Beyond the "Deep Reef Refuge" Hypothesis: A Conceptual Framework to Characterize Persistence at Depth

The rapid deterioration of coral reefs worldwide has led to a growing interest in identifying areas that can offer protection against adverse conditions including coral reef communities at intermediate (similar to 15 to 30 m) and mesophotic (>= 30 m) depths. However, various concepts regarding the protective potential of deeper coral reef communities, and subsequent roles in overall reef resilience and persistence, remain poorly defined. Herein, we organize these ideas into an initial conceptual framework and review for scleractinian corals how these ideas may be supported by the limited empirical data that is currently available. We distinguish between the concepts of "depth refuges," "depth refugia," and "depth resilience areas," based on the nature (i.e., avoidance versus resilience) and temporal scope of protection. Although past examples have confirmed the role of mesophotic coral ecosystems as short-term, ecological "refuges," there is thus far little support that they comprise long-term "refugia." In contrast, the concept of "deep resilience areas," reef communities that persist long-term through disturbance by resistance and recovery, remains largely unexplored. In terms of the functional roles of such protected areas in the overall coral reef ecosystem, we distinguish between the concepts of "reseeding" and "local persistence." The potential to actively reseed shallow reefs may be ecologically important, but only for a small proportion of shared biodiversity, whereas the potential to promote persistence of local biodiversity may apply across a broad range of coral reef species. Although empirical evidence remains very limited, we hope that the incipient conceptual delineations presented here provide a constructive reference for further discussion and research into the ecological importance of deep reef communities