Climate change and other disturbances are causing the loss of scleractinian coral reefs. Underneath shallow reefs and despite light decreases with depth, Mesophotic Coral Ecosystems (MCEs) are characterised by photosynthetic corals, and associated reef communities, at depths between 30 and 150 m. MCEs interest lies in increasing their knowledge and potential role as shallow reefs refugia, yet most MCEs remain unexplored, with locations like French Polynesia understudied. This thesis is part of the DEEPHOPE project (a collaboration between scientists and Under The Pole) to shed light on the MCEs in French Polynesia with ~800 Trimix Rebreather dives below 100 m depth. The expedition allowed this thesis to test (1) the diversity patterns and coral cover with depth; (2) the acclimation and adaptation of scleractinian corals to depth; and (3) whether mesophotic depths are impacted by thermal coral bleaching. Chapter 1 introduces coral reefs and MCEs worldwide and the context for French Polynesia to set the departing hypotheses. It also explains the methodology to collect the ecological, physiological, environmental and taxonomical data. Chapter 2 shows that MCEs are hotspots of scleractinian generic diversity in French Polynesia despite coral cover decreasing with depth. Hotspots of diversity because the genus richness follows a mid-domain effect with a peak of (alpha) α-diversity between 20 and 60 m and because spatial (beta) β-diversity increases with depth, meaning that while shallow depths are similar across locations, mesophotic depths are different and unique. Chapter 3 shows that although coral cover decreases with depth generally, mesophotic depths are also home to unexpectedly high (by models) coral cover communities for the depth. These were in some cases more highly coral-covered than shallow reefs and associated with moderate to steep slopes and low sedimentation. It also shows the foundations for predictive environmental modelling of coral cover and community structure (with coral morphologies) in mesophotic depths.Chapter 4 shows the strategies of two scleractinian species to cope with low-light environments increasing chlorophyll concentration, symbiont density (with no specific shifts in Symbiodiniaceae composition), and modifying morphological skeleton to keep up with photosynthesis. Results suggest no nutritional plasticity (switch to heterotrophy) even if both species reduced the δ13C with depth.Chapter 5 demonstrates that mesophotic coral communities escaped a thermal coral bleaching event in French Polynesia, with subsequent mortality decreasing from 35% at 6 m to less than 5% at 20 m to non-existent after 40 m. This bleaching reduction was observed among all genera from their upper to their lower depth distribution range, suggesting that MCEs might act as short-term refugia from bleaching events. Chapter 6 also considers some results of DEEPHOPE to discuss the depth zonation and community structure, the spatial differences across islands and with other worldwide locations, and some “terms” used in research, considering quantitative abundances. The importance of MCEs with implications in conservation, the limitations, and future directions to improve MCEs’ research. Overall, this thesis increases the knowledge of MCEs in French Polynesia from a quantitative perspective, suggesting the potential of this region for MCEs and setting the basis for future studies. Also, to support MCEs’ importance because, in some way or the other, it shows evidence that mesophotic depths are key depths for conservation, with highly diverse and covered communities less affected by thermal bleaching. With the uncertain future of coral reefs with climate change, MCEs could play an ecological role in the ecosystem-wide coral reef resilience. Although our outcomes are at the scale of French Polynesia, similar quantitative studies are necessary for other locations to substantiate the global character of the findings of this thesis.

Depth range
6- 120 m

Mesophotic “mentions”
357 x (total of 48223 words)

Classification
* Presents original data
* Focused on 'mesophotic' depth range
* Focused on 'mesophotic coral ecosystem'

Fields
Biodiversity
Community structure
Ecology
Physiology
Molecular ecology

Focusgroups
Scleractinia (Hard Corals)
Symbiodinium (zooxanthellae)
Overall benthic (groups)

Locations
French Polynesia

Platforms
Rebreather