Trait-based approaches reveal that deep reef ecosystems in the Western Indian Ocean are functionally distinct

Tropical deep reefs (>30 m) are biologically and ecologically unique ecosystems with a higher geographic reach to shal-low (<30 m) reefs. Yet they are poorly understood and rarely considered in conservation practices. Here, we characterise benthic and fish communities across a depth gradient (10-350 m) in remote coral atolls in Seychelles, Western Indian Ocean. Using taxonomic and trait-based approaches we present the taxonomic and functional composition of shallow and deep reef communities, with distinct communities and traits dominating different depths. Depth-related changes in community metrics (taxa richness, abundance and biomass) and functional diversity metrics (richness, dispersion, and evenness) indicate complex relationships across different biological components (fish, benthos) that differ between shallow and deep reefs. These in turn translate into different patterns of reef resilience against disturbance or species in-vasions with depth. Notably, deep reefs host on average fewer and less abundant taxa but with higher functional contri-bution and originality scores, some of which are of conservation concern. Overall, the results highlight the unique nature of deep reefs that requires their explicit consideration in conservation and management activities.

Automated summary

Tropical deep reefs (>30 m) are unique ecosystems with a wider geographic reach compared to shallow reefs, and they require explicit consideration in conservation and management activities. This study characterizes the benthic and fish communities in shallow and deep reefs in Seychelles, Western Indian Ocean, revealing distinct communities and traits dominating different depths. The results highlight the complexity of relationships between different biological components and the differences in reef resilience against disturbance or species invasions with depth.