Abstract
Abstract Background Mesophotic coral ecosystems (MCEs) are tropical and sub-tropical reefs between 30 m and potentially >150 m depth, the maximum for photosynthetic hard corals. The definition’s upper boundary is ecologically arbitrary. Recently, research has focused on the deep reef refugia hypothesis suggesting MCEs can be protected from shallow-water threats, potentially acting as a local source for re-colonisation of shallow reefs. This led to recent calls to increase their protection. It remains unclear whether the current MCE definition reflects changing biodiversity with depth, and so whether protecting MCEs based on this definition will protect shallow reef species. We ask where shifts in ecological community structure occur across the shallow-mesophotic depth gradient. We consider to what extent MCEs as currently defined protect shallow reef taxa. Research on coral reef depth gradients has a long history. Research relevant to MCEs has been published under a variety of terms. We will use the systematic review framework to collect older data sources, increasing accessibility by depositing the meta-data in an online library for researchers and managers. Methods A systematic review will be conducted, searching online databases, grey literature and personal libraries of experts. The primary question was formulated after consulting an advisory committee. Inclusion criteria discriminate among studies by sampling depths and community data. Critical appraisal of studies will consider key criteria concerning internal validity. We shall identify where more biodiversity and community-level data are required, determined by whether a meta-analysis is possible. Considering how to structure a meta-analysis once community metric and variability data have been collected will help to advise future data collection. Provided enough data are extracted, we shall conduct a meta-analysis examining changes in species richness, abundance and biomass across the depth gradient. If ecological community level data are present, we shall conduct an additional meta-analysis looking at community turnover with depth.
Highlights
Mesophotic coral ecosystems (MCEs) are tropical and sub-tropical reefs between 30 m and poten‐ tially >150 m depth, the maximum for photosynthetic hard corals
Mesophotic coral ecosystems (MCEs) are currently defined as reefs ‘characterised by the presence of lightdependent corals and associated communities typically found at depths ranging from 30–40 m and extending to over 150 m in some tropical and subtropical regions
When a MCE survives the damage occurring on an adjacent shallow reef, recruits from depth could settle in the shallows and support recovery [9]
Summary
Searches Searches shall be limited to studies providing an English title and abstract. Any literature detected as relevant to MCEs shall be recorded and reported to http://www.mesophotic.org, regardless of whether it is included in the final dataset concerned with ecology This prevents the loss of physiology and genetic studies from the search and their addition to the online resource. The bibliography of retained papers will be downloaded and reviewed using the study inclusion criteria; papers retrieved in this way shall not be subject to further citation chasing to minimise the influence of knowledge bias, the tendency of authors to cite the same paper pool repeatedly. This increases the chance our searches will bring back relevant data.
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