SCIENTIFIC PROGRAM

Authors

Raick X., Di Iorio L., Lecchini D., Gervaise C., Hédouin L., Under The Pole Consortium, Pérez-Rosales G., Rouzé H., Bertucci F., Parmentier E.

Abstract

Contrarily to shallow-water coral reefs, mesophotic coral ecosystems remain largely unknown because they are not easily accessible to humans. Biodiversity associated with corals is therefore also little known, and only sparse information is available on fish communities at these depths.

In order to fill this gap, the present work aimed to characterize the diversity of fish communities in mesophotic coral reefs and determine how this acoustic fish diversity varies depending on the depth and the type of island in six Polynesian islands: three atolls (Rangiroa, Raroia and Tikehau) and three high islands (Bora Bora, Mangareva and Moorea). For this purpose, passive acoustics (study of sounds) was used.

The fieldwork was realized by deep divers from UNDER THE POLE. At each island, three different depths (−20, −60 and −120 m) were sampled simultaneously on the external slope of the reef. At each depth, an underwater long-term acoustic recorder was deployed during 72 h to record fish sounds. In addition, for each island and depth, photo-quadrats (30 quadrats per depth) were realized to characterize the benthic cover (e.g., sand, living scleractinian coral, dead coral, gorgones, black coral, etc.).

Results of the present study show that Polynesian underwater soundscapes are rich, from shallow-water coral reefs to deeper reefs. Fish sounds exhibit a stratification driven by depth, with maximum acoustic diversity at -20 meters. Diversity then decreases down to -60 meters, and is equivalent between -60 and -120 meters. The depth of -60 meters appears to be a transition zone since certain acoustic communities of fish from -20 and -120 meters are found there. This zone is explained by a progressive transition from one assemblage of coral species to another one at this depth. Thus, fish sound diversity reflects indirectly different benthic features. In addition to the differences related to depth and associated benthic cover, part of the acoustic variability is explained by the type of island (high islands or atolls).

In conclusion, acoustic monitoring of fish assemblages reveals subtle differences in benthic cover. The present study opens perspectives in the monitoring of mesophotic coral ecosystems using passive acoustics, which is a promising tool to follow temporal changes in the depths.

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Authors

Godefroid M., Dubois P., Under The Pole Consortium, Hédouin L.

Abstract

Les écosystèmes coralliens mésophotiques (30-150m de profondeur) sont des oasis de biodiversité. Les travaux antérieurs du programme DEEPHOPE ont révélé que la diversité des coraux constructeurs de récifs (c’est-à-dire les coraux scléractiniaires) est étonnamment plus élevée dans la zone mésophotique (40-60m) que dans les récifs de surface, et que ces coraux mésophotiques échappent au blanchiment corallien (voir nos publications par Pérez-Rosales, 2021 & 2022).

Reef-building corals co-exist with antipatharians, commonly known as black corals. Antipatharians are particularly abundant at mesophotic depths, where they can form dense aggregations called Marine Animal Forests. These habitats act as spawning, nursery and feeding areas that support diverse animal communities, and create biodiversity hotspots.

Understanding the capacity of mesophotic corals to cope with heat stress is important to predict how climate change will shape the future of the reefs, from shallow to mesophotic areas.

The collaboration between UNDER THE POLE, the Université Libre de Bruxelles, and the French National Centre for Scientific Research (CNRS) allowed to assess for the first time the thermal sensitivity of corals from lower mesophotic zone at 85 m depth (45m at the deepest in previous studies). Coral sensitivity to global warming was compared to the ones of shallower corals at 25m depth.

Sampling of corals (Reef-building corals: Pachyseris speciosa, black corals: Stichopathes sp.) was carried out in Mo’orea (French Polynesia) at 25m and 85 m depth by technical divers from UNDER THE POLE. Corals were installed in 200 L-aquaria to be exposed to different temperature treatments (23°C to 32°C) for 10 days. Thermal performances and metabolic responses of corals were determined to assess their capacity to cope with heat stress.

We revealed that mesophotic reef-building corals have a higher capacity to encompass large range of temperatures compared to their congeners at 25 m depth. It is likely because mesophotic colonies are exposed to more variable seawater temperatures created by the rise of deep cold and nutrientrich waters being uplifted up the reef slope (called internal waves). In Mo’orea, temperature variability increases with depth, with a temperature range of 4.6 °C at 90 m vs 2.9 °C at 20 m.

The present study also shows that black corals have lower thermal window of performance compared to reef-building corals in response to heat stress. It confirms our previous results revealing that black corals have poor thermal acclimatization capacity. Global warming could have significant consequences for this ecosystem-engineer species (see our publication by Godefroid et al., 2022).

En conclusion, la plus grande performance thermique des coraux mésophotiques met en évidence le rôle de la zone mésophotique en tant qu’oasis de biodiversité, résistante à un stress thermique. Les écosystèmes mésophotiques sont probablement plus tolérants au réchauffement climatique que leurs homologues moins profonds et leur prise en considération sera un atout important à l’avenir pour la gestion et la conservation de la biodiversité.

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Authors

Pérez-Rosales G., Hernandez-Agreda A., Bongaerts P., Rouzé H., Pichon M.,  Carlot J., Torda G., Under The Pole Consortium,  Parravicini V, Hédouin L.

Abstract

While mesophotic coral ecosystems have been suggested as biodiversity hotspots, potential short-term thermal refuges and larval sources for damaged shallow reef areas, they have been overlooked in conservation efforts because they are still largely understudied due to logistical and cost challenges. To advance coral reefs’ conservation strategies, it is crucial to identify mesophotic coral reefs showing high coral cover and the environmental factors that influence such coral ‘hotspots’.

For this purpose, UNDER THE POLE and scientists from the DEEPHOPE expedition explored eight islands across French Polynesia over a wide depth range (6 to 120 m). The evaluation of the 2880 photoquadrats revealed that 30% of mesophotic studied sites (20 out of 64 sites) exhibited higher coral cover than predicted (Bayesian models). The highest coral cover at the mesophotic depths was observed in Gambier islands, with > 40% coral cover at 90 m and 120 m versus the average expected values based on the model of less than 15% at 90 m and 10% at 120 m. The most influencing factors for the likelihood of high coral coverage are the light, the type of substrate (hard substrates) and the reef slope (moderate to steep slopes of 30 to 70°).

The present work unveils the presence of unexpectedly and unique high coral cover communities at mesophotic depths, highlighting the importance of expanding the research on mesophotic depths for coral reef conservation.

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Authors

Pérez-Rosales G., Pichon M.,  Rouzé H., Villeger S., Torda G., Bongaerts P., Carlot J., Under The Pole Consortium, Parravicini V, Hédouin L.

Abstract

Mesophotic coral ecosystems might be crucial for the conservation of coral reefs.

This new article reveals that coral diversity is unexpectedly higher in the mesophotic range (40-60m) than in shallow reefs. Indeed, this mid-zone hosts coral assemblages typical of both ranges, the shallow and deep reefs; thus, it could act as a safeguard of biodiversity.

At the same time, while the diversity of shallow reefs is similar among islands, the diversity of mesophotic reefs differs from site to site. These pioneering findings challenge our perception of conservation needs.

We provide new perspectives that if we seek to preserve the coral biodiversity, efficient conservation of coral reefs requires considering mesophotic depths and site-specific measures. This is particularly relevant in the light of ever-increasing human pressures and climate change effects, which seem to be less severe with increasing depth.

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Authors

Godefroid M., Hédouin L., Mercière A., Under The Pole Consortium, Dubois P.

Abstract

An understanding of all components of coral ecosystems is essential to predict effects of global warming. Antipatharians, also called black corals, are components of mesophotic coral ecosystems (MCE). Antipatharians are present in almost all oceans of the world and their abundance reach a peak at mesophotic depths. Antipatharians can play a key role in MCE by forming black coral forests, which act as spawning, nursery and feeding areas that support diverse animal communities, and create biodiversity hotspots.

The collaboration between UNDER THE POLE, the Université Libre de Bruxelles, and the French National Centre for Scientific Research (CNRS) allowed to fill the gap of knowledge on black coral sensibility to global warming. We provided the first assessment on the susceptibility of black coral to thermal stress, by working on the mesophotic antipatharian Stichopathes sp. from Mo’orea (French Polynesia).

Sampling was carried out between 70 and 90 m depth by technical divers from UNDER THE POLE. Corals were installed in 200 L-aquaria to be exposed to four temperature treatments (26°C control temperature, 27.5°C, 29°C and 30.5°C) through a 16-d laboratory exposure. To assess their physiological capability to cope with heat stress, metabolic, biochemical, and cellular responses of corals were determined.

The optimal temperature, i.e. allowing the black coral Stichopathes sp. to efficiently perform its biological functions, was determined around 28.3 °C. The first signs of stress were observed around 29°C, revealing a narrow thermal window of performance for this coral.

These results suggest that black corals have poor thermal acclimatization capacity. A 1°C increase could have significant consequences for this foundation species.

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Authors

Pérez-Rosales G., Rouzé H., Torda G., Bongaerts P., Pichon M., Under The Pole Consortium, Parravicini V., Hédouin L.

Abstract

The mesophotic zone rises as a refuge for corals from accelerating global warming.

Climate change and consequent global warming are causing the disappearance of reefbuilding corals worldwide because of coral bleaching. While bleaching episodes significantly impact shallow waters, little is known about their impact on mesophotic coral communities.

During the heatwave that affected Polynesian reefs in 2019, scientists from the “DEEPHOPE” expedition monitored with UNDER THE POLE the bleaching effects along an extreme depth range (6 to 90 m), a worldwide premiere. This pioneering survey reveals that the incidence of coral bleaching decreases with depth, with no bleaching observed at lower mesophotic depths (>= 60m).

The mesophotic zone (between 30 – 150 m) rises as a potential refuge and may play a role in providing extra time for corals from escalating global warming compared to the surface.

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Authors

Rouzé H., Galand P.E., Medina M., Bongaerts P., Pichon M., Pérez-Rosales G., Torda G., Moya A., Under The Pole Consortium, Raina J.B., Hédouin L.

Abstract

A new vision of coral reefs

Coral reefs are one of the most productive and diverse ecosystems on Earth. Hard coral, which builds reefs, relies mainly on a symbiotic relationship with microalgae (zooxanthellae) to power, among other things, the energetic needs of calcification. During the symbiosis, the coral provides limited inorganic nutrients, while algae share essential organic compounds (e.g., sugars) derived from their photosynthetic activity. Thus, the symbiosis between reefbuilding corals (photosynthetic scleractinian corals) and microalgae underpins the health and productivity of tropical coral reef ecosystems.

Although coral ecosystems extend into the depths, this symbiotic relationship has mainly been studied in shallow waters (<30 m) because of technical limitations imposed by traditional scientific scuba diving. We do not know how deeper corals, inhabiting large and vastly underexplored mesophotic coral ecosystems, modulate their symbiotic associations to grow in environments that receive less than 1% of surface light.

To better understand how reef-building corals can survive so far away from their presumed light optimum, the DEEPHOPE program brough together the scientific expertise of an international consortium and the diving exploration expertise of UNDER THE POLE. This program made it possible, for the first time, to study of scleractinian corals on a wide depth gradient (6 to 172 m) and geographic scale (5 archipelagos of French Polynesia) in order to push the limits of our knowledge of corals.

This study allowed the discovery of the deepest photosynthetic hard coral at 172 m depth. The analyzes of symbiotic communities show that the coral, identified as Leptoseris hawaiiensis, survives at such depth by hosting Symbiodiniaceae, predominantly of the genus Cladocopium, but also a single species of endolithic algae from the genus Ostreobium. These algae in the coral skeleton are adapted to photosynthesize in near-darkness using shorter wavelengths compared to other zooxanthellae and optimize light capture. Morphologically, L. hawaiiensis is characterized by a thin flat skeleton, which is optimal for light harvesting and reducing skeletal carbonate deposition. Thus, this coral species uses different strategies to acclimate and survive in extreme low light conditions.

The present results report a new depth record for photosynthetic scleractinian corals at 172 m and provide a new vision of coral reefs. These ecosystems have been extensively studied in shallow waters, which represents less than one-fifth of the total depth range of the tropical coral reefs. Depth is a dimension that can no longer be ignored in future studies to understand coral ecosystems. A better knowledge of coral ecosystem functioning is necessary for the implementation of effective management and preservation measures of these threatened ecosystems.

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