Aim: Research on Mesophotic Coral Ecosystems (MCEs) has increased exponentially in recent decades, and the significance of this ecosystem has been recognised both in terms of biodiversity and distribution. However, this research has mostly focussed on corals and is globally sporadic, with the Indian Ocean remaining largely unexplored and overall MCEs under protected. Hence, baseline data on MCE benthic communities is lacking, but nonetheless essential for developing adequate management strategies. Here, we assess the variation in diversity and community structure of MCEs along the depth gradient and among sites in the Indian Ocean and the environmental parameters that are potentially driving these differences. Location: Egmont Atoll, Chagos Archipelago, central Indian Ocean. Time period: Present. Major taxa studied: Marine benthic invertebrates, plants. Methods: Using a Remotely Operated Vehicle, we collected video transects between 15 and 160 m. We analysed the diversity and composition of benthic communities and used DistLM analysis to determine the environmental drivers of the community structure over the depth gradient and between sites. Results: We observed distinct benthic communities along the depth gradient, with a strong community break observed at ~60 m. We also identified a clear zonation of the benthic taxa with depth. This zonation was primarily driven by downward irradiance and temperature, with additional environmental processes playing a minor role in structuring the benthic communities. Main conclusions: We show that MCEs in the Chagos Archipelago are distinct communities, and their distribution is driven primarily by irradiance and temperature. Our results highlight the variability in benthic community structure of MCEs, both along the depth gradient and at local geographical scales for the study region. This study showcases the uniqueness of MCEs and will aid in predicting their distribution, potential refugia for shallow reefs, and in developing evidence-based protection for MCEs, to maintain overall marine biodiversity.

Title: Light and temperature drive the distribution of mesophotic benthic communities in the Central Indian Ocean Short title: Drivers of mesophotic reef distribution Author list: Clara Diaz1†; Kerry L. Howell1; Edward Robinson1; Philip Hosegood1; Adam Bolton1; Peter Ganderton1; Peter Arber1; Martin J. Attrill 1; Nicola L. Foster1 1 School of Biological and Marine Sciences, University of Plymouth, Drake Circus, Plymouth, PL4 8AA, United Kingdom. † Corresponding author: Clara Diaz, [clara.diaz@plymouth.ac.uk](mailto:clara.diaz@plymouth.ac.uk), 6th floor Davy building, School of Biological and Marine Sciences, University of Plymouth, Drake Circus, Plymouth, PL4 8AA, United Kingdom Raw data files are provided at this DOI: doi:10.5061/dryad.pzgmsbcs8 and is composed of 2 excel documents: Final\_meso\_data\_with\_metada is the annotated morphospecies abundance per image. The second sheet called "metadata" provide the abbreviations meaning and general information about the morphospecies abundance data. The collection and annotation methods can be found in the main manuscript under the methods section. Final\_env\_data\_with\_metadata is the document containing the environmental variables associated with each image. The second sheet called "metadata" provide the units; abbreviations meaning; and general information about the environmental data. The collection methods can be found in the main manuscript under the methods section. A benthic species catalogue is currently published: Diaz C, Foster NL, Howell KL. Shallow and mesophotic benthos reference image database for the Chagos Archipelago, central Indian Ocean. Zenodo. 2023. DOI:10.5281/zenodo.7948517

Data collection methods can be found in the main manuscript, under the methods section.