskenderun, southeastern coast of Turkey). If we were able to integrate the light regime into the definition of the mesophotic zone, we could provide a robust estimation of its bathymetric range and spatial extent not only for the Mediterranean Sea but for any geographical location. A better understanding of the mesophotic domain of the Mediterranean Sea, starting from its spatial and bathymetric definition, is, moreover, crucial to orient future research and explorations, and support proper management plans and conservation actions. From late 90s, the advances in underwater technologies along with the growth in computing performances permitted the direct observation of benthic ecosystems populating intermediate to deep situations. Visual surveys performed with manned and unmanned vehicles represent plastic The most conservative time-based method (with intervals of 10 s) is, then, used to analyze a set of 25 visual surveys collected at mesophotic depth in the Mediterranean Sea with the aim of characterizing their taxonomic and functional diversity. More than 290 taxa are identified, occurring in 5 categories of assemblages presenting different levels of both taxonomic and functional diversity. The role of environmental variables and geographic location in influencing the presence and the composition of the surveyed mesophotic assemblages is also explored, highlighting that the environmental setting might be the primary factor controlling the distribution and the structure of the assemblages at mesophotic depths. A predictive model is, successively, implemented and used to predict the distribution of areas suitable for the presence of deep-water oyster reefs assemblages, which show high levels of taxonomic and functional diversity but are still largely unexplored in the Mediterranean Sea. Finally, the Gulf of Mexico is used as case study to test if the physical approach to map the mesophotic zone could be applied to different geographical location and whether the environmental setting influences the composition of mesophotic assemblages also beyond the Mediterranean borders. The results suggests that the proposed light-based method can be successfully applied also to estimate the bathymetric and spatial extent of the mesophotic zone in extra-Mediterranean regions and that environmental variables might play a central role in shaping mesophotic assemblages structure on a global scale. coral reef built by non-symbiotic scleractinians. Scientific Reports, 9(1), 3601.
doi:10.48676/unibo/amsdottorato/10134 fatcat:lp2ona6wnrggthrlmfdbhyemmy