Vulnerability and resilience of mesophotic coral ecosystems to climate change

Globally, coral reefs generate substantial social and economic benefits. Based on well-researched shallow-water coral reefs, it is widely accepted that shallow reefs are under imminent threat, with 99% expected to be lost within 50 years under a 1.5-2°C rise in global temperature. Mesophotic coral ecosystems (MCEs) are less well-researched and our understanding of these ecosystems is limited. MCEs are light-dependent ecosystems that reside between 30-150m and provide many of the same social and economic benefits (e.g. ecosystem services of food provision, climate regulation) as shallow-water reefs. At these deeper depths, temperature stratification bathes MCEs in cooler temperatures than at the surface, providing a possible spatial refuge for shallow-water coral species against rising sea surface temperatures. MCEs may also offer a compensatory role in supporting biodiversity, ecosystem function and ecosystem service provision following the decline of shallow-water reefs. However, there are fundamental gaps in our knowledge and understanding of MCEs in terms of their diversity, distribution and vulnerability to the impacts of warming temperatures, stemming from the logistical and financial challenges of studying ecosystems at depth. Our recent research in the central Indian Ocean (IO) demonstrates that the previously-held assumptions of mesophotic corals being immune to elevated temperatures were ill-founded. We observed extensive coral bleaching at depths of 60-90m during the 2019 Indian Ocean Dipole (IOD) event. This bleaching was due to the large-scale deepening of the thermocline throughout the central and western IO. Thus, MCEs may be as vulnerable to climate change as shallow-water reefs, and the role that these ecosystems will play in the future functioning and biodiversity of coral reefs globally, as shallow-water reefs decline, is uncertain. This project will provide a step change in our understanding of the vulnerability and resilience of MCEs in the IO and how these ecosystems respond to changes in temperature that are predicted to become more widespread and severe with climate change. Our objectives are: O1) Describe the distribution, diversity and health of mesophotic coral ecosystems over the depth gradient (30-150m). O2) Determine the relative impact of hydrodynamic processes and associated biogeochemical changes in modulating the environment at mesophotic depths, and how this impacts MCE diversity and health. O3) Quantify the horizontal and vertical connectivity of mesophotic coral ecosystems on a local and regional scale. O4) Characterise the reproductive strategies and life history characteristics of keystone coral species from mesophotic depths. O5) Assess the thermal performance of early life history and adult stages of keystone mesophotic coral species. O6) Determine the physiological response and acclimation potential of early life history stages of keystone mesophotic corals reared at shallow-water depths, and vice versa. O7) Build and strengthen our existing network of researchers and managers across the IO region to develop a standardised approach (methodology and equipment) to monitor long-term changes on MCEs and their response to climate change, and to inform policy and management. The improved knowledge gained here will constitute a cornerstone for future efforts in assessing the impact of deterioration in coral reefs globally, providing a standardised methodology for long-term monitoring of MCEs, an open-access, global repository for data on MCE health, and an in situ bleaching monitoring system. With this new information it will be possible to integrate MCEs more broadly into marine policy and management, as important functional ecosystems that require protection, to improve resilience to climate impacts.