Understanding the importance of Mesophotic Coral Ecosystems (MCEs)

Mesophotic Coral Ecosystems (MCEs)

What inspired your focus on Mesophotic Coral Ecosystems (MCEs), in particular, as opposed to shallow or deep sea reef systems?

• Shallow coral reefs have been the focus of research for centuries due to their accessibility, ecological importance, and economic value, as well as their high vulnerability to anthropogenic pressures.
• Deep-sea ecosystems, while more challenging to study, have still received significant scientific attention over time, first through bottom trawling and, more recently, with advanced deep-sea technologies.
• MCEs, however, represent a comparatively underexplored zone between these two extremes. They occur at depths where sufficient light supports photosynthesis and complex ecological communities, yet they remain difficult to access with traditional diving methods. Research on MCEs only began in earnest around two decades ago, and the limited baseline knowledge about these systems provides an important opportunity to address fundamental ecological questions. That sense of mystery and opportunity for exploration is what drew me into this field of study.

What are some of the most surprising factors you’ve discovered about life in MCEs?

• The vertical complexity of these habitats is remarkable. They exhibit extensive three-dimensional structures dominated by large gorgonians and antipatharians, while maintaining a reef-like framework close to the seabed.
• The occurrence of photosynthetic scleractinian corals at depths exceeding 100 meters is particularly striking, as is the observation that these deep corals can also undergo bleaching.
• Additionally, the presence of large predators and shallow-reef-associated fish species at mesophotic depths highlights the ecological connectivity between shallow and deeper reef systems.

EAF-Nansen Tanzania Expedition

What has been the most significant finding from your time on the EAF-Nansen Tanzania expedition?

• Tanzanian MCEs exhibited both similarities and differences to those of Chagos.
• Benthic communities were diverse, with fewer large gorgonians but a notable presence of massive barrel sponges.
• Photosynthetic scleractinians were relatively scarce, although one colony was documented at a depth of 108 meters — among the deepest recorded in the region.
• Fish diversity was high, but abundance appeared lower than in Chagos, and no sharks were observed within the MCE depth range, only in deeper habitats.

How might the findings from this expedition contribute to our understanding of MCEs?

• The benthic assemblages displayed high spatial variability, likely influenced by differences in substrate type, geomorphology, and hydrodynamic conditions among sites.
• This expedition provided the first systematic exploration of Tanzanian MCEs, revealing substantial biodiversity beyond the shallow reef zone.
• Both ROV footage and collected specimens indicate the potential discovery of new species, which will contribute to refining biodiversity baselines and improving understanding of Indian Ocean mesophotic ecosystems.
• These data will directly contribute to MCEs conservation, notably by the implementation of MPAs in the surveyed zones.

Chagos Archipelago

Your work largely takes place in the Chagos Archipelago, the island group in the Indian Ocean – why is this a significant area of interest?

• The Chagos Archipelago is one of the most remote and least disturbed coral reef regions globally. Apart from the restricted military presence on Diego Garcia, the archipelago is uninhabited and designated as a no-take marine protected area.
• Its isolation and protection status make it a valuable reference site (or a “natural laboratory”) where ecosystem changes can be attributed primarily to natural variability and global environmental change, rather than direct local anthropogenic impacts.

What key findings can you share from your work in the Chagos Archipelago?

• The MCEs of Chagos support a high level of benthic diversity, with species richness in some cases comparable to that of shallow subtropical reefs.
• Their community composition differs significantly from that of shallower reefs, indicating that MCEs are distinct ecosystems rather than subsets of shallow reef assemblages.
• These habitats appear to be widely distributed across the archipelago, predicted to occur on all atolls, islands, and submerged banks.
• Despite their depth, MCEs are not insulated from climate-driven stress. During field surveys, we recorded coral bleaching between 60–90 meters depth, even when no bleaching was observed in the shallows. Subsequent visits indicated partial recovery, suggesting some degree of resilience within these deeper coral communities.